Chapter IV: Isomers of (CH)sX (X = N, P, As and SiH) and C4Si2H6

Chapter IV: Isomers of (CH)*X (X = N, P, As and SiH) and C4Sil&

4.0: Abstract

Ah mlho HF, MP2 and CCSD(T) and hybnd denslty functional theory (B3LYP)

calculat~ons were performed on the valence Isomers of benzene, group V heterobenzenes

and s~labenzene A benchmark study on the benzene valence lsomers was done at varlous

levels of theory In each of the Group V heterobenzene potentla1 energy surface, ten

mlnlmum energy structures were located The broad relatlve energy orderlng of the

valence lsomers of benzene and the group V heterobenzenes rematn the same Flfteen

valence lsomers were Identified on the sllabenzene potentla1 energy surface, out of whlch

twelve were found to be minima, two were transltlon states and the other was found Lo be

a second order saddle polnt A stark contrast 1s notlced In slla- and d~s~lahen/ene valence

Isomers compared to those of group V heterobenzenes, several mlnlma on the potential

energy surface are observed whlch are wlth~n 30 kcallmol Two new valence lsomcrs

were located, namely Vla, and Vlb,, they were found to 11e only about 20 kcallmol

hlgher than the global mlnlma, sllabenzene The reactlvlty of the sllabenzene Isomers IS

assessed based on the absolute chemical hardness values Benchmark calculat~ons on the

three dlsllabenzenes were performed at vanous levels lncludlng the HF. B3LYP. MP2

CCSD(T), CASSCF and CASPT2 uslng varylng sues of bass sets Based on the results.

B3LYP method w ~ t h 6-3IG' level was chosen to model the other d~s~lahewene lsomcrs

Totally, 78 statlonary polnts were ldentlfied of whlch, 61 were characterlrcd ds mlnlmd

on the potent~al energy surface Several unconvent~onal structures are hlghly colnpctltlvc

In energy compare to the global mlnlma

chapter N Isomvs o f (CHW (X = N. P. As and s i H ) d C+SI& 107

4.1: Introdaction

The valence Isomers of benzene, whlch differ only by carbon-carbon connccanty

of the SIX CH umts, represent the most Important sub-class of benzene wmers among

more than 200 posslble Isomers, owng to the enormous expenmental and theoret~cal

Interest I J Scheme 4 1 depicts the five fundamental valence lsomenc forms, whch arc

benzene (B), benzvalene (V), Dewar benzene (D). pnsmane (P) and blcyclopropenyl (C)

Recently, Johnson and Daoust confirmed that trans-Dewar benzene and Motblus benzene

are also mlnlma on the (CH)6 potentla1 energy surface3 In addlt~on to these Isomers,

Balahan's henzmob~usstnpane, tw~sted pnsmane proposed by Karl and Bauer, and Claus'

benzene, all of wh~ch have the ~dentlcal connectlvlty, are other poss~b~l~t~es (Scheme 4 2).

however, these are chemically unreallstlc '* The syntheses of all the classrcal Isomers

except for trans-Dewar benzene (T) have been achieved, and these formed the subjects of

a number of theoretical and expenmental Investcgatlons

Scheme 4.1

benzene (B). D, benzvalene (V), C,, Dcwar benzene (D), C,,

pnsmane (P), D,, 3,3'-btcyclopropmyl ( C), C,, rrons-Dewar benzene (T). C,,

Scheme 4.2

Chapter IV h m a - s of (CHN (X - N, Pa As and SIH) md &St+& 108

Phosptume, srsabmzene and nlabenzene, multed tn replacing the methlne

p u p by P. As and SIH respectively were also demonstrated as aromatlc compounds "' Most of the valence Isomers of phosphlnme, B1P (Scheme 4 3) were synthesized and

were found to undergo ~nterestlng rearrangement react~ons and possess novel blndlng

capablhtles as hgands to transltron metal templates lo' ' I b Translent generation of the

Dewar pyndlne (DIN) through W lrradlat~on of pyndlne 1s reported recently I' It 1s to

be noted that the substltuted Dewar pyndlnes and azapnsmanes are well known

However, the data on aza-benzvalenes (VIN, V2N or V3N) and 3.3'-aza-b~cyclopropenyl

Isomers or then substltuted Isomers are scarce Although, arsabenzene IS known for qulte

some t~mqthe chem~stry of 11s other valence lsomenc forms IS relatively unknown ' I

Scheme 4.3

B IX V I X V2X V3X DIX

D2X P I X C IX C2X T2X

X=N,PandAs

The analogy between carbon and slhcon appears to be stra~ghtfonvard, desp~te the

substant~al contrasts In the chemlstly of hydrocarbons and their s~hcon subst~tuted

counterparts The s~hcon-organ~c compounds have been favonte huntlng grounds for

the theoretical and computat~onal chemistry for more than three decades The slmplest

sll~con analogue of benzene - sllabenzene (B1) - has been a subject of great Interest for

qulte some tune (Scheme 4 4)2126 Although the parent compound of sllabenzene

remalned an elusive specles, an unambiguous charactenzat~on of substltuted sllabenzene

1s ach~eved employtng the strategy of employng bulky groups attached to the SI

Chapter N: Isomers of (CHN (X = N. P. As md SiH) md &Si& 109

center!334 Nonetheless, various spectroscopic studies on the identification of silabatzene

and its isomerization to other products were known for quite some time.2s26 The wnccpt

of silaaromaticity is one of the most intensely studied topics in the last three decades.""

A large number of theoretical studies wnfess aromaticity on silabenzene, where one of

the methine groups is replaced by s ~ H ! ~ ~ ~ ~ ~ ~ The synthesis and characterization of

substituted silabenzene was unambiguously achieved only recently, while the data on the

other valence isomeric forms is scarce!' However, the infra-red, electronic and

photoelectronic spectroscopic characterization of silabenzene and I-methylsilabenzene in

argon matrices were reported quite a while ago!"he strategy of employing bulky

substituents resulted in a fluny of synthetic accomplishments in the silaaromatics, which

are otherwise elusive in their pristine form.23"' Maier and co-workers reported the

photochemical isomerizations of silabenzene to Dewar silabenzene, in a matrix isolation

study." While the silabenzene was unambiguously characterized, the characterization of

the isomerization product, benzvalene was based only on the NMR data, which was not

definitive, as acknowledged by the authors themselves, and these class of compounds are

too reactive for elaborate experimental investigation using the conventional techniques

such as X-ray diffraction.'! Chandresekhar and Schleyer have done calculations on B1,

Dl and other three isomers of silabenzene." Recent studies indicate that silabenzene is

only slightly less aromatic than benzene.27.30-32 Wakita et al along with their experimental

investigation on the isomerization of silabenzene to silabenzvalene reported B3LYP16-

31G* energetics on the five (Bl, V2, V3, Dl, DZ) valence isomeric forms?"

Among the disilabenzene isomers, hexamethyl substituted 1,4-disilabenzcne (3)

has been synthesized as early as 1987 and several photochemical reactions were

studied." Also, recent studies indicate the existence of silabenzene and 1,bdisilabenzene

as ruthenium complexes.36 Ando et al, have suc~essfully synthesized 40, which yields 25

and 49 on heating, and proposed that the possible common intermediate for both the

conversions to be 42?l Even though 37 and 46 are not valence isomers, we have

considered in this study as their derivatives have been synthesized recently?8e39

Considering the experimental interest in the isomerization reactions among the various

forms of disilabenzene, we ventured into a detailed theoretical study on disilabenzene

isomers. Baldridge and Gordon reported theoretical studies on the three disilabenzenes, 1,

2 and 3 uslng the HF method wth the STO-3G bas~s set "O In 1985. Chandrasckhar and

Schleyer have reported the HF13-21G* energehcs of 3, 10.11 and 25 " Recently, Ando

and w-workers used the HF method wth the 6-3 IG* bass set to evaluate the energet~cs

of 3, 25, 49, 40 and 42 " Yoshzawa and w-workers reported a detaled computat~onal

study on the lsomenzat~on reactlon between 13-d~s~labenzene (3) and 1.4-dls~la Dcwar

benzene (25) uslng the dens~ty funct~onal B3LYP and CASSCF procedures employing a

double-< bas~s set 42 The last part of the chapter concentrates on the vanous d~stlabenzene

Isomers, whlch are classified as monocychc, b~cycltc, tncycl~c and tetracycl~c Isomers

(Scheme 4 5 - 4 8)

Scheme 4.4

chnpter I V Isomers o f ( C H H (X = N. P, As and SIH) and &Sl2H6 111

Scheme 4.5

c': Q /

Sl

Chapter I V Isomers of (CHM (X = N. P, As ond SIH) d C.SI~H( 112

Chapter IV Isomers of (CHM O( - N. P. As and SIH) and C.SIII+ 113

Scheme 4.7

Scheme 4.8

I:'::

Chapter I V I s o w s of (CHhX (X = N. P. AS and SIH) and C.SI& 114

The chemistry of the valence Isomers of &(X = N, P. AS, and SIH) where all the

methtne groups are replaced by ~sovalent groups was extenstvely studled by

computat~onal means 4' However, httle attention 1s pald to the theoret~cal stud~es on the

valence Isomers of group V heterobenzenes, sllabenzene and d~s~labenzene, desplte the

expenmental Interest m these classes of compounds Examlnatlon of the equ~llbnum

geometnes, charactenzatlon of the mlnlma on the potentla1 energy surface, and the

relative energles and thelr comparison w ~ t h the parent benzene counterparts provlde a

basis to assess the vanatlon Induced by the heteroatom Benchmark calculat~ons are done

on benzene to test the sultablllty of the popular theoret~cal models by comparing with the

ava~lable experimental and other hlgh level theoret~cal calculat~ons Ah mir~o and hybnd

density funct~onal theory calculat~ons were performed on the group V heterobenzene,

sllabenzene and d~s~labenzene valence lsomers and other related lsomers of d~s~labenzene

and the results are presented In the same order

4.2: Valence lsomers of Group V Heterobenzenes

Thls sectlon presents the computat~onal results obta~ned for the group V

heterobenzene valence Isomers Deta~led calculat~ons on benzene valence Isomers, and

companson of the geometric parameters and energetics computed by vanous procedures

with the hlgh level calculat~ons and expenmental data are presented first The effect of

replacement of methlne groups of the benzene valence lsomers by lsovalent group V

elements (N, P and As) on the equil~bnum geometnes and the relatlve energles is studled

