Conformational stability and molecular structure of dimethyl (methylthio) phosphine from electron diffraction studies and ab initio calculations

Journal of Molecular Structure, 268 (1992) 143-154 Elsevier Science Publishers B.V., Amsterdam

143

Conformational stability and molecular structure of dimethyl (methylthio )phosphine from electron diffraction studies and ab initio calculations J.R. Durig, D.A. Barron and J.F. Sullivan Department of Chemistry, University of South Carolina, Columbia, SC 29208 (USA)

D.G. Anderson, S. Cradock and D.W.H. Rankin Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh, EH9 355 (UK) (Received 30 October 1991)

Abstract The molecular structure of dimethyl(methylthio)phosphine, (CH,),PSCHs, has been determined from a combined gas-phase electron diffraction study at 20°C and ab initio calculations utilizing both the 3-21G* and 6-31G’ basis sets. Two distinct conformers, trans and cis or nearcis (the cis conformer has the thiomethyl group eclipsing the non-bonded electron pair on the phosphorus atom and the trans conformer has this methyl group staggered with respect to the two methyl groups attached to the phosphorus atom) were identified with the isomeric composition of 78? 5% of the cis form. The differences between the geometrical parameters were fixed at the values obtained from the ab initio calculations. The determined heavy atom structural parameters for the cis conformer are r(P-S) ~2.111 f0.002, r(P-C) =1.862 f0.002, r(S-C) = 1.769fCj.006, L PSC = 102.3 f 2.6, L CPC = 96.7 f 0.8 and L CPS = 96.8 f 1.5 where the distances are in A and the angles in degrees. The parameters which differ for the trans conformer are r(P-S) = 2.103 ? 0.002, L PSC = 108.5 + 2.5 and L CPS = 99.9 f 1.5. The rms amplitudes of vibration with their associated uncertainties have been determined for some of the distances. In addition, the geometry has been calculated by ab initio Hartree-Fock gradient calculations with geometry optimization at the 3-21G* and 6-31G* levels, and the computed structural parameters are in reasonable agreement with those obtained from the electron diffraction data. Features of the structure are compared with those of similar molecules.

INTRODUCTION

We [l-9] have been investigating the conformational stability of organophosphorus molecules of the general formula CHBCHzP ( Z)Yz where Y is hydrogen, halogen or a methyl group and Z is oxygen, sulfur or just the nonbonded electron pair. From our far-infrared studies [5] of ethyl phosphine, it Correspondence to: Professor J.R. Durig, Department of Chemistry, University of South Carolina, Columbia, SC 29208, USA.

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144

has been determined that the trans conformer is more stable than the gauche form by 232 -t 2 cm-‘. This value agrees satisfactorily with the ab initio results which vary from 359 cm-l, obtained from the 3-21G* basis set without optimization, to 117 cm-‘, calculated with the 6-31G* basis set with optimization [7]. We have also carried out conformational studies of CH&H2PF2, CH&H2PC12 and CH3CH2P(CH,),. For the first two molecules [3,6], again the trans conformers are found to be more stable (for CH,CH,PF, the trans rotamer is more stable by 56 ? 22 cm-’ and for CH,CH,PCl, by 53 ? 38 cm-l) than the gauche conformers, whereas for ethyldimethylphosphine [8], the gauche conformer is more stable than the trans form by 134 + 32 cm-l. From these data, it appears that there are both electronegativity and steric effects contributing to the conformational stability of these P (III) molecules. A study [lo] of the vibrational spectra of CHBOPC1:!in solid, liquid and solution phases showed that only a single conformation was present in the crystalline solid, but that two distinct forms coexisted in the liquid and solution phases; the form stable in the solid was shown to be trans, and the other form, 0.2 It 0.1 kcal mol-’ less stable, was assigned as gauche. An electron diffraction study in the gas phase [ 111 was unable to determine the conformational composition unequivocally, but a reasonable POC angle (about 125’ ) was obtained if the trans conformer was assumed. For CHBOPF2, a microwave study [ 121 gave a reasonable POC bond angle (124” ) for a molecule with trans conformation assigned to the most prominent transitions; there were, however, many other unassigned transitions and it seems likely that a range of conformations may coexist in the vapor. More recently, an electron diffraction study [ 131 using the microwave results as additional data confirmed that the electron diffraction pattern was consistent with its existence in a single form with trans conformation. This is consistent with a study [ 141 of the vibrational spectra of the gas, liquid and solid phases, which concluded that only one conformer was present. CH,0P(CH3)2 was shown [ 151 to exist predominantly as one conformer, gauche, in the gas, as a mixture of gauche and trans in the liquid, and exclusively as the trans form in the annealed solid. In a related study [ 161 of the vibrational spectra of CH,SP(CH3)2, it was shown that two conformers coexist in the gas and liquid. Raman polarization data strongly suggested that both forms had a mirror plane of symmetry and they were assigned as the perfect cis and trans conformers, with the trans form (with a larger dipole moment) favored by intermolecular forces in the condensed phases. An electron diffraction study [ 171 of the conformation and structure of CH3SPF2 suggested that a single conformation with a torsional angle of 106 (9) ’ (perfect cis is 0’) gauche is 60’) trans is 180” ) gave the best fit to experimental data. However, other possible conformers with torsional angles of 19” and 171’ fitted the data equally well and it is impossible to be sure that some mixture of conformers would not have led to an even better fit.

