The crystal and molecular structure of thioformaldehyde trimer

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1 The crystal and molecular structure of thioformaldehyde trimer J. E. FLEMING AND H. LYNTON Department of Chemistry, Lrniocrsity of Victoria, Victoria, British Columbia Received August 11, 1966 Crystals of thiofornlaldehyde trimer, (CH~S)J, are orfiorhombic, space group Pw~n2~ with a = A (o/dn = A), b = A (U/V'N = A), c = A (a/di\i = A). There are two molecules in the unit cell with sulfur and carbon atoms in positions (4b) and (2a). The atomic parameters for sulfur and carbon have been determined from a three-dimensional analysis using observed and calculated differential syntheses with isotropic temperature factors. No absorption corrections were applied to the intensity data and no attempt has been made to establish the hydrogen positions. The final discrepancy index is R = The molecule has the chair configuration and shows no significant pifference between the lengths of any S-C bonds. The mean S-C bond distance is A and the standard deviation of the mean is A. This value is in good agreement with the commonly accepted value of f A for the S-C paraffinic bond. Canadian Journal of Chemistry. Volume 45, 353 (1967) INTRODUCTION An X-ray single crystal investigation of (CHzS)3 was carried out in 1937 by Moerman and Wiebenga (I), who derived approximate values for the atomic parameters of the sulfur and carbon atoms from trial and error methods using relative intensities. These parameters indicated that the molecules possessed the chair configuration. This molecular arrangement was also proposed by Hassel and Viervoll (2) from their study of the electron diffraction pattern of the molecules in the gaseous phase. They reported a S-C dist+nce of 1.81,! and a S-S distance of 3.05 A. It is evident, from a consideration of their results for a- and p-trithioacetaldehyde given in the same paper (2), that there exists a misprint in the angles quoted for the trimer, and these should read S-C-S = 114.5", C-S-C = 106.5", instead of C-S-C = 114.5" and S-C-S = 106.5". Sutton (3) has calculated the correct S-C-S angle from the bond lengths quoted above, but also gives the original incorrect value without apparently realizing that it is a case of the labels for the angles having been interchanged. More recently, the space group and cell dimensions for this compound have been redetermined by R/lortillaro, Credali, RIammi, and Valle (4) during their investigation of its isomorph, triselenoformaldehyde. A specimen of trithioformaldehyde was made available to the authors by Dr. N. S. Isaacs of this department, and it was decided to carry out a three-dimensional structure analysis. CRYSTALLOGRAPHIC DATA The original sample was recrystallized from acetone to give short colorless columns. A preliminary examination was carried out photographically to determine the space group and cell dimensions. The cell dimensions were later re-measured with greater accuracy on a Buerger single crystal equi-inclination diffractometer, manufactured by the Charles Supper Co. Nickel-filtered Cu Kcu radiation was used with a Philips xenon-filled proportional counter as detector. The preliminary cell dimensions were used to calculate the approximate diffractometer settings, then accurate T, p, and Y values were measured for several high order axial and non-axial reflections in the two zero layers hko and Okl. None of the reflections which were intense enough to be located accurately with the diffractometer had a sufficiently high 0 value for the KCXI, Kaz doublet to be resolved. The crystal data for (CHzS)3 is given in Table I, and is in general agreement with that previously reported (1, 4). The space group was not unambiguously determined until the initial Patterson synthesis had

