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Ab initio study of torsional potentials in 2,2’-bithiophene and 3,4’- and 3,3’-dimethyl-2,2’-bithiophene as models of the backbone flexibility in polythiophene and poly(3-methylthiophene)

V. Hernandez and J. T. Lopez Navarretea) Departamento de Quimica Fhica, Universidad de Malaga, 29071-bfalaga, Spain

(Received 30 November 1993; accepted 30 March 1994)

Ab initio molecular orbital theory is employed to calculate the gas-phase barrier to internal rotation in 2,2’-bithiophene. Ground state geometries are fully optimized at the restricted Hat-tree-Fock level of theory using the 3-21G* and 6-31G ** basis sets. Methylation in @positions modulates the geometry, the inter-ring twist angle and the conformational properties of thiophene dimers. These methyl substitution effects have been assessed by calculations on the 3,4’ and 3,3’-dimethyl derivatives in a number of selected conformations. A meaningful picture of the molecular relaxation on rotation is attained by allowing for full geometry optimization at both levels of calculation.

I. INTRODUCTION

Polyconjugated conducting polymers are a class of materials which show very large nonlinear optical responses and become good electrical conductors when suitable doped.tm5 Several works have focused on the synthesis and characterization of polythiophene (PTh) and poly(3-alkylthiophenes) (P3ATh’s). Unlike PTh, P3ATh’s are processable if the alkyl side chains are sufficiently long; i.e., longer than butyl group.6 This processability makes the P3ATh’s particularly interesting for various applications. A thorough understand- ing of the physical properties of polythiophene requires knowledge about their structure. Very often the direct investigation of molecular properties and structure-property rela- tionships is limited by the low degree of order attained in these materials, because of mislinkages, saturated sites, and conformational distorsions.

The molecular origin of the electrical properties of these polyconjugated materials requires that rr delocalization fa- vors intramolecular charge carrier hopping; when this hopping is hindered by some kind of barrier (chemical defects, conformational distorsions, chain ends, etc.) intermolecular hopping may occur. Such contribution depends in turn on the supramolecular organization of the material. Disentangling the contributions of these two phenomena is not easy. Very recently Zerbi et al7 have faced this task by analysing spec- troscopically a series of ,Q?‘-bridged-2,2’-bithiophenes in which the conformation is partially frozen and held fixed by a suitable functionalization.

Thiophene oligomers may show optical and electrical properties comparable to those of the polymer; their use for molecular electronics and optical devices has recently been suggested.*-” By example, it has been shown that

“Author to whom correspondence should be addressed.

sexithiophene presents a field-effect mobility higher than that of polythiophene.” This improvement in the molecular properties has been attributed to the better structural organization (typically in layers) of the oligomers, since charge transport in these short chain molecules occurs through face-to-face intermolecular exchange. Alkyl-thiophene oligomers with a well-defined structure have also been used instead of mono- mers as starting materials for electrochemical preparation of polyalkylthiophenes. l1

Intramolecular delocalization of r electrons in the five- membered heteroaromatics depends on the extent of the overlapping of the pZ orbitals of the carbon atoms in the (Y positions. Such overlapping is modulated by the conformation. It would be important to get information on molecular parameters such as the inter-ring bond lengths or torsional angles, when substituents are present in the thiophene rings.

There have been some theoretical studies dealing with the geometry and the conformation of unsubstituted and alkyl-substituted thiophene oligomers,“-i5 as these properties critically affect the mean conjugation length that can be achieved in the polymer chain and are, consequently, crucial in determining the optical and electronic properties.‘6*‘7

Recently Kofranek et aZ.,*’ Samdal et al.,‘* and Diste- fano et al. t9 have reported ab initio Hartree-Fock calculations on gaseous 2,2’-bithiophene using different basis set levels. To gain a deeper insight into the influence of ring substituents on the polymer properties, we present here a theoretical analysis of the effects induced by methyl groups on the conformational and geometrical properties of 2,2’-bithiophene. The main purpose of the present work is to investigate theoretically the equilibrium conformations and torsional potentials in three molecules which are models for PTh and P3MeTh; 2,2’-bithiophene (2Th), 3,4’-dimethyl- 2,2’-bithiophene (2Th34), and 3,3,‘-dimethyl-2,2’- bithiophene (2Th33) (scheme I).

J. Chem. Phys. 101 (2), 15 July 1994 0021-9606/94/i 01(2)/l 369/9/$6.00 0 1994 American Institute of Physics 1369

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1370

II. CALCULATIONS

All calculations were performed using the GAUSSIAN-90 program2o on a Convex 240 at the CICA Computer Center of Sevilla, (Spain). Ground state geometries were optimized at the restricted Hartree-Fock level of theory. Two standard basis sets have been used; i.e., 3-21G* (Ref. 21) and 6-31G**.22 The first basis includes a set of d-symmetry po- larization functions for the second-row elements. The second basis includes a set of six second order (d-type) Gaussian primitives to the description of each heavy (nonhydrogen) atom and a single set of Gaussian p-type functions to each hydrogen atom.

Calculations on 2Th and 2Th33 were performed assuming that both molecules belong to the Cl,, and CZv symmetry point groups in the anti and syn conformations, respectively. A C2 symmetry was imposed to all twisted conformers. Geo- metrical parameters for 2Th34 were allowed to vary indepen- dently apart from planarity of the ring and methyl groups internal geometries, kept fixed to the values optimized for 2Th33 in the orthogonal conformation. The absence of symmetry in the 2Th34 molecule and the required computer time

V. Hemandez and J. T. Lopez Navarrete: Ab initio study of torsional potentials

preclude for the moment to perform 6-3 lG** calculations on conformations other than the absolute minimal one. In future we are interested in extending the present calculations to other conformers. Possible influence of electron correlation on the potential energy curves will be also investigated.

Calculations of gas phase barriers to internal rotation were done by fixing the torsion dihedral angle 4 (CsC2C9Cte angle in scheme II)

at selected values of 0” (syn), 45”, 90”, 145”, and 180” (anti). The energy differences are always relative to the corresponding absolute minimum conformation. The geometries were completely optimized along the torsional potential curve to account for the molecular relaxation, yielding a physically meaningful picture of nonrigid rotation. Perturbations caused by the introduction of methyl groups in @positions will be summarized throughout the text.

