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Determination of the molecular structure of ethylphosphonothioic dichloride by gas-phase electron diffraction and ab initio calculations
51
Journal of Molecular Structure, 244 (1991) 51-58 Elsevier Science Publishers B.V., Amsterdam
DETERMINATION OF THE MOLECULAR STRUCTURE OF ETHYLPHOSPHONOTHIOIC DICHLORIDE BY GAS-PHASE ELECTRON DIFFRACTION AND AB INITIO CALCULATIONS
D.G. ANDERSON, S. CRADOCK, G.A. FORSYTH and D.W.H. RANKIN Department of Chemistry, University of Edinburgh, (Gt. Britain)
West Mains Road, Edinburgh, EH9 355
J.F. SULLIVAN, T.J. HIZER and J.R. DURIG Department of Chemistry, University of South Carolina, Columbia, SC 29208 (U.S.A.) (Received 11 June 1990)
ABSTRACT
The molecular structure of ethylphosphonothioic dichloride, CHsCH,P (S)Cl,, in the gas phase has been determined from an electron diffraction study. The gas consists of an equilibrium mixture of two conformers, in which the methyl group is trans or gauche with respect to the sulfur atom, but it is not possible to measure the proportions of the two forms because of the similarity of the sulfur and chlorine scattering powers, and of the P=S and P-Cl bond lengths. Ab initio calculations, at the 3-21G* level, were therefore performed, and these indicate that the conformation should be 82% gauche and 18% trans. These proportions were then used in the electron diffraction refinements, and the calculated differences between geometric parameters, most notably in the PCC angles, are also incorporated. Refined parameters ( ra) for the major conformer include r (PCl) 203.0(l), r(P=S) 189.7(2), r(P-C) 180.8(5), r(C-C) 150.5(6) pm, LPCC 114.4(8), LClPC(mean) 103.1(5), LCPS 116.1(12) and ~ClPCl102.0(4)“.
INTRODUCTION
We have recently studied a wide variety of ethyl phosphines [l-4] and the related oxides [ 5,6], sulfides [ 7-91 and borane adducts [ 10-131, by a number of different spectroscopic and structural techniques, with particular interest in the conformations adopted by such molecules. In every case a mixture of trans and gauche conformers was found; in some cases the trams form is more stable (CH,CH,PX, whereX=H [l],F[2], C1[3]; CH,CH,P(BH,)X, where X=H[lO], F[ll], C1[12]; CH,CH2P(0)X, where X=C1[5], F[6]; and CH3CH2P(S)Xz where X=F [7] ), whereas for the others the gauche form is more stable (CH&H,PMe, [ 41 and its borane adduct [ 131 and sulfide [ 81 (the oxide has not yet been investigated), and CH,CH,P (S ) Cl2 [ 91) . It is apparent from the above results that only CH3CH2P (S ) Clz among the compounds stud0022-2860/91/$03.50
0 1991-
Elsevier Science Publishers B.V.