2 CANADIAN JOURNAL OF CHEMISTRY. VOL TABLE I Crystal data for (CH2S)s System Molecular weight Space group a b C v Orthorhombic L), (by flotation) X - Do i.59 g cm-3 p, absorption coefficient for Cu Ka cm-i Systematic absences h0l:h t 1 = 2n +1 been solved as the systematic absences are STRUCTURE ANALYSIS compatiblewith Pmn21, P2lnm, and Pmnm. All computations were carried out on a The last space group requires the molecule I.B.M computer, using the Unified to be planar; this possibility was rejected Set of Crystallographic Programs written in the light of the evidence available for the by Ahmed, Gabe, Mair, and Pippy (5). molecule existing in the chair arrangement. Initially all data were corrected for Lorentz polarization factors and put on an arbitrary INTENSITY DATA scale. The crystal chosen for the intensity data collection was about 0.2 mm long and had a roughly square cross section, with an average thickness of 0.1 mm. The long The positions of the sulfur atoms were found from a three-dimensional sharpened Patterson synthesis, using the orthodox heavy atom technique. Solutions in each direction corresponded to the c crystallo- of the permissible space groups were graphic axis. The diameter was below the calculated optimum thickness, 2/p, but no larger satisfactory single crystals could be attempted, and it was found that only in Pmn21 was there an acceptable solution, the sulfur atoms being in positions (4b) and found. No absorption corrections were Pa>. applied to the intensity data. A Wilson plot, using the scattering factor With Cu Ka radiation and the multipre curves for sulfur and carbon given in film pack technique, equi-inclination Weis- Volume 111 of the International Tables for senberg photographs were taken of the X-ray Crystallography (B), placed all data hko, hkl, hk2, hk3, hk4, and Okl layers. on an absolute scale. The carbon atoms The common reflections in the Okl layer were located from a three-dimensional were used to put all data on the same Fourier synthesis, and improved sulfur arbitrary scale. The intensities of all reflec- and carbon positions found from a second tions were estimated by visual comparison three-dimensional Fourier synthesis. These with a calibration strip prepared using a atomic positions gave a discrepancy index suitable reflection from the intensity crystal of R = itself. The atomic coordinates were refined by There were calculated to be 388 per- means of observed and calculated differmitted independent reflections within the range of Cu Ka radiation: 334 reflections were within the range covered by the photographs, and of these 312 were above the minimum intensity limit for observation. ential syntheses, with initially one isotropic temperature factor for all atoms. After several cycles had reduced the discrepancy index to R = 0.10, and the atomic shifts had become small, separate values for the temperature factors of the sulfur and carbon

3 FLEMING AND LYNTON: CRYSTAL AND MOLECULAR STRUCTURE 355 TABLE I1 Fractional atomic coordinates, and their e.s.d.'s (A) -- - Atom x Y z "(x) "(Y) (2) S(1) s(2) atoms were introduced. When the refinement was halted the largest atomic shift indicated was A. RESULTS The final atomic coordinates are listed in Table I1 and the final electron densities, mean curvatures, and isotropic temperature factors in Table 111. TABLE I11 Electron densities (e A+), mean curvatures (e A-6), and isotropic temperature parameters (if2) Atom PO PC pol1 pol' Biso A final set of structure factors1 was computed and these gave a discrepancy index of R = for the observed reflections only, and R = when the unobserved reflections were included. A more detailed analysis of the agreement of individual reflections was carried out (7) and is summarized in Table IV. Only three observed reflections, all of low intensity, and two unobserved reflections show relatively high discrepancies. The e.s.d.'s (estimated standard deviations) for the atomic Darameters were calculated according to the expression of Cruickshank and Robertson (8), using the observed data only. The sums required had been computed during the agreement summary analysis. The results are included in Table I I. 'These data have been placed in the Depository of Unpublished Data. Photocopies may be obtained upon request to: Depository of Unpublished Data, National Science Library, National Research Council, Ottawa, Canada. TABLE IV Agreement summary (overall discrepancy index, R = 0.100) Category Limits Number 312 observed reflections (1.1 < IF,\ < 46.5) 22 unobserved reflections (1 F,l,,, = 6.4) *IFthl = threshold amplitude = 0.2 to 1.7. The standard deviations of the interatomic distances were estimated for atomic coordinate errors using the equation of Ahmed and Cruickshank (9), and for unit cell dimension errors using the expression where XI, YI, 21 are the fractional atomic coordinates of the first atom, ~(a), ~(b), U(C) are the standard deviations of the mean for the cell dimensions, and cos a, cos P, cos r are the direction cosines of the line joining the atoms. The interatomic distances and the total standard deviations are listed in Table V. The standard deviations of the angles were estimated according to the expression m cos e - 1 PI.2(e) = [ lmsin 0 I2u2(l)