4.2.1: Computational Details

All the calculat~ons were done uslng the Gausslan 98 sulte of programs44 The

gwmetnes of all the structures were fully opt~m~zed wlthln the symmetry constraints

~nltlally at the HFl6-3lG* level 45 Further refinement of geometnes 1s done at B~LYP*"

and MP2 levels uslng thc 6-31G* bass set The effect of addlng a ser of polarization

functions to the peripheral hydrogens on the geometry of these compounds 1s tested by

dolng optlmlzatlon uslng the 6-31G** basis set at the MP2 level The MP216-31G' and

MP216-31G** geometnes are virtually ~dent~cal lndlcat~ng that 6-31G* 1s adequate In

glvlng proper equlllbnurn geometnes The nature of the statlonary po~nts obtalned was

charactenzed by frequency calculat~ons at HF and B3LYP levels, which des~gnate all the

valence isomers considered In the study as mlnlma The present and prevlous stud~es

Chapter IV: Isomus of (CUM (X = N. P. As and SiH) and C4SizH6 115

indicate that MP2 gives better equilibrium geometries; hence MPU6-31G* optimized

geometries were taken for further single point calculations at CCSD(T)/6-31G* and

MP216-31 1+G0* levels. A recent study on the diphosphinines and their valence isomers

indicates that a single determinantal approach is adequate to get reliable results for this

class of compounds." The enthalpy correction to the total energy is obtained from the

vibrational frequency data obtained at the B3LYP level. The best estimates for the

relative energies were obtained for all the molecules considered in this study, using eqn.

4.1.

AE = AEccswr, + AE(~~2,6311ffi*.- M P U ~ ~ I G * ) + AH - ... eq. 4.1.

AH is the enthalpy correction factor obtained by frequency calculations at the

B3LYPI6-31G* method. Although, the CCSD(T) method is a reliable theory, the basis

set of 6-31G* quality is not adequate. Performing CCSD(T) calculations with triple-<

quality basis sets uniformly for all the species considered in this study is beyond our

available computational facilities. Therefore our scheme of best estimates, which

accounts for the CCSD(T) basis set deficiency at MP2 level, is designed to circumvent

this problem. This scheme seems to be in excellent agreement with the high level

calculations on benzene isomers and the available experimental results (vlde infro).

4.2.2: Results and Discussion

The discussion on the valence isomers of benzene is done as a reference as well as

to test the suitability of the adopted computational tools in modeling the systems under

~tudy. The equilibrium geometries of all the valence isomers are compared and contrasted

among themselves as well as with the reference pristine isomers. Similarly, the relative

energies of the valence isomers of benzene are taken as reference to assess the relative

energy ordering in its group V counterparts. The discussion on the relative stability

orderings is presented next.

4.2.2.1: Equilibrium Geometries

Figure 4.1 depicts the MP2/6-31G* optimized geometries of the valence isomers

of benzene along with the experimental data, wherever available. The MP2 geometries

are in excellent agreement (maximum deviation of 0.012A in bond length) with the

QCISDl6-311G* geometries.7 The computed geometries are in reasonable agreement

with the available experimental numbers, and will form our reference values to estimate

RDpter IV: Isomers of (CHW (X = N. P, As and SIH) and C4Si2H6 116

the perhubahon m the skeletal bond lengths upon replacing the methine groups w~th the

lsovalent atoms. A went computat~onal study, wh~ch cons~dered the five classical

valence Isomers, c o n f i s the sultablllty of MP2 level In obtaining reliable geometnes ' The less explored trans-Dewar benzene (T) Isomer has a substantially shorter central

slngle bond and fauly longer double bond lengths '

Figure 4.1 The pnnc~pal geornetnc parameters of the valcnce Isomers of benzene obtalned at the MP216-31Gt level Expenmental parameters are glven In parenthesls wherever avalable The bond lengths are In A and the angles are In degree

The pnnc~pal geometnc parameters of the substituted benzenes (BIX),

benzvalenes (VlX, V2X and V3X), Dewar benzenes (DlX and DZX), pnsmanes (PIX),

b~cyclopropenyls (ClX and C2X) and trans-Dewar benzenes (T2X) obta~ned at the HF,

B3LYP and MF'2 levels of theory w~th 6-31G* basls set are glven In F~gures 4 2.4 3.4 4,

4 5, 4 6 and 4 7 respectively The numbering for the lndlv~dual pos~t~onal Isomers has to

be dec~phered h m Scheme 4 1 Wlule Hartree-Fock method consistently underest~mates

bond lengths for well known reasons, B3LYP and MP2 are In farly good agreement w~th

Chaptv W. ISOMS o f (CHW (X = N, P, As md SiH) and C,Si21+ 117

each other in most of the cases. In general B3LYP method overestimates bond lengths

especially for P and As valence isomers, with maximum deviation of 0.033 A in the bond

lengths and 1 .So in bond angles. Employing 6-31G9* basis set at the MP2 level did not

result in any noticeable changes in geometric parametm, with almost identical results to

first three decimal places in bond lengths (maximum deviation 0.005 A) and to the first

decimal for bond angle (maximum deviation 0.2'). The discussion on the structures will

be based on the MP216-31Gt geometries throughout the rest of the study unless

otherwise stated.

Figure 4.2: The principal geometric parameters of pyridine (BIN), phosphinine (BlP) and arsabenzene (BIAS) obtained at the HF (ordinary), B3LYP (underlined) and MP2 (bold) levels using the 6-31G' basis set. All values are given in A.

Pyridine (BIN), phosphinine (BIP) and arsabenzene (BIAS) are found to have

bond lengths corresponding to aromatic compounds, a result which is consistent with

previous studies. The C 4 bond lengths in the three compounds are similar to the C-C

bond length in benzene with a maximum deviation of only 0.005 A in BIAS. In addition

to this, the C-X bond lengths in these compounds are in between the normal C-X single

and double bond lengths witnessing full delocalization in terms of bond length

equali~ation.~~ In general, among the benzvalene isomers, the substitution pattern or the

type of substitution does not seem to perturb the skeleton significantly in a majority of

the cases. Among the benzvalene valence isomers all the C-C bonds in the positional

Chapter I V . Isomers of (CHH (X = N, P, As ond 8H) and C,S2H6 118

~somcrs are very smtlar and are closer to the correspond~ng pnstlne molecules

Obv~ously, the C-X bonds are d~fferent due to the d~fferent atomlc tad11 of the substituted

atom X The bndge-head C-C bond of the b~cyclobutane molety In VZN 1s substant~ally

shorter compared to the parent molecules, while every other Isomer. VZP. VZAs, V3N,

V3P and V3As have bond lengths s ~ m ~ l a r to the unsubstltuted benzvalene

A closer look at the Dewar benzene pos~t~onal isomers ~nd~ca te that the

heteroatom subst~tut~on does not make any s~gn~ficant perturbat~on to the skeleton In

terms of bond lengths In general. the computed C-C, srngle and double bond lengths are

essent~ally ~dent~ca l to the parent molecule lrrespect~ve of the subst~tuent slze and slte

The angle between the two planes (0) 1s slmllar, except In DIP and Dl As, when P and As

are subst~tuted at the bndge head posltlon result~ng in reduct~on of 0 by approx~mately

10' The pnsmane valence Isomers also do not show any slgnlficant devlat~ons In the

bond lengths compared to the parent molecules In CZN, the C-N s~ngle bond length IS

found to be longer than the normal C-N bond length Even In the same compound, the C-

C s~ngle bond IS computed to be much less compared to the C-N s~ngle bond

Interestingly, the bond lengths In CIX and CZX (X = P and As) are found to be

comparable to the correspond~ng standard bond lengths

In all T2X, the bndg~ng bond IS substant~ally shrunk compared to the bndglng

bond In the correspond~ng crs-lsomer (DZX), w ~ t h TZN e x h ~ b ~ t ~ n g the maxlmum effect

Sim~lar to the sltuatlon In parent Isomers, most other bond lengths exhlb~t exactly the

opposlte trend, I e , elongation when compared to the correspond~ng DZX Isomers Thc

C-N s~ngle bond length in TZN 1s substant~ally elongated, and T2P and T2As also show

s ~ m ~ l a r trends albeit to a smaller extent T h ~ s feature IS lnd~catlve of straln 111 the system

and lndlcates the p o s s ~ b ~ l ~ t y of nng openlng through C-X cleavage Therefore, the present

analys~s ~nd~ca tes that the replacement of methlne group by N. P, or As on the skeletons

of the valence Isomers of benzene does not Induce not~ceable skeletal perturbat~ons

Figure 4.3 The pnnctpal geometric parameters of the benzvalene tsomen (VIX, VZX and V3X) obta~ned at the HF (ord~nary), B3LYP (underlined) and MP2 (bold) levels ustng the 6-31G* bas~s set The bond lengths are ~n A and the angles are in degree

Chapter IV: Isomcrs of (&I)& (X : N. P. As rmd SiH) md C,SilH6 120

DIN. C, (0) DIP. C, (0) D l h , C, (0)

Figure 4.4: The principal geometric parameters of the Dewar benzene isomers (D1X and DZX) obtained at the HE (ordinary), B3LYP (underlined) and MP2 (bold) levels using the 6-31G* basis set. The bond lengths are in A and the angles are in degree.

PIN. C, (0) PIP, c, (0) PIAs. C, (0)

Figure 4.5: The principal geometric parameters of the prismane isomers (PIX) obtained at the HF (ordinary), B3LW (underlined) and MP2 @old) levels using the 6-31G* basis set. All values are given in A.

