Journal of Electron Spectroscopy and Related Phenomena, 57 (1991) 165-187 Elsevier Science Publiihers B.V., Amsterdam

165

A valence- and inner-shell electronic and photoelectron spectroscopic study of the frontier orbitals of 2,1,3- benzothiadiazole, C6H4SNz, 1,3,2,4_benzodithiadiazine, CBH4S2N2, and 1,3,5,2,4-benzotrithiadiazepine, C,H,S,N,

A.P. Hitchcock”, R.S. DeWitte”, J.M. Van Esbroeck”, P. Aebi”, C.L. Frenchb, R.T. Oakleyb and N.P.C. Westwoodb “Department of Chemistry, McMuster University, Hamilton, Ont. LAS 4Ml (Canada) bGueEph Waterloo Centre for Graduate Work in Chemistry, Guelph Campus, Department of Chemistry and Biochemistry, University of Guelph. Guelph, Ont. NlG 2Wl (Canada)

(Received 6 May 199 1)

Abstract

He1 photoelectron spectroscopy, inner-shell electron energy loss spectroscopy involving the S2p, 528, Cls and Nls edges, and Sls synchrotron radiation photoabsorption spectroscopy have been used to probe the occupied and unoccupied valence levels of 2,1,3-benzothiadiazole, 1,3,2,4&n- zodithiadiazine, and 1,3,5,2,4-benzotrithiepine. The term values obtained from the Sls, 52s and Nls oscillator strength spectra are sensitive to the aromatic/anti-aromatic character in the thiazyl ring, whereas the first ionization potentials for the latter two molecules remain relatively constant. The combination of valence photoelectron and core excitation results, aided by semi- empirical (MNDO ) molecular orbital calculations, provides a useful probe of the frontier molec- ular orbitals of these aromatic/anti-aromatic molecules.

INTRODUCTION

All planar binary sulphur nitride rings have 4n + 2 n-electron counts [ 11. This observ&ion has sparked a continued debate as to whether the concept of aromaticity is pertinent to the study of these systems. Topological resonance energies (per electron) for various known and unknown systems have been calculated 121, and the variations between 4n and 4n+ 2 systems rival those found in organic chemistry; a planar 12 n-electron S,N, molecule, for example [3 1, is predicted to be strongly anti-aromatic. The absence, however, of any experimental probe for measuring the degree of aromatic character has left these theoretical ideas untested. The rapid growth in heterocyclic sulphur ni- trogen chemistry during the last decade has provided novel examples of both aromatic and anti-aromatic benzo-SN molecules. For example, in addition to the long-known 10 n-electron 2,1,3-benzothiadiazole, 1, the 12 x-electron,

0368-2048/91/$03.50 0 1991 Elsevier Science Publishers B.V. All rights reserved.

166

1,3,2,4-benzodithiadiazine, 2 [4 J, and 14 x-electron 1,3,5,2,4benzotrithiadi- azepine, 3 [4,5], have recently been characterized. Of the three, 2 is both re- duced and oxidized most easily, and is also thermodynamically most suscep- tible to addition of norbornadiene.

A B

a = 1 N> N

1 acN>s N

While the above results are qualitatively useful, their interpretation in terms of analytical models of aromaticity is hampered by the fact that 2 and 3 are not members of an homologous series. For example, the molecular structure of 1 reflects a heavy contribution of the quinoidal valence formulation (B ) [ 61, while the structures of 2 and 3 both favour the sulphur diimide representations (A). The ground state electronic structures of l-3 have been examined [ 51,

and, consistent with the observed molecular geometries, the results of MNDO calculations on the two molecules reveal only minor interaction (especially for 3 ) between the organic and inorganic subunits. More recently the excited state manifold of 3 has been probed by MCD spectroscopy [7]. The molar elliptic- ities of the B and L bands are consistent with somewhat restricted electron delocalization around the 11-atom perimeter.

In order to probe more fully the electronic structure of these molecules, in particular the distributions and energies of the high-lying occupied and Iow- lying unoccupied molecular orbitals, i.e. those of chemical importance, we have carried out studies of these molecules using two forms of electron spectroscopy. Experimental investigations of the occupied valence level structure of moie- cules is best achieved with ultraviolet photoelectron spectroscopy (UPS ). This has been done for a variety of sulphur-nitrogen molecules and radicals, which often have extremely rich electronic structure [S-11]. In many cases the or- dering of energy levels is often supported by the use of ab initio calculations, or for larger systems, semi-empirical methods such as MNDO; Koopmans’ theorem is assumed to hold, given the infeasibility of performing calculations beyond this level on molecules of this size. As it turns out, semi-empirical MNDO calculations, with their limitations acknowledged, can provide reason- able estimates of the occupied orbital energies [ lOa, 12 1. Electron transmis-

167

sion (ET) spectroscopy, or inner shell excitation by electron energy loss (IS- EELS ) or X-ray absorption (XAS ) spectroscopies provide probes for the unoccupied levels. Indeed, localized core excited states act as a site-specific probe of unoccupied electronic structure. In principle, core excitation can give information on the spatial distribution of the final orbital, i.e. orbital mapping [ 131. Recently, an ISEELS investigation of borazine, the aromatic analogue of benzene, provided direct evidence that it consist of a delocalized x-system rather than a proposed, alternative, localized ionic structure [ 141.

