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On the stereoelectronic effects governing the rotational isomerism

of 1,2-di-haloethanes

Felipe R. Souza a, Matheus P. Freitas a,*, Roberto Rittner b

a Departamento de Qumica, Universidade Federal de Lavras, C.P. 3037, 37200-000 Lavras, MG, Brazilb Chemistry Institute, State University of Campinas, P.O. Box 6154, 13084-971 Campinas, SP, Brazil

a r t i c l e i n f o

Article history:Received 18 April 2008Received in revised form 3 June 2008Accepted 3 June 2008Available online 10 June 2008

Keywords:

Conformational analysisHyperconjugationClassical effects1,2-Di-haloethanes

a b s t r a c t

The rotational isomerism of 1,2-di-haloethanes, namely 1,2-difluoro- (1), 1,2-dichloro- (2), 1,2-dibromo-(3) and 1,2-diiodo-ethane (4), was theoretically evaluated in this work. Our DFT calculations showed thatthe gauche conformer is prevalent in 1, whilst the trans form is largely favoured in the remaining com-pounds, at the isolated state. NBO analyses revealed that the fluorinated compound experiences thegauche effect, which is due to a CAH/CAF* hyperconjugation. Classical steric/electrostatic repulsionand hyperconjugative interactions are more competitive in 2, and the CAX/CAX* electron delocalization(X = Br and I) strongly favours the trans conformer in 3 and 4.

2008 Elsevier B.V. All rights reserved.

1. Introduction

The rotational isomerism of compounds containing the 1,2-difluoroethane fragment has been extensively studied and the syn-clinal preference often attributed to thegauche effect, the tendencyof keeping close certain substituents [15]. The origin of this effectfor such compounds is assumed to be due to antiperiplanar CAH/CAF* hyperconjugation, though other recent interpretations havebeen applied to different systems having vicinal electronegativesubstituents [6,7]. In fluorinated organic compounds, the halogenbehaves as a fluoride, because of the great ability of CAF antibond-ing orbital in receiving electrons, according to the resonance struc-tures ofFig. 1.

However, the other CAhalogen bonds (halogen = Cl, Br and I)are much better electron acceptors than CAF, which is influencedby the CAhalogen interaction matrix and halogen electronegativity

[8]. The issue about this apparent paradox may be properly ad-dressed when the donor orbital, the CAH bond, is taken into ac-count; which is supposed to be a better electron donor than CAF,and this explains the gauche preference in 1,2-difluoroethane andanalogues. Rablen et al. [9] has also invoked similar hyperconjuga-tive interactions to justify the conformational preferences of a ser-ies of 2-substituted-1-fluoroethanes.

NMR results for 1,2-dichloro- and 1,2-dibromopropane suggestthat the trans conformer is largely favoured in various media [10],in disagreement with the expected behavior if one considers CACl*

and CABr* as better electron acceptors than CAF* and assumesCAH as a good electron donor. Clearly, competing effects are then

operating in such systems, but probably not only classical steric/electrostatic interactions andCAH hydrogen bonding, as previouslyreported [11]. Thus, in order to completely understand andquantifytheelectronacceptor/donorabilityin 1,2-di-haloethanes, whichcanbe extended to analogous systems, as well as to establish a parallel-ism among halogenated substituents, the rotational isomerism of1,2-difluoro- (1), 1,2-dichloro (2), 1,2-dibromo- (3) and 1,2-diiodo-ethane (4) was theoretically investigated in this work.

2. Computational methods

Potential energy surfaces (PES) were built for 1-4 at the B3LYP/6-31 g(d,p) level (B3LYP/LANL2DZ for 4) by scanning the XCCX

dihedral angle (X = halogen) in steps of 10. Subsequently, minimafor each compound (gauche and trans conformers) were then opti-mized at the B3LYP/aug-cc-pVTZ level (B3LYP/aug-cc-pVTZ-PP[12]for 4). Natural bond orbital (NBO) analysis calculations [13],including deletion of all Rydberg and antibonding orbitals, as wellas the determination of total NBO steric exchange energies, werecarried out at the same level of theory, using the Gaussian 03 pro-gram [14].