Chapter I V Isomers of (CHM (X = N. P, AS and SIH) a d C,SI~Y 121

F~gure 4.6 The pnnc~pal geornetnc parameters of the b~cyclopropenyl Isomers (CIX and CZX) ohtamed at the HF (ordinary), B3LYP (underlmed) and MP2 @old) levels usrng the 6-31G* basis set All values are glven In A

Figure 4.7 The pnnc~pal geornetnc parameters of the trans-Dewar benzene Isomers (T2X) obta~ned at the HF (ordinary), B3LYP (underlmed) and MP2 @old) levels uslng the 6-3 lG* bas,$ set The bond lengths are ~n A and the angles are ln degree

Chapter I V Isomers of ( C H M (X = N P As and SIH) and C1S~2H6 122

4.2.2.2: Relative Energies

In t h ~ s sechon taklng the valence lsomers of benzene, on wh~ch the expenmental

or highly rel~able theoret~cal data 1s ava~lable, the performance of vanous levels of theory

is assessed In glving the rehable energetlcs for thls class of compounds A comparison

will enable us to assess the strengths and drawbacks of vanous levels of theory and help

us In chooslng a rlght cho~ce of method, whlch can be appl~ed on the valence lsomers of

the compounds under study The relat~ve energles for all the valence lsomers of benzene

ohtamed at different levels of theory are glven In Table 4 1 Previously reported relat~ve

energles from G2 methods and expenmental heats of format~on wherever ava~lable are

also glven In the same table 6 7 Expectedly, the HF method was not good for quantltat~ve

results and the dynamic electron correlation 1s essent~al In obtaming rehable energetlcs

Surpr~smgly, the B3LYPl6-31G* method also cons~derably overestimate the stab~llty of

the benzene wh~ch results In the consistent underestlmatlon of all other Isomers Thls

problem may be traced to the Ilmitat~ons of the B3LYP, as well as most DFT based

methods, In comparing the energles between x-delocal~zed and locallzed structures along

the potent~al energy surface V~rtually same values are obta~ned at MP216-31GL and

MP216-31G** levels for all the lsomers Very s~m~lar relat~ve energles are obtalned at

CCSD(T)I6-31G* level The Inadequate basis set, 6-31G*, used at the CCSD(T) level IS

remed~ed as the bass set correct~on 1s done at the MP2 level (eqn 4 1) Thus, excellent

agreement 1s observed between the best est~mates of the relat~ve energles (eqn 4 1) and

the reported hlgh level calculat~ons Therefore, the relatlve energles obta~ned lor the

valence lsomers of pyndine (BIN), phosphln~ne (B1P) and arsabenzene (BIAS) by t h~s

procedure are llkely to be very slmllar at further h~gher levels of theory

The relatlve energles obta~ned at varlous levels of theory for all the valence

Isomers of pwdme (BIN), phosphn~ne (BIP) and arsabenzene (BIAS) considered are

glven In Tables 4 2, 4 3 and 4 4 respect~vely Both the HF and the B3LYP levels

consistently overestimate the stabil~ty of the delocallzed con~ugated systems While

Ctmpter I V Isomers of (CH)3( (X = N. P. As and SIH) and CISI~H) 123

adding a set of polanzat~on funct~ons on per~pheral hydrogens, golng to 6-31 I+G**

qual~ty b a s s set marg~nally Improves the energet~cs, espec~ally for the 3,3'-

bicyclopropenyl and trans-Dewar benzene Isomers In contrast, MP2 g~ves a cons~stently

better agreement and the bans set qual~ty over the 6-31G* have only a mlnor

~mprovement In the relat~ve energles Therefore. dynam~c electron correlat~on 1s essent~al

In ob ta~n~ng the accurate energet~cs, and the bas~s sets of double-< quahty augmented

w ~ t h a set of polanzat~on funct~ons are expected to y~eld rel~able energles " However. In

t h ~ s class of compounds the conxent~onal ah ~ n ~ t t o methods perform much better than the

currently popular dens~ty funct~onal theory based methods

The d~scusslon on energetlcs w~ll be based on the best estlmates unless otherw~se

specified The plot of the best estlmates of all the valence lsomers of benzene, pyndlne,

phosph~nine and arsabenzene 1s glven In F~gure 4 8 As reported earher, the five valence

lsomers of benzene span a w ~ d c range of stab~lrt~es up to 150 4 kcal mol ' w ~ t h ~rnns-

Dewar benzene (T) be~ng the least stable (Table 4 1 ) ' ( I 7 T h ~ s d~fference between the

energles of the most stable and the least stable Isomer decreases wh~le go~ng from

benzene farn~ly to the p y r ~ d ~ n e fam~ly, wh~ch 1s 147 0 kcal mol ' T h ~ s further decreases

to a greater extent for the valence lsomers of phosph~n~ne (121 8 kcal mol ' ) and reduces

a httle for the valence isomers of arsabenzene ( 1 13 3 kcal mol ') (Tables 4 2, 4 3 and

4 4) In all the class of compounds cons~dered here benzene Isomer (BIX) IS computed to

be the most stable and the I~OIIJ-Dewar benzene Isomer (T2X) to be the least stable

Isomer The valence lsomers of phosph~n~ne follow the same relatlve energy order~ng as

that of arsabenzene Absolutely no crossover In the relat~ve cncrgy ordcr~ngs IS found In

golng from the valence Isomers of phosphln~ne to arsabenzene (F~gure 4 8) Although the

framework 1s ch~efly respons~ble for the relat~ve stab~ht~es of the subst~tuted valence

Isomers, the energy perturbat~ons by skeletal subst~tut~ons are substantla1 among the

benzvalene lsomers VZN, where the heteroatom IS In the sp2 centcr, 1s the most stable

among the azabenzvalenes whereas the correspond~ng PlAs subst~tuted Isomer,

Chaptv IV : Isomers of (CH)& (X = N. P. As and SiH) and C,SI&+~ 124

VZPNZAs is the least stable. Similarly, in case of Dewar benzene, the stability ordering

of D1X and DZX is reverse when we go h m X = N to P or N to As. In contrast, in the

3.3'-bicyclopropenyl isomers, the reversal in the stability is not seen between C1X and

C2X when going 'om X = N to P or As. However, the energy difference between C l X

and C2X for X = N is 37.0 kcal mol-' which is only 6.0 and 7.6 kcal mol" for X = P and

As respectively. The N substitution always prefer to occupy the sp2 center compared to

the sp3, as is evident from the higher stability of V3N, D2N and CZN compared to their

other positional isomers. In contrast, the P and As substitutions prefer the sp3 centers in

benzvalene and Dewar benzene isomers. However, the bicyclopropenyl isomers, the sp2

center substituted isomers are marginally more stable. The relative energy orderings of

the phosphinine and arsabenzene valence isomers are identical and the energy gap

between various isomers decreases slightly in going from the former to the later.

Figure 4.8: The plot of the best estimates of the relative energies of the valence isomers of benzene and Group V heterobenzenes.

Chapter IV Isomers of (CHW (X = N, P, As ond SOH) and CISIZH~ 127

chapter I V Isomers of ( C U M (X = N, P. As and SIH) and CCSa2H6 128

chapter IV: Isomers of (CHkX (X = N. P. As and SiH) and C,Si,& 129

4.3: Valence Isomers of Silabenzene

A total of eleven (CH)sSiH isomers are considered, the stationary points located,

their nature on the silabenzene potential energy surface, their relative energies and the

reactivities assessed by the chemical hardness values are discussed in this section.

4.3.1: Computational Details

All the structures (Scheme 4.4) were fully optimized within the symmetry

constraints at the B3LYP level of theory with the 6-31G* basis set initially. The

stationary points thus obtained were characterized based on the frequency calculations.

The geometries were further refined with the cc-pVDZ and 6-31 1 +G** basis sets at the

B3LYP level. The geometries were also evaluated at the MP216-31G** level. These were

followed by single point calculations at the MP216-31 I+G** and CCSD(T)/6-31G*

levels on the MP216-31G** optimized geometries. However, 6-31G* basis set is

probably not adequate at the CCSD(T) level and therefore we have considered a bigger

basis set at MP2 level and this scheme (equation 4.2) was quite impressive in giving good

fits with the higher level calculations (Section 4.2.2.2).

AE = A E c c s q ~ ~ + AE(MPM-31 I+G.. - ~ ~ 2 1 6 - 3 1 ~ ' . ) + AH ... eq. 4.2

AH is the enthalpy correction factor obtained by frequency calculations at the

B3LYPl6-31 1+G** method. Harmonic frequencies were computed using 6-31G*, cc-

pVDZ, and 6-31 1+G** basis sets at the B3LYP level. The differences in the computed

harmonic frequencies obtained using various basis sets were very small. Thus the basis

set employed to obtain the harmonic frequencies is adequate. Most of the B3LYP

optimizations were carried out using the Jaguar 4.1 program package initially.52

However, for the sake of uniformity all the reported calculations were done using the

Gaussian 98 suite of programs.M

4.3.2: Results and Discussion

All the valence isomeric forms of silabenzene given in Scheme 4.4 were fully

optimized within the symmetry constraints initially at the B3LYP level with 6-31G* basis

set. All the structures were characterized as minima showing all real frequencies except

for the benzvalene isomer, V1. Attempts to locate V1 lead to one or other of the

stationary points, V l a and V l b and both of them turned out to be first order saddle

points. The normal modes corresponding to the imaginary frequencies are followed and

Chapter I V : Isomers o f (CUM (X = N, P, As ond SiH) and C4Si2H6 130

minimum energy struchuw, Vla, and Vlb., were obtained. The two new structures

Vla, and Vlb, are distinctly distorted when compared to benzvalene and correspond to

bicyclic structures. Energetically, these two isomers lie closer to the benzenoid

compound, B1 compared to the other isomers (vide infra). This prompted us to

investigate the esthetically appealing C5, pyramidal structure, PY, where the Si is bound

to all the five carbons and a stationary point was located. The frequency calculation

characterizes this as a second order saddle point. Following one of the two imaginary

frequencies lead to the first order saddle point, Vla and Vlb, which further, lead to

minima (Vla, and Vlb,). Although, it is necessary to have two electrons less to

stabilize such a nido system, the relatively low energy of PY and its connectivity to the

novel minimum energy structures, Vla. and Vlb,, prompted us to explore this part of

the potential energy hypersurface in detail. The cyclopentadienyl moiety is virtually flat

In all the cases, PY, Vla, Vlb, Vla, and Vlb,, depicted in Scheme 4.9. While SiH is

bound in ~l~-fashion in PY, it is in ~l'-fashion in Vla and Vlb and ?12-fashion in Vla,

and Vlb,. Similarly, a closer examination of the optimized structure of CZ reveals that,

C-Si single bond was broken in the Si containing three membered ring, leading to an

open structure, trans-C2 with a divalent Si (Scheme 4.10). The corresponding cis isomer,

cis-CZ was located and characterized as a minimum on the potential energy surface.