52
ied so far shows a different conformational preference from its parent phosphine, CH3CH2PC12,and we have complemented our earlier vibrational study [9] with a combined electron diffraction/ah initio investigation intended to provide confirmation of the change in preference and further information about the energy difference between the conformers. A preliminary electron diffraction study showed that without the ab initio calculations it was not possible to determine the conformational composition of the vapor because of the similarity of the scattering powers of the chlorine and sulfur atoms. EXPERIMENTAL
Electron diffraction data were collected on Kodak electron image plates using the gas-phase electron apparatus at Edinburgh [ 141. The sample and nozzle temperatures were 335 and 355 K, respectively. Optical densities were measured using a Joyce-Loebl MDM6 microdensitometer [ 151. Five plates from the long (256 mm) and three from the short (96 mm) camera distance were used in subsequent refinements of the structure. The camera distances and electron wavelength were calibrated using scattering data for benzene. Data reduction [ 151 and least-squares refinements [ 161 of the structure were carried out as described previously, using standard scattering factors [ 171. In Table 1 are listed the s ranges and intervals, weighting points used to set up the off-diagonal weight matrices, and other experimental data. AB INITIO CALCULATIONS
The LCAO-MO-SCF restricted Hartree-Fock calculations were performed with the program GAUSSIAN-86 using Gaussian-type basis functions [ 181. For the tram and gauche conformers the energy minima with respect to the nuclear coordinates were obtained by using the gradient method of Pulay [ 191. The 3-21G* basis set was employed for these calculations. At this level of calculation the calculated distances should be accurate to 2.0 pm and the angles to 2 ‘. Initially we obtained the optimized structural parameters for the truns conformer since there are fewer variables. Subsequently, the optimized strucTABLE 1 Data analysis parameters Camera distance (mm)
AS
S,in
s-x (nm-‘)
SW1
SW2
Correlation parameter
Scale factor
Wavelength (pm)
256.13 95.82
2 4
20 100
148 328
40 140
126 270
0.354 0.089
0.821(5) 0.646(20)
5.673 5.674
53 TABLE 2 Molecular parameters (distances in pm, angles in degrees) Parameter
tram
gauche ED
Ab initio
ED
Ab initio
r(P-Cl) r(P=S) r(P-C) r(C-C) r(C-H) (methyl) r(C-H)(methylene)
203.0(l) 189.7(2) 180.8(5) 150.5(6) 110.5(8) 110.8(8)
202.2” 190.1 180.7 154.9 108.2” 108.5”
202.9(l) 189.8(2) 180.9(5) 150.1(6) 110.5(8) 110.9(8)
202.1 190.2 180.8 154.5 108.2” 108.6”
L PCC
114.4(8) 103.4(5) 102.7(5) 116.1(12) 102.0(4) 108.0” 109.5” 58.6(61) 180.0”
112.8 102.9 102.1 117.6
118.3(8)
116.7
104.1(5) 114.9(6) 102.0(4) 108.0” 109.5” 180.0” 180.0’
103.5 116.4
LCl(8)PCb LCl(9)PCb L CPS L ClPCl LHCH LCCH r(SPCC) 7( PCCH)
110.3” 60.0 178.8
110.0” 180.0 180.0
“Average value. bDependent parameter. “Fixed value.
tural parameters for the tram conformer were utilized to obtain a potential surface scan in which only the CCPS torsional dihedral angle was varied from 180” (tram position) to 360” (cis position) in 10” increments. The potential surface scan predicted a second minimum at approximately 300” which corresponds to the gauche conformation. At this point all structural parameters including the dihedral angle were optimized. The resulting total energy value of - 1727.856712 Hartree for the gauche conformer compared to the total energy of - 1727.856057 Hartree for the tram conformer shows that the gauche conformer is more stable by 144 cm-l. The structural parameters obtained from the calculations at this level are given in Table 2 where they can be compared to those obtained from the current electron diffraction study. The ab initio calculated parameters at the 3-21G* level are reasonably consistent with those obtained from the electron diffraction data. STRUCTURE ANALYSIS