Ill. RESULTS AND DISCUSSION

The importance of the use of a large basis set for conformational energy calculations by ab initio molecular orbital (MO) methods has been claimed repeatedly.23-30 The energy difference between rotamers calculated at the HF level using a crude basis set often does not agree with the experimental energy difference, since such a calculation has a large error and cannot reproduce the small energy difference correctly. Calculated conformational energies using a reasonably large basis set have, however, a high accuracy and show good agreement with the experimental values.23-30 On the other hand, the traditional use of a rigid rotor model usually results in a rather rough artificial potential function on which some nonphysical additional conditions are imposed. This approach is particularly risky in systems bearing

J. Chem. Phys., Vol. 101, No. 2, 15 July 1994


V. Hemandez and J. T. Lopez Navarrete: Ab initio study of torsional potentials

TABLE I. Bond lengths (A) and angles (deg) of 2,2’-bithiophene.

1371

4 Basis set

0 90 180 44.3 43.9 146.3 147.4

3-2lG* 6-31G** 3-21G* 6-31G** 3-21G* 6-31G** 3-21G* 6-31G** 3-21G* 6-31G** Expt.a Expt.b

c2-s, 1.737 I.740 1.735 1.739 1.734 1.739 1.735 1.739 1.738 1.741 1.733 c3-c2 1.355 1.354 1.353 1.351 1.350 1.349 1.353 1.351 1.354 1.352 1.370 G-C, 1.432 1.431 1.434 1.433 1.435 1.434 1.434 1.433 1.433 1.433 1.452 G-C4 1.347 1.344 1.348 1.345 1.348 1.345 1.347 1.344 1.347 1.343 1.363 %Sl 1.719 1.723 1.720 1.724 1.719 1.723 1.721 1.725 1.721 1.721 1.719 G-C2 1.459 1.467 1.460 1.467 1.469 1.475 1.458 1.465 1.456 1.464 1.456 b-C3 1.068 1.073 1.069 1.074 1.069 1.074 1.069 1.074 1.070 1.074 1.124 H7-C4 1.069 1.074 1.069 1.074 1.069 1.074 1.069 1.074 1.069 1.074 1.123 Q-C, 1.067 1.071 1.067 1.071 1.068 1.071 1.067 1.071 1.067 1.071 1.122 c,-s,-c, 91.80 91.76 91.55 91.53 91.50 91.49 91.61 91.58 91.67 91.66 91.7 c3-c2-s, 110.60 110.41 111.04 110.81 111.17 110.86 110.98 110.77 110.71 110.54 111.8 c4-c3-c2 113.17 113.38 112.93 113.18 112.91 113.20 112.96 113.17 113.17 113.26 111.9 c,-c,-c, 112.57 112.67 112.48 112.57 112.37 112.48 112.51 112.65 112.51 112.62 112.3 %c2-c3 127.04 127.32 126.79 127.07 127.22 127.78 127.82 128.29 128.87 128.32 126.3 kC3-c4 123.23 123.12 123.96 123.88 124.07 123.98 123.70 123.60 123.27 123.17 125.0 H,-C,-C, 123.47 123.60 123.58 123.70 123.64 123.75 123.52 123.63 123.48 123.60 123.63 HB-CS-C4 127.33 128.06 127.12 127.84 126.96 127.78 127.15 127.88 127.28 128.01 127.87

1.717 1.357 1.433 1.357 1.717 1.480 1.08

1.08 92.02

120.1

‘From Ref. 18. bFrom Ref. 33.

bulky side groups which make the internal rotation sterically hindered. Meaningful comparison with experiment should be only possible if deviations from rigid rotor behavior are small, i.e., there are not other coupled floppy degree of free- dom. We think that RHF/3-21G* and ECHF/6-31G** calculations including full geometry optimization offer reasonable compromises between reliability and applicability, since gen- erally the more accurate the method, the smaller the size of the molecule that can be feasible studied.

Tabulated in Tables I-III are the structural parameters for the studied conformers of 2Th, 2Th33, and 2Th34 (see scheme I for atom numbering). Plotted in Fig. 1 are the

3-21G* and 6-31G** internal torsion potential functions for 2Th, allowing the molecular geometry to relax on changing qb. The corresponding internal torsion potentials for 2Th33 (3-21G* and 6-31G**) and 2Th34 (only 3-21G* results) are plotted in Figs. 2 and 3, respectively. The variations of the Mulliken atomic charges of 2Th33 (3-21G* and 6-31G**) and 2Th34 (3-21G*) as a function of the molecular conformation are summarized in Tables IV and V.

A. 2,2’-bithiophene

Some experimental data on the conformational prefer- ences of 27% have been reported by Bucci er ~1.~~ These

TABLE II. Bond lengths (A) and angles (deg) of 3,3’-dimethyl-2,2’-bithiophene.

4 Basis set

c2-s,

c3-c2

c4-c3

c5-c4

GSI G-C2

G-C3

H7-C4

WC, H17-C6

HI8-C6

CT*-s,-cs c3-c2-s,

c4-c3-cz cs-c,-c, +c2-c3

c,-q-c, H7-C4-C3 H8-CS-C4 H,,-C,-C3

Hl8-C6-C3

0 45 145 180 94.0 94.2

3-21G* 6-31G** 3-21G* 6-31G** 3-21G* 6-31G** 3-21G* 6-31G** 3-21G* 6-31G**

1.760 1.760 1.750 1.752 1.736 1.741 1.743 1.747 1.750 1.753 1.366 1.366 1.360 1.360 1.353 1.354 1.362 1.362 1.367 1.366 1.442 1.441 1.442 1.441 1.440 1.440 1.438 1.439 1.437 1.438 1.341 1.339 1.344 1.342 1.347 1.344 1.345 1.342 1.343 1.341 1.707 1.710 1.712 1.714 1.717 1.720 1.712 1.716 1.712 1.714 1.486 1.491 1.473 1.479 1.468 1.475 1.470 1.476 1.467 1.475 1.514 1.510 1.511 1.508 1.511 1.508 1.514 1.511 1.513 1.510 1.069 1.074 1.070 1.074 1.070 1.074 1.069 1.074 1.069 1.074 1.067 1.071 1.067 1.071 1.067 1.071 1.067 1.071 1.067 1.071 1.081 1.082 1.082 1.083 1.083 1.084 1.082 1.083 1.082 1.083 1.080 1.082 1.081 1.082 1.084 1.085 1.083 1.084 1.084 1.085