Several attempts to locate a stationary point corresponding to TI failed at both B3LYP

and MP2 levels and the putative structures collapsed to Dl upon optimization. Attempts

were made to locate T I at the HF, B3LYP and MP2 levels, and in each case all the

putative structures converged to the cis-Dewar benzene and thus indicating that T I

structure does not correspond to a stationary point on the potential energy surface of

(CH)$iH. Thus the present study identifies fifteen important stationary points on the

potential energy surface of silabenzene where thirteen are minima, two are transition

states and one structure is a second order saddle point. All the fifteen structures were then

reoptimized and recharacterized by frequency calculations at the B3LYP level using the

cc-pVDZ and 6-311+G** basis sets. The geometry optimization were also carried out at

the MP216-31G8 level. The assignment of the stationary points with higher basis sets was

found to be identical with the 6-31G* basis set. The equilibrium geometries are discussed

first and then the relative energies, vibrational spectra and the reactivity.

dropter IV : Isomers of (CHH (X = N, P. As and SIH) and C.SI~HI 131

Scheme 4.9

H ,

Scheme 4.10

Chapter I V ISO~CTS of (CHM (X = N, P. AS and SIH) and C,S1&I6 132

4.3.2.1: Equilibrium Geometries

Figure 4 9 deplcE the pnnclpal optlm~zed geometnc parameters of all the

structures cons~dered m the study at B3LYPl6-31G8, B3LYPIcc-pVDZ. B3LYP16-

311+G**, and MP216-31G** levels of theory In general, the geometnes obtalned at the

B3LYP and MP2 levels are m good agreement w~th each other However, B3LYP

method consistently overestimates the C-SI slngle bond lengths w~th all the bas~s sets

compared to the MP2 level The geometnc parameters obtalned uang the 6-31G* and the

6-31 1+G** bass sets are essennally ~dent~cal and therefore, 6-31G* quality bas16 set may

be assumed to be qulte adequate for the geometnes The d~scuss~on on the equlllbnum

geometnes from here throughout the rest of the text will be based on those obtalned at the

MP216-31G** level

The C-C bond lengths In sllabenzene, B1 are equal and also the C-SI bond length

IS found to Ile between the C-SI slngle and double bond lengths The bond length

equallzat~on tn skeletally substituted benzenes IS elaborately discussed In Chapter 111 In

Vla and Vlb, the five membered nng formed by the carbon atoms 1s found to be

virtually planar All the C-C bond lengths are closer to the aromatlc bond lengths whereas

the C-SI bonds are substant~ally elongated It occurred to us that the sltuatlon m~ght

correspond to the one where a cyclopentad~enyl anlon 1s coordlnated In a rl'-fash~on w~ th

SIH' cap Indeed, the natural population analysls at the HFl6-31G*llMP2/6-31G'

lnd~cates that there 1s a charge polanzat~on of +O 4546 In Vla and +O 4678 In V l b on

SIH group lndlcatlng that the five membered rlng has substant~al aromat~c stab~llzat~on

However, the benzvalene isomers only VZ and V3 retaln the skeleton and V1 does not

correspond to a statlonary polnt and spontaneously collapses to Vla and V l b T h ~ s 1s In

contrast w~ th the valence Isomers of pyndlne, phosph~nlne and arsabenzene, where all the

three benzvalene Isomers were charactenzed as mmmma (Sect~on 4 2 2) These tn.0

structures are transltlon states, the five membered nng formed by the carbon atoms In the

corresponding mlmmum energy structures, Vla, and Vlb, exhlblts slm~lar properties It

IS lnterest~ng to note that the SIH unlt 1s coordlnated to only two carbon atoms The

bndglng C-C bond of the cyclobutane molety m V2 1s 1 523 A, whlch 1s elongated

compared to 1 453 A In the pnstlne compound at the same level of theory Thls may be

due to the release of stran caused by the presence of Sl In the blc~clobutane moiety In

Chapter I V : Isomers o f (CHW (X = N, P, As and SIH) and C4Si+i6 133

V2. The comparable bond length in V3 with the corresponding C-C bond length in

pristine benzvalene confirms this.

In C1, trans-C2 and cis-C2, the bond lengths in the three membered rings are

shorter when compared to the corresponding standard bond lengths. The bridging C-C

bond in T 2 is substant~ally shrunk compared to the C-C bond in the cis isomer, D2,

whereas all other bonds are elongated. 1401

V l b . C , ( I ) Vl a,. C, (0)

Figure 4.9: The principal geometric parameters in the valence isomers of silabenzene obtained at the B3LYP level using 6-31G* (normal), cc-pVDZ (underlined) and 6- 311+G** (italics) basis sets and MP216-31G** level (bold). All values are given in A. The number of imaginary frequencies is given in parenthesis, the point group is also given.

Choptcr I V : Isoms of ( C H M (X : N, P. As and SIH) and &sa2u6

Figure 4.9 (contd.): The pnnc~pal geometric parameters in the valence ~somers of silabenzene obtained at the B3LYP level using 6-31Gf (normal), cc-pVDZ (underlined) and 6-31 1+G** (itahcs) bas~s sets and MP216-31G** level (bold). All values are given in A. The number of imaginary frequencies is given in parenthesis, the point p u p is also given.

Chaptcr I V Isomers of (CHhX (X = N, P, As and SIH) and C,Sl2H6 135

4.3.2.2: Relative Energies

Table 4 5 glves the relatlve energles obta~ned at vanous levels of theory,

~ncludmg the best estlmates calculated uslng equatlon 4 2 The trends obtalned at vanous

levels of theory are essentially ~dentlcal w ~ t h rnlnor quanntatlve d~fferences The

tendency of the dens~ty funct~onal methods to overestimate the stab~llzatlon of the n-

delocal~zed structure compared to the nondelocallzed structures IS reflected In the present

case also " The d~scuss~on on the relat~ve energles will be based on the best estlmates,

throughout the rest of thls sect~on unless othenv~se spec~fied

Table 4.5 The relative energles obtained at vanous levels of theory and the best estlmates of the relatlve energles, of the valence Isomers of s~labenzene All values are given In kcallmol -- --

B3LYPl B3LYPI B3LYPI MP2I MP21 CCSD(T)/ Best Shucture 6-31G* cc-p\'DZ 6-31 ItGI* 6-31G** 6-31 l+G**' 6-3 IG*' ~s t lmatc~ ' B1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Vla 25 2 240 268 205 202 222 210 1'1 b 30 5 290 315 280 275 289 275 Vla, 23 4 22 2 25 0 20 1 199 212 205 Vlb, 276 265 291 244 242 254 249 V2 480 496 490 419 404 432 404 V3 78 0 78 4 79 3 69 5 68 9 71 1 68 8 Dl 427 449 434 392 385 375 355 D2 72 1 729 729 659 649 648 624 PI 89 1 902 913 831 815 848 813 C1 85 4 879 840 842 829 822 782 trans-C2 85 1 84 6 85 1 890 89 6 83 4 82 1 cis-C2 88 0 874 880 919 924 860 846 T2 1323 131 8 1323 1280 1253 1248 1203 PY - 612 590 619 594 563 617 558 Slngle polnt calculat~ons on MP216-31G** opt~m~zed geomeines I l e best estlmates are calculated uslng equatlon 4 2 The enthalpy correction obtalned at the B3LYPl6-31 1+G8* level, scaled w~th a factor of 0 98

Flgure 4 10 ~llustrates the vanatron of the relatlve energles of the vanous Isomers

cons~dered In the study The best estlmates of the relatlve energles for the comspondlng

alence lsomers of benzene were also glven In the sanle figure for cornpanson The two

translt~on states (Vla and Vlb) and the pyram~dal structure (PY) are excluded, as they do

not Correspond to mlmrna on the potentla1 energy surface However, all s~labenzene

Chaptcr IV komcrs of ( C H N (X = N. P, AS and SIH) end C,SI~H( 136

~somers Ite closer to the reference compound, s~labenzene mndaatlng that the suhst~tut~on

reduces the energy gap among the vanous valence Isomers Thus, the transltlon state

structures, V l a and V l b he ~ u s t 21 0 and 27 5 kcallmol h ~ g h energet~cally above the most

stable s~labenzene, B1 It IS to be noted that the correspond~ng energy d~fference between

benzene and benzvalene 1s as h ~ g h as 71 7 kcaUmol The two correspond~ng non-class~cal

valence lsomenc mlnlmum energy structures, Vla. and Vlb, are less stable than

s~labenzene only by around 20 and 25 kcaVmol respect~vely The non-classmal nature of

Vla, and Vlb, coupled w ~ t h the11 prox~m~ty to the s~labenzene warrants expenmental

attempts towards them V3 IS found to be thermodynam~cally less stable than the other

benlvalene Isomers, the reason be~ng the weak rr-bondlng a b ~ l ~ t y of SI w ~ t h C The

framework seems to broadly declde the relat~ve energ~es to a greater extent compared to

the substltutlon pattern Thus the relatlve energy ordenng IS qu~te s ~ m ~ l a r to those

observed In the valence Isomers of benzene There 1s a stnk~ng d~fferentlat~on In the

stab~l~zat~on of the pos~t~onal Isomer, depending on the nature of the replacement slte,

thus replacement by SI at the saturated scte 1s ovenvhelm~ng preferred compared to the

unsaturated sltes Thus, although the skeletal replacement by SI causes substantla1

stablhzat~on among the valence Isomers of s~labenzene In general, the stabll~zat~on 1s

rnarglnal In the lsomers contalnlng C=SI S~m~larly the weak C=SI" 55 bond In D2 makes

~t energet~cally above Dl C1, wh~ch has two straned three membered nngs, IS expected

to be less stable than trums-CZ and cis-C2, both hav~ng only one three membered nng

But, C1 IS observed to 11e below both the C2 Isomers, whlch may be due to the presence

of d~valent s ~ l ~ c o n In the latter The C=SI present In the trans-Dewar benzene Isomer, TZ

renders lt as a least stable lsomer and IS computed to Ile about 120 kcallmol w ~ t h respect

to s~labenzene Interestingly, PY, whtch 1s a hlgher order saddle po~nt was computed to

11e only 55 S kcallmol h~gher and becomes more stable than many other valence Isomers

o i sllabenzene T h ~ s adds to the amazlng list of contrasts that were witnessed In the

chemlstnes of organlc and sllaorganlc compounds In all cases, there 1s a s~gnlficant

destab~l~zat~on In the Isomers where the C=SI exlsts, a result In aaeement wlth the

class~cal double bond rule

thaptcr I V : ISOMS of (CHW (X : N, P. AS md SiH) 4 C4Si2& 137

Structure

Figure 4.10: The correlation of the best estimates of relative energies of the valence isomers of silabenzene. The relative energies of corresponding benzene valence isomers are given for comparison.

