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An electron diffraction investigation of the molecular structure of ethyldimethylphosphine in the gas phase
Journal of Molecular Elsevier
Science
Structure,
Publishers
B.V.,
145 (1986) Amsterdam
127-134 - Printed
in The Netherlands
AN ELECTRON DIFFRACTION INVESTIGATION OF THE MOLECULAR STRUCTURE OF ETHYLDIMETHYLPHOSPHINE GAS PHASE
IN THE
J. R. DLJRIG and J. F. SULLIVAN
Department STEPHEN
of Chemistry,
University of South Carolina, Columbia, SC 29208
Department of Chemistry, University of Edinburgh, Edinburgh EH9 3JJ (Gt. Britain) (Received
(U.S.A.)
CRADOCK
26 November
West Mains Road,
1985)
ABSTRACT The molecular structure of ethyldimethylphosphine has been investigated at 22°C by electron diffraction in the gas phase. Two distinct conformers, gauche and trans, were identified; there is about 60% of the gauche isomer at this temperature. The important structural parameters (ra, distances in a, angles in degrees) with their uncertainties in parentheses, were found to be, for the gauche conformer; r(C-C) = 1.559(5), r(C-P) = 1.848(2), LCCP 112.3(35)“, f_C,PC, = 101.5(35)“, L&PC, = 99.6(20)“, with a CP torsional angle of 114(8)” from the trans position (defined as having the ethyl group trans to the phosphorus lone pair). The only parameter that is significantly different for the trans conformer, apart from the dihedral torsional angle, is LCCP = 107.6(27)“. INTRODUCTION
For some time we have been investigating the conformational stability of molecules of the type CH3YP(Z)X, where Y = CH2, 0 or S; Z = BH,, 0, S or the non-bonded electron pair and X = H, F, Cl or a CH3 group [l-lo]. These studies have been aimed at discovering whether the conformational stabilities can be rationalized on the basis of the gauche effect or whether non-bonded (steric) interactions play a dominant role. For the most part they have been carried out using vibrational spectroscopy, but such studies do not allow one to determine the structures of the various conformers. More recently we have also used microwave spectroscopy, which can give some structural information for investigating conformational problems. In our studies of ethyl phosphine [ 1,2] and ethyldifluorophosphine [ 7,8] we found the trans conformer to be more stable than the gauche (by some 275 f 10 cm-‘, 786 f 29 cal mol-‘) in the first case, but the gauche conformer to be more stable (by about 80 cm-‘, 229 cal mol-‘) in the fluoride. For ethyldichlorophosphine [9] we found that the gauche conformer is only slightly more stable than the tram form, whereas in the liquid the tram form 0022-2860/86/$03.50
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Elsevier
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Publishers
B.V.
128 is more stable than the gauche, presumably because of intermolecular forces in the liquid. Since the methyl group has an electronegativity similar to that of hydrogen but a size close to that of chlorine, we have recently studied the vibrational spectra of ethyldimethylphosphine [lo], hoping to determine its conformational stability and hence distinguish between the effects of electronegativity and size. The gauche conformer appears to be more stable than the tram from this study, by 134 f 32 cm-’ (383 + 92 cal mol-i) in the liquid phase; however, the proportions of the two conformers could only be estimated from the relative intensities of the corresponding peaks in the vibrational spectra, and the identification of the two conformers was based on the depolarization data from the Raman spectrum and on normal coordinate calculations. We felt that an electron diffraction study of this molecule would provide confirmation of the conformational composition of the gas, as well as structural parameters which would be useful for comparisons with the other ethyl-substituted phosphines we have studied. The results of this investigation are reported herein. EXPERIMENTAL The sample of ethyldimethylphosphine was obtained from Organometallics, Inc., and purified using a low-temperature fractionating column. The sample was stored over activated Linde 3 a molecular sieves in a sample tube equipped with a greaseless stopcock and kept cold in an ethanol-Dry Ice slush until used for the electron diffraction studies. The diffraction experiments were performed on the Oregon State University apparatus with the nozzle tip at 35°C and a sample-bath temoerature of 22°C. Other experimental conditions included an r3 sector, 8 X 10 inch Kodak projector slide plates of medium contrast which were developed for 10 min in D-19 developer diluted 1: 1,0.40 to 0.44 PA beam currents, exposure times of 60-70 s (long camera) and 180 s (intermediate camera), electron wavelengths of 0.05716 to 0.05718 a and nominal nozzle-to-plate distances of 750 mm (long) and 300 mm (intermediate). The ambient pressure in the diffraction chamber was 3.8 X 10” Torr (long camera) and 6.2-7.0 X 10m6 Torr (intermediate camera). Data were obtained from four or five plates at each distance in the usual manner [ll] and atomic scattering subtracted as described by Huntley et al. [ 121 using the complex scattering factors of Schafer et al. [ 131. The useful data covered the range 2.0 < s < 13.4 8-l and 6.0 < s < 33.6 8-l for the long and intermediate distances, respectively. Data reduction and refinements were carried out using programs that have been described previously [ 12, 141. The reduced total scattering intensities with final difference curves (observed-calculated intensity) are shown in Fig. 1. The scale factors and correlation parameters and the weighting points used in setting up the off-diagonal weight matrices for the refinement are listed in Table 1.