145

In view of the apparent differences in conformational composition between methoxy- and methylthiophosphines, we have determined the structure of CH3SP ( CH3)z using a combination of electron diffraction in the gas phase and ab initio calculations, which we show to be highly effective in overcoming some of the correlations to be expected in systems containing several closely similar interatomic distances.

EXPERIMENTAL

Dimethyl(methylthio)phosphine-do was prepared by method I previously described by See1 and Velleman [ 181. The product was purified prior to use on a low-temperature, low-pressure fractionating column. The sample identity and purity were verified by comparison of its mid-infrared spectra with that previously reported [ 181. Electron diffraction data were collected on Kodak Electron Image plates using the gas-phase electron apparatus at Edinburgh [ 191. The sample and nozzle were both at room temperature. Optical densities were measured using a Joyce-Loebl MDM6 microdensitometer [20]. Three plates from the long (286 mm) and three from the short (128 mm) camera distance were used in subsequent refinements of the structure. The camera distances and electron wavelengths were calibrated using scattering data for benzene. Data reduction [20] and least-squares refinements [21] of the structure were carried out as described previously, using standard scattering factors [ 22 1. Table 1 gives the s ranges and intervals, weighting points used to set up the off-diagonal weight matrices, and other experimental data. The molecular intensity curves and final differences are shown in Fig. 1, and the radial distribution curve with final difference in Fig. 2. TABLE 1 Camera distances, s-ranges and weighting points, refined scale factors, correlation parameters and electron wavelengths Camya distance (mm) As (A;‘) %xlin Q-9 SW1(4-l) 3% (4-l) s,,, (A-‘) Scale factor Correlation Electron wavelength (A)

285.7 0.2 2.0 4.0 12.4 14.6 0.744(12) 0.117 0.05668

128.3 0.4 8.0 10.0 28.6 33.6 0.684(33) 0.463 0.05670

AI

v-

A

-*

Fig. 1. Observed molecular scattering intensities and final weighted differences for nozzle-to-plate distances of (a) 128 and (b) 286 mm.

Fig. 2. Observed and final difference radial distribution curves P( r) /r, for MeSPMe2. Before Fourier inversion the data were multiplied by s exp (-0.002 s’)/(Z,-f,) (2,-f,). AB INITIO CALCULATIONS

The LCAO-MO-SCF

restricted Hartree-Fock calculations were performed

147

with the program GAUSSIAN 86 using Gaussian-type basis functions [ 231. For the trans, cis and near-cis conformers, the energy minima with respect to the nuclear coordinates were obtained by using the gradient method of Pulay [ 241. Both the 3-21G* and 6-31G* basis sets were employed for these calculations, with significant differences found in the potential surfaces derived using them, i.e. a trans/near-cis conformational pair for the 3-21G* level and a trans/cis conformational pair for the 6-31G* basis set level. The structural parameters obtained for the calculations at these levels are given in Table 2, where they

Fig. 3. Potential function governing internal rotation of dimethyl(methylthio)phosphine as determined by ab initio calculations with the 3-21G* basis set obtained by optimization at the cis, near-& and trans minima and maximum by relaxation of all of the geometric parameters.