4.3.2.3: Chemical Hardness

Absolute chemical hardness (7) has been used as a measure of kinetic stability or

the reactivity of organic ~ o m ~ o u n d s . ~ ~ ~ ~ ' Within the Koopman's approximation, hardness

( 7 ) is defined as half of the magnitude of the energy difference between the energies of

the HOMO and the LUMO.

7 = (ELIJMO - EHOMO)R . . .eq. 4.3

The frontier orbital energies and the chemical hardness computed at B3LYPl6-

311+G8* level is given in Table 4.6. While the thermodynamic stabilities of the

compounds under study are controlled by the skeleton of their structure, the kinetic

stability seems to be dictated by the bonding type of Si atom. C1, V2, PI and Dl, where

the Si occupies the sp3 centre, are kinetically stable than the other isomers. Whereas B1,

D2, V3, Vla,, T2 and V2b, are more reactive; in all these cases Si is tri-coordinated.

Compounds containing divalent Si, rransC2 and cis-C2 are found to be the least stable.

SO the kinetic stability exactly follows the order: Isomers containing tetracoordinated

Chapter IV: 1~0tners of (CHhX (X = N, P. AS and SiH) and C4SizH6 138

Si>isomers containing hicoordinated Si>isomers containing dicoordinated Si. The

relative stabilities and the hardness values do not have a linear relationship. Thus, quite a

few valence isomers (Vl, Dl, P1 and V1) have much higher hardness values than the

benzvalene isomers, Vla, and Vlb,, which are very stable energetically, according to

the hardness criteria correspond to least stable compounds.

Table 4.6: The frontier orbital energies along with the Mulliken symbols and the absolute chemical hardness (7) of the valence isomers of silabenzene obtained at the B3LYP16- 3 1 I S * * level. All values are given in eV.

Structure €HOMO ELUMO tl

B1 -6.02 (B,) -1.14 (B!) 2.44

Vln. -6.24 (A') -2.16 (A") 2.04

Vlb, -5.70 (A') -2.36 ( A ) 1.67

V2 -6.58 (A") -0.75 (A") 2.91

V3 -5.23 (A") -l.IO(A") 2.07 Dl -6.80 (A') -1.32 (A') 2.74

D2 -5.81(A) -1.35(A) 2.23 PI -5.87 (A") -0.30 (A') 2.78

C1 -6.78 (A') -0.78 ( A ) 3 .OO

trans-C2 -5.88 (A') -2.62 ( A ) 1.63

cis-C2 -5.81 (A') -2.74 (A") 1.54

TZ -5.32 (A) -1.83 (A) 1.75

4.4: Isomers of Disilabenzene

The present study reporls B3LW and CCSD(T) computed results of

Jisilabenzene valence isomers and some related structures (Scheme 4.5-4.8). By no

means we have considered all the possible isomers. See, e.g., benzene has more than 200

isomers. Thus C4SiZH6 will have well over 1000 isomers. But, in this study a systematic

attempt is made to consider most of the stable isomeric forms. The structures considered

are classified as (a) monocyclic, (b) bicyclic, (c) tricyclic and (d) tetracyclic. The

equilibrium geometries of all the stationary points located 011 the potential energy surface

and their relative energies are discussed in the above order. Comparisons were made with

the corresponding benzene, silabenzene and diphosphabenzene valence isomers in some

w t w I V Isomers of (CHhX (X = N. P. As and SIH) and C , S , ~ H ~ 139

4.4.1: Computational Details

All the structures glven In Schemes 4 5-4 8 were fully optlmlzed w~thln the

symmetry constra~nts at the B3LYP level"47 w ~ t h 6-31G+ bas~s set The statlonary polnts

obtalned uslng the default gradlent procedures were charactenzed based on the frequency

calculat~ons The normal modes corresponding to the lmaglnary frequencies of the

transltlon states and hlgher order saddle polnts were followed and the true mlnlma were

located Slngle polnt energy calculat~ons at the B3LYPIcc-pVTZ level on the B3LYP16-

31G* optlmlzed geometnes were done to estlmate the effect of bass set on the relat~ve

energies Couple cluster method was found to y~eld excellent energetics for thls class of

compounds 4951 Therefore, we have performed CCSD(T) slngle polnt calculat~ons w ~ t h

the 6-31G* bass set on the B3LYP opt~mlzed geometr~es The best est~mate, AE glven In

following equatlon 1s expected to y~eld results slmllar to CCSD(T)lcc-pVTZ level, even

though the effect of Increase In the qual~ty of the basls set 1s evaluated only at the B3LYP

level

AE = AEcrsnii + AE(BILYPIC~ PVTZ BXYPM JIG.) + AH eq 4 3

Enthalpy correction values (AH) are Included from the frequency data obtalned at the

B3LYPl6-31G* level All the theoret~cal methodolog~es employed are based on the slngle

detenn~nantal approach Hence, we have performed CASSCF and CASPTZ" calculat~ons

with 6-3 IG* and cc-pVDZ bass sets by lncludlng the x-system In the actlve space for I ,

2 and 3 The coefficients of the major Slater deternunant are found to be more than 0 92

In all the cases at the CASSCF level e~ther wlth 6-31G* or cc-pVDZ bas~s set Thls

~nd~cates that non-dynam~c electron correlat~on 1s not declslve and single determlnantal

approaches adequately descrlbe the electron~c structure and bondlng In thls class of

compounds All denslty functional theory calculat~ons were performed uslng the Jaguar

4 1 program package Gauss~an 98 su~te of program was used to perform the CCSD(T).

CASSCF and CASPT2 calculat~ons " The graphical Interface program, Moplot was used

to examlne the equll~bnum geometnes 59

4.4.2: Results and Discussion

A total of s~xty SIX Isomers were consldered ln~t~ally (Scheme 4 5-4 8) Upon

geometry opt~m~zatlon and charactenzat~on of the nature of the statlonary po~nts of all the

structures consldered. 61 of them were confirmed as the true mlnlma possessing all real

Chapter I V . ISOM of (CUM (X N, P, AS d SIH) and C ~ S I ~ H ~ 140

harmonic fnquencles However, some are found to be transltlon states, some are lugher

order saddle polnts and some do not correspond to a statlonary point on the dlsllabenzene

~otentlal energy surface The normal modes corresponding to the tmaglnary fiequencles

of the saddle polnts were followed In each case and the mlnlmum energy structures were

ohtamed. whlch Increases the total number of statlonary polnts to 78 Among them, 61

were charactenzed as mlnlma, 12 as transltlon states, 4 as second order saddle polnts and

one structure was found to be a thlrd order saddle polnt The equ~l~bnum geometnes of

the opt~mlzed statlonary po~nts and thelr nature on the potentla1 energy surface are

discussed first They-are arranged m the follow~ng order, vlz monocycl~c, blcycl~c.

tncycl~c and tetracycllc Isomers Then, the relatlve energles of the vanous isomers are

d~scussed Vanous factors, controlling the relatlve energy ordenngs, are addressed bnefly

In general as well as In each group of the pos~t~onal lsomers

4.4.2.1: Equilibrium Geometries

4.4.2.1.1 : Monocyclic Isomers

F~gure 4 11 deplcts the important geometnc parameters of the dls~lahenzenes and

other monocycllc Isomers obtnned at the B3LYPl6-31G' level For the dlmlabenzenes

(1-3), the reported bond lengths obtalned at the B3LYPicc-pVTZ level are given m

parentheses The geometnc parameters obtalned uslng the two bass sets are very slmllar

w~th a maxlmum dev~at~on of 0 007 A

We also have cons~dered some monocycl~c structures (4-15) where the rr-

lelocahzat~on m the three dlsllabenzenes 1s d~smpted and concomltantly one or both the

SI atoms become dlvalent Among the monocycltc Isomers w~th one d~valent sllrcon (4-

11 ), the planar forms of those structures where the dlvalent slllcon 1s present adjacent to a

CHI group are found to be trans~tlon states (5, 7.8 and 11) The corresponding mrnlmum

energy structures (Sm, 7m, 8m and I lm) were then rdentrfied Importantly, at the

CCSD(T) level, 8m and l l m he hgher in energy compared to 8 and 11 respectively

Th~s reveals that the planar forms are very l~kely to be mlnlma at the CCSD(T) level

~ndlcatlng that the B3LYP method incorrectly designates the nature of the planar

molecules ~h~ lndlcates that the out-of-plane dlstortron may be traced to the llmltat1on

of B3LYp method and slmllar observabon was made earher All the m~lecules with two

dlvalent slllcon atoms are found to be h~gher order saddle Points (12-15) The mmlmum

Chapter IV : ISOWS of (CHM (X = N, P, As and SiH) and C4SizHs 141

energy structures 12m. 13m and 14m correspond to pyramidal like structures, with one

of the Si atoms forming the apex of the pyramid. Unlike in the previous case, the planar

forms lie much higher in energy compared to the non-planar structure. 16, where one of

the hydrogens is bridged between the two Si atoms, is found to be a transition state. The

Si-Si bond length is too long (2.922 A) and the connection between the two may be

traced mainly through the bridging hydrogen atom. The normal mode of the imaginary

frequency corresponds to the puckering of the bridged hydrogen, which was then

followed and 16m was obtained. Therefore, 16 may be treated as a transition state

interconnecting the two identical forms of 16m. In 16, all the C-C and C-Si bonds are

almost comparable to the m a t i c bond lengths, whereas in the minimum energy

structure (16m) the bonds are localized. Similar to 16, the Si-Si bond length in 17 is

substantially elongated with the bridging hydrogens strongly bound to both the silicon

atoms. 18 is found to be a transition state and following the normal mode corresponding

to the imaginary frequency yielded 48.