92.7 1 92.83 92.06 92.13 91.43 91.45 92.08 92.10 92.28 92.37 109.61 109.47 110.74 110.58 111.71 111.52 110.70 110.59 110.11 110.01 112.27 112.31 111.87 111.92 111.89 111.90 112.11 112.06 112.36 112.30 114.16 114.42 113.81 114.07 113.05 113.37 113.52 113.83 113.65 114.00 134.5 1 134.79 132.80 133.03 127.08 127.89 128.88 129.52 129.27 129.92 117.17 117.12 119.78 119.68 123.65 123.23 120.78 120.44 119.85 119.44 122.02 122.13 122.25 122.45 123.03 123.08 122.55 122.62 122.38 122.44 127.46 128.27 127.09 127.86 126.89 127.60 127.25 128.01 127.36 128.17 109.66 109.79 110.02 110.16 110.48 110.60 109.90 109.96 109.83 109.86 111.44 111.96 111.34 111.82 110.83 111.34 111.39 111.88 111.44 111.91

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1372 V. Hernandez and J. T. Lopez Navarrete: Ab inifio study of torsional potentials

TABLE III. Bond lengths (A) and angles (deg) of 3.4’~dimethyl-2,2’-bithiophene.

4 0 45 108.6 116.2 145 180 4 0 45 108.6 116.2 145 180 Basis set 3-21G* 3-21G* 3-21G* 6-31G** 3-21G* 3-21G* Basis set 3-21G* 3-21G* 3-21G* 6-31G** 3-21G* 3-21G*

cz-Sl 1.744 1.739 1.735 1.740 1.740 1.745 c9-s13 1.744 1.738 1.735 1.739 1.737 1.741 c3-c2 1.362 1.357 1.354 1.355 1.358 1.360 Go-C9 1.356 1.351 1.349 1.348 1.354 1.356 c4-c3 1.438 1.439 1.439 1.440 1.439 1.440 cll-clo 1.439 1.440 1.441 1.441 1.438 1.435 cs-c4 1.345 1.346 1.347 1.344 1.346 1.344 c,2-c,, 1.346 1.347 1.348 1.346 1.347 1.347 %-St 1.714 1.715 1.716 1.720 1.715 1.712 c12-s,3 1.721 1.722 1.723 1.726 1.723 1.720 c9-c2 1.462 1.464 1.468 1.474 1.463 1.461 HI,-C10 1.065 1.069 1.070 1.075 1.070 1.070 c6-c3 1.509 1.512 1.511 1.509 1.513 1.512 %-HI, 1.508 1.508 1.508 1.504 1.508 1.508 H7-C4 1.070 1.070 1.070 1.074 1.070 1.069 H16-C12 1.068 1.068 1.068 1.072 1.068 1.068 H8-C5 1.067 1.067 1.068 1.071 1.067 1.067 %.o-C15 1.083 1.083 1.083 1.084 1.083 1.083 H17-C6 1.083 1.083 1.083 1.084 1.083 1.083 Hz,-C,, 1.084 1.084 1.084 1.085 1.084 1.084 Hl8-C6 1.084 1.084 1.084 1.085 1.084 1.084 C,-S,,-C,, 91.81 91.47 91.36 91.40 91.58 91.73 c,-s,-c5 91.93 91.58 91.42 91.52 91.69 91.76 c,o-cq-s,3 110.07 110.71 110.98 110.61 110.47 110.03 c3-c2-s, 110.86 111.49 111.79 111.58 111.36 110.96 c,,-c10-c9 113.88 113.63 113.55 113.98 113.85 114.12 c,-c,-c* 112.00 111.76 111.76 111.71 111.78 112.01 c,2-c,,-c,0 111.84 111.62 111.49 111.39 111.53 111.57 c5-c4-c3 113.55 113.36 113.14 113.56 113.45 113.31 I-I,,-C,,-C,, 121.83 122.91 123.48 123.37 123.03 122.44 c,-c,-c3 129.20 128.23 127.26 127.52 126.22 126.34 C,5-C,,-C10 122.82 123.04 123.13 123.33 123.06 122.44 C6--c3-c, 121.21 122.28 123.37 122.72 121.81 121.28 H16-C12-CL1 126.91 126.61 126.50 127.28 126.76 126.85 H,-C,-C, 122.58 122.78 122.94 122.93 122.66 122.54 H,-C,,-C,, 110.48 110.48 110.48 110.60 110.48 110.48 H8-C5-C4 127.26 127.04 126.91 127.66 127.07 127.24 H21-C15-Cll 110.83 110.83 110.83 111.34 110.83 110.83 H17-C6-C3 110.48 110.48 110.48 110.60 110.48 110.48 H18-C6-C3 110.83 110.83 110.83 111.34 110.83 110.83

authors investigated the 2Th molecule as partially oriented in the nematic phase of a liquid crystal solvent. Nuclear mag- netic resonance (NMR) measurements indicated the anti conformation to be most stable, with a barrier to interconversion between the anti and syn forms of about 522 kcal/mol. A qualitative analysis of the data suggested that the stability difference between the anti and syn forms is very small, on the order of 0.2 kcal/mol. The conformational weight of the anti conformer was estimated to be about 70%. In the solid state at 133 K, 2Th has an anti configuration as determined from x-ray diffraction expeximents.32

In an early gas phase electron diffraction33 study of 2Th, it was concluded that the molecule as a whole does not seem to be planar. The twist angle was estimated to be 146”. It was

2.5 , I

2.0

s 8 1.5 5 F!. $3 1.0

Ia 0.5

0 30 150 180

not unlikely that another minimum existed on the potential energy curve at approximately 95”, with a flat potential between the two minima. Ab initio ST0-3G,‘2(a)V34 STO-3G* (Ref. 35) and 3-21G [Ref. 12(b)] and semiempirical calculations predicted syn and anti conformations, with the anti arrangement as the most stable. On the contrary to what sometimes experimentally proposed, no local minimum was found outside the coplanar conformations. Only the semiempirical AM1 internal rotation potential showed an overall good agreement with calculations,14(a)

experiments.37 The MNDO on the other hand, predicted a 120” anti-like

conformation. In a recent paper Samdal et aZ.‘* have performed a new

gas phase electron diffraction study on 2Th, claiming for the

0 30 60 90 120 150 180 Torsional Angle (Degree)

FIG. 1. Torsional potential energy vs inter-ring torsional angle of 2,2’-bithiophene. Results form ab initio full geometry optimizations at the 3-21G* and 6-31G** levels of calculations.