On the basis of ab initio calculations, the composition of CH3CH2P ( S)CIZ was predicted to be 82% of the gauche conformer, with the remainder trans. The model for the gauche conformer assumed CJV symmetry in the C-CH3 group. Fourteen independent parameters were used, five of which were the
54
bond distances r(P-Cl), Y(P=S), r(P-C), r(C-C) and T(C-H) (the latter defining the methyl C-H distance), and six were valence bond angles, L PCC, LClPC, LCPS, LClPCl, LCCH (methyl) and LHCH (methylene). The methylene group and its adjacent bonds were assumed to have local C,, symmetry. The differences between the C-H bond lengths in the methylene and methyl groups was fixed at the calculated value, with r(C-H) (methylene) = Y(C-H) (methyl) + 0.3 pm. The ab initio calculations predicted that the bisector of the ClPCl angle did not lie in the SPC plane, and this deviation was described by the twelfth parameter, the displacement of the PC4 group in its own plane. Two torsional parameters were defined, the first describing the twist of the P-C bond, relative to the position in which the C-C bond eclipsed the P=S bond, and the second described the position of the CH3 group relative to a configuration in which one C-H bond eclipsed the P-C bond. In the tram conformer, differences in bond lengths relative to the gauche conformer were fixed at the calculated values, with Ar(P-Cl) = -0.1 pm, Ar(P=S)= +O.l pm, Ahr(P-C)= +O.l pm and Ar(C-C)= -0.4 pm, Ar(CH) (methyl) =O.O pm and AY(C-H) (methylene) = +0.4 pm. Similarly, with the bond angles, ALPCC= +3.9”,ALClPCl= + 1.0” and ALCPS= - 1.2”. Angles ClPCl, CCH and HCH were assumed to be the same in both conformers, as were the torsional parameters delining the conformations of the CH3 TABLE 3 Interatomic distances ( ra) and amplitudes of vibration (u) Atom pair
F,(P-Cl) r,(P=S) F3(P-C) F4(C-C) r,(C-H) (methyl) r,(Cl(Es)*-3)
F,(Cl(S)~~~S) r,(Cl...Cl) r,(Cl.a.S) F,,(C(4)...S) r,,(C(4)**C1(9)) F,,(P~~C(4)) r,,(C(l).*C1(8)) F14(C(l)..C1(9)) F1,(C(4)~.Cl(8))
r(pm)
u(pm)
gauche
tram
gauche
trans
203.0( 1) 189.7(2) 180.8(5) 150.5(6) 110.5(8) 331.4(3) 332.1(3) 315.0(10) 315.5(22) 359.1(103) 440.9(9) 278.6(7) 301.1(5) 299.5(5) 352.1(116)
202.9(l) 189.8(2) 180.9 (5) 150.1(5) 110.5(8) 331.3(3) 331.3(3) 314.8(10) 313.7(22) 451.0(10) 339.1(17) 284.3 (7) 302.4(5) 302.4(5) 339.1(17)
5.1(l) 4.5a 5.2’ 4.2(9) 9.0” 7.9(3) 7.9d 7.2d 8.6d 22.0(30) 10.6(4) 6.3(5) 7.2’ 7.2’ 21.5g
5.lb 4.5” 5.2” 4.2” 9.0” 7.9d 7.9d 7.2d 8.6d 10.6” 11.9d 6.3’ 7.2’ 7.2’ 11.9d
“Fixedvalue.‘Tiedtou(P-Cl).“riedtou(C-C).dTiedtou(Cl(8)...S).”Tiedtou(C(4)...Cl(9)). fTied to u(P**eC(4)). @Tiedto u(C(4)*..S).
55 TABLE 4 Least-squares correlation matrix” LPCC r(P-C) r(C-C) L ClPC LCPS ?sPCC u(C(4).*.S)
L ClPC
L CPS
L ClPCl
TSPCC
53 -61
81 -95 -55 61
-70
-57 -62 49 40
-90 -40 45
“Only elements with absolute values> 40% are listed.
I
a I\
b
I
40
I
160
I
s/nm-1
Fig. 1. Observed molecular scattering intensities and final weighted differences for nozzle-to-plate distances of (a) 96 and (b) 256 mm.
56
groups. A fifteenth independent parameter was used to describe the torsion angle about the P - C bond in the trans conformer, in which the bisector of the /_.C1PCI lay in the SPC plane, and the final parameter defined the proportion of the gauche conformer present. In the refinements of the structure, the P-C torsional parameter in the trans conformer was fixed at 180 °, and the common parameters /__HCH, /__CCH and the C-C torsion angle were fixed at 108.0, 109.5 and 180 °, respectively. The proportion of the gauche conformer present was fixed at the calculated value, 0.82, and the deviation of the bisector of the CIPC1 angle was fixed at 0.48 °. All other parameters were allowed to refine (the C-C torsion refined to ca. 185 °, but with an e.s.d, of +_20 °, so it was decided to fix it at the calculated value). Of the major amplitudes of vibration, u(P---S) (with u ( P - C ) tied to it) refined to an unreasonably small value, 2.9 ( 1 ) pm, and refining each ampli-
100
I 300
1
I 500
r/pm
Fig. 2. Observed and final difference radial distribution curves, P(r)/r, for CH3CH2P (S) C12.Before Fourier inversion the data were multiplied by s exp ( - 0.00002s 2) / (Zp - f p ) (Zcl - [ c l ) .