4.4.2.1.2: Bicyclic Isomers

The equilibrium geometries of the bicyclic isomers (Scheme 4.6) considered in

the present study are discussed in this section. Figure 4.12 gives the principal geometric

parameters of the bicyclic isomers obtained at the B3LYPi6-31G* level. In our previous

study on the silabenzene valence isomers, we identified two bicyclic structures where a

SiH unit is bound to a cyclopentadienyl species in a r12 fa~hion.~'' These unusual

s*.uctures were found to be the next stable species to silabenzene in the (CH)5SiH

potential energy surface. We have considered similar structures, 19-24 (Scheme 4.6) out

of which, stationary points corresponding to 19, 20.22 and 24 could be located and were

found to be minima. Initial structures of 21 and 23 upon optimization collapsed to 48

indicating that these are not stationary points on the potential energy surface. In 24, the

C-Si bond with the r12-bound SiH moiety seems to be too long and in contrast Si-Si is

quite normal indicating that its connectivity is close to that found in 18. The greater

stabilization in 24 compared to 18 may be traced to the higher delocalization in the five

.nembered ring in the former.

The stationary points corresponding to the six Dewar benzene isomers (25-30)

were obtained and the frequency calculations indicate that all the Dewar benzene isomers

Chnptcr I V I s o m o f (CHhX (X = N, P, As and SIH) and C,SlzH6 142

arc nuluma except 28 In all the cases except ~n 25, the bndgng bond IS longer compared

to the cowpond~ng standard bond lengths, a feature whlch 1s observed In the parent

Dewar benzene itself The SI-SI d~stance m 27 1s computed to be too short However, the

bond order calculated usmg Atoms m Molecules AIM)^ at the B3LYPl6-31G8 level of

theory 1s only 0 04 suggestmg that bond between the two SI atoms does not exlst The

normal mode of the rmaglnary frequency In 28 corresponds to the out-of-plane d~stortion

of the hydrogen atoms connected to the two SI atoms The mlnlmum energy structure

correspondlng to 28 IS obwned following the dlrectlon of the lmagtnary frequency

normal mode 28m 1s found to have a twlsted double bond where the hydrogens

connected to the SI atoms stay out-of-plane Th~s IS s~mllar to the sltuatlon ~n the slhcon

analog of ethylene where the D2h fonn of H2S1=S1H2 IS a transltlon state, the

correspondlng mlnlmum energy structure tnvolves substantial pyramidal~zat~on at the SI

center b' Thus there seem to an Inherent tendency to have puckered geometry for the

SI=SI, but considering the fact that the planar 1,2-dls~labenzene (1) corresponds to

mmima, thls puckering could be prevented when a stronger symmetrlzlng force 1s

operatlve All of our attempts to locate a statlonary compound correspondlng to 32 were

fut~le, upon geometry optlmizatlon the lnltlal structures collapsed to 48 The frequency

calculations designate the five statlonary polnts for the trans-Dewar benzenes as mlnlma

on the potent~al energy surface In all the Isomers except m 33, the bndgmg bond 1s short

compared to that In the correspondlng as-compound Most of the computed peripheral

bond lengths In the trans-Dewar benzene Isomers are longer compared to those in the cry

Isomers, which 1s lnd~catlve of hlgher stratn In the trans lsomers S~milar to 27, despite

the Si-SI Interatomic dlstance of 2 476 A In 33, the bond order calculat~on does nor

support any bond between the SI atoms

Among the b~cyclopropenyl Isomers (40-45), 43 1s computed to be a transltlon

state The normal mode of the lmaglnary frequency corresponds to the out-of-plane

dlstortlon of two hydrogens connected to the SI atoms Opt~mizlng 43 wlth the two

hydrogen atoms out-of-plane, lead to I-43m, where one of the C-SI bonds becomes too

long to glve a monocycl~c compound, w~th only one three membered nng In the process,

the sp3 carbon has become an sp2 c h n and one of the SI atoms becomes dlvalent The

correspondlng cls Isomer, c43m was also opt~mized and was charactenzed as a mlnlma

C h a p t ~ L W W S of (CHM (X = N. P, As and SiH) Md C4SilH6 143

SimilWl~, 42 On optimization leads to a monocyclic compound, r-42 with one divalent

silicon; the cis compound, c-42 was also then identified. ~ 0 t h 44 and 45 upon

optimization collapses to an acyclic compound, n-44 with two divalent silicon atoms; the

corresponding cis-tram isomers (cc-44 and 13-44) were then characterized as minima on

the disilabenzene potential energy surface. This conIrast with the parent bicyclopropenyl

can be explained based on the instability of divalent carbene in contrast to the divalent Si.

The propensity of three membered rings containing a tricoordinated Si for ring opening

may be understood based on the following points: (1) it relieves strain in the three

membered ring, (2) one C-Si n-bond is replaced by C-C n-bond, (3) it leads to divalent

Si. Isomers, 37 and 46, where all the hydrogens are replaced by tertiary butyl groups have

been synthesized recently and photochemical rearrangement reaction has been observed

between these two c o m p o ~ n d s . ' ~ ~ ~ ~ In both cases especially in 46, the Si-C single bond

lengths are slightly shortened compared to the standard bond length. 38 is computed to be

minima while its positional isomer, 39 is a transition state. Following the nonnal mode

corresponding to the imaginary frequency and optimizing th~s collapsed to 22.

4.4.2.1.3: Tricyclic Isomers

The principal geometric parameters of all the tricyclic obtained at the B3LYP/6-

31G* level are given in Figure 4.13. Initially, efforts were made to locate the stationary

points for the seven possible disilabenzvalene isomers (48-54) depicted in Scheme 4.7.

However, exhaustive attempts could not yield a stationary point corresponding to 53. All

the putative structures of 53, upon optimization relaxed to 48. Thus, the computations

Imply that 53 is not a stationary point on the disilabenzene potential energy surface. All

other stationary points were characterized as minima, except 54, which was characterized

as a transition state. Seven disilabenzvalenes were located, out of which six were

characterized as minima. In 48 the Si atom, which is in the bridgehead of the

bicyclobutane moiety, is weakly bound to two carbon atoms and the other silicon.

However, this isomer is the one, which is energetically more stable, compared to the

other benzvalene isomers. The five membered ring formed by one of the Si atoms and the

four cabon atoms is virtually planar and the bond distances are substantjally short

compared to the standard single bond lengths. Thus, 48 may be considered

as an S ~ H + ion bound to the silacyclopentadienyl species in an ?' fashion to the CCSi

Chaptv I V : IsomuS of (CHW (X = N. P. As and SiH) and C,SizH6 1 44

ort ti on. The computed bond lengths in 49, 51 and 52 exhibit normal bond distances

except for the bridging bond of the bicyclobutane moiety in 49, which is slightly

elongated. In 49, the Si-Si distance is computed to be only 2.639 A. In 50, the computed

Si-Si and C-C single bond lengths are shorter compared to the normal bond lengths. The

frequency calculation characterizes 54 as a transition state, and the imaginary frequency

normal mode corresponding to out-of-plane distortion of the hydrogens attached to the

Si=Si, similar to the situation in 28 and 43. Following this normal mode, the minimum

energy structure 54m is reached where the silicon center is pyramidal. In both 54 and

541% the bridging bond of the bicyclobutane moiety is found to be very short compared

to the corresponding C-C bond in other isomers.

Benzvalene isomer, 54, with one hydrogen bridging between the two Si atoms

(55) was computed to be a second order saddle point. Following the normal modes of the

imaginary frequencies the structure 55m was obtained, which has a tetravalent and a

divalent Si. However, similar structure, 56 with two bridging hydrogens was conlputed to

be a minimum energy structure. In 56, the Si-Si bond length is too long compared to the

normal Si-Si single bond distance. Similar to the structures, 19-24, we have considered

57-63, where two SiH units are bound to a cyclobutadien~ moiety in various possible

modes. Among these structures, 61 and 63 are minima; 57,58 and 60 are computed to be

transition states. Many attempts to locate a stationary point corresponding to 59 failed

and all the initial geometries collapsed to 20 upon optimization. 57m and 58m

correspond to distorted forms of 57 and 58 respect~vely where, two C-SI bonds are

significantly elongated. These compounds can be viewed as cyclobutenes, where t u o

hydrogens connected to sp' carbon atoms are substituted by SiH units. D~s ton~ng the

skeleton of 60 following the normal mode of the imaginary frequency and opt1n1171ng

lead to 20. Initial structures of 62 on optimization leads to a molecule where one of the SI

is tetravalent with two hydrogens connected and the other is divalent Si (Flgure 4.13).