FIG. 2. Torsional potential energy vs inter-ring torsional angle of 3,3’-dimethyl-2,2’-bithiophene. Results form ab initio full geometry optimi- zatioos at the 3-21G* and 6-31G** levels of calculations.



V. Hernandez and J. T. Lopez Navarrete: Ab initio study of torsional potentials 1373

2.0

9 E $

1.5

p 1.0

8 0.5

0 45 90 135 180 Torsional Angle (Degree)

FIG. 3. Torsional potential energy vs inter-ring torsional angle of 3,4’dimethyl-2,2’-bithiophene. Results form ab initio full geometry optimizations at the 3-21G* level.

uncertainties connected to the conformation in the previous investigation.“” Their new experimental data show the existence of two conformations, anti-like and syn-like, with torsional angles of 146” and 36” and conformational weights 56% and 4~4%~ respectively. These experimental data are in agreement with ab initio results using standard 3-21G* (Refs. 18,19) and 6-31G* (Ref. 18) basis.

Contradictions, however, seem to exist between the theoretical 3-21G* results reported in Refs. 18 and 19. In the work by Samdal et al. the perpendicular conformation, where rr conjugation is completely absent, was calculated to be more stable by about 1.1 kcal/mol that the highly conjugated syn form. This fact was attributed mainly to a steric effect between the two sulfur atoms, since the nonbonded S-S distance was calculated to be lower than the corresponding van der Waals distance of 3.70 A. In the calculations by Distefano er &.,I9 however, the energies of both conformers were estimated to be quite close.

More reliable theoretical information on the internal rotation in 2Th, based on ab initio 3-21G* and 6-31G** full geometry optimizations, are now presented in Fig. 1. Struc- tural parameters on changing 4 are summarized in Table I,

TABLE IV. Mulliken charges of 3,3’-dimethyl-2,2’-bithiophene.

TABLE V. Mulliken charges of 3,4’-dimethyl-2,2’-bithiophene.

4 0 45 108.6 116.2 145 180 Basis set 3-21G* 3-21G* 3-21G* 6-31G** 3-21G* 3-21G*

Sl 0.462 0.462 0.464 0.306 0.459 0.457 G -0.292 -0.305 -0.319 -0.273 -0.303 -0.299 c3 -0.037 -0.035 -0.027 0.051 -0.044 -0.046 c4 -0.221 -0.224 -0.227 -0.118 -0.221 -0.219 CS -0.483 -0.479 -0.477 -0.359 -0.480 -0.479 C6 -0.619 -0.602 -0.605 -0.341 -0.613 -0.629 H7 0.247 0.246 0.246 0.153 0.246 0.247 H8 0.269 0.267 0.266 0.180 0.267 0.268 c9 -0.273 -0.284 -0.299 -0.239 -0.283 -0.272 Go -0.230 -0.212 -0.199 -0.094 -0.223 -0.237 Cl, -0.048 -0.050 -0.053 0.029 -0.048 -0.044 Cl2 -0.490 -0.485 -0.481 -0.387 -0.489 -0.497 s13 0.454 0.456 0.455 0.299 0.461 0.469 HI4 0.254 0.248 0.253 0.163 0.255 0.255 Cl5 -0.596 -0.597 -0.598 -0.338 -0.598 -0.597 HI6 0.262 0.261 0.260 0.174 0.260 0.260 HI, 0.220 0.214 0.212 0.121 0.217 0.222 HI8 0.228 0.222 0.233 0.141 0.238 0.239 HIP 0.228 0.237 0.237 0.145 0.235 0.239 H20 0.216 0.217 0.216 0.125 0.216 0.216 H21 0.223 0.222 0.223 0.131 0.224 0.224 Hz2 0.223 0.224 0.224 0.133 0.224 0.224

together with the corresponding electron diffraction values reported by Almenningen et al.33 and by Samdal e? al.‘*

A detailed study of the optimized geometrical parameters for the anti and syn rotamers (see Table I), and for the optimized structures between them, reveals that the inter-ring C,-C, bond length and the C,-C,-C3 bond angle undergo the maximum changes on rotation. The magnitude of these changes are almost the same for the two different levels of calculation.

Comparatively, the 6-31G** geometry of each rotamer is essentially the same and does not differ significantly from the 3-21G* one. Concerning the bond lengths, the biggest differences between the two series of data take place for the C&J, distances which are longer by about 0.006-0.008 A in the 6-31G** calculations. Small changes are also found between the two sets of bond angles; the differences are always less than lo.

0 45 4

145 180 94.0 94.2

Basis set 3-21G* 6-31G** 3-21G* 6-31G** 3-2lG* 6-3lG** 3-2lG* 6-3lG** 3-2lG* 6-3lG**

Sl 0.453 0.314 0.454 0.307 0.459 0.303 0.463 0.311 0.472 0.321 c2 -0.279 -0.268 -0.291 -0.276 -0.317 -0.283 -0.303 -0.261 -0.294 -0.247 c3 -0.051 0.056 -0.040 0.063 - 0.023 0.059 -0.042 0.035 -0.050 0.023 C4 -0.221 -0.127 -0.224 -0.125 -0.229 -0.120 -0.222 -0.116 -0.217 0.111 C5 -0.484 -0.365 -0.479 -0.358 -0.475 -0.356 -0.483 -0.366 -0.491 -0.376 c6 -0.607 -0.340 -0.615 -0.340 -0.603 -0.340 -0.614 -0.348 -0.630 -0.367 H7 0.246 0.152 0.245 0.152 0.245 0.153 0.246 0.153 0.246 0.154 Ha 0.270 0.183 0.267 0.181 0.266 0.179 0.266 0.180 0.267 0.181 HI, 0.217 0.126 0.217 0.125 0.211 0.121 0.216 0.122 0.221 0.126 HI8 0.229 0.134 0.233 0.129 0.230 0.137 0.237 0.145 0.238 0.148 H,9 0.229 0.134 0.234 0.142 0.236 0.146 0.236 0.144 0.238 0.148



