Tetrathiafulvalene: gas-phase molecular structure from electron diffraction

Journal of Molecular Structure, 317 (1994) 213-271 0022-2860/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved

213

Tetrathiafulvalene: gas-phase molecular structure from electron diffraction Istvan Hargittai”>*, Jon Brunvollb, Maria KolonitC, Vladimir Khodorkovskyd ‘Institute of General and Analytical Chemistry, Budapest Technical University and Structural Chemistry Research Group of the Hungarian Academy of Sciences, H-1521 Budapest, Hungary bPhysical Chemistry Division, University of Trondheim, N-7034 Trondheim, Norway ‘Structural Chemistry Research Group of the Hungarian Academy of Sciences, Ebbiis University, H-1431 Budapest, Hungary ‘Department of Chemistry, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel

(Received 8 July 1993) Abstract The gas-phase molecular structure of 2,2-bi-1,3-dithiole (tetrathiafulvalene) was determined by electron diffraction and compared with the crystal molecular structure. A nonplanar “boat” structure was found to give the best agreement with experimental electron diffraction results. The electron diffraction results were found to be consistent overall with those from X-ray crystallographic studies.

Introduction

Analysis

The crystal and molecular structure of 2,2-bi-1,3dithiole, or tetrathiafulvalene, has been known for some time [ 1,2]. This compound, as well as some of its derivatives, has been found to possess a unique molecular packing mode giving rise to a family of so called organic metals - charge-transfer complexes with acceptors and non-stoichiometric cation-radical salts. Strong intermolecular interactions are a usual and important feature of this class of compounds and we found it of interest to determine its gas-phase molecular structure and compare it with the crystal molecular structure. It is noteworthy that there are no structural data on free five membered ring 1,3-dithiole derivatives in spite of the relatively rich information available on the molecular geometry of volatile sulfur compounds [3].

The electron diffraction photographs were recorded with an EG- 1OOAapparatus of the Budapest Group [4] using a stainless steel nozzle system modified from a previous design [5]. The nozzle temperature was about 160°C. Nozzle-to-photographic plate distances of about 50 and 19 cm were used. The nominal accelerating voltage of the electron beam was 60 kV. The electron wavelength (X = 0.04928 A) was calibrated with a TlCl powder pattern [6]. Eight and seven plates were used for analysis from the 50 and 19cm camera distances, respectively. The ranges of intensity data used were 2.250 I s 5 14.OOOA-’ and 9.75 5 s I 35.25 A-’ with data intervals of 0.125 and 0.25 A-‘, respectively (s = 47rX-’ sin(Q/2), where 0 is the scattering angle). Experimental and calculated molecular intensities and radial distributions are shown in Figs. 1 and 2. A least-squares refinement of the geometrical

* Corresponding SSDZ

author.

0022-2860(93)07873-U

I. Hargittai et al./J. Mol. Struct. 317 (1994) 273-277

274

C6H4S4,

I

Tetrathlafulvalene

E-T

Fig. 1. Molecular intensities and difference curves.

and vibrational parameters was based on the molecular intensities [7]. The atomic inelastic and elastic scattering factors and phase shifts were taken from refs. 8 and 9. Mean vibrational amplitudes (Z) and correction terms for ra,/rcr conversion (1*/r - E, e.g. ref 10) were calculated by normal coordinate analysis. Valence force constants from Bozio et al. [ll] were utilized. A range of values for the out-of-plane and torsional force constants were tested because of their uncertainty. Considering the overall molecular symmetry, three models were tested (Fig. 3), one all-planar (D2,,) and two nonplanar, the “boat” (C2,) and the “chair” (C,,). The central carbon and

I

f(r)

sulfur atoms lie in a plane (A, according to the designation by Cooper et al. [l]) in all three models. In addition, there are two symmetryrelated planes (B) through the sulfur atoms and the end carbon atoms. The two planes B are in the same plane as A in the DZh model; they are related by a symmetry plane in the C2, model, and by a twofold axis of rotation in the C2t,model. The nonplanarity of the C2, and C2h model was defined by the dihedral angle between planes A and B, called 6SS. In addition, seven distance and angle parameters described the geometry; these are identified in Table 1. The numbering of atoms is also given in Fig. 3. The calculated radial distributions for the

C6H4S4.

Tetrathlafulvolene

E-T

Fig. 2. Radial distributions

and their difference curve.

275

I. Hargittai et a/./J. Mol. Struct. 317 (1994) 273-277

B _

-

Fig. 3. Numbering models.