-180

-120

-60

DIHEDRAL

60

0

ANGLE

120

180

($)

Fig. 4. Potential function governing internal rotation of dimethyl(methylthio)phosphine as determined by ab initio calculations with the 6-31G* basis set obtained by optimization at the cis and trans minima and maximum by relaxation of all of the geometric parameters. The dashed line was obtained by varying the dihedral angle but holding all the structural parameters fixed at the optimized values for the cis conformer.

Parameter

2.103 f 0.002 1.862 _+0.002 1.769 4 0.006 1.081+ 0.002 1.081+ 0.002 1.081 f 0.002 1.081+ 0.002 1.08lk 0.002 108.5 k2.4 96.7 f0.8 99.9 kO.9 113.1 k2.0 113.1 f2.0 113.1 k2.0 110.8 f4.0 110.8 f4.0 108.9 108.9 105.7 105.7 105.7 180’ 120” 120”

2.111 f0.002 1.862 _+0.002 1.769 + 0.007 1.081 AI0.002 1.081& 0.002 1.08lk 0.002 1.081 f 0.002 1.081 f 0.002 102.3 k2.5 96.7 f0.8 96.8 k0.9 113.1 k2.0 113.1 f2.0 113.1 52.0 110.8 k4.0 110.8 k4.0 108.9 108.9 105.7 105.7 105.7 180” 120’ 120”

2.126 1.852 1.821 1.082 1.081 1.085 1.086 1.083 101.4 99.6 100.8 112.7 108.8 109.7 106.6 111.2 109.1 109.6 108.6 108.7 108.2 180.0 120.2 120.6

Near-cisb 2.104 1.844 1.826 1.082 1.080 1.084 1.087 1.084 107.3 99.9 103.5 113.4 108.6 109.6 106.3 111.5 108.6 110.2 108.1 109.0 108.2 180.0 120.1 120.5

Cis

3-21G”

Trans

Near-cis

2.118 1.846 1.828 1.080 1.081 1.085 1.085 1.086 101.1 99.3 99.9 112.4 109.3 109.5 106.9 110.9 109.1 109.8 108.7 108.6 108.2 173.8 120.0 120.9

6-31G’

Ah initio

Electron diffraction

Trans

2.118 1.851 1.820 1.082 1.081 1.084 1.086 1.084 107.8 100.1 103.9 113.7 108.4 109.5 106.1 111.8 108.5 110.0 108.2 108.9 108.0 180.0 120.2 120.4

Trans

Structural parameters, rotational constants and dipole momenta for the trans, cis and near-cis conformers of dimethylmethylthiophosphine”

TABLE 2

6

(E + 852 ) Hartrees

120c 15.7 fl.6

120” 186

121.6 20.0 4881 1997 1729 0.8048 - 0.2493 0.5881 1.0275 0.9759335

122.3 180.0 4053 2156 2052 0.7973 0.0000 - 2.9308 3.0374 0.9752204 122.0 0.00 4890 1938 1718 0.7217 0.0000 0.5881 0.9309 5.0894548

122.5 180.0 4047 2124 2014 0.8004 0.0000 - 3.0145 3.1189 5.0880023

“Bond lengths in A, bond angles in degrees, rotational constants in MHz and dipole momenta in Debyes. bValuesfor non-symmetric structural parameters of the near-cis conformation are the averages of the two slightly different parameters. ‘Assumed symmetric methyl groups. dD denotes a dummy atom defined as the bisector of the L C,P,C,.