1374 V. Hernandez and J. T. Lopez Navarrete: Ab initio study of torsional potentials

In our opinion the overall agreement between our theoretical data and the two sets of experimental data are not quite good. Concerning the set reported by Almenningen et a1.,33 these authors interpreted their results assuming mirror-symmetry in the rings. An approximate force field estimated from that of tbiophene-2-aldehyde was used by Samdal et al. ‘* to derive correction parameters. The same authors pointed out the necessity to improve the force field by using ab initio methods. Furthermore, from electron diffraction data, the positions of light atoms, particularly hydrogen, cannot be exactly located in the presence of heavy atoms. Also, it is difficult to resolve interatomic distances of similar length. At this stage, it is not easy to discriminate between the theoretical and the experimental data. From the theoretical standpoint, the full geometry optimizations that we have performed should provide a reliable framework, at least with the use of the 6-31G** basis set. From the experimental standpoint, on the other hand, it is a hard task to select the best molecular model on which to found the fur- ther interpretation of the experimental data by least-square fittings.

the other hand, practically matches the van der Waals distance of 2.4 A.

The above results on the flexibility of 2Th as a model of PTh chains can be rationalized by the ease of the sulfur atom to vary its electron density. The likelihood of resonance interactions with substituents in the cr-position involves a de- creased participation of the sulfur electron pairs in the thiophene nucleus resonance.40 This largely fluctuable contribution from the unshared electrons of the sulfur to reso- nant structures should translate into much more deformable geometries than in other polyaromatic or polyheteroaromatic systems.

It is worthwhile to notice that allowing the structure to relax on changing 4 does not lead to large changes in the bond lengths and bond angles. From our results it can be stated that in 2Th the geometrical relaxation has little influence on the absolute values of the total energy. This will not be the situation for the methyl derivatives since the bulky side groups cause a great steric hindrance (see below).

Concerning the theoretical barrier to rotation of 2Th (see Fig. l), the two ab initio calculations are in excellent agreement with each other and indicate that the anti-like C2 minimum conformation (3-21G*, 146.3” and 6-31G**, 147.4”) lies about 0.34-0.35 kcaYmo1 below the CZh anti conformer. It is thus easy to understand that, in the solid state, packing can lead to an almost coplanar situation.32 Another stable syn-like conformer is calculared also at torsional angles of 44.3” (3-21G*) and 43.9” (631G**). The anti-like minimum conformer is more stable by 0.63 (3-21G*)-0.75 (6-31G**) kcal/mol than the syn-like one.

These two minimal conformations closely correspond to the electron diffraction’* minima occurring at 36” and 146”. The delocalization of the w-electrons would force the molecule to be coplanar, while the steric repulsions would favor a twisted conformation. The balancing of such competing forces yields gas phase structures out of planarity. The barrier height we calculate at the perpendicular conformation is 1.46 (3-21G*)-1.69 (6-31G**) kcal/mol. Since rr conjugation is negligible in the perpendicular geometry the increase in total energy remains small. This result can be explained by the inter-ring o-n hyperconjugation that stabilizes the perpendicular form and tends to compensate for the loss of direct rr conjugation.38’39 For the perpendicular case, the rr and cr MO’s localized on each ring have u tails or components on the other molecular fragment. This effect can also justify the fact that the C9-C2 inter-ring bond distance is essentially unaffected by the torsion (see Table I).

Turning to the NMR measurements by Bucci et ~l.,~’ our barrier height for the perpendicular situation is about three times smaller than that experimentally measured (522 kcal/ mol). Polar solvents could preferentially stabilize the C2,-syn or Cz-twisted conformers (i.e., those conformers having permanent dipole moment) with respect to the C,,-anti one. Furthermore, it is known that a change of the liquid crystal environment used to orient the molecules usually leads to structure deformations of the solute molecules. The best experimental conditions are reached in liquid crystal mixtures in which the absolute value of the dipolar 13C-‘H coupling constant of methane (used as an internal reference) is relatively small. At this stage, it is difficult to evaluate how the nematic liquid crystal can influence the conformation of the 2Th molecule.

Our ab initio results and those previously reported’8”9 demonstrate clearly that d-symmetry functions are required on the S atoms in order to correctly predict the optimal conformations in PTh chains. These polarizations functions provide for nonuniform displacements of electronic charges away from the nuclear centers.

B. Dimethyl-2,2’-bithiophenes

The total energy of the syn conformation is 1.34 (3-21G*)-1.56 (6-31G**) kcal/mol higher than that of the anti conformer, probably because of a small steric hindrance between the hydrogen atoms linked to the inner Ppositions and mainly due to the nonbonding interactions between the lone pairs of the sulfur atoms. In fact, in the syn structure the S,-& (3-21G*, 3.319 A; 6-31G**, 3.325 A) distance is shorter than the van der Waals value of 3.70 A. The He-H,, (3-21G*, 2.384 A; 6-31G **, 2.403 A) interatomic length, on

In an early paper, Souto Maior et ~l.t’(~) studied the re- giochemistry of substitution in P3MeTh by comparing the properties of the stereoregular poly(3,3’-dimethyl- 2,2’-bithiophene) with those of the conventional polymer obtained from direct polymerization of 3-methylthiophene. The regiospecific head-to-head polymer had its maximum absorption in the visible region at considerably shorter wave- lenght (417 nm) than its nonregiospecific counterpart (508 run); thus as the energy of the r-fi absorption indicates, poly(3,3’-dimethyl-2,2’-bithiophene) exhibits a lower degree of conjugation than poly(3-methylthiophene). In the past few years McCullough et ~1.~’ have developed another regiospecific method that produces P3ATh’s that contain almost ex- clusively head-to-tail couplings (>93%). In the UV-VIS, the poly(3-dodecylthiophene) (P3DDTh) prepared by this method shows shifts to lower energy of the absorption maximum of up to 14 nm (450 nm) in solution and 46 nm (526



nm) in the solid state, with other intense lower energy peak with shifts of up to 129 nm (609 nm) from P3DDTh prepared with FeCl,.