129
Fig. 1. Reduced molecular scattering intensities and final difference curves (arbitrary tensity units) for long (upper trace) and intermediate (lower trace) camera distances. TABLE
1
Weighting Camera 750 300
in-
points,
distance
s-intervals, (mm)
(long) (intermediate)
STRUCTURAL
MODEL
scale factors
and correlations
As (A-‘)
swl (A?)
sw2 (a-r)
Scale factor
Correlation
0.2 0.4
4.0 8.0
12.0 30.0
0.871(10) 1.136(14)
0.422 0.402
AND ANALYSIS
With 16 atoms, there are up to 120 distinct distances between pairs of atoms in any one conformer, though this large number is reduced if C, symmetry is assumed, as for the tram conformer expected here. Our model considers a mixture of the trans form with a fixed zero degree torsional angle about the PC bond with a second conformer whose torsional angle is free to refine, and is expected to be close to 120”, corresponding to the gauche conformer. To keep the analysis as simple as possible, most of the structural parameters are assumed to be the same for the two conformers, the exceptions being the PCC bond angle and the methyl torsional angle of the ethyl group. The proportion of the second conformer was also variable, though it
130
could not be included as a refining parameter. All the C-H and P-C bonded distances were assumed to be equal, and methyl groups were assumed to have local CSV symmetry about the C-P or C-C bonds. No rotational constants appear to be known for this molecule, and we have not included the vibrational corrections (which could be calculated from the known vibrational frequencies using a harmonic force-field analysis) needed to define an r, structure that could be used to generate a set of B, rotational constants. The results reported here are calculated on an r, basis, without any correction for shrinkage effects. REFINEMENT
AND RESULTS
The data proved to be consistent with the expected mixture of conformers. The radial distribution curve (Fig. 2) shows a clear peak at about 4.2 ,iX that must arise from the longer C ***C!, distance in an unsymmetrical conformer, but it is also necessary to include some tram conformer to fit the data well, though no discrete peak in the radial distribution curve can be ascribed solely to this conformer. The overall fit is extremely good, with Rfactors of RG = 0.053, RD = 0.037. The minimum R-factor was obtained with the proportion of the second conformer set at 0.57; we estimate the uncertainty in this as kO.15. The methyl torsional angles in the ethyl groups of the two conformers were fixed after a series of refinements had shown
Fig. 2. Radial distribution curve and final difference curve, with the most important distances contributing to P(r)/r. Before Fourier inversion the intensity data were multiplied by (s) exp[--0.002
s’/(ZP
-
fp)(.%
-
fdl.