3. Results and discussion

The conformational equilibrium of the fluorinated compound 1was theoretically investigated in the present study and the results

0166-1280/$ - see front matter 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.theochem.2008.06.003

* Corresponding author. Tel.: +55 35 3829 1891; fax: +55 35 3829 1271.E-mail address: matheus@ufla.br (M.P. Freitas).

Journal of Molecular Structure: THEOCHEM 863 (2008) 137140

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compare reasonably well with those previously presented in theliterature [3], i.e. the gauche conformer is more stable than thetrans conformer, and thus experiences thegauche effect, whose ori-gin is attributed to an attractive antiperiplanarrCH?r*CF hyper-conjugation. According to our calculations, the gauche conformeris 1.0 kcal mol1 more stable than the trans conformer (Table 1),and the referred hyperconjugative contribution is of ca.4.5 kcal mol1 (Table 2). Influence of such interaction on the con-formational equilibrium of 1 may be properly visualized in thePES of Fig. 2a; the gauche conformer of a hypothetical system,where all interactions involving both r*CF orbitals are removed,becomes a metastable conformer, i.e. it does not appear as a globalminimum anymore, such as in the real system. This evidences the

significant contribution of the electron delocalization involving ther*CF orbitals for the gauche stabilization, although the NBO stericresults also seem to favour the gauche conformer (Table 1) by ca.1.2 kcal mol1. Moreover, the hypothetical structuresyn is loweredin energy (in comparison with the real system) after removinginteractions ofr*CF orbitals (Fig. 2a), suggesting that synperipla-narrCH?r*CF interaction is a secondary force ruling the rotationabarrier in 1. In fact, NBO analysis for the optimized transition states(TS, eclipsed structures) shows that the synperiplanarrCH?r*CFinteraction for the TS structure with CAH bond eclipsed to CAF

is ca. 2.6 kcal mol1, significantly larger than the remaining syn-periplanarinteractions (Table 3). This may be confirmed by analyz-ing the FCCF dihedral angle of the gauche conformer of 1, asalso discussed elsewhere [3]; which is about 11.7 larger than theusual 60, approaching vicinal CAH and CAF bonds and then max-imizing syn-type CAH/CAF* overlap, and it is also larger than thosefor the remaining studied compounds (Table 1). Also, the gaucheconformer appears as a transition state after deletion of all hyper-conjugative interactions (the syn and trans conformers are the sta-ble forms in this hypothetical case), indicating that the sum ofother hyperconjugative interactions are important in describingthe conformational isomerism of1.

Thus, the r*CF orbital acts as an important electron acceptor,

which definitely rules the conformational preferences of1. How-ever, the corresponding antibonding orbital for the other haloge-nated compounds is assumed to be a significantly better electronacceptor, although their trans conformer has shown to be stronglyfavoured in the gas state (trans is more stable by 1.6, 2.4 and3.4 kcal mol1 in 2, 3 and 4, respectively; Table 1). This suggeststhat gauche steric/electrostatic interactions compete in a greaterextent with the rCH?r*CX hyperconjugation (X = Cl, Br and I) inthe gauche form, and that a better electron donor thanrCF, namelyrCX, is importantly operating in these systems. In order to confirm

Table 1

Energies (kcal mol1) and structural parameters obtained through the DFT calculations, for the optimized conformer structures of compounds 14