Barbarella et uZ.“(~)*~* have paid a lot of attention to the molecular characterization of a series of oligomethylth- iophenes of well-defined structure by means of i3C NMR spectroscopy. These authors concluded that oligothiophenes are very mobile systems. Only the presence of two methyl groups in a head-to-head substitution pattern freezes the molecule in solution in a local twisted anti-like conformation. Nethertheless, also in this case a fully coplanar arrangement may be reached in the solid state even if with severe bond and angle deformations, Distefano et ~1.‘~ have concluded from ultraviolet photoelectron and electron transmission spectroscopic data and ab initio calculations that the values for the prevailing rotamers of gaseous 2Th34 are about 140” and less than 50” (values quite close to those of 2Th). Their data on 2Th33 suggest, however, a largely tilted gas phase conformation.

We have calculated the ab initio 3-21G* and 6-31G** geometrical parameters for six rotamers of 2Th33 at frozen twist angle values of 0”, 45”, 90”, 145”, and 180”. The molecular geometry was also fully optimized at both theoretical levels without imposing any constraint to the inter-ring dihedral angle. A completely equivalent series of ab initio 3-21G* data have been obtained for the 2Th34, while at the 6-31G** level only the absolute minimal conformation was computed. For this last molecule both methyl group internal geometries were assumed as invariants on changing 4, kept fixed to the 3-21G* optimized values for the 2Th33 orthogonal conformer. It was thought to represent a good compro- mise between the quality of the results and the required computer time since, as it was previously verified in the case of 2Th33, methyl geometry changes very little on rotation. The optimized geometrical parameters for all the 2Th33 and 2Th34 conformers are summarized in Tables II and III, respectively. Figure 2 shows the 3-21G* and 6-31G** evolu- tions of the relative conformational energies for 2Th33, while Fig. 3 plots the 3-21G* torsion potential for 2Th34.

Once again, one observes that the more sophisticated polarized basis, 6-31G**, provides for each conformer nearly the same geometry as the 3-21G* basis. Shown in Tables II and III are also the 3-21G* and 6-31G** structures of the absolute equilibrium configuration of 2Th33 and 2Th34, respectively. As in the case of 2Th, in 2Th34 almost all bond lengths remain nearly constant on changing 4. Only the inner C,-St, Cs-C,, C9-S,,, and C,-Cl0 bonds de- crease by about 0.009 %, on going from the syn to the orthogonal arrangement of the rings, whereas the interannular CZ-C9 bond increases by 0.007 A. In our opinion these little geometrical modifications are not surprising. Contrary, we believe that this persistence in the values of the molecular parameters must be ascribed to the c-m inter-ring overlapping effect and the deformability of the sulfur-containing molecules, rather than to the inability of the 3-21G* and 6-31G** basis to correctly describe the internal rotation in these conjugated systems.

The geometrical perturbation induced by the introduction of a methyl group can be inferred from the structures of

the 2Th and 2Th34 orthogonal conformers (compare, for in- stance, the 3-21G* data in Tables I and III). The C2-C3 and Cd-C3 bonds slightly stretch by 0.003-0.005 A upon substitution (from 1.350 and 1.435 A in 2Th up to 1.353 and 1.440 8, in 2Th34, respectively). Methylation also narrows the ring valence angle localized at the substituted carbon atom, namely C4-Cs-C2 angle reduces from 112.91” in 2Th down to 111.82” in 2Th34. The variations of the 6-31G** data for 2Th and 2Th33 follow just the same trends (compare rel- evant parameters for the orthogonal and nearly orthogonal conformers in Tables I and II, respectively).

Unlike 2Th where structure relaxation did not lead to large changes in the molecular parameters, in the 3,3’-dimethyl derivative steric hindrance causes significant changes on rotation. Particularly, a strong dependence on the inter-ring dihedral angle is calculated on going to the syn- like conformations (see Table II). The biggest differences between the planar syn, 45’ syn-like and nearly orthogonal forms for the ring framework concern the C&St and C3-C2 bond lengths which are longer by about 0.024 and 0.013 8, respectively, in the coplanar arrangement. Small modifications also take place on going to the anti-like situation. The C,-C,-C, valence angle opens up meaningfully by about 7.5” (3-21G*) in the syn structure with respect to the value reached in the completely decoupled orthogonal situation (127.0”), whereas the C9-C2 bond elongates by 0.018 A.

It is worth mentioning that while the inter-ring bond length (C&-C,) and valence angle (C,-C&s) greatly vary along the syn-rotation, they * practically remain unchanged along the anti sense of twisting, as if the nonbonding interactions between the lone pairs of the two sulfur atoms play a major role in determining the final conformation. The C6-C3-C4 angle undergoes also large variations along both senses of rotation. All these geometrical modifications have their common origin in the steric and electrostatic repulsions between nearby atoms. Thus, although the Mulliken popula- tion analysis is sensitive to the basis set used and other methods have been proposed,20,43 some insight can still be gained from the calculated atomic charges.

The electrostatic picture for 2Th33 provided by the 6-31G** basis is found to be almost independent from the molecular conformation, namely the largest differences do not exceed 10% (see Table IV). The most noticeable information derived from these computations is the distinctive role played by the electrostatic interactions in both coplanar situations. In the anti conformer, the repulsive interactions between the sulfur atom and the two nearest methyl hydrogen atoms (S,, 0.321; His and Hi9, 0.148) are partially com- pensated by the attractive interaction with the methyl group carbon, due to the existence of the opposite sign charge density over this last atom (C,, -0.367). Electrostatic interactions render the syn conformer less stable because of the same kind of electron density on close mirror-symmetric atoms.

The introduction of two methyl groups either in the 3 and 3’ or in the 3 and 4’ positions leads to a more tilted molecular backbone with respect to unsubstituted 2Th. The conformational energies, relative to the most stable conformer, are plotted vs the C3C2C9C10 twist angle value in Fig.

V. Hernandez and J. T. Lopez Navarrete: Ab initio study of torsional potentials 1375


1376 V. Hernandez and J. T. Lopez Navarrete: Ab inifio study of torsional potentials

2 for 2Th33 (3-21G* and 6-31G** data) and in Fig. 3 for 2Th34. Both torsional potentials differ too much from that of 2Th. The absolute minima are found to occur at 108.6” in 2Th34 and at w-94” in 2Th33.