EXPERIMENTAL

Starting materials

2,1,3-Benzothiadiazole, 1, was obtained commercially (Aldrich), 1,3,2,4- benzodithiadiazine, 2, and 1,3,5,2,4_benzotrithiadiazepine, 3, were prepared according to literature methods [ 5 1.

ISEBLS

The inner-shell electron energy loss spectra were obtained using techniques and apparatus described previously [ 15 1. Briefly, the spectra are the result of inelastic electron scattering through a small angle ( x 2” ) by vapour phase samples, with a high velocity incident electron beam (&= 2.5 keV plus the energy loss). Under these electron scattering conditions primarily electric di- pole transitions are excited, with intensities (after minor correction for kine- matic factors [ 161) that are very similar to those of the corresponding soft X- ray photoabsorption spectra. Because of the very low vapour pressure of 3 it was placed directly in the collision chamber of the spectrometer, which was then heated by circulating hot water. Care was taken in recording the Nls spectra to ensure there were no contributions from the strong Nk-+rt* tran- sition (401.1 eV) of Nz.

The Cls spectra were calibrated with the ti feature of CO, at 290.74 eV [ 171. The Nls spectra were calibrated with respect to the it* feature of N, at 401.1 eV [ 18 1. In these cases the calibration was carried out on a stable mixture of the gas and the sample. The S2p/S2s spectra were calibrated internally to the Cls spectra. All results were converted to absolute optical oscillator strengths (fvalues) by normalization of the isolated continuum core ionization signal to atomic oscillator strengths [ 19 ] using a procedure described and tested pre- viously [ 16 J .

X-ray absorption spectra

The Sls spectra were recorded using gas ionization and total ionization yield

168

2.0

0.0

-2.0

4

-8.0

-10.0

-12.0

-14.0

{@VI

- 7ro =2 -

TV I bl- -5.0

a2- .._ -_

- 1

_-

__--- bl -------m -7.0

WI

_G__a2-_-_----

bl----____

Fig. 1. Computed (MNDO) n-orbital energies for HNSNH and benzene, along with the a- and x- orbital energies of 1 (a-levels are offset to right). On the right are shown the average term values (TVs) of the core excitations to virtual z*NS and X* (CC) orbitala, as well as the experimental IPs. The term value scale is calibrated from the LUMO.

detection at the double crystal beam line of the Canadian Synchrotron Radia- tion Facility located at the Synchrotron Radiation Centre (SRC ) , Stoughton, WI. The monochromated X-ray flux was passed through a thin Mylar window into a 23 cm long ionization chamber which contained the sample at its equi- librium vapour pressure. For 1 and 2 the room temperature vapour pressure ( x 50 m Torr ) gave adequate ionization signals. For 3 the vapour pressure was insufficient, so the spectrum shown was measured as the sample current in- duced by the X-ray beam incident on the solid material as a coating on a con- ducting carbon tape. The energy scale was calibrated using the a,, line at 2486.0 eV in the gas phase spectrum of SFs [ 201. The oscillator strength was set at 25 eV above the estimated Sls ionization potential (IF) to the literature value for Sls ionization of atomic sulphur (9.6 x 10e4 eV_l) [ 191. {Note that the units for the vertical scale in Fig. 9 are one-tenth that of the other scales on the core excitation figures in the paper. )

169

2.0

0.0

-10.0

-12.0

-14.0

W

--__ - --__ --_ - Exp’t

TV

-5.0

Fig. 2. Computed (MNDO) n-orbital energies far HNSNSH and benzene, along with the IT- and n-orbital energies of 2 (u-levels are offset to right). See Fig. 1 for details.

HeI photdectron spectra

Two photoelectron spectrometers were used. 1 and 2 were vapourized at 25 and 40” C respectively, into the ionization chamber of a home-built instrument [ 211 with digital data acquisition via a PC/XT [ 221. Resolution was 40 meV during the experiments, and spectra were calibrated with the known (IPs of CHJ and Ar [23]. 1 has been studied previously by UPS [24-261, but it is included here for completeness. The more involatile compound 3 was investi- gated using a PS 16/18 spectrometer with a probe temperature of 60” C. Res- olution was k: 40 meV with calibration of IPs using CHBI and Nz [ 221.

Computation

Semi-empirical calculations employing the MNDO hamiltonian for heats of formation, optimized structures, and eigenvalues of 1,2 and 3 were performed

170

2.t

OS

-8.1

-10.1

-12.1

-14.t

(W

l-

)-

I-

)-

D-

D-

D-

)

7ru bi I

- *- - .’

I b- #0’ aI m-------m -___ _______b,------- -

G c- c-

_-________a*-______

b,-____--

m a*-___---

--._ --__

I.--

-bt m

-_ -__ - --a_

it:> -

-- --

Exp’t I_-___ - ---_ b I-

i

TV

-5.0

-7.0

W)

0 c N-Y/\ P \\

N-5 Al ‘/

Fig. 3. Computed (MNDO) n-orbital energies for HSNSNSH and benzene, along with the 6- and z-orbital energies of 3 (a-levels are offset to right}. See Fig. 1 for details.

on a 20 MHz 386 PC running MOPAC (Version 5.0) [ 27 ] a. Pull optimization was performed within C,, symmetry for 1 and 3, and within Cs symmetry for 2.