Fig. 3. Perspective views of (a) the gauche conformer and (b) the trans conformer of CH3CH2P (S) C12.
tude separately still gave a low value for u (P=S ), and 9.1(2 ) pm for u (P-C), which is too large, so they were fixed at 4.5 and 5.2 pm, respectively. All other major amplitudes were allowed to refine. The final R-factor RG was 0.065. In Table 2 are listed the final parameters, in Table 3 the interatomic distances and amplitudes of vibration, and in Table 4 the major elements of the least-squares correlation matrix. The refined and calculated parameters are generally in good agreement, although r( C-C) and L PCC showed significant discrepancies. The molecular scattering intensity curves and radial distribution curves are shown in Figs. 1 and 2, and in Fig. 3 the perspective views of the two conformers are shown. DISCUSSION
The present study confirms our earlier results [9] that suggested that the gauche form of CH&H2P (S)Cl, is more stable than the tram form. The ab
initio calculations, using the 3-21G* basis set, give an energy difference of 411 cal mol-l (144 cm-‘) between the two conformers, implying a composition of 82% gauche and 18% trans. The vibrational study showed both forms to be present in the liquid, but only the gauche form was found in the crystalline solid. The present electron diffraction results are unable to provide information about the conformational composition of the vapor, because of the close similarity of the scattering factors of the Cl and S atoms, but the observed scattering is quite compatible with the conformational composition suggested by the ab initio calculations. With the differences in molecular structure parameters fixed at values suggested by the calculations the refined structural parameters are in reasonable agreement with the calculated values except for the carbon-carbon distance. An ab initio calculation [20] on CH,CH,PH, using the 3-21G* basis set gave a C-C bond distance 1.1 pm longer than the experimental value [ 1] of 153.6 (2) pm, based on microwave data using 13 isotopic species. This suggests that the C-C distance in the present case should be slightly shorter than calculated, in the range 153-154 pm, rather than the experimental values of 150.5(6) and 150.1(6) pm, respectively, for the gauche and tram forms. Microwave data [ 111 on 9 isotopic species of CH,CH,P (S)F, gave a CC bond length of 153.2 (6) pm very similar to that in CH3CH2PH2, so the value in the present case is also expected to be around 153.5 pm. The discrepancy is puzzling, and we propose to initiate a microwave study of several isotopic species of CH,CH,P (S)Cl, in an effort to resolve the discrepancy between the experimental and expected values. The CH3CH2PC12/CH3CH2P (S)Cl, pair thus remains unique among the ethylphosphines so far studied in that the conformational preference changes on addition of a sulfur atom to the phosphorus. We have, however, shown [ 31 that the energy difference between gauche and tram forms in CH3CH2PC12 is
58 very small (about 50 cm- ’ ) in the gas, and that the gauche form predominates in the gas phase even though the truns form is more abundant in the solid. The difference between CH,CH,PCl, and CH3CHpP (S)Cl, may therefore be more apparent than real, and reflects only small changes in the relative energies of the two conformers. ACKNOWLEDGMENTS
We thank the Science and Engineering Research Council for financial support and for provision of the densitometer facility at the Daresbury Laboratory. We thank the NATO Scientific Affairs Division for assistance through Collaborative Research Grant No. 102-87. We also thank R.J. Harlan for assistance with the ab initio calculations.