4 356 CANADIAN JOURNAL OF I ZHEMISTRY. VOL TABLE V Interatomic distances and their e.s.d.'s (A) Interatomic distance Length e.s.d. Bonding where I, m, and n are the interatomic distances forming - the triangle - in which the angle e is opposite side n, and a(l), a(m), a(n) are the total e.s.d.'s for these distances. The angles and their standard deviations are!listed in Table VI. The general appearance of the trithioformaldehyde molecule is shown in Fig. 1. TABLE VI Angles and their e.s.d.'s (") Angle Measure e.s.d. Bond angles ~(2)-~(1)-c(al) S(2)-C(2)-S(1) C(1)-S(2)-C(2) S(2/)-C(1)-S(2) Angles in sulfur triangle S(2)-S(1)-S(2') S(1)-S(2)-S(2') Angles in carbon triangle C(2)-C(1)-C(2') C(1)-C(2)-C(2') Interplanar angles S(1)-X- Y C(1)- Y-X DISCUSSION The chair-shaped molecular configuration suggested by earlier workers is confirmed in detail. The angle between the triangle defined by the atoms S(1), C(2), C(2') and the central plane defined by the atoms C(2), C(2'), S(2'), S(2) is calculated to be " (e.s.d. = 0.72"), whereas that between the triangle C(1) S(2) S(2') and the central plane is " (e.s.d. = 0.98'). The three values found for the S-C bond distance do not differ significantly from S(?) FIG. 1. The thioformaldehyde trimer molecule. The mirror plane passes through the atoms S(l) and C(1) and the points X and Y. one another (for each pair P > 0.05 using Cruickshank's expression (8)). The weighted mean S-C bond distance is A and the standard deviation of the mean is A. This value compares with that of f A given by Sutton (10) as the mean for the paraffinic S-C bond. The distances between sulfur atoms in the molecule are not significantly different from one another, nor are those between any two carbon atoms in a molecule, so each of these sets defines a triangle which may be taken as equilateral. There are three types of closest intermolecular distance: that between two sulfur atoms, that between two methylene groups, and that between a sulfur atom and a methylene group. The corresponding values for these distances, with the e.s.d.'s given in parenthesis, are: A (0.005 A) ; A (0.009 A) ; and A (0.006 A). The first distance is slightly longer and the second and third somewhat shorter than those calculated from the Van der Waal?' radii of 1.85 A for a sulfur atom, and 2.00 A for the methylene group (11). ACKNOWLEDGMENTS We acknowledge the financial support of the National Research Council of Canada for this work, and we thank Dr. N. S. Isaacs for suggesting the problem and preparing the sample used in the analysis. REFERENCES 1. N. F. MOERMAN and E. H. WIEBENGA. Z. Krist. Abt. B, 97,323 (1937) HASSEL and H. VIERVOLL. Acta Chem. Scand. 1,149 (1947). 3. L. E. SUTTON. Tables of interatomic distances

5 FLEMIKG AND LYXTON: CRYSTAL AND MOLECULAR STRUCTURE 357 and configurations in molecules and ions. The Chemical Society, London M L. MORTILL~~RO, L. CREDALI, hf. MAMMI, and G. VALLE. J. Chem. Soc. 807 (1965). 5. F. R. AH~IED, E. J. GABE, G. A. MAIR, and M. E. PIPPY. A unified set of crystallographic programs for the I.B.M Computer. Divisions of Pure Physics and Pure Chemistry, National Research Council, Ottawa, Canada. September International tables for x-ray crystallography. Vol The Kynoch Press, Birmingham, England Table3.3.lA. 7. F. R. AHMED and W. H. BARNES. Acta Cryst ). 8. D.' W. J.'CRUICKSHANK and A. P. ROBERTSON. Acta Cryst. 6,698 (1953). 9. F. R. AHMED and D. W. J. CRUICKSHANK. Acta Cryst. 6, 385 (1953). 10. L. E. SUTTON. Tables of interatomic distances and configurations in molecules and ions. Suppl The Chemical Society, London S22s. 11. L. PAULING. Nature of the chemical bond. 3rd ed. Cornell Univ. Press, Ithaca, N.Y p. 260.