-

Pt

PC

pb

Pia

dihedral H ~o.~~C.,P,/H,,,,,C,,~P~ ~(CW%P,D)~ A B c

150

can be compared to those obtained from the current electron diffraction study. The ab initio calculated parameters at both the 3-21G* and 6-31G* levels are consistent with those obtained from the electron diffraction data. At the 3-21G* level the near-cis conformer is calculated to be 157 cm-’ more stable than the trans conformer. At the 6-31G* basis set level the cis conformer is calculated to be 319 cm-l more stable than the trans form. These energy differences are consistent with the electron diffraction data where the amount of the near-cis conformer is found to be 78 5 5%. The potential energy curves as a function of the torsional angle, calculated at the 3-21G* and the 6-31G* levels, are illustrated in Figs. 3 and 4. STRUCTURAL ANALYSIS

The ab initio calculations, which do not include any effects of vibrational averaging over large-amplitude torsional motions, showed that dimethyl (methylthio)phosphine was expected to exist as a mixture of cis (81% ) and trans (19% ) conformers, rather than the gauche/trans mixtures apparently found for methoxyphosphines. They also showed that the only structural parameters that varied significantly with the PS torsion angle were the PS bond length and the PSC and SPC bond angles. Accordingly, the model adopted for the analysis of the electron diffraction data included two conformers, with different values of the torsion angle, rPS, and the PSC and SPC bond angles, but common values for the remaining bond distances (rPC, rCH and rCS) and bond angles ( L PCP, L PCH and L SCH). All methyl groups were assumed to have CBVsymmetry about the fourth bond to carbon, and the PC:! group was assumed to have CpVsymmetry. The mole fraction of one of the two conformers was variable but could not be refined as an independent parameter. The CP and CS torsion angles were also refinable parameters. With the above assumptions, and with no assumption of overall C, symmetry for either conformer, each form has 59 independent interatomic distances. There are therefore 118 distances for which amplitudes of vibration need to be defined. Amplitudes of vibration were initially set at values suggested by experience with related compounds, and those corresponding to major peaks in the radial distribution curve were later allowed to refine; the values remained reasonable. Table 3 gives the final list of bonded distances and amplitudes, plus data for all non-bonded atom pairs including at least one non-hydrogen atom; Table 4 shows the correlation matrix for refining parameters and amplitudes. It was found not to be possible to allow the bond length and angle parameters of the two conformers to refine together without further constraint. Accordingly, we applied loose constraints suggested by the ab initio results at the 631G* level, so that rPS was 0.008 A longer in the near-cis form than in the

151 TABLE 3 Refined interatomic distances and amplitudes of vibration (A) for dimethyl (methylthio)phosphine Distance

Amplitude”

Near-cis

Trans

P-S P-G.5 s-c3 C-H (all) S.C,, P..C3 C4..C5 C,..G,,

2.103(2) 1.862(2) 1.769(6) 1.081(2)

0.037(7) 0.078(4)

2.976(22)

3.039(21)

3.030(52)

3.149(47) 2.782(16)

4.062(35) 4.370(20)

P...H

3.328(35) 2.492(26)

2.809(36)

0.057(3)

2.988(49)

3.761(24) 3.870(28) 2.380(53) 3.043(19)

3.092(20)

3.199(22) 3.993(11)

3.311(21) 4.041(9)

0.094(12) 0.074(20) 0.150 fixed (cis) 0.175 fixed (trans.) 0.112(22)=~18 0.150 fixed 0.112 fixed 0.150 fixed 0.112 tied to ~18 0.150 fixed 0.151 fixed 0.150 fixed

2.735(39)

0.180 fixed

3.266(26) 3.722(18) 3.842(63) 3.968(44) 4.362(45) 4.384(40) 4.387(38) 4.604(25) 4.829(21) 4.835(50) 4.939(63) 5.006(21) 5.188(34) 4.253(52)

2.813(48) 3.017(43) 3.392(42) 3.687(33) 4.297(31) 4.391(35)

0.200 fixed

“In almost all cases the amplitudes were assumed to be identical for the two conformers.

trans form, while the PSC bond angle was 6.4” smaller. Tolerances of + 0.001 A and 1 O, respectively, were used on these loose constraints. In addition, the CPS bond angle was set 3.1’ smaller in the near-cis form. With these loose constraints, the refinement converged satisfactorily, giving a minimum R-factor of 0.075 (Ro) when the proportion of the near-cis form was 78%. The two conformers were a perfect trans form, with a torsion angle of 180” (fixed) and a near-cis form with a refined torsion angle of 15.7 _+1.6”.