131
which values gave the minimum R-factor. All other parameters were free to refine in the final stage, except for the methylene HCH angle, which remained fixed at 108.8” throughout. The final parameters are shown in Table 2, and the interatomic distances contributing to the radial distribution curve are listed in Table 3, with the amplitudes of vibration (Zi) refined or assumed. Amplitudes for all bonded distances and for all non-bonded distances not involving hydrogen atoms were allowed to refine. Amplitudes for a set of 2-bond P* - H distances near 2.45 a were finally fixed after refining, and the same was done for a set of 3-bond Pa* *H and Cm* -H distances in the range 2.7-3.8 8, each set being assumed to have a common 1value. Other distances involving hydrogen atoms were assigned fixed amplitudes. The most striking feature of the results, apart from the confirmation of the conformational composition deduced from the vibrational spectrum, is the large change in the PCC bond angle as the PC torsional angle changes from (tram) towards 120” (gauche). The direction of the change suggests that steric forces on the ethyl group are greater in the gauche conformer, for which the methyl group is closer to the phosphorus lone pair. The reverse effect has been suggested for ethyldifluorophosphine, where the limited microwave data [7] have shown a greater PCC bond angle in the tram form. It appears that an electron diffraction study of the fluoro-compound would be useful. Alternatively, microwave data of more isotopic species could be used to determine the structure of the fluoro-compound more precisely. The C-C bond appears longer than expected at 1.56 8; the corresponding peak in the radial distribution curve is weak and overlapped by the much stronger C-P bonded distance peak, but the correlation between the two refining distance parameters is low (0.21) and we have no reason to question the correctness of the C-C distance. It is known that the C-C single bond distance is significantly shorter in ethyl fluoride [ 151 (l-513(4)) and chloride [ 161 (1.520(3)) than in ethane [ 171 (1.534(2)) and other alkanes (1.540 a). The refined amplitude of 0.060(5) a is somewhat larger than expected from other C-C single bonds (0.050-0.054 .& seems to be a typical range) but the discrepancy is only 1.5 e.s.d. from the mid-point of the expected range. It l
TABLE
2
Model parameters
and estimated
r( P-C)
1.848(2) A 1.559(5) a 1.097(3) a 99.6( 20)” 101.5(35)” 112.3(35)” 107.6(27)”
tic-C) r( C-H) LC,PC, LC,PC, LPCC, LPCC,
*e, Ethyl;
standard
m, methyl;
g, gauche;
deviations
(in parentheses)a
LPCH, LCCH, CH,P, or PCbX(,) PC,,,(,) CH,Ct.w,) CH,Cw,) Proportion
t, trans; tor, torsional
angle.
gauche
110.5(9)” 107.5(21)” 29.8( 50)” 114.1(80)” 0 fixed -6” fixed 30” fixed 0.57 c 0.15
132 TABLE
3
Interatomic (a) Distances
distances
and amplitudes
common
to both
of vibration
conformers
Atoms
r
1
C-P
1.8485 1.5592 1.0970 2.863 2.824 2.178 2.159 2.458 2.436 2.936 3.045 3.780 2.731 2.752 2.755 3.294 3.502 3.686
0.064( 1) (I,) 0.060(5) 0.080(S) (I,) 0.085(7) (I,) 0.085 (I,) 0.12 f* 0.12 fc 0.132 f 0.132 f
C-C C-H c.*c! c**c C.-H C..H P..H P..H H*..P
H...C
(b) Distances Atoms
C*.P C.-C C..H
unique
(A)’
0.243
f
0.243
f
to conformers
gauche
tram
r
1
r
1
2.834 3.342 4.249 2.776 2.837 2.956 3.104 3.124 3.214 3.556 3.807 4.236 4.306 4.328 4.464 4.586 4.620 4.976 5.126
0.085 (I,) 0.083(20) 0.071(15) 0.20 f* 0.15 f* 0.15 f* 0.15 f* 0.18 f* 0.18 f* 0.20 f* 0.15 f* 0.20 f* 0.18 f* 0.18 f* 0.20 f* 0.20 f* 0.18 fc 0.20 f* 0.18 f*
2.756 3.016
0.085 (1,) O.OSS( 33) (I,)
2.402 2.838 2.864 3.069 3.706 3.801 3.811 4.110
0.18 0.20 0.20 0.15 0.18 0.18 0.15 0.20
f* f* f* f* f* f* f* f*
aThe symbol f indicates that an amplitude iKas fixed after refining to this value, f* that it remained fixed at this initial value. Non-bonded C*mH distances are listed in increasing order. Non-bonded H.-H distances are not listed; a copy of a list of these distances can be obtained from one of the authors (SC.).