Distefano et a1.l9 have proposed the existence of two stable rotamers in gaseous 2Th34 at values of the inter-ring twist angle of 140” (anti-like) and 50” (syn-like) as the result of their combined spectroscopic and theoretical study. Our fairly large basis set calculations do not support the conclusions reached by these authors. No other stable minimum was found outside the nearly orthogonal (108.6”) arrangement of the rings. The driving force seems to be the steric repulsion between the lone pairs on the sulfur atom of one ring and the 3-methyl group on the other. We do not expect that electrostatic repulsions could justify such a large distor- sion from planarity. In fact, although the positive charge on the sulfur is calculated to reach a sizable value [3-21G*, 0.45-0.47; 6-31G**, about 0.30 (see Table V)], the sum of Mulliken charges on the methyl group is rather small [about 0.07 (see also Table V)]. Nevertheless, the overall torsional potential in 2Th34 is still rather flat.

Methyl substitution causes rather small changes in the thiophene ring geometry and only the conformational properties are deeply modified because of the steric hindrance. Accordingly, the thiophene ring bond lengths and valence angles for the three orthogonal systems are almost the same within 0.006 8, in distances and 1.1” in angles. Contrary to this theoretical result, methylation dramatically influences the overall shape of the torsional potentials, leading to a large tilt from planarity.

In 2Th34 the torsional potential curve was found to be still flat; only moderate geometrical changes take place on rotation. Such a relatively low barrier values point out once more that coplanar or slightly tilted conformations can be reached when intermolecular forces (electrostatic interactions with polar solvents or crystal packing) are switched on, despite the presence of substituents.

From the shape of the 3-21G* and 6-31G”” torsional energy curves of 2Th33 we learn that, in the case of a head- to-head junction, the backbone flattening can only take place from a syn-rotation with large modifications in the molecular geometry and high energetic cost. The flattening seems to be only reachable from an anti-like twisting around the bond connecting the two thiophene moieties, even when the energy is not too low. In the case of a heail-to-head junction, such energetically expensive syn-like flattening does not favor the electrostatic interactions with polar solvents.

IV. CONCLUSIONS

We have discussed ab initio quantum-chemical calculations on the torsional potentials of three conjugated dimers, namely 2,2’-bithiophene, 3,4’-dimethyl-2,2’-bithiophene, and 3,3’-dimethyl-2,2’-bithiophene, which are models of a conjugated polymer of high current interest, poly(3- methylthiophene). Our purpose was to obtain theoretically valid information on the gas-phase conformations in methyl- substituted thiophene chains in the neutral state. We have performed two levels of ab initio molecular orbital calculations, i.e., RHF/3-21G* and RHF/6-31G**, on the model dimers to assess optimal conformations and torsional potentials. Our results show that except the head-to-head junction, the barrier heights for rotation around the inter-ring single bond are on the order of a few kcal/mol, thus indicating that such chain should be rather flexible.

From the calculated 3-21G* and 6-31G** torsional energies for 2Th33 we conclude that the flattening of such a block in a random polymer is only possible from an anti-like twisting around the bond connecting the two thiophene moieties. The barrier we calculate for the syn situation now reaches 12.7-13.7 kcal/mol; it is almost seven times as large as when the unsubstituted 2,2’-bithiophene rotates. Neither the solvation energy nor the gain of rr conjugation could energetically compensate for the syn-like twisting. This highly hindered rotation has consequences in the large modifications of the structural parameters on going from the orthogonal to the syn conformations. If one compares the inter- ring distance in the syn conformer of 2Th33 with the corresponding values in the other two model systems, 2Th and 2Tl-134, the bond is longer in 2Th33 by about 0.024- 0.027 A. The reasons for this elongation in 2Th33 are the electrostatic and steric repulsions, which are maximum in the mirror-symmetric syn coplanar conformation. The easy deformability of the sulfur-containing rings and the inter-ring u-r populations overlapping probably soften all geometrical modifications.

ACKNOWLEDGMENTS

The research reported in this communication has been financially supported by the Junta de Andalucia (Group No. 6072). V. H. thanks the Ministerio de Education y Ciencia of Spain for an individual grant. We also appreciate the always kind assistance of the technicians at the Centro Informatico de la Comunidad Andaluza (CICA) of Sevilla (Spain).

Gaseous 2,2’-bithiophene is shown to possess a minimum for a = 146” anti-like conformation and a second stable syn-gauche conformer at about 44”. The difference in total energy of the anti-planar form relative to the absolute minimum is only 0.33 kcal/mol, indicating that the anti conformation is certainly allowed in the solid state. The backbone chains are flexible enough to adopt a helical conformation in solution since the syn-gauche conformer is stabilized by electrostatic interactions with polar solvents. Structural parameters vary very little on changing 4, thus indicating that internal rotation in FTh is not largely hindered.

‘Handbook of Conducting Polymers, edited by T. A. Skotheim (Dekker, New York, 1986). Vols. 1 and 2.

‘Conjugated Polymers, edited by J. L. Bredas and R. Silbey (Kluwer Aca- demic, Dordrecht, 1991).

3 Nonlinear Optical and Electroactive Polymers, edited by P. N. Prasad and D. R. Uhich (Plenum, New York, 1988).

4H. Naarmann, Synth. Met. 41-43, 1 (1991). ‘K. Muller and U. Scherf, Synth. Met. 55-57, 739 (1993). 6R. Sugimoto, S. Takeda, H. B. Gu, and K. Yoshino, Chem. Express 1,635

(1986). ‘G. Zerbi, R. Radaelli, M. Veronelli, E. Brenna, F. Sannicolo, and G. Zotti,

J. Chem. Phys. 98, 4531 (1993). s(a) F. Gamier, G. Horowitz, and D. Fichou, Synth. Met. 28, C705 (1989);

(b) 2. G. Xu, D. Fichou, G. Horowitz, and E Gamier, Adv. Mater. 3, 150 (1991).

‘K. Yoshino, Synth. Met. 28, C669 (1989).



V. Hemandez and J. T. Lopez Navarrete: Ab inifio study of torsional potentials 1377

“(a) G. Horowitz, D. Fichou, X. Z. Peng, Z. G. Xu, and F. Gamier, Solid State Commun. 72, 381 (1989); (b) G. Horowitz, X. Z. Peng, D. Fichou, and F. Gamier, J. Appl. Phys. 67, 528 (1990).