RESULTS AND DISCUSSION

Any discussion of the valence electronic configurations of 1,2 and 3 is in part predicated upon a consideration of the occupied and unoccupied valence manifolds. Figures l-3 show these energy levels obtained from the MNDO calculations for the three molecules of interest. Also shown are the a-orbital energies of HNSNH, HNSNSH, HSNSNSH and C6H6, these being the mo- lecular building blocks from which the three molecules are constructed. These calculations are new versions of those previously described [ 5 ], and employ the new parameters for sulphur [ 281 which give much improved heats of for-

*This version of MOPAC, coppiled with NDP F&ran 386, running under the PharLap DOS extender, gives a performance comparable to an Apollo DN4500 without an FPA.

171

TABLE 1

Experimental and calculated (MNDO, in parentheses) bond lengths (A> for 1,2 and 3”

Bond lb 2c 3d

C&b

cb-cc

G-G

G-C6

c-s

C-N

N-S(C)

S-N(C)

N-S-N bv. 1

1.46 (1.453) 1.29

(1.368) 1.46

(1.458) 1.41

(1.487)

1.34 (1.327)

-

1.60 (1.626)

1.387(N), 1.387(S) (1.414(N), 1.397(S)) 1.407(N), 1.403(S)

(1.411(N), 1.413(S)) 1.381

(1.398) 1.394

(1.432) 1.796

(1.711) 1.423

(1.395) 1.693

(1.607) 1.544

(1.553) 1.546

(1.542)

1.421 (1.438)

1.349 (1.387) 1.396

(1.417) 1.387

(1.408) 1.731

(1.690)

1.609 (1.572)

1.538 (1.519)

*Carbon atoms lettered alphabetically from the quaternary carbons. bFkf. 6. ‘Ref. 4. dR4?f. 5.

mation. The principal structural conclusions remain the same, i.e. the calcu- lated bond lengths and angles follow those obtained by X-ray diffraction (Ta- ble l), and support the dominance of the quinoidal formulation (B) for 1 and the sulphur diimide structure (A) for 2 and 3 [4-61.

Although the absolute magnitudes of the eigenvalues for the occupied and (especially) the unoccupied levels must be taken cum grano salis, the trends evident in Figs. l-3 illustrate some essential features of relevance to this work. Of particular importance is the perturbation of the frontier x-orbitals of the three sulphur diimides HNSNH, HNSNSH and HSNSNSH occasioned by the attachment of the benzene ring. Taken collectively, we view these differences as reflecting the relative merits of C (Pplr)-N (2pn) versus either C (2px)- S (3~7~) or N (2plt)-S (3pa) overlap. Accordingly, fragment orbital mixing is heaviest, and structural changes the most pronounced, in 1, least in 3 and, intermediate in 2. Both the relative and absolute values of these calculated orbital energies will be analysed in the light of the results of the inner shell and valence level spectroscopy to be described below. The effect of the NSN chro- mophore has been discussed before in the context of the UV-visible spectrum of several organic sulphur diimides [29 1.

172

I . . . . I . . . . 1.. 200 300 400

Energy Loss <eV>

Fig. 4. Electron energy loss spectra of 1 (C,H,SN, ), 2 ( CBH,SzN, ) and 3 (C&H,S,Nz) recorded with 2.5 keV final electron energy, 2” scattering angle and 0.7 eV fwhm resolution. The intensity of the Cls edge jump (I (300 eV)-I(280 eV)) has been assigned a value of one in each case to visually demonstrate that the S2p intensity scales with the number of S atoms in these species. The approximate locations of the S2pz,z, S2s, Cls and Nls ionization thresholds are indicated by the bars at the beginning of the hatched regions.

Inner-shell excitation

Overviews of the energy loss spectra of 1, 2 and 3 spanning the S2p, S2s, Cls and Nls excitation regions (Fig. 4) indicate the true signal-to-background (S : B ) ratio at each core edge and illustrate the basic sensitivity of ISEELS to elemental composition. The S2p ionization intensity relative to the Cls and Nls ionization intensities scales approximately with the number of sulphur atoms in each molecule (1: 2 : 3 for the molecules 1,2 and 3) as expected from the elemental aspects of inner-shell excitation.

The Cls oscillator strength spectra {Fig. 5, the energies, estimated term values (TV) and proposed assignments in Table 2) of 2 and 3 are similar to each other with the main Cls-,n* peaks occurring at nearly the same energy. Both Cls spectra are generally similar to that of benzene and other aromatic ring compounds [ 14,301. The features around 285.2 and 287.6 eV in both mol- ecules (identified as llry and 27r* in Table 2 ) can be attributed to core hole

173

l”~-.‘-.*~‘..‘.‘*.l 280 290 300 310

Energy Cd0

Fig. 5. Oscillator strengths for Cls excitation derived from electron energy loss spectra of 1,2 and 3 (see Fig. 4 for details). The hatched lines indicate estimates of the Cls IPs (see Tables).

states with considerable benzene-based composition. In the orbital picture ob- tained from the MNDO calculations the lx*-2n* separation is x 2.2 eV (Figs. 2 and 3). However, the Cls spectrum of 1 is markedly different (Fig. 5 and Table 3 ), as it contains two strong low-lying resonances with the second peak of even greater intensity than the first, rather than a single dominant low-lying lr peak as seen in 2 and 3. This makes it difficult to specify the W-2n* separation in 1 which is calculated to be smaller, 1.6.eV, than those of 2 and 3. The melting point, IR data, mass spectrum and UPS (below) all indicate that the ISEELS spectrum was obtained on a pure sample. Thus the most probable explanation for the apparently unusual spectrum of 1 is that the qui- noid form (A) dominates and that, in contrast to the benzene-like spectra of 2 and 3, that of 1 consists of lower energy Cls (C-C) + Ir* (C-C ) and higher energy Cls(C=N) -+n*( C-N) transitions which differ in energy largely be- cause of an appreciable chemical shift between the two Cls levels but also, because of the difference in energy of the optical orbitals of n* (C-C) and n* (C-N) character. Although the Cls XPS spectrum of 1 has not been re- ported to our knowledge (the Nls spectrum of 1 in the solid state is known [31] ), the chemical shift for a ring carbon attached to N is 1.3 eV in phenyl- amines and 1.6 eV in phenyl isocyanides [32 1. Thus, based on the shift be-