A

B

&,

of atoms and the projections

of three

Dzh model indicated that the model-dependent distances, and in particular the longest C. . C distances, were too long compared with the experimental distribution. The situation improved considerably when the refinement was based on an ra structure rather than on an ra structure. Accordingly, at least part of the difficulty with the planar model may be ascribed to the consequences of perpendicular vibrations. Introducing nonplanarity further improved the agreement between the experimental and theoretical distributions, and slightly better agreement was observed for the C,, model than for the C2t, model, as witnessed by the R-factors, while the difference was not discernible on the radial distributions. We note also that the R-factor variation was much more pronounced for the small angle region than for the rest of the intensity data in these refinements. This is consistent with the importance of the model-dependent long non-bonded distances. Their variation is shown in Table 2. The drastic

Table 1 Bond lengths (A) and angles (deg) of tetrathiafulvalene Gas-phase electron diffractiona

X-ray crystallographyb

ra

Tg Independent parameters

C-H C7=C8 ACC Cl-S3 ACS s3-Cl -s4 C8=C7-H 6SS”

1.077 1.338 0.016 1.758 0.019 114.2 123.5 13.5

1.105 1.348 [O.OlO] 1.767 [0.014]

0.008 0.004 0.006 0.004 0.6 1.1 4.2

0.91 - 0.95 1.314 (3) to.0351 1.756 (2) [0.024] 114.5 2.1

parameters Cl=C2 c7-s3 Cl-s3-c7 S3-C7-C8

1.354

1.358

0.005

1.739 94.5

1.753

0.004 0.6

117.6

a Present work; q, estimated

total errors (see e.g. ref. 12). b Ref. 1. ’ Dihedral angle between the S3ClS4 (A) and S3C7C8S4 (B) planes.

0.3

1.732 (2) 94.4 118.3

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I. Hargittai et a1.j.l. Mol. Struct. 317 (1994) 273-277

Table 2 Variation of some model-dependent

and model-independent

Planar (&) ra

nonbonded

distances (A)

Nonplanar ra

“Chair” (C&

“Boat” (C,,)

ra

ra

ra

r.

6.39

Model-dependent Cl.. C9 c7. Cl0 c9. S3 ClO. ‘S3 Cl..C9

6.51 6.37 5.27 4.88 3.92

6.47 6.33 5.25 4.86 3.90

6.44 6.30 5.23 4.84 3.89

6.44 6.30 5.23 4.84 3.88

6.25 5.24 4.84 3.89

6.40 6.26 5.24 4.84 3.88

Model-independent s3. 35 s3.. c36 s3.. Cs4

4.40 3.25 2.96

4.40 3.26 2.96

4.40 3.26 2.96

4.40 3.26 2.95

4.40 3.26 2.96

4.40 3.27 2.96

R

0.0613

0.0448

0.0379

0.0362

0.0361

0.0347

changes occurred when going from the planar ra to the planar I, refinement and then on to the nonplanar models. There seemed to be no noteworthy change in the ra/ru comparison for the nonplanar models. Whereas model-dependent distances may change as much as 0.1 A in these refinedistances, such as ments, model-independent S3. . S4, change little more than 0.01 A.

Results and discussion

The geometrical parameters from this analysis, in terms of ra bond lengths and angles and dihedral angles, as well as rg bond lengths, are presented in Table 1, along with the results of the X-ray crystallographic study by Coppens et al. [l]. There is an overall consistency between the two sets of parameters. All electron diffraction bond lengths are larger than the X-ray results, the ra bond lengths being somewhat closer to them, as they are, indeed, more consistent with regard to their physical meaning. The non-planar model proved to be superior to the planar form and the deviation from planarity seemed to be markedly more pronounced in the gas

than in the crystal. Of the “chair” and “boat” forms, the “boat” gave somewhat better agreement with the experimental electron diffraction results than the “chair”. Mixtures of the two forms have also been tested; however the pure “boat” form gave the best agreement. Acknowledgments

This research was supported by the Dozor Foundation (Philadelphia) and by the Hungarian National Scientific Research Foundation (OTKA, No, 2103). References W.F. Cooper, N.C. Kenny, J.W. Edmonds, A. Nagel, F. Wudl and P. Coppens, J. Chem. Sot., Chem. Commutt., (1971) 889. W.F. Cooper, J.W. Edmonds, F. Wudl and P. Coppens, Cryst. Struct. Commun., 3 (1974) 23. I. Hargittai, Structure of Volatile Sulphur Compounds, Reidel, Dordrecht, 1985. I. Hargittai, J. Tremmel and M. Kolonits, Hung. Sci. Instrum., 50 (1980) 31. J. Tremmel and I. Hargittai, J. Phys. E, 18 (1985) 148.

1. Hargittai

et al./J. Mol. Struct. 317 (1994) 273-277

6 W. Witt, Z. Naturforsch., 19 (1964) 1363. 7 B. Andersen, H.M. Seip, T.G. Strand and R. Stolevik, Acta Chem. Stand., 23 (1969) 3224. 8 C. Tavard, D. Nicolas and M. Rouault, .I. Chim. Phys. Phys.-Chim. Biol., 64 (1967) 540. 9 R.A. Bonham and L. Schafer, International Tables for X-ray Crystallography, Vol. IV, Kynoch, Birmingham, UK, 1974, Chapter 2.5.

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10 I. Hargittai, in I. Hargittai and M. Hargittai, (Eds.), Stereochemical Applications of Gas-Phase Electron Diffraction, Part A, VCH, New York, 1988, Chapter 1. 11 R. Bozio, I. Zanon, A. Girlando and C. Pecile, J. Chem. Phys., 71 (1979) 2282. 12 M. Hargittai and I. Hargittai, J. Chem. Phys., 59 (1973) 2513.