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
19 20
P. Groner, R.D. Johnson and J.R. Durig, J. Mol. Struct., 124 (1986) 363. 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, C.G. James, A.E. Stanley, T.J. Hizer and S. Cradock, Spectrochim. Acta, Part A, 44 (1988) 911. 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. J.R. Durig, T.J. Hizer and R.J. Harlan, J. Phys. Chem., in press. J.R. Durig, R.D. Johnson, H. Nanaie and T.J. Hizer, J. Chem. Phys., 88 (1988) 7317. 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.D. Odom, P.A. Brletic, S.A. Johnston and J.R. Durig, J. Mol. Struct., 96 (1983) 247. J.R. Durig, J.J. Rizzolo, J.F. Sullivan, M.-S. Chang, T.J. Hizer and J.D. Odom, J. Mol. Struct., 156 (1987) 267. J.D. Odom, T.J. Hizer, A.E. Stanley, T.L. Tonker and J.R. Durig, Spectrochim. Acta, Part A, 44 (1988) 631. J.R. Durig, T.J. Hizer and J.D. Odom, J. Mol. Struct., 159 (1987) 85. 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. De Frees, R. Seeger, R.A. Whiteside, D.J. Fox, E.J.M. Fleuder and J.A. Pople, Gaussian-86, Carnegie-Mellon Quantum Chemistry Publishing Unit, Pittsburgh, PA, 1984. P. Pulay, Mol. Phys., 17 (1969) 197. J.R. Durig, MS. Lee, R.J. Harlan and T.S. Little, J. Mol. Struct. (Theochem), 200 (1989) 461.
Journal of Molecular Structure, 244 (1991) 51-58 Elsevier Science Publishers B.V., Amsterdam
DETERMINATION OF THE MOLECULAR STRUCTURE OF ETHYLPHOSPHONOTHIOIC DICHLORIDE BY GAS-PHASE ELECTRON DIFFRACTION AND AB INITIO CALCULATIONS
D.G. ANDERSON, S. CRADOCK, G.A. FORSYTH and D.W.H. RANKIN Department of Chemistry, University of Edinburgh, (Gt. Britain)
West Mains Road, Edinburgh, EH9 355
J.F. SULLIVAN, T.J. HIZER and J.R. DURIG Department of Chemistry, University of South Carolina, Columbia, SC 29208 (U.S.A.) (Received 11 June 1990)
ABSTRACT
The molecular structure of ethylphosphonothioic dichloride, CHsCH,P (S)Cl,, in the gas phase has been determined from an electron diffraction study. The gas consists of an equilibrium mixture of two conformers, in which the methyl group is trans or gauche with respect to the sulfur atom, but it is not possible to measure the proportions of the two forms because of the similarity of the sulfur and chlorine scattering powers, and of the P=S and P-Cl bond lengths. Ab initio calculations, at the 3-21G* level, were therefore performed, and these indicate that the conformation should be 82% gauche and 18% trans. These proportions were then used in the electron diffraction refinements, and the calculated differences between geometric parameters, most notably in the PCC angles, are also incorporated. Refined parameters ( ra) for the major conformer include r (PCl) 203.0(l), r(P=S) 189.7(2), r(P-C) 180.8(5), r(C-C) 150.5(6) pm, LPCC 114.4(8), LClPC(mean) 103.1(5), LCPS 116.1(12) and ~ClPCl102.0(4)“.
INTRODUCTION
We have recently studied a wide variety of ethyl phosphines [l-4] and the related oxides [ 5,6], sulfides [ 7-91 and borane adducts [ 10-131, by a number of different spectroscopic and structural techniques, with particular interest in the conformations adopted by such molecules. In every case a mixture of trans and gauche conformers was found; in some cases the trams form is more stable (CH,CH,PX, whereX=H [l],F[2], C1[3]; CH,CH,P(BH,)X, where X=H[lO], F[ll], C1[12]; CH,CH2P(0)X, where X=C1[5], F[6]; and CH3CH2P(S)Xz where X=F [7] ), whereas for the others the gauche form is more stable (CH&H,PMe, [ 41 and its borane adduct [ 131 and sulfide [ 81 (the oxide has not yet been investigated), and CH,CH,P (S ) Cl2 [ 91) . It is apparent from the above results that only CH3CH2P (S ) Clz among the compounds stud0022-2860/91/$03.50
0 1991-
Elsevier Science Publishers B.V.