Parameters 1 2 3 4 gauche trans gauche trans gauche trans gauche trans

ERel 0 1.01 1.60 0 2.43 0 3.41 0EHyperconjugation 224.20 219.53 147.64 145.46 136.61 135.58 113.42 113.46ESteric 70.04 71.27 58.16 56.97 52.71 51.83 48.64 47.62rCX () 1.39 1.39 1.81 1.81 1.97 1.98 2.17 2.19rCC () 1.50 1.52 1.51 1.51 1.51 1.50 1.51 1.50rCH

a () 1.09 1.09 1.09 1.08 1.09 1.08 1.09 1.08rCH

b () 1.09 1.08 1.08 1.08\CCX () 110.78 108.16 112.85 109.09 113.47 108.99 114.44 109.45\CCH

a () 109.72 111.13 109.34 111.79 109.62 112.57 109.84 113.27\CCH

b () 110.91 111.71 112.44 112.96\HCH () 109.73 109.69 109.55 109.96 109.77 110.41 109.61 110.55\HCX

a () 107.86 108.31 106.31 106.98 105.14 105.93 104.05 104.81\HCX

b () 107.77 106.89 106.10 105.49hHCCX

a () 169.29 61.23 171.96 61.88 171.98 62.79 172.37 63.45hHCCX

b () 47.91 50.53 49.59 49.66hXCCX () 71.72 180.00 69.95 180.00 70.83 180.00 71.07 180.00

a For CAH trans to the vicinal CAH bond.b For CAH trans to the vicinal CAF bond.

Table 2

Antiperiplanarhyperconjugative interactions (kcal mol1) for the conformers of 14

Interaction 1 2 3 4

gauche trans gauche trans gauche trans gauche trans

rCH ? r*CH 2.26 (2) 2.09 (4) 2.80 (2) 2.73 (4) 2.84 (2) 2.87 (4) 2.94 (2) 3.09 (4)rCH ? r*CX 4.49 (2) 5.62 (2) 6.19 (2) 6.44 (2) rCX ? r*CH 0.66 (2) 1.70 (2) 2.27 (2) 2.85 (2) rCX ? r*CX 1.31 (2) 3.75 (2) 5.27 (2) 7.09 (2)LPX ? r*CH 0.61 (2) 0.87 (2) 0.85 (2) 0.74 (2)

LPX ? r*CX 1.43 (2) 1.87 (2) 1.93 (2) 1.76 (2)

H

FF

H

H

HH

H

F

H

F-

H+

F

H

H

H

F-

H+

Fig. 1. Resonance structures of 1,2-difluoroethane, as a result of hyperconjugation.

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this, NBO analysis was performed to account for the steric energiesand hyperconjugative interactions in 24 (Table 1). It has beendemonstrated that hyperconjugative interactions are usually over-estimated by NBO [15], but our results are especially useful to ex-plain the trends along the series of the halogenated compounds.Whilst the steric hindrance between halogens in 24 destabilizesthe gauche conformer, the hyperconjugation energies, whichstrongly favoured thegauche conformer in 1, shows a progressivelyinversion on going from 2 to 4. This is due to the strong contribu-tion of the rCX?r*CX interaction, especially in 4, where it corre-sponds to ca. 7.1 kcal mol1.

The effect of deleting both r*CX orbital and all hyperconjuga-tions on the conformational isomerism of 24 may be illustratedthrough the analysis of the torsional curves in Fig. 2(bd). Thebehaviour of PESs differs a lot from that observed for 1. The curvesof the real system and those corresponding to the hypothetical sys-

tem with ther

*CX orbitals removed are almost congruent for 2.This shows that there is not a marked influence of r*CCl-basedinteractions on the rotational isomerism of 2, since therCH?r*CCl interaction (in gauche) is somewhat equivalent totherCCl?r*CCl interaction (in trans). Thus, other electron delo-calization processes (as demonstrated by the appearance of justone minimum in the PES withall hyperconjugative interactions de-leted) and classical steric/electrostatic effects must be governingthe rotational isomerism of 2. However, the figure changes for 3and 4, in which a great increase in energy for the unstable syn formis observed after removing interactions ofr*CBr andr*CI orbitals,illustrating that r*CX-based interactions in the syn transitionstate, such as those presented in Table 3, strongly contribute forthe rotational barrier in 3 and 4. In addition, a hypothetical con-former gauche becomes prevalent after the removal of interactionsinvolving such orbitals. This reveals that r*CBr and r*CI orbitals