152 TABLE 4 Least-squares

correlation

matrix~

rPS, rPS, rPC rPS,

91

100

56

53

FPC

rCS Scale lb Scale 2b L CPC L CPS L PSC, L PSC, L PCH L SCH UC.~~C uP...H

uCP

100 for dimethyl(methylthio)phosphine” LPSC,

LPSC,

-54 100 97 90 80 -88

97 100 92 80 -88

90 92 100 70 -76

-92

-85

-83

-12

-93

uCH UPS LCPS

LPCH

uC...S uP...H

100 -68 -81 71 68 68 80 80 70 100

-90 -92 -89 -84 85 60 82

-85 -83 -72 -93 95 54 100

*Only elements whose absolute magnitude exceeds 50% are shown. There are no such elements connecting the two blocks given above. bScale 1 and Scale 2 are intensity scale factors refined to give the best overall intensity match between observed and calculated molecular scattering curves for camera distances 1 and 2, respectively.

Fig. 5. Perspective

view of the near-cis conformer

of MeSPMe,.

The cis molecular structure is illustrated in Fig. 5. The refined bond lengths for the two conformers agree well with those found in the ab initio calculations, except that the CS bond length is about 0.06 A shorter than calculated. This may reflect some minor deficiency in the basis set used for S in the calculations, or may be due to the difficulty of fully defining two closely similar bond distances (P-C and S-C) which contribute to a single peak in the radial distri-

153

bution curve. The calculated value is close to that found [ 171 in CH,SPF,, where there is no overlapping peak due to P-C bonds. The bond angles at P are also rather smaller than calculated, while apparently the HCP and HCS bond angles in the methyl groups are a few degrees larger than calculated. Attempts to improve the fit by allowing the methyl torsional angles to vary were ineffective. The refined PS torsional angle of 15.7 (16) o in the near-cis form is very close to the value found in the ab initio calculations at the 3-21G* level (20” ); at the 6-31G* level no reduction of total energy was achieved by allowing the torsional angle to deviate from the ideal cis position of 0 ‘. DISCUSSION

Overall, our results reveal a high degree of consistency between the calculated and experimental structures. There is some suggestion that the CS bond is not as long as calculated, and the perfect cis conformer found at a global minimum energy in the calculations at the 6-31G* basis set level is not confirmed either by the calculations at the 3-21G* level or the experimental data. Nevertheless, the ab initio calculations provided an excellent source of additional information, without which we would have been unable to solve the conformational problem using electron diffraction data alone. It is interesting that the two conformers found are consistent with the trans and near-cis forms, rather than the trans and gauche forms suggested for related methoxyphosphines. The distinction between perfect cis (z= 0’ ) , nearcis (zx 20” as here) and gauche (rw 60’ ) may not be clear-cut, as it is logically possible for the potential energy surface to be fairly flat over a wide range of torsional angles, but it would be worth calculating energy surfaces as a function of torsional angles for other phosphines to investigate the situation in more detail. The apparent discrepancy between the ab initio and electron diffraction results reported here and the vibrational study [ 161, in which both conformers were reported to possess mirror symmetry, is almost certainly due to the effects of large-amplitude torsional motions in the cis conformer. This results in the effective symmetry for the remaining vibrations, as the energy of the perfect cis form is only a few cm-’ higher than that of the near-cis equilibrium form and individual molecules will move freely from one side of the low barrier to the other. Moreover, even if the perfect cis conformation does represent a global minimum of energy, as suggested by the 6-31G* level calculations, the electron diffraction results would be expected to show an effective torsion angle deviating from zero because of the effects of the large-amplitude vibrations, which bring non-bonded atoms closer together on average, even in a torsional motion with a single minimum. The present data cannot clearly exclude this possibil-