52
ied so far shows a different conformational preference from its parent phosphine, CH3CH2PC12,and we have complemented our earlier vibrational study [9] with a combined electron diffraction/ah initio investigation intended to provide confirmation of the change in preference and further information about the energy difference between the conformers. A preliminary electron diffraction study showed that without the ab initio calculations it was not possible to determine the conformational composition of the vapor because of the similarity of the scattering powers of the chlorine and sulfur atoms. EXPERIMENTAL
Electron diffraction data were collected on Kodak electron image plates using the gas-phase electron apparatus at Edinburgh [ 141. The sample and nozzle temperatures were 335 and 355 K, respectively. Optical densities were measured using a Joyce-Loebl MDM6 microdensitometer [ 151. Five plates from the long (256 mm) and three from the short (96 mm) camera distance were used in subsequent refinements of the structure. The camera distances and electron wavelength were calibrated using scattering data for benzene. Data reduction [ 151 and least-squares refinements [ 161 of the structure were carried out as described previously, using standard scattering factors [ 171. In Table 1 are listed the s ranges and intervals, weighting points used to set up the off-diagonal weight matrices, and other experimental data. AB INITIO CALCULATIONS
The LCAO-MO-SCF restricted Hartree-Fock calculations were performed with the program GAUSSIAN-86 using Gaussian-type basis functions [ 181. For the tram and gauche conformers the energy minima with respect to the nuclear coordinates were obtained by using the gradient method of Pulay [ 191. The 3-21G* basis set was employed for these calculations. At this level of calculation the calculated distances should be accurate to 2.0 pm and the angles to 2 ‘. Initially we obtained the optimized structural parameters for the truns conformer since there are fewer variables. Subsequently, the optimized strucTABLE 1 Data analysis parameters Camera distance (mm)
AS
S,in
s-x (nm-‘)
SW1
SW2
Correlation parameter
Scale factor
Wavelength (pm)
256.13 95.82
2 4
20 100
148 328
40 140
126 270
0.354 0.089
0.821(5) 0.646(20)
5.673 5.674
53 TABLE 2 Molecular parameters (distances in pm, angles in degrees) Parameter
tram
gauche ED
Ab initio
ED
Ab initio
r(P-Cl) r(P=S) r(P-C) r(C-C) r(C-H) (methyl) r(C-H)(methylene)
203.0(l) 189.7(2) 180.8(5) 150.5(6) 110.5(8) 110.8(8)
202.2” 190.1 180.7 154.9 108.2” 108.5”
202.9(l) 189.8(2) 180.9(5) 150.1(6) 110.5(8) 110.9(8)
202.1 190.2 180.8 154.5 108.2” 108.6”
L PCC
114.4(8) 103.4(5) 102.7(5) 116.1(12) 102.0(4) 108.0” 109.5” 58.6(61) 180.0”
112.8 102.9 102.1 117.6
118.3(8)
116.7
104.1(5) 114.9(6) 102.0(4) 108.0” 109.5” 180.0” 180.0’
103.5 116.4
LCl(8)PCb LCl(9)PCb L CPS L ClPCl LHCH LCCH r(SPCC) 7( PCCH)
110.3” 60.0 178.8
110.0” 180.0 180.0
“Average value. bDependent parameter. “Fixed value.
tural parameters for the tram conformer were utilized to obtain a potential surface scan in which only the CCPS torsional dihedral angle was varied from 180” (tram position) to 360” (cis position) in 10” increments. The potential surface scan predicted a second minimum at approximately 300” which corresponds to the gauche conformation. At this point all structural parameters including the dihedral angle were optimized. The resulting total energy value of - 1727.856712 Hartree for the gauche conformer compared to the total energy of - 1727.856057 Hartree for the tram conformer shows that the gauche conformer is more stable by 144 cm-l. The structural parameters obtained from the calculations at this level are given in Table 2 where they can be compared to those obtained from the current electron diffraction study. The ab initio calculated parameters at the 3-21G* level are reasonably consistent with those obtained from the electron diffraction data. STRUCTURE ANALYSIS