Clwpter I V . Isomers of (CHM (X = N, P. As and SOH) and C I S I ~ H ~ 145

4, C,; 17.5 (0) 5, C,; 27.2 (1) 5m, CI; 25.1 (0)

6 , C,; 30.0 (0) 7, C,; 21.0 (1) 7m, C1; 20.9 (0)

8, C,; 16.3 (1) 8m, C1; 16.1 (0) 9, C,; 17.5 (0)

I'tgure 4.1 1 (contd.) The prlnclpal geometric parameter? of Lhc mooocycl~c Isomers ohtnlned at the B3LYPl6-31G* level Those ohtalned at the BiLYPIcc-pVTZ are glven 111 parentheses only for 1, 2 and 3 All values are glven In A The polnl groups, best emnate of the relat~ve energles and number of tmaglnary tiequencies are glven

10, C2v; 13.8 (0) 11, C,; 34.4 (1) l l m , C1; 34.7 (0)

2 j40

14, C,; 34.4 (2) 14m, C1; 23.8 (0)

Figure 4.11 (contd.) The pnnc~pal geometnc parameters ol the m o n o ~ y ~ l l ~ Isomer\ obtalned at the B3LYPl6-3IG* level Those obtalned at the B3LYP/cc-pV7Z arc glvcn In parentheses only for 1, 2 and 3 All values are glven In A The po~nt groups, best estlmate of the relat~ve energles and number of imaginary frequcnc~es are glven

Chapter I V . Isomers of (CHM (X = N. P, As and SIH) and C I S I ~ H ~ 147

16, C,; 47.0 (1) 16m, CI; 16.6 (0) 17, C2,; 32.8 (0)

18, C,; 71.7 (1)

Figure 4.11 The pnnc~pal geometric parameters of the rnonocycl~c Isomers obtatned at hc B3LYPi6-31G* level Those ohtamed at the B3LYP:cc-pVTL are glvcn In ~drenthcscs only for 1, 2 and 3 All values are given In A The polnt group, best estlmate IS the relatlve energles and number of Irnagmary Srequencles are glven

Chapter I V : Isomers of (CHhX (X = N, P. As ond SIH) and C4SlzHb 148

19, C,; 30.8 (0) 20, C,; 35.3 (0) 22, C1; 22.2 (0)

27, C1; 21.9 (0) 28, C,; 49.3 ( 1 ) 28m, Cl. 47 4 (0)

29, C2; 64.8 (0) 30, C,; 61.1 (0) 31, Czh, 88.5 (0)

Figure 4.12 (contd.) The pnnc~pal geometnc parameters (11 the ~ I L Y L I I C isomers

obtmned at the B3LYPl6-31G* level All values are glven In A The point groups, bcst estlmate of the relatlve energles and number of Imaginary frequencies are glven

Chapter I V : Isomers of ( C H M (X 1 N, P. As and SIH) and C4S~&t6 149

36, c,; 106.1 (0) 37, C2h; 26.0 (0) 38, C,; 66.0 (0)

40, czh; 48.6 (0) 41, cI; 75.0 (0) C-42, c,; 56.8 (0)

t-42, C,; 54.4 (0) 43, C,; 98.5 ( I ) c-43,, C,; 85.9 (0)

Figure 4.12 (contd.) The pnnc~pal geometnc parameters of the b ~ c y c l ~ c Isomers ohtamed at the B3LYPi6-31G* level All values are glven In A The polnt groups, best estlmate of the relat~ve energ~es and number of lmaglnary frequenc~es are glven

Chapter I V . Isomers of (CH)3( (X = N. P, As and SiH) and C ~ S I Z H ~ 150

SiH 13x9

1 362 slms: 824 776&i8i

Figure 4.12 The pnnc~pal geometric parameters of the ~ I L Y L ~ I L IwIIior\ obtillned at thc B3LYPl6-31G' level All values are glven In A Thc polnt groups, ho~t c\tlmate ol tho relatlve energles and number of lmaglnary frequencies 'Ire g1vc11

4.4.2.1.4: Tetracyclic Isomers

All the three structures were located and character~zed as mlnlma on the potentla1

energy surface The Important geometric parameters of the three prlsmane Isomers

obta~ned at the B3LYPl6-31G* level are deplcted In Flgurc 4 14 In all the three Isomers,

the straln due to the replacement of two SI atoms In the parent prlsmane molety IS

reflected on the bond angles and d~hedral angles and not on the bond lengths However,

all the C-Si bond lengths are sl~ghtly elongated, an lndtcatlve of the straln present In the

Chapter IV: Isomers of (CHM (X = N. P. As and SIH) md G S I Z H ~ 151

51, '2,; 38.3 (0) 52, C,; 48.4 (0) 54, Czv; 60.2 (1)

Figure 4.13 (cootd.) The principal geometric parameters of the tricyclic Isomers obta~ned at the B3Ll-P/6-31G* level All values are glven In A The point groups, best estlmate of the relati\ e energles and number of maginary frequenc~es are glven

Chaptv IV . Isomcrs of (CHM (X = N. P, As and SIH) and C4S2H6 152

61, CzV; 109.3 (0) 62, C,; 72.8 (0) 63, CZ; 72.6 (0)

Figure 4.13 The pnnclpal geometnc parameters of the t n c y c l ~ ~ Iwtners ohtalned at the B3LYPl6-31G* level All values are glven In ,A Thc polnt group\, best cstlmate ot'thc relatlve energles and number of ~maglnary frequcncle~ arc given

64, C,; 56.3 (0) 65, C2v; 67.0 (0) 66, C2, 56.0 (0)

Figure 4.14 The principal geometr~c parameters of the t e t rncy~ l l~ Isomers obtained at the B3LYPi6-31G* level All values are glvcn In A The polnt groups. bust estlmate of the relatlve energles and number of lmaglnary frequen~~es are glverl

C b p t ~ IV I S O ~ W S of (CHbX (X : N, P. As and SIH) and C,!j~~b 153

4.4.2.2: Relative Energies

The relatlve energ~es of the three dlsllabenzenes have been studled extensively by

theoretical calculat~ons of vanous levels of soph~st~catlon, the results of whlch are glven

In Table 4 7 The complex~t~es In choosing the rehable computat~onal procedures for

s~labenzenes was thoroughly lnvestlgated by Baldndge et al and thelr systematic and

elaborate calculat~ons on a serles of sllabenzenes inferred that B3LYP level w ~ t h cc-

pVTZ qual~ty bas~s set IS the rel~able approach for getting the correct energetlcs

compared to the conventional HF or MP2 methodolog~es~' B3LYPicc-pVTZ method,

wh~ch was shown to y~elds results comparable to that of the CCSD(T)/cc-pVTZ level IS

chosen as the method of cholce In thls study to model the other non-planar Isomers

cons~dered In the study Consldenng the lack of mult~determmantal nature of the

wavefunct~ons, the CASSCF and CASPT2 approaches do not seem to be appropnate for

t h ~ s class of compounds Table 4 7 reflects that the qualitat~ve agreement of MP2 and

B3LYP IS much better w ~ t h h~gher level calculat~ons compared to that of CASPT2

The B3LYPl6-31G*, B3LYPIcc-pVTZ, CCSD(T)I6-31G* relatlve energ~es and

thc best estlnlates of the relatlve energies obtalned uslng equatlon I are given In Table

4 8 The B3LYP level computed relattve energles uslng the 6-31G* and the cc-pVTZ

hasls sets are m close agreement wlth each other with a maximum deviation of 3 5

kcallmol The qual~tat~ve trend of the relatlve s tab~l~t~es obtalned at all the levels of

theory employed IS essent~ally the same The dlscuss~on on the energetlcs wlll be based

on the best estimates of the relatlve stabllitles method unless othenvlse stated

Chapter IV: Isomers of (CHW (X = N, P, As and SiH) a d C,S,H, 154

Table 4.7: The relative energies of 2 and 3 with respect to 1 obtained at various levels of theory. All values are given in kcal/mol.

Method 2 3

HFl6-3 IG* -2.6 13.4

Thls work. Taken from Ref. 27.

" From Chapter 111.. Slngle point calculations on the MP216-31G** optimized geometries. Single point calculations on the B3LWl6-3 IG* optimized geometnes. Single point calculat~ons on the B3LWIcc-pVTZ optimized geometries.

Chaptv IV: Isomers of ( C H H (X : N. P. As and S i n ) ond C4Si2H, 155

Table 4.8: The relative energies (kcaVmol) of the isomers of disi labeme obtained at the B3LYP and CCSD(T) levels, the best estimates of relative energies. The number of ~maginaty frequencies (NIMG) obtained at the B3LYPl6-31Gg level is also given.

B3LYPl CCSD(T)I Best SbUcmre NIMG !-%g =-pVTZ1 6-31C*' bsflmatcb

1 0 0.0 0.0 0.0 0.0 2 3 4 5 Sm 6 7 7m 8 8m 9 10 11 l l m 12 1 2m 13 13m 14 14m 1s 15m 16 16m 17 111 19 20 22 24 25 26 27 28 28m 29 30 31

66 0 63 2 62 3 58 9 56 0 Slngle polnt calculations on B3LYPi6-31G* opt~m~zed geometnes

Chapter IV Isomers o f ( C H M (X = N, P, As and S I H ) and C4S~,H6 157

4.4.2.2.1: Monocyclic Isomers

Among the three dlsllabenzenes (Figure 4 11). only the denvatlve of 1.4-

d~sllabenzene, 3 1s expenmentally known and rearrangement reactions have b m studred

Interestlngly, thls 1s the least stable Isomer among the three dlsllabenzenes 1.3-

dlsllabenzcne (2) 1s found to be more stable than 12-dtsrlabenzcne (1) by about 3

kcalimol Thls 1s In contrast to the sltuatlon In d~phosphabenzenes, where 1.2-

dlphosph~ntne was found to be the global mlnlma 48 The relatlve stablhty ordenng of the

skeletally subst~tuted benzenes has been one of the most lntngulng aspects and several

mutually Independent factors were found to be respons~ble (See Chapter 111)

Among the monocycl~c isomers, where the rr-delocal~zat~on 1s disrupted (4-IS),

10 1s found to be the most stable one and IS found to he only about 5 kcal/mol above tts

aromatlc counterpart. 3 Whereas, 4 1s less stable than 11s dlsllabenzene counterpart, 1 by

about 17 kcaVmol T h ~ s lndlcates that ~t 1s more expensive to dlsmpt the rr-delocal~zat~on

In 1 than In 3 Among those structures, where one of the slhcon 1s dlvalent and the other

St 1s tncoordlnated (5-9, l l ) , 7m, 8m and 9 are more stable than the rest The orlgtn may

be traced to the preference of SI atom to occupy 1,3 posltlon for electrostat~c reasons

Among the planar forms of the compounds conta~nlng two dlvalent slllcon atoms (12-IS),

14 1s the most stable one where the two SI atoms occupy 1,3 posltlons These structures

were computed to be hlgher order saddle points Among the non-planar Isomers, 13m 1s

the most stable, where one of the SI atoms 1s bound to the other SI and three carbon

atoms 16m 1s less stable than 1 by 16 kcalimol even though they possess equivalent

skcleton Thls may be due to the fact that the brldgtng bnngs In locallzat~on In the

benzene skeleton, whlch 1s reflected In the geometries Thls effect 1s furthered In the

d~br~dged structure, 17 whlch l ~ e s around 32 kcaYmol htgher than 1

4.4.2.2.2: Bicyclic Isomers

In addltlon to the class~cal valence lsomenc analogues, Dewar benzene and

blcyclopropenyl, several other Isomers were cons~dered In tbls category S ~ m ~ l a r to the

case In a monosllabenzene Isomer, where SIH 1s coordinated to cyclopentadlenyl molety