“(a) L. Laguren-Davidson, C. Van Pham, H. Zimmer, and H. B. Mark, Jr., 1. Electrochem. Sot. Electrochem. Sci. Technol. 135, 1406 (1988); (b) R. M. Souto Maior, K. Hinkelmann, H. Eckert, and F. Wudl, Macromolecules 23. 1268 (1990).

“(a) J. L. Bredas, G. B. Street, B. Themans, and J. M. Andre, J. Chem. Phys. 83, 1323 (1985); (b) J. L. Bredas and A. J. Heeger; Macromolecules 23. 11.50 (1990).

13C. X. Cui and M. Kertesz, Phys. Rev. 40, 9661 (1989). “(a) J. T. Lopez Navarrete, B. Tian, and G. Zerbi, Synth. Met. 38, 299

(1990); (b) J. T. Lopez Navarrete and G. Zerbi, J. Chem. Phys. 94, 957 (1991).

‘“M. Kofranek, T. Kovar, H. Lischka, and A. Karpfen, J. Mol. Struct. (THEOCHEM) 259, 181 (1992).

16J M Tour, R. Wu, and J. S. Schumm, J. Am. Chem. Sot. 113, 7064 (i99i).

“E. E. Havinga and E. W. Meijer, J. Am. Chem. Sot. 113, 5887 (1991). ‘*S. Samdal, E. J. Samuelsen. and H. V. Volden, Synth. Met. 59, 259 (1993). 19G Distefano M. D. Colle, D. Jones, M. Zambianchi, L. Favaretto, and A.

Modelli, J. Whys. Chem. 97, 3504 (1993). =MI. J. Frisch, M. Head-Gordon, J. B. Foresman, G. W. Trucks, K. Ragha-

vachari, H. B. Schlegel, M. Robb, J. S. Binkley, C. Gonzalez, D. J. De- frees, D. J. Fox, R. A. Whiteside, R. Seeger, C. F. Melius, J. Baker, L. R. Kahn. J. J. l? Stewart, E. M. Fluder, S. Topiol, and J. A. Pople, GAUSSLAN 90 (Gaussian, Inc.. Pittsburgh, PA, 1990).

*‘W. J. Pietro. M. M. Francl, W. J. Hehre, D. J. Defrees, J. A. Pople, and J. S. Binkley, J. Am. Chem. Sot. 104, 5039 (1982).

‘*M. M. Franc& W. J. Pietro, W. J. Hehre, J. S. Binkley, M. S. Gordon, D. J. Defrees, and J. A. Pople, J. Chem. Phys. 77, 3654 (1982).

UK. Raghavachari, J. Chem. Phys. 81, 1383 (1984). z4A. L. Aljibury, R. G. Snyder, H. L. Strauss, and K. Raghavachari, J. Chem.

Phys. 84, 6872 (1986). “K. B. Wiberg and M. A. Murcko, J. Am. Chem. Sot. 110, 8029 (1988). %K. B. Wiberg and M. A. Murcko, J. Comput. Chem. 9, 488 (1988).

*‘K. B. Wiberg and K. E. Laidig, J. Am. Chem. Sot. 109, 5935 (1987). ‘*M. Head-Gordon and J. A. Pople, J. Chem. Phys. 97, 1147 (1993). 29S. Tsuzuki and K. Tanabe, J. Chem. Sot. Faraday Trans. 87,3207 (1991). %S. Tsuzuki, T. Uchimam, K. Tanabe, and T. Hirano, J. Phys. Chem. 97,

1346 (1993). 3’ P. Bucci M. Longeri, C. A. Veracini, and L. Lunazzi, J. Am. Chem. Sot.

96, 130; (1974). 32G J Visser G. J. Heeres, J. Wolters, and A. Vos, Acta Crystallogr. Sec. B

24,467 (19’68). 33A. Almenmngen, 0. Bastiansen, and P Svendsas, Acta Chem. Stand. 12,

1671 (1958). 34D. Jones, M. Guena, L. Favaretto, A. Modelli, M. Fabrizio, and G. Dis-

tefano, J. Phys. Chem. 94, 5761 (1990). 3sV. Barone F Lelj, N. Russo, and M. Toscano, J. Chem. Sot. Perkin Trans. 9 .

2, 907 (1986). 36(a) V. Galasso and N. Trinajstic, Tetrahedron 28, 4419 (1972); (b) R.

Abu-Eittah and F. Al-Sageir, Int. J. Quantum Chem. 13,565 (1978); (c) A. Skancke, Acta Chem. Stand. 24, 1389 (1970).

37V. Hemandez and J. T. Lopez Navarrete, Synth. Met. 51, 211 (1992). ‘sE. Orti, J. Sanchez-Martin, and F. Tomas, J. Mol. Struct. (THEOCHEM)

108, 199 (1984). “J P Daudey G . .

36, 3399 (PiSO;. Trinquier, J. C. Barthelat, and J. P. Malrein, Tetrahedrom

@R. G. Charles and H. Freiser, J. Am. Chem. Sot. 72, 2233 (1950). 4’(a) R. D. McCullough and R. D. Lowe, J. Chem. Sot. Chem. Commun.

1992, 70; (b) R. D. McCullough, R. D. Lowe, M. Jayaraman, P. C. Ew- bank, D. L. Anderson, and S. T. Nagle, Syntb. Met. 55, 1198 (1993).

42(a) G. Barbarella, A. Bongini, and M. Zambianchi, Tetrahedron 48, 6701 (1992); (b) Adv. Mater. 3, 494 (1991); (c) G. Barbarella, M. Zambianchi, A. Bongini, and L. Antolini, ibid. 4, 282 (1992).

43(a) J. P. Foster and F. Weinhold, J. Am. Chem. Sot. 102, 7211 (1980); (b) A. E. Reed, R. B. Weinstock, and F. Weinhold, J. Chem. Phys. 83, 735 (1985); (c) A. E. Reed and F. Weinhold, ibid. 83, 1736 (1985); (d) A. E. Reed, F. Weinhold, and L. A. Curtis, Chem. Rev. 88, 899 (1988).


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