133
may be that the greater apparent amplitude here reflects slightly different bond lengths in the two conformers, but the difference must be very small, of the order of 1 e.s.d. (0.005 A). We must point out that our assumption of a zero torsional angle for the tram conformer and our neglect of shrinkage corrections associated with the torsion about the PC bond may well be responsible for some distortion of the apparent CC bond length and PCC bond angles. In the gauche conformer, some of the shrinkage effects may be taken up by the deviation of the torsional angle from 120”, though the large e.s.d. on this refining parameter and its high correlations with other parameters and amplitudes (Table 4) mean that the discrepancy is not statistically significant. Fixing the PC torsional angle in the gauche form at 120” led to a rather poorer fit to the data (Ro = 0.058), but to very little change in the other refining parameters which were all less than 1 e.s.d. in every case. The P-C bonded distance, which is very well-determined at 1.848(2) A, is of course a weighted mean of the presumably slightly different P-Cmethyl The apparent amplitude and P-Cethyi bond distances in the two conformers. of 0.064(l) ,4 is rather higher than expected. For example the spectroscopically calculated value [X3] is 0.052 A in trimethyl phosphine and the refined values [ 191 were found to be 0.054(3) and 0.045(3) A in trimethylphosphine sulfide, respectively. The apparent excess amplitude may indicate some spread of the true bonded distances about the mean, which could be as much as 0.02 A, much larger than the statistical uncertainty of the mean itself. The apparent C-H bonded distance is again a weighted mean of up to 11 potentially-different actual C-H bond distances in each conformer; our data are clearly unable to allow us to investigate small differences such as those found spectroscopically [ 201 in similar but simpler molecules. Inspection of the radial distribution curve shows that the geometry of the gauche conformer can be specified more precisely than that of the tram form because of the distinct peak at 4.2 A referred to earlier, whereas there is no distinct peak due to long non-bonded distances in the tram form, all of which appear in the congested region near 3.0 A. TABLE
4
Least squares correlation matrix X 100 Only off-diagonal elements whose absolute
L C,PC,
IC,PC, LPCC, LPCH, 1, 1, 1, 1,
LPCC,
LPCC,
-64 -54 100 53
-94
83
70 52
LCCH,
value exceeds tCH,P
tpc,
-54 96 -59
50 are given. I,
I,
-78
-62 69
Scale 1
Scale 2
53
77 54
83
52
83
100 63
100
134 The proportion of the gauche and tram conformers found here is indistinguishable from the 2:l ratio expected for two forms of equal energy on statistical grounds. The discrepancy between the present results and those deduced from the vibrational spectrum [lo] must indicate that the assumption made in the earlier study that the intrinsic intensities of the corresponding normal modes for the two forms are the same is not a very good approximation. Our present results lead to an energy difference between the two forms of 70 t 100 cm-’ (200 f 300 cal mol-‘) which is considerably lower than the value obtained spectroscopically from the liquid. Nevertheless, the vibrational study provided an invaluable basis for the present study, as it showed that two conformers are present in the gas phase, and gave an indication as to which they are and of their relative abundances, allowing us to formulate the molecular model in a suitable fashion. ACKNOWLEDGEMENTS
We are happy to thank Professor K. Hedberg of Oregon State University for his help in obtaining the data, and Dr. D. W. H. Rankin of the University of Edinburgh for the use of his data reduction and refinement programs. This work was supported by grants from the National Science Foundation (Grant CHE-83-11279 to the University of South Carolina and CHE-84-11165 to the Oregon State University) and the NATO Scientific Affairs Division through their collaborative research program under Grant No. 140/82. REFERENCES 1 J. R. Durig and A. W. Cox, Jr., J. Chem. Phys., 64 (1976) 1930. 2 J. R. Durig and A. W. Cox, Jr., J. Phys. Chem., 80 (1976) 2493. 3 J. D. Odom, P. A. Brletic, S. A. Johnston and J. R. Durig, J. Mol. Struct., 96 (1983) 247. 4 R. L. Odeurs, B. J. van der Veken, M. A. Herman and J. R. Durig, J. Mol. Struct., 117 (1984) 235. 5 J. R. Durig and B. J. Streusand, Appl. Spectrosc., 34 (1980) 65. 6 J. R. Durig and J. S. DiYorio, J. Chem. Phys., 48 (1968) 4154. 7 P. Groner, J. S. Church, Y. S. Li and J. R. Durig, J. Chem. Phys., 82 (1985) 3894. 8 J. S. Church, Ph.D. Thesis, University of South Carolina, 1980. 9 A. E. Stanley, Ph.D. Thesis, University of South Carolina, 1982. 10 J. R. Durig and T. J. Hizer, J. Raman Spectrosc., 17 (1986) 97. 11 K. Hagen and K. Hedberg, J. Am. Chem. Sot., 95 (1973) 1003. 12 C. M. Huntley, G. S. Laurenson and D. W. H. Rankin, J. Chem. Sot., Dalton Trans., (1980) 954. 13 L. Schafer, A. C. Yates and R. A. Bonham, J. Chem. Phys., 55 (1971) 3055. 14 S. Cradock, J. Koprowski and D. W. H. Rankin, J. Mol. Struct., 77 (1981) 113. 15 P. Nosberger, A. Bauder and Hs. H. Gunthard, Chem. Phys., 1 (1973) 418. 16 R. H. Schwendeman and G. D. Jacobs, J. Chem. Phys., 36 (1962) 1245. 17 T. Iijima, Bull. Chem. Sot. Jpn., 46 (1973) 2311. 18 B. Beagley and A. R. Medwid, J. Mol. Struct., 38 (1977) 229. 19 C. J. Wilkins, K. Hagen, L. Hedberg, Q. Shen and K. Hedberg, J. Am. Chem. Sot., 97 (1975) 6352. 20 D. C. McKean, J. Mol. Struct., 113 (1984) 251.