154

ity and the discrepancies between the 3-2lG* and 6-31G* calculations, together with the equivocal experimental data, confirm that the potential energy surface near the cis conformation is very close to flat. Based on these results, ab initio calculations should be carried out for CH,OP (CH,), and such a study is currently under way. ACKNOWLEDGMENT

The authors gratefully acknowledge partial support for this study by the NATO Collaborative Research Grant No. 870102.

REFERENCES 1

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

24

J.R. Durig and T.J. Hizer, J. Mol. Struct., 145 (1986) 15. J.R. Durig and T.J. Hizer, J. Raman Spectrosc., 18 (1987) 415. J.R. Durig, J.S. Church, C.M. Whang, R.D. Johnson and B.J. Streusand, J. Phys. Chem., 91 (1987) 2769. J.R. Durig, R.D. Johnson, H. Nanaie and T.J. Hizer, J. Chem. Phys., 88 (1988) 7317. P. Groner, R.D. Johnson and J.R. Durig, J. Chem. Phys., 88 (1988) 3456. J.R. Durig, C.J. James, A.E. Stanley, T.J. Hizer and S. Cradock, Spectrochim. Acta, Part A, 44 (1988) 911. J.R. Durig, M.S. Lee, R.J. Harlan and T.S. Little, J. Mol. Struct. (Theochem), 200 (1989) 41. J.R. Durig and T.J. Hizer, J. Raman Spectrosc., 17 (1986) 97. J.R. Durig, C.G. James and T.J. Hizer, J. Raman Spectrosc., 21 (1990) 155. A.B. Remizov, F.S. Bilalov, S.A. Katsyuba and E.N. Ofitserov, J. Appl. Spectrosc., 37 (1982) 1029. N.M. Zaripov, J. Struct. Chem., 23 (1982) 292. E.G. Codding, C.E. Jones and R.H. Schwendeman, Inorg. Chem., 13 (1974) 178. M.J. Davis, D.W.H. Rankin and S. Cradock, J. Mol. Struct., 238 (1990) 273. J.R. Durig and B.J. Streusand, Appl. Spectrosc., 34 (1980) 65. B.J. van der Veken, T.S. Little, Y.S. Li, M.E. Harris and J.R. Durig, Spectrochim. Acta, Part A, 42 (1986) 123. J.R. Durig, D.F. Smith, D.A. Barron, R.J. Harlan and H.V. Phan, J. Raman Spectrosc., 23 (1992). D.E.J. Arnold, G. Gundersen, D.W.H. Rankin and H.E. Robertson, J. Chem. Sot., Dalton Trans., (1983) 1989. F. See1and K. Velleman, Chem. Ber., 105 (1972) 406. C.M. Huntley, G.S. Laurenson and D.W.H. Rankin, J. Chem. Sot., Dalton Trans., (1980) 954. S. Cradock, J. Koprowski and D.W.H. Rankin, J. Mol. Struct., 77 (1981) 113. A.S.F. Boyd, G.S. Laurenson andD.W.H. Rankin, J. Mol. Struct., 71 (1981) 217. L. Schafer, A.C. Yates and R.A. Bonham, J. Chem. Phys., 55 (1971) 3055. M.J. Frisch, J.S. Binkley, H.B. Schlegel, K. Raghavachari, C.F. Melius, R.L. Martin, J.J.P. Stewart, F.W. Bobrowicz, C.M. Rohlfing, L.R. Kahn, D.J. DeFrees, R. Seeger, R.A. Whiteside, D.J. Fox, E.M. Fleuder and J.A. Pople, GAUSSIAN-S, Carnegie-Mellon Quantum Chemistry Publishing Unit, Pittsburgh, PA, 1984. P. Pulay, Mol. Phys., 17 (1969) 197.