11: a tll fashlon, 22 and 24 he only about 25 kcaVmol hlgher than the global mlnlma The

charge analysts lnd~cates that the five membered nng formed by one SI and the four

carbon atoms IS negatively charged and the SIH possesses Posltlve charge The high

Chapter I V Isomcrs of (CHM (X = N. P. As and SIH) a d C.SlrHa 158

stab~l~ty of this class of compounds may be attributed to the ammatlc stab~hzatlon of the

five membered nng

Among the Dewar benzene Isomers (25-30), 25 IS the most stable Isomer and l ~ e s

only about 21 kcaVmol hgher than the global mlnlma In ttus Isomer, both the s~llcon

atoms are tetracoord~nated It 1s to be noted that the parent Dewar benzene 1s less stable

than henzene by about 75 kcallmol 27 and 26, where one of the SI 1s coord~nated and the

other 1s tncoord~nated, Ile energet~cally h~gher compared to 25 The least stable lsomers

are the ones where both the SI atoms are tncoord~nated However, 27 IS energet~cally

competltlve to 25 In splte of the fact that one of the SI atoms IS tncoord~nated In case of

d~phosphahenzenes, all the Dewar benzene Isomers Ile very close to each other (44 8-52 4

kcaWmol), whereas Dewar d~sllabenzenes span a w~de range of stabll~t~es from 18 3 to

64 8 kcaVmol As observed In case of the valence lsomers of benzene, sllabenzene and

Group V heterobenzenes, trans-Dewar benzenes are the ones those are least stable among

the valence Isomers However, one of the trans-Dewar benzene Isomer, 33 l~e s below few

b~cyclopropenyl lsomers (41, 43, c-43m and t-43m) Among the trans-Dewar benzene

lsomers (31-36). 33 1s the least stable lsomer and 1s found to be less stable than the global

rnlnlma by only 76 kcaVmol whereas the parent trans-Dewar benzene hes as high as 150

kcalimol above benzene

As observed In the Dewar benzene Isomers, the envlronrnent In wh~ch the SI atom

1s present seems to control the relatlve s t ab~ l~ t~es of the b~cyclopropenyl Isomers One

mlght expect that n-45, wh~ch does not have even a s~ngle three membered nng would be

more stable than the other lsomers hav~ng one or two stmned three membered nngs But,

tt-45 IS found to be less stable than 40, wh~ch has two three membered nngs The relat~ve

stabll~ty ordenng seems to be controlled by the pos~tlon of the Sl atoms T h ~ s lndlcates

that the subst~tutlon pattern plays an important role In dec~d~ng the relat~ve stab~l~ty of the

Isomers m t h ~ s class of compounds In addltlon to the stra~n ~n the skeleton Th~s 1s tn

contrast to the d~phosphtn~ne Isomers, since the most stable b~cyclopropenyl lsomer 1s the

one where both the phosphorous atoms occupy sp2 centers and the least stable Isomer 1s

the one where the P atoms are present In sp3 centers 37 and 46, whose denvat~ves were

synthes~zed recently, are unstable than the global mlnlma only by 29 and 52 kcaVmol

Chopter IV Isomers of (CHM ( X - N, P, As ond SIH) Md C4S01H6 159

respect~vely Both of them have two str;uned four and three membered nngs respectively,

but are energet~cally competltrve w ~ t h the other Isomers

4.4.2.2.3: Tricyclic Isomers

In t h ~ s sect~on, the relat~ve stab~htres of benzvalene lsomers and some other

tncychc non-classical structures are discussed As observed In the Dewar benzene and

b~cyclopropenyl lsomers, molecules hav~ng two tetracoord~nated SI atoms are stable

compared to those contalnlng tncoordinated Si atoms T h ~ s can be attnbuted to the weak

rr-bondmg a h ~ l ~ t y of SI either w ~ t h C or w ~ t h another SI 48, where one of the SI atom 1s

bound to the five membered nng formed by the other s~hcon atom and the four carbon

atoms in a ~l' fashion, IS computed to the most stable Isomer among the benzvalene

isomers 55m, where one of the SI 1s tetravalent and the other 1s dlvalent, IS more stable

than 54m, where both the Si atoms are tncoordlnated, otherw~se shanng an equivalent

skeleton, by 9 2 kcalimol T h ~ s ~nd~cates that SI atoms prefer to be more stable when In

tetravalent and d~valent form compared to be In two tncoordlnated form The drbndged

structure, 56 lies 8 kcalimol blgher In energy than the correspond~ng non-bridged isomer,

54m Among the two SIH unit capped cyclobutadlenyl systems (57-63), the trans

compounds, 57 and 58 are more stable than the correspond~ng CIS compounds 60 and 61

The Isomer, where the two SIH unlts are coord~nated from elther s~des to the oppos~te

comers ofbutadlenyl specles, 63 is found to be most stable among these lsomers

4.4.2.2.4: Tetracyclic Isomers

64 and 66 I I C close to each other w ~ t h 65 lying about 10 kcal mol ' lower

cornpared to the other two Thus, the relat~ve energy ordenng of the pnsmane lsomers 1s

as follows 66 z 64 < 65

4.5: Conclusions

Thls chapter presents a comprehens~ve theoret~cal study on the valence lsomers of

group V heterobenzenes, silabenzene and d~sllabenzene, In add~t~on to some C4SlzHb

lsomers Ten valence Isomers were located on each of the potentla1 energy surface of

(CH)5X (X = N, P and As) and found to be mlnlma The planar benzene analog 1s the

lowest energy Isomer ln all cases and the relative energy ordenng of the vanous classes

of posit~onal isomers resembles that of benzene ~n most cases The relatlve energles

obtnned uslng the MP2 method are consistently in better agreement w ~ t h coupled cluster

Chapter I V Isomers of (GI)& (X = N. P. As md SIH) and C,SIZH~ 160

compared than those obtained at HF or B3LYP levels The hybnd dens~ty funchonal

B3LYP method sl~ghtly overestunates the stablllty of the planar stmctuns wth

delocal~zed rt-systems The geometries obtaned at the MP2 and the B3LYP levels are

very slmllar In most cases and are In good agreement wlth the avalable expenmental

results

Out of the fifteen lsomers Identified on the (CH)sSiH potentla1 energy surface,

twelve of them were charactenzed as minlma on the potentla1 energy surface, two of

them were transltlon states and one 1s a second order saddle polnt The spread of the

relatlve energies of vanous sllabenzene lsomers is substant~ally smaller compared to the

corresponding benzene valence lsomers Interestingly. wh~le benzene hes above the next

stable valence lsomer by about 70 kcal/mol, Vla, is only about 20 kcallmol higher In

energy than the most stable silabenzene (Bl) However, the relat~ve energy ordenng in

silabenzene valence lsomers 1s very slmllar to that In benzene valence lsomers In general

bamng some exceptions The hardness (q) values taken as a measure of reactlvlty

indicates that compounds havlng tetracoordmated SI atoms are reactlve compared to

those havlng tn- and dl-coordmated Si The apparent d~spar~ty In energet~c and hardness

cntena In determlning the stab~l~ty and the hlgh reactlvlty of thls class of compounds are

~nd~cative of lntncacles involved in making pred~ct~ons

S~xty SIX dls~labenzene lsomers were cons~dered, which ~ncludes (CH)I(SIH)?

valence Isomers and many other related Isomers, seventy eight statlonary polnts were

located Of these, slxty one were charactenzed as mlnlmum energy structures, 12 as

transltlon states, 4 as second order saddle polnts and 1 as a thlrd order saddle point In all

the minlmum energy structures of the b~cyclopropenyl Isomers where tncoordlnated Si

atom 1s connected to an sp3 carbon, the three membered nng opens up to y~eld

monocycl~c or acycllc isomers In Dewar benzene, benzvalene and b~cyclopropenyl

isomers, those compounds w~th Sl=Si bond, with the hydrogens connected to them are In-

plane, are computed to be transltlon states The environment in wh~cb the Si atoms are

present and the strain In the skeleton are some guldlng factors In explaining the observed

stab~ht~es In most of the cases, the lsomer where both the slhcon atoms are

tetracoordlnated IS found to be the most stable among the pos~tlonal lsomers So, the

slhcon atom has an Inherent preference to be tetracoordlnated compared to tncoordlnated,

Chapter I V Isomers of (CHW ( X = N, P, As and SIH) and C,SI~H~ 161

whlch IS attnbuted to the weak n-bonding ahrllty of SI w~th C or w~th another SI

Companson of the relat~ve energles of the valence Isomers of d~s~lahenzene and

d~phosphabenzene reveals that the posltlonal lsomers In d~s~labenzene span a wlde range

of relat~ve stah~l~tles compared to the d~phosphabenzene lsomers Thls study h~ghllghts

the nchness of C4S12H6 Isomers and importantly ~nd~cates how close are these lsomers to

each other energetically Thls leads to a sltuatlon where fac~le rearrangement among

vanous lsomerlc forms owlng to the h~gh reactlvlty of this class of compounds

The energy gaps between the varlous valence lsomers ~nvolv~ng P. As and SI

atoms are smaller compared to the benzene valence Isomers. ~ndlcat~ng smoother

rearrangements Also, the smaller magnitudes of the harmon~c trequencles correspond~ng

to skeletal reorganlzatlons. comparcd to the henzene, ~ndlcate that skeleral

rearrangements among the valence lsomers of s~lahenzene are more fac~le compared to

the parent benzene The success w~tnessed In the synthesis of phosphln~nc valence

lsomers should tr~gger synthet~c attempts towards thelr u a - and arsa- analogs and these

valence lsomers have the potentla1 to d~splay lnlrlgulng Isorner~tatton reacttons We lee1

that the present study and the recent advances In the synthes~s of phosphln~ne valence

lsomers and some aza analogs and bulky group protected s~laaromat~cs should enthuse

the expenmental~sts to explore the r~ch potentla1 of the rearrangement reactions of t h ~ s

class of compounds

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