Science
Structure,
Publishers
B.V.,
145 (1986) Amsterdam
127-134 - Printed
in The Netherlands
AN ELECTRON DIFFRACTION INVESTIGATION OF THE MOLECULAR STRUCTURE OF ETHYLDIMETHYLPHOSPHINE GAS PHASE
IN THE
J. R. DLJRIG and J. F. SULLIVAN
Department STEPHEN
of Chemistry,
University of South Carolina, Columbia, SC 29208
Department of Chemistry, University of Edinburgh, Edinburgh EH9 3JJ (Gt. Britain) (Received
(U.S.A.)
CRADOCK
26 November
West Mains Road,
1985)
ABSTRACT The molecular structure of ethyldimethylphosphine has been investigated at 22°C by electron diffraction in the gas phase. Two distinct conformers, gauche and trans, were identified; there is about 60% of the gauche isomer at this temperature. The important structural parameters (ra, distances in a, angles in degrees) with their uncertainties in parentheses, were found to be, for the gauche conformer; r(C-C) = 1.559(5), r(C-P) = 1.848(2), LCCP 112.3(35)“, f_C,PC, = 101.5(35)“, L&PC, = 99.6(20)“, with a CP torsional angle of 114(8)” from the trans position (defined as having the ethyl group trans to the phosphorus lone pair). The only parameter that is significantly different for the trans conformer, apart from the dihedral torsional angle, is LCCP = 107.6(27)“. INTRODUCTION
For some time we have been investigating the conformational stability of molecules of the type CH3YP(Z)X, where Y = CH2, 0 or S; Z = BH,, 0, S or the non-bonded electron pair and X = H, F, Cl or a CH3 group [l-lo]. These studies have been aimed at discovering whether the conformational stabilities can be rationalized on the basis of the gauche effect or whether non-bonded (steric) interactions play a dominant role. For the most part they have been carried out using vibrational spectroscopy, but such studies do not allow one to determine the structures of the various conformers. More recently we have also used microwave spectroscopy, which can give some structural information for investigating conformational problems. In our studies of ethyl phosphine [ 1,2] and ethyldifluorophosphine [ 7,8] we found the trans conformer to be more stable than the gauche (by some 275 f 10 cm-‘, 786 f 29 cal mol-‘) in the first case, but the gauche conformer to be more stable (by about 80 cm-‘, 229 cal mol-‘) in the fluoride. For ethyldichlorophosphine [9] we found that the gauche conformer is only slightly more stable than the tram form, whereas in the liquid the tram form 0022-2860/86/$03.50
0 1986
Elsevier
Science
Publishers
B.V.
128 is more stable than the gauche, presumably because of intermolecular forces in the liquid. Since the methyl group has an electronegativity similar to that of hydrogen but a size close to that of chlorine, we have recently studied the vibrational spectra of ethyldimethylphosphine [lo], hoping to determine its conformational stability and hence distinguish between the effects of electronegativity and size. The gauche conformer appears to be more stable than the tram from this study, by 134 f 32 cm-’ (383 + 92 cal mol-i) in the liquid phase; however, the proportions of the two conformers could only be estimated from the relative intensities of the corresponding peaks in the vibrational spectra, and the identification of the two conformers was based on the depolarization data from the Raman spectrum and on normal coordinate calculations. We felt that an electron diffraction study of this molecule would provide confirmation of the conformational composition of the gas, as well as structural parameters which would be useful for comparisons with the other ethyl-substituted phosphines we have studied. The results of this investigation are reported herein. EXPERIMENTAL The sample of ethyldimethylphosphine was obtained from Organometallics, Inc., and purified using a low-temperature fractionating column. The sample was stored over activated Linde 3 a molecular sieves in a sample tube equipped with a greaseless stopcock and kept cold in an ethanol-Dry Ice slush until used for the electron diffraction studies. The diffraction experiments were performed on the Oregon State University apparatus with the nozzle tip at 35°C and a sample-bath temoerature of 22°C. Other experimental conditions included an r