174

TABLE 2

Energies, term values and proposed assignments for features in the carbon la spectra of 2 and 3

2

Feature E (eW

3 Assignment

Feature E (eV) Z’cH Tcs (eV) (eV)

1 sh 284.44” 2 285.34”

3 287.5 4 sh 289.6 5 290.0

IPb 290.5 IPb 291.0 IPb 292.0

6sh 293(l) 7 294.0 (5) 8 301(l)

6.1 - 5.2 5.7

3.0 - 0.9 1.4 0.5 1.0

- 2.5 -3.5

-11

1 sh 6.2 2

3 sh 4.5 4 2.4 5 2.0 6

IPb lPb

7 294.3(5) -2.8 8 301(l) -11

284.3 6.2 - 285.1” 5.4 5.9 285.6 4.9 5.4 287.6 2.9 3.4 289.3 1.2 1.7 290.7 0.3

290.5 291.0

a*(N-S)? 17ic &(C-S) 27?, a* (C-S) Rydberg Rydberg

Estimated CH IP Estimated CS IP Estimated CN IP

@(C-N) ti (C-C)-lC dc(c-c)-2=

“Calibration: 2 (-5.40(5) eV relative to Cls+z* in CO, (290.74 eV); 3 (-5.64(6) eV relative to Cls++fl in CO,). bIPs estimated from experimental XPS values of related compounds: CsHe, -290.3 eV; C,H,NH,(CH), -290.2 eV; CN, -291.3 eV, &H,NC, -290.9 eV, -292.5 eV) [32]. ‘These continuum features correspond to those characteristic of Cls+ti(C-C) transitions in benzene and other 6 n-electron aromatics [ 14,301.

tween the two main peaks in the Cls spectrum of 1, we believe that there is a chemical shift of up to 2.4 eV between the two carbons attached to the heter- acyclic ring and the other four carbons. A smaller chemical shift is then likely to occur for the C-N and C-S carbons in 2 and 3. Based on XPS IPs of related molecules [ 321 (see footnotes to tables) we estimate ( t 0.5 eV) the C-H IP to be 290.5 eV in all three molecules, the C-N IP in 1 and 2 to be 292.0 eV and the C-S IP in 2 and 3 to be 291.0 eV. A similar pattern of two very strong resonances, with the higher energy peak of greater intensity, has been observed in the Cls spectrum of benzoquinone [ 34 ] , a molecule which is the paradigm of the quinoid structure. As for 1, distinct Cls-t~*(C-C) and Cls( C-O ) + 7~* (C=O) transitions, separated primarily by the Cls chemical shift, have been used to explain the spectrum. Interestingly, the low energy region of the Cls spectrum of acrylic acid [35] is also similar to that of 1 and benzoquinone, for similar reasons.

There are other differences in the Cls spectra of these three species in the 288-294 eV region. They can be explained in terms of the different numbers

175

TABLE 3

Energies, term values and proposed assignments for features in the carbon Is spectra of 1

Feature E TCH (eVJ (eV)

TCN

(eV) Assignment (final orbital)

CH CN

1 2 sh 3 sh 4 5 sh IP(CH) 6 IP(CN) 7 8 sh 9w 10 br

284.44” 6.1 - 234.8 5.7 - 286.5 4.0 5.5 286.9 3.6 5.1 290.2 0.3 1.8 290.5b 290.8 1.2 292.0b 293.4 - 2.9 -1.4 294.8 - 4.3 -2.8 298.3 7 301.4 (5) 10

1x* In*

271’ 17P w*(C-N) Rydberg

Continuum onset Rydberg

la*(CC)

Double electron excitation Zdc(CC)

Continuum onset la* (CC)

“Calibration: -6.29(5) eV relative to Cls+rt* in COz. “IPs estimated from experimental XPS values of related compounds: C6H6, - 290.3 eV; C&,NH2 (CH), - 290.2 eV, CN, -291.3 eV, G&NC, - 290.9 eV, 292.5 eV [32 1. Note that the solid phase Cls XPS spectrum of naphthabis(thiadiazole) exhibits a chemical shift of 1.3 eV between the Cls (CN) and Cls (CH) peaks, in good agreement with this estimate [33].

1

Fig. 6. Schematic diagrams of the ground state HOMOs and LUMOs of 1,2 and 3 predicted by MNDO.

176

Nls

390 400 410 420 430 Energy (eV)

Fig. 7. Oscillator strengths for Nls excitation derived from electron energy loss spectra of 1, 2 and 3 (see Fig. 4 for details).

of C-N and C-S bonds in the three compounds. Thus, according to the bond length correlation [36] one expects Cls+a*(C-N) transitions close to the core ionization threshold (around 290 eV ) . This is probably the origin in 2 of the additional breadth and intensity on the low energy side of the 294 eV fea- ture relative to the corresponding feature in 3. Similarly, Cls-+b* (C-S) tran- sitions are expected around 286-287 eV [ 371. Thus the presence of twice as many C-S bonds in 3 compared with 2 gives rise to the stronger dt (C-S) signal observed as the high energy shoulder (285.6 eV) on the Cls+lt* tran- sition in 3. We note that there is a peak with a TV of 6.2 eV observed for the lower symmetry 2; from the Nls and S2s/2p spectra (below) this is assigned to a Cls+ K* (N-S ) transition, terminating in a state which has some contri- bution from the benzene ring, unlike the corresponding LUMO in 3 which is strongly localized on the thiazyl fragment (Fig. 6 ) .

The Nls, S2s and Sls spectra are the most interesting with regard to the question posed in initiating this research since they should be sensitive to the aromatic/anti-aromatic character of the thiazyl ring .through their sampling of the N2p and S3p characters of the I1L (N-S) LUMO. The Nls spectra of 1, 2 and 3 are comparea in Fig. 7 with the energies, estimated term values and

177

TABLE 4

Energies, term values and proposed assignments for features in the nitrogen 1s spectra of 1,2 and 3

1 2 3 Assignment

Feature E T Feature E T Feature E T

1

2 3 4 IPb 5

398.6” 6.9 1 397.76” 7.8 1 398.6” 6.9 a*(N-S) 2 sh 400.1 5.4 2 sh 406.7 4.8 3s Rydberg

401.3 4.2 3 401.1 4.4 3 401.6 3.9 @(N-S) 402.7 2.8 4 402.8 3.7 3p Rydberg 404.2 1.3 4 405.3 0.2 Rydberg 405.5 IPb 405.5 IPb 405.5 Estimated 409.5 -4.0 5 407.4 -1.9 dc (N-C)

“Cabbration: 1, -2.53(6) eV relative to Nls-+R* in Nz (401.1 eV); 2, -3.43(5) eV relative to Nls+x* in N,; 3, -2.57(6) eV relative to Nls+TE* in N,. bIPs estimated from experimental XI’S values of related compounds: Nr, - 469.9 eV; N&, -465.3 eV; N& -405.6 eV [32]. In the solid state the Nls IP of 1 (399.5 eV) is 0.4 eV greater than that of solid pyridine 133 1. (Other aromatic thiadiazoles show similar Nls IPs in the solid state [33].) Based on the gas phase Nls IP of pyridine (404.9 eV 13211, this predicts the gas phase Nls IP of 1 to be 405.3 eV.

proposed assignments summarized in Table 4. The Nls+ Ir* (N-S) transition occurs at 0.84 eV lower energy in the Nls spectrum of 2 when compared with 1 or 3; the calculated (MNDO) difference in the LUMO energies is 0.73 and 0.71 eV respectively. This result is consistent with the greater aromatic char- acter, larger t~,ti) splitting and thus higher energy fl orbital in the more aromatic 1 and 3 species. Another characteristic of the Nls spectra (Fig. 7) is the systematic decrease in the intensity of the broad peak around 409 eV at- tributed to Nls-, a* (N-C > transitions. This scales in proportion to the num- ber of C-N bonds in these species (2 : 1: 0 in 1,2 and 3). We assign the rela- tively strong feature observed around 402 eV in each species to Nls+b* (N-S ) transitions.

The S2s and the near-edge region of the S2p spectra of 1,2 and 3 are pre- sented in Fig. 8 (energies, term values and proposed assignments are compiled in Tables 5 and 6). The main feature in the S2s spectrum of 2 is attributed to an unresolved overlap of S2s+7r*(N-S) and S2s-+dr (N-S) transitions. This occurs about 0.4-0.8 eV lower than the corresponding feature in the S2s spectra of 1 and 3, in accord with the calculated LUMO energies, within our assump- tion that the S2s IP of each species is the same. Although the background slopes, and thus the intensities, of the S2s spectra presented in Fig. 8 are sen- sitive to the details of the difficult background subtraction, it appears that the intensity of the S2s+dc (N-S) transitions increases in the order 1,2,3, con- sistent with the LUMO S3p character predicted by the cahzulations (Fig. 6).

178

Enargy

S2s

E b II I 12 3

Fig. 8. Oscillator strengths for the S25 spectrum and the discrete region of the S2p excitation spectrum derived from electron energy loss spectra of 1,2 and 3 (see Fig. 4 for details),

The Sls spectra are presented in Fig. 9 while energies, estimated term values and proposed assignments are given in Table 7. Sls excitation should access similar final states to those in S2s excitation. Relative to the S2s spectra, the shape and detailed structure of the Sls spectra are much better defined. This is due both to the greater edge jump and to the smaller natural linewidth. Even so, comparison of the Sls and S2s spectra shows that they are very similar, particularly in the discrete structure below the respective IPs. At each edge, 1 has a low energy tail and a small peak/large peak pattern; 2 has a large peak/ smaller peak pattern; while 3 has a single broad peak. The similarity of the Sls and S2s spectra indicates that there were no problems with photo- or sur- face-induced modifications of the sample in the sealed ionization chamber.

The low energy shoulder of the Sls spectrum of 1 is attributed to transitions to the LUMO which has mainly Ir* (C-C) character but also considerable de- localization onto the S-N ring (see Fig. 5). This feature is in effect the signa- ture of the strong quinoid character of 1 as seen from the heteroatom ring. The second peak is assigned to Sls + n* (S-N) transitions. Its estimated term value (6.8 eV) is quite similar to that for the first peak in the Nls spectrum of 1. A

179

TABLl35

Energies, term values and proposed assignmenta for features in the sulphur 2p and 2s spectra of 1 and 3

Sulphur 2p

Feature 1 3 Assignment

E (eV) T(3/2) T(1/2) E T(3/2) W/2) 3/2 l/2 (eV) (eV) (eV) (eV)

1 164.73 2 166.3 3 167.7’ 4 168.7 5 169.9 IP (3/Z) 170.5b IP (l/2) 171.6b 6 175.4 7 178.2 8 188(l) 9 201(2)

5.8 4.2 2.8 1.8 0.7

-4 -7

-17 -30

5.2 3.8 2.8 1.6

sh 165.1 166.48’

ah 168.0 169.5 171.2 171 172 173.2 174.8 176 192(2)

5.9 4.5 5.5 3.0 4.0 1.5 2.5

0.8

- 1.2 3d resonance -2.8 3d resonance -5 3d resonance

-20 EXAFS

n+(N-S) dC(S-N) n*(S-N) 4s (R, ) o+‘(S-N) % 4s (R,) Rs Rz Estimated

Sulphur 2s

Feature 1 3 Assignment

E (eV) T(eV) E (eV) T (eV)

1 229.2 6.8 229.6(3) 6.4 n*(N-S) IP (estimated) 236 236 2br 239(l) -3 238( 1) -2 a+ (N-C)?

YX!p calibration: 1, - 116.7 (1) eV relative to the 284.44 eV Cls peak; 3, -124.3(l) eVrelativetoCls-rn* in CO,. The 52s calibration of 3 wae taken from a spectrum containing both S2p and S2s sign&. bin the solid state the S2p3,2 IP of 1 is 0.6 eV above that for Solid thiophene [33 1, Based on the gas phase S2p of thiophene (170.0 eV) [ 32 1, this predicta the gas phase S2p,,, IP of 1 to be 170.5 eV.

strong transition is found at similar energy in the Sls spectra of all three spe- cies. The third peak in 1, which is the strongest feature in the spectrum, is tentatively assigned as dc (S-N) throughout the three species. Excitations to a second orbital of ti (S-N) character may also make important contribu- tions, thus explaining the very strong intensity at 2474.4 eV in 1.

The intense, sharp, first feature in the Sls spectrum of 2 is attributed to Sls-t ti (S-N ) transitions. It is followed by a broader feature attributed to the overlap of dc (S-N) resonances. Previous studies have shown that a* (S-C ) resonances occur with term values of 4.2-4.8 eV, with very little dependence on bond length [ 383. cP (S-N) resonances would be expected to occur at slightly higher term values and to show a similar independence of bond length.

The Sls spectrum of 3, which was recorded for the solid rather than the gas, is less structured than those of 1 or 2. In part this may be attributed to inter- molecular interactions in the solid state. In addition we believe the first broad, asymmetric peak to arise from the overlap of n* (S-N), dr (S-C ) and a* (S-N)

180

TA3LE 6

Energies, term values and proposed assignments for features in the sulphur 2p and 2s spectra of 2

Sulphur 2p

Feature E (cV) T(3/2) (eV)

T(W) (eV)

Assignment

3/2 l/2

1 ah 163.8 2 164.8 3 166.1 4 167.68 5 169.1 6 170.6 IPb 171 IP 172 7 174.2 8 178.2(5) 9 191(l)

7.2 . 7.2

5.1 5.4

0.4

a*(N-S)

@(N-S)

Rydberg

Estimated

R*(N-S)

b*(N-S)

Rydherg

-3 Double excitation -6 S3d

-20 S3d

Sulphur 2s

Feature E (eV) T (eV) Assignment

1 228.8(2) 7.2 ti(N-S) 2 sh 231(l) 5 ti (N-S) IP 236 3 239(l) -3 bc (N-C)?

“SZp calibration: -123.2(2) eV relative to Cls+n* in COx. bIPs estimated from experimental XPS values of related compounds [ 321; S2N2, - 172.44 eV ( 2paIz), 236.6 eV (2s). The S2paf2 IP of 1 is estimated to be 170.5 eV (see Table 5).

resonances and that this overlap results in a single broad band without clear shoulders in contrast to the more structured spectra of 2 and 3 which have fewer transitions expected in this energy range. A number of broad features are observed at higher energy above 2479 eV, the estimated Sls IP [ 39 1. These probably arise from a combination of Sls-+ S3d resonances, double excitations and extended X-ray absorption fine structure features. More detailed assign- ments of these continuum structures probably require core hole decay studies of the sort recently carried out for SF, [ 401.

The S2p spectra should be dominated by excitations to the d” (N-S) orbital, since this will have a larger S3s character than n* (N-S). In all three species the discrete features are a complex overlap of two sets of S2p excitations to orbitals with large S3s character, separated by the 1.1 eV spin-orbit splitting. The S2p spectra (Fig. 6) also reflect the trends discussed above for the Nls, S2s and Sls spectra in that the overall shape of the S2p spectra of 1 and 3 are

181

4-

3-

O- 1....1....1....1....~~..‘1

2460 2470 2480 2490 2500 251

Photon Energy GaV)

Fig. 9. Oscillator strengths for Sls excitation of 1,2 and 3 derived from X-ray total ionization yield spectra recorded at the OCMR-CSR.F double crystal beam line at SRC, Stoughton, WI. The hatched lines indicate the estimated Sls II’s [39].

similar and significantly different from that of 2. The S2p discrete region of 2 is considerably more structured than that of 1 or 3. The lesser amount of structure in 3 may simply reflect the larger number of sulphurs and the pres- ence of two sites of different symmetry (in 2 and 3 ) . The latter should give rise to greater spectral congestion owing to a larger number of transitions from possibly chemically shifted S2p levels to various unoccupied orbitals with S2s character. The fact that a rather sharp, low-lying feature is seen in 1, but does not have a counterpart in 2 or 3, may reflect the clearer character of S2p excitation in this species which cannot have any complications from chemical shifts of the core levels. As with the Nls spectrum, the similarity of the 1 and 3 spectra and their contrast with 2 would seem to be related to the aromatic/ anti-aromatic character of these heterocyclic rings.

Thus the picture of the unoccupied levels of 1,2 and 3 that emerges from the core excitation spectra is one of z* orbitals with decreasing TVs from ti (N-S), through the (split) la* benzene-like level to the 2ti benzene-like level; the 7t* (N-S) orbital in 2 lies below that in 3 by about 1 eV. Interspersed be- tween these benzene-like levels are dr (N-S) and a* (C-S) levels, the latter

182

TABLE 7

Energies, estimated term values and proposed assignments for features in the Sls spectra of 1,2 and 3

1 2 3 Assignment

Feature E (eV) G)

Feature E (eV) TeV)

Feature E T (eV) (cV)

1 sh 2

3 4 sh 5

IP 2479.0 IP 2479.0 IP 2479.0 Estimatedb

6 2484.9 -6 6 7 2493.1 -14 7 8 2504.4 -24 8

2471.1 2472.7

2474.4 2475.7 2478.8

8.9 6.3 1

2sh 4.6 3 br 3.3 4 sh 0.2 5

2472.9 6.1 1 2474.0 5.0 2475.6 3.4 2 sh 2476.3 2.2 2478.0 1.0

3 2484.0 -1 Double excitation 2484.1 -5 4 2483.1 -4 S3d? 2493.3 - 13 5 2489.3 -10 S3d? 2496.8 -18 6 2495.7 -17 EXAFS?

7r* (C-C) 2473.2 5.8 x* (S-N)

a* (S-C) 2474.9 4.1 dc (S-N)

4~ (Rydberg) 5~ (Rydbecd

“T= IP - E. Note that the accuracy of these values is dependent on the accuracy of the estimate of the Sls IPs. bValue estimated from the Sls IPs ofother species, e.g. RSH, - 2477.8 eV; RSR, 2478 eV (R = CH, or C2H,; Me,SO, 2480.4 eV [39]. There do not appear to be any data for SN compounds so the accuracy of this estimate is difficult to evaluate.

being identified by their greater intensity in 3. The Cls spectrum of 1 provides clear evidence for distinct ti (C-C ) and R* (C-N ) upper levels, consistent with a strong quinoid character of this molecule. The calculated eigenvalues for the ground state unoccupied orbitals (Table 8) are generally consistent with these trends. Table 8 also includes the eigenvalues of the ground state occupied or- bitals which are of relevance to the He1 spectrum.

Hdphotoekctron spectra

The photoelectron spectra of 1, 2 and 3 are shown in Fig. 10, with the ex- perimental IPs compiled in Table 9. As is evident from both Table 8 and Fig. 10 only distinct bands up to about 12 eV can be assigned with any certainty. The UPS spectrum of the 10 n-electron malecule 1 has been observed before [ 24-261, and in the last instance was assigned with the aid of ab initio calcu- lations. The sharp first IP at 8.95 eV with some associated vibrational struc- ture, relatively high in binding energy, is associated with the a,, z orbital (node at S), which is N-N antibonding. The second IP arising from an orbital (b,)

183

TABLE 8

Calculated (MNDO) occupied/unoccupied orbital energies (eV) for 1,2 and 3”

1 2 3

Unoccupied 3.33 b, 1.96 a2 1.69 aI 1.22 b, 0.50 bl 0.17 a2

- 1.35 b,

Occupied -9.53 a2 - 9.90 b,

- 11.99 al - 12.15 b, - 12.28 bI - 12.33 a2 - 13.71 aI - 13.93 bz - 14.15 aI - 15.43 bI - 15.55 b2

3.40 a’ 2.31 a” 1.85 a’ 1.16 a’ 0.46 a’ 0.36 a”

- 0.07 a” - 2.06 an

- 8.05 arr - 9.50 an

- 10.57 a” - 10.90 a’ - 12.26 a’ - 12.66 a’ - 12.83 a’ - 13.15 a” - 13.39 a’ - 13.88 a’ - 14.72 a’ - 15.47 a“ - 15.73 a’

3.55 b2 2.10 a2 1.96bz 1.36 a, 0.86 aI 0.14 b2

-0.02 b, -0.32 a2 - 1.33 bI

- 8.15 a2 - 8.93 b,

- 10.01 a2 - 10.54 b2 -11.04 b, - 12.17 a1 - 12.65 a, - 13.02 a2 - 13.24 bz - 13.32 a, - 13.50 b, - 13.91 bz - 15.27 b, - 15.45 b,

“HOMO and LUMO distributions are shown specifically in Fig. 6. Correlation lines in Figs. l-3 indicate approximate orbital constitution of remaining z-orbit&.

comprising principally the S lone pair, is close in energy (9.50 eV), with no vibrational fine structure. Beyond this there is some disagreement with the assignments [24-261, but in the absence of other information we are inclined to follow the ab initio results [ 241.