0 20 40 60 80 100 120 140 160 180

0

1

2

3

4

5

6

7

8

F

F

F

F

Relativee

nergy,

kcalmol-1

Dihedral angle F-C-C-F,

No deletion

No Hyperconjugation

C-F* deleted

0 20 40 60 80 100 120 140 160 180

0

2

4

6

8

10

12

Cl

ClCl

Cl

Relativeenergy,

kcalmol-1

Dihedral angle Cl-C-C-Cl,

No deletion

No Hyperconjugation

C-Cl* deleted

0 20 40 60 80 100 120 140 160 180

0

2

4

6

8

10

12

14

16

18

Br

Br

Br

Br

Relativeenergy,

kcalmol-1

Dihedral angle Br-C-C-Br,

No deletion

No Hyperconjugation

C-Br* deleted

0 20 40 60 80 100 120 140 160 180

0

5

10

15

20

25

30

35

40

I

I

I

I

Relativeene

rgy,

kcalmol-1

Dihedral angle I-C-C-I,

No deletion

No Hyperconjugation

C-I* deleted

Fig. 2. Dihedral angle dependencies of the energy for the real systems (no deletion), hypothetical systems with all hyperconjugative interactions absent (nohyperconjugation) and hypothetical systems with interactions involving r*CX orbital removed.

Table 3

Hyperconjugative interactions (kcal mol1) for the transition states of 14

Interaction hFCCF (1) hClCCCl (2) hBrCCBr (3) hICCI (4)

0 126.74 0.05 117.06 0.05 117.05 0.04 114.41

rCH?r*CHa 0.66 0.66 0.82 0.78 0.89 0.82 0.94 0.85

rCH?r*CHb 0.62 0.79 0.58 0.82 0.66 0.85 0.74 0.83

rCH?r*CXa 2.61 1.09 0.82 0.50

rCH?r*CXb 0.83 0.84 2.12 1.87 2.60 2.41 2.99 2.88

rCX?r*CHa 0.54 0.74 1.12

rCX?r*CHb 0.57 0.87 1.19

rCX?r*CXa

rCX?r*CXb 1.09 1.39 1.66

a Synperiplanar.b Anticlinal.

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are especially important to receive electron density from the corre-sponding bonding orbital in the trans conformer.

In order to find structural evidences for such interactions, bonddistances and angles were analyzed taking into account the reso-nance structures shown in Fig. 1, despite the expected additionaleffects of steric hindrance between halogens in the gauche con-former leading to some incongruent results. These resonancestructures indicate that some of the geometric parameters are lar-ger for the gauche conformer of 1 and smaller for 3 and 4, whilenegligible differences are observed between gauche and trans con-formers for 2. This is true for the CAX bond length, while almostthe same values were obtained for CAH and CAC bond lengths.Moreover, the HACAH angle is larger for the trans conformer of2, 3 and 4 in relation to the gauche conformer, in opposition tocompound 1, and similarly with the CACAH and HACAX angles(with exception of compound 1). These results correlate well withour NBO energies.

4. Conclusions

We have found that ethane fragments containing electronega-tive atoms or good electrons acceptors, as vicinal substituents, donot follow the general rule of the gauche effect. Classical effectsand the electron donor ability are features strongly recommendedto account for the rotational isomerismof 1,2-di-haloethanes. In 24, the rCH?r*CX interaction in gauche, which is up to now thegeneral source of stabilization by hyperconjugation, proves to beof similar size only as rCX?r*CX in trans.

Acknowledgements

Authors thank FAPEMIG (Grant No. CEX 415/06) and FAPESP(Grant No. 2005/59649-0) for the financial support of this research.

FAPEMIG is also gratefully acknowledged for the studentship(F.R.S.), as is CNPq for the fellowships (M.P.F. and R.R.).

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