On the basis of ab initio calculations, the composition of CH3CH2P ( S)CIZ was predicted to be 82% of the gauche conformer, with the remainder trans. The model for the gauche conformer assumed CJV symmetry in the C-CH3 group. Fourteen independent parameters were used, five of which were the
54
bond distances r(P-Cl), Y(P=S), r(P-C), r(C-C) and T(C-H) (the latter defining the methyl C-H distance), and six were valence bond angles, L PCC, LClPC, LCPS, LClPCl, LCCH (methyl) and LHCH (methylene). The methylene group and its adjacent bonds were assumed to have local C,, symmetry. The differences between the C-H bond lengths in the methylene and methyl groups was fixed at the calculated value, with r(C-H) (methylene) = Y(C-H) (methyl) + 0.3 pm. The ab initio calculations predicted that the bisector of the ClPCl angle did not lie in the SPC plane, and this deviation was described by the twelfth parameter, the displacement of the PC4 group in its own plane. Two torsional parameters were defined, the first describing the twist of the P-C bond, relative to the position in which the C-C bond eclipsed the P=S bond, and the second described the position of the CH3 group relative to a configuration in which one C-H bond eclipsed the P-C bond. In the tram conformer, differences in bond lengths relative to the gauche conformer were fixed at the calculated values, with Ar(P-Cl) = -0.1 pm, Ar(P=S)= +O.l pm, Ahr(P-C)= +O.l pm and Ar(C-C)= -0.4 pm, Ar(CH) (methyl) =O.O pm and AY(C-H) (methylene) = +0.4 pm. Similarly, with the bond angles, ALPCC= +3.9”,ALClPCl= + 1.0” and ALCPS= - 1.2”. Angles ClPCl, CCH and HCH were assumed to be the same in both conformers, as were the torsional parameters delining the conformations of the CH3 TABLE 3 Interatomic distances ( ra) and amplitudes of vibration (u) Atom pair
F,(P-Cl) r,(P=S) F3(P-C) F4(C-C) r,(C-H) (methyl) r,(Cl(Es)*-3)
F,(Cl(S)~~~S) r,(Cl...Cl) r,(Cl.a.S) F,,(C(4)...S) r,,(C(4)**C1(9)) F,,(P~~C(4)) r,,(C(l).*C1(8)) F14(C(l)..C1(9)) F1,(C(4)~.Cl(8))
r(pm)
u(pm)
gauche
tram
gauche
trans
203.0( 1) 189.7(2) 180.8(5) 150.5(6) 110.5(8) 331.4(3) 332.1(3) 315.0(10) 315.5(22) 359.1(103) 440.9(9) 278.6(7) 301.1(5) 299.5(5) 352.1(116)
202.9(l) 189.8(2) 180.9 (5) 150.1(5) 110.5(8) 331.3(3) 331.3(3) 314.8(10) 313.7(22) 451.0(10) 339.1(17) 284.3 (7) 302.4(5) 302.4(5) 339.1(17)
5.1(l) 4.5a 5.2’ 4.2(9) 9.0” 7.9(3) 7.9d 7.2d 8.6d 22.0(30) 10.6(4) 6.3(5) 7.2’ 7.2’ 21.5g
5.lb 4.5” 5.2” 4.2” 9.0” 7.9d 7.9d 7.2d 8.6d 10.6” 11.9d 6.3’ 7.2’ 7.2’ 11.9d
“Fixedvalue.‘Tiedtou(P-Cl).“riedtou(C-C).dTiedtou(Cl(8)...S).”Tiedtou(C(4)...Cl(9)). fTied to u(P**eC(4)). @Tiedto u(C(4)*..S).
55 TABLE 4 Least-squares correlation matrix” LPCC r(P-C) r(C-C) L ClPC LCPS ?sPCC u(C(4).*.S)
L ClPC
L CPS
L ClPCl
TSPCC
53 -61
81 -95 -55 61
-70
-57 -62 49 40
-90 -40 45
“Only elements with absolute values> 40% are listed.
I
a I\
b
I
40
I
160
I
s/nm-1
Fig. 1. Observed molecular scattering intensities and final weighted differences for nozzle-to-plate distances of (a) 96 and (b) 256 mm.