For 2 and 3 the three uppermost occupied levels in each case are x as indi- cated by the relatively sharp first few IPs, although the distribution of atomic orbital contributions is different in each case (Fig. 6), reflecting the addition of successive sulphur atoms, and the additional z electrons associated with these increasingly electron rich systems. Thus the simple N-N antibonding combination in the 10 x-electron molecule 1 now has some S3p character mixed in for the 121~ system 2, and even more for the 141~ 3. In the case of 2, the HOMO is an asymmetric distribution of N-S a-antibonding, whereas for 3 the HOMO is a unique a2 orbital comprising two separated, antibonding N-S frag-

3 I Ar

8 IO I2 14 16 ev lONlZAT,ON POTENTIAL

Fig. 10. He1 photoelectron spectra of 1,2 and 3.

ments, resulting in a sharper photoelectron band with some vibrational struc- ture (1070 + 450 cm-l). For 3, this IP is 0.7 eV lower than that in the parent 1,3,5,2,4_benzotrithiadiazepine molecule [ 411, indicating the destabilizing ef- fect of attaching a benzene ring relative to a simple olefmic linkage.

The second IP of 3 (8.54 eV) is associated with a large S p7c contribution, whereas the corresponding band of 2 has a larger contribution from the ben- zene ring, as reflected in the higher IP (9.30 eV) , and slightly broader band. The third distinctive band in both 2 and 3 with a maximum at 10.3 and 9.9 eV respectively, contains two IPs (low energy shoulders are observed on both bands), attributable to a 0 plus a R orbital, the latter having a large benzene component. The higher IPs then become somewhat more cluttered, and the listing/correlation of experimental and calculated IPs in Tables 8 and 9 simply reflect the best match; by and large the distribution of experimental IPs follows the calculated values fairly closely, albeit giving IPs a little too high. This is quite typical for these kind of molecules as illustrated by an analysis of MNDO calculated and observed IPs for some 12 SN molecules with the NSN moiety, which have been unambiguously assigned [ lOa,12].

185

TABLE 9

Experimental IPs (eV) for 1,2 and 3”

1 2 3

8.95 7.84 7.88

9.50 9.30 8.54

10.66 11.32 11.7 (sh)

12.74 13.53

14.76 15.75 16.53

10.1 {ah) 9.6(sh) 10.33 9.87

11.65 12.22 (sh) 12.65

13.94

15.10 14.0 15.87 14.7

11.0 11.4

12.5 13.0 {sh )

“The spacing in the columns reflects the distribution of IPs in the He1 photoelectron spectra.

CONCLUDING REMARKS

As mentioned initially, the structural parameters of 1,2 and 3 reveal vary- ing degrees of quinoidal involvement which can be traced to the extent of in- teraction between the benzene ring and the N2S, (x= 1,2,3 ) fragment. These same factors also influence the orbital energies of the three molecules, offset- ting, for example, the oscillation expected for the first IPs of an homologous series of 10, 12 and 14 x-perimeters. The similarity of the first IPs of 2 (7.84 eV) and 3 (7.88 eV) can be understood within this context. On the basis of aromaticity one would expect the first IP of 3 to be increased relative to that of 2 [ 421 b. However, the absence of the ideal homologous pattern makes such an expectation unrealistic. By virtue of its capability to incorporate some qui- noidal stabilization (it can form one C=N bond), the ground state of 2, and hence its HOMO, is stabilized relative to that of 3. This result is in accord with the calculated IPs for 2 and 3, which show a spread of only 0.1 eV (Figs. 2 and 3, and Table 8). The aromaticity of 3 is therefore not immediately apparent from the occupied levels. In contrast to 3, the quinoidal structure of 1 stabilizes

bA classical example of the oscillation in first IPs expected for an homologous series of 4n and 4n+ 2 z-systems is observed in the fluorophosphazenes (F2PN), (n = 3-6) [42 1.

186

the orbital energies relative to that expected from the aromatic structure, and leads to a low-lying HOMO. The virtual levels appear to retain the relative ordering expected from a perimeter model. The LUMO energy of 3, as deter- mined by ISEELS and predicted by calculation, lies well above that of 2. The strong contribution of the quinoid form of 1 and the predominant benzenoid form of 2 and 3 is unambiguously demonstrated by the Cls spectra.

From the analysis of the UPS, ISEELS and synchrotron XAS data it is apparent that the experimental results, and the trends shown therein, are fol- lowed reasonably closely by the semi-empirical calculations. As these mole- cules are not part of an homologous series, both the quinoidal versus non- quinoidal character and the aromatic/anti-aromatic concepts need to be con- sidered in the analysis. These results refine the picture of frontier orbital ener- gies based on simple perimeter model notions.

ACKNOWLEDGEMENTS

We thank the Natural Sciences and Engineering Research Council of Can- ada for financial support in the form of operating grants (to A.P.H., R.T.O. and N.P.C.W.) and an NSERC Summer Research Scholarship (to J.M.vE.). A.T. Wen is thanked for his assistance with spectral acquisition. We thank the CSRF team, particularly RX. Yang, for their assistance with the new double crystal beamline used for the Sls measurements. This facility is funded by the Ontario Centre for Materials Research and operated by the Canadian Syn- chrotron Radiation Facility. The Synchrotron Radiation Facility in Stough- ton, WI is funded by NSF and operated by the University of Wisconsin- Madison.

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