56
groups. A fifteenth independent parameter was used to describe the torsion angle about the P - C bond in the trans conformer, in which the bisector of the /_.C1PCI lay in the SPC plane, and the final parameter defined the proportion of the gauche conformer present. In the refinements of the structure, the P-C torsional parameter in the trans conformer was fixed at 180 °, and the common parameters /__HCH, /__CCH and the C-C torsion angle were fixed at 108.0, 109.5 and 180 °, respectively. The proportion of the gauche conformer present was fixed at the calculated value, 0.82, and the deviation of the bisector of the CIPC1 angle was fixed at 0.48 °. All other parameters were allowed to refine (the C-C torsion refined to ca. 185 °, but with an e.s.d, of +_20 °, so it was decided to fix it at the calculated value). Of the major amplitudes of vibration, u(P---S) (with u ( P - C ) tied to it) refined to an unreasonably small value, 2.9 ( 1 ) pm, and refining each ampli-
100
I 300
1
I 500
r/pm
Fig. 2. Observed and final difference radial distribution curves, P(r)/r, for CH3CH2P (S) C12.Before Fourier inversion the data were multiplied by s exp ( - 0.00002s 2) / (Zp - f p ) (Zcl - [ c l ) .
Fig. 3. Perspective views of (a) the gauche conformer and (b) the trans conformer of CH3CH2P (S) C12.
tude separately still gave a low value for u (P=S ), and 9.1(2 ) pm for u (P-C), which is too large, so they were fixed at 4.5 and 5.2 pm, respectively. All other major amplitudes were allowed to refine. The final R-factor RG was 0.065. In Table 2 are listed the final parameters, in Table 3 the interatomic distances and amplitudes of vibration, and in Table 4 the major elements of the least-squares correlation matrix. The refined and calculated parameters are generally in good agreement, although r( C-C) and L PCC showed significant discrepancies. The molecular scattering intensity curves and radial distribution curves are shown in Figs. 1 and 2, and in Fig. 3 the perspective views of the two conformers are shown. DISCUSSION
The present study confirms our earlier results [9] that suggested that the gauche form of CH&H2P (S)Cl, is more stable than the tram form. The ab
initio calculations, using the 3-21G* basis set, give an energy difference of 411 cal mol-l (144 cm-‘) between the two conformers, implying a composition of 82% gauche and 18% trans. The vibrational study showed both forms to be present in the liquid, but only the gauche form was found in the crystalline solid. The present electron diffraction results are unable to provide information about the conformational composition of the vapor, because of the close similarity of the scattering factors of the Cl and S atoms, but the observed scattering is quite compatible with the conformational composition suggested by the ab initio calculations. With the differences in molecular structure parameters fixed at values suggested by the calculations the refined structural parameters are in reasonable agreement with the calculated values except for the carbon-carbon distance. An ab initio calculation [20] on CH,CH,PH, using the 3-21G* basis set gave a C-C bond distance 1.1 pm longer than the experimental value [ 1] of 153.6 (2) pm, based on microwave data using 13 isotopic species. This suggests that the C-C distance in the present case should be slightly shorter than calculated, in the range 153-154 pm, rather than the experimental values of 150.5(6) and 150.1(6) pm, respectively, for the gauche and tram forms. Microwave data [ 111 on 9 isotopic species of CH,CH,P (S)F, gave a CC bond length of 153.2 (6) pm very similar to that in CH3CH2PH2, so the value in the present case is also expected to be around 153.5 pm. The discrepancy is puzzling, and we propose to initiate a microwave study of several isotopic species of CH,CH,P (S)Cl, in an effort to resolve the discrepancy between the experimental and expected values. The CH3CH2PC12/CH3CH2P (S)Cl, pair thus remains unique among the ethylphosphines so far studied in that the conformational preference changes on addition of a sulfur atom to the phosphorus. We have, however, shown [ 31 that the energy difference between gauche and tram forms in CH3CH2PC12 is
58 very small (about 50 cm- ’ ) in the gas, and that the gauche form predominates in the gas phase even though the truns form is more abundant in the solid. The difference between CH,CH,PCl, and CH3CHpP (S)Cl, may therefore be more apparent than real, and reflects only small changes in the relative energies of the two conformers. ACKNOWLEDGMENTS
We thank the Science and Engineering Research Council for financial support and for provision of the densitometer facility at the Daresbury Laboratory. We thank the NATO Scientific Affairs Division for assistance through Collaborative Research Grant No. 102-87. We also thank R.J. Harlan for assistance with the ab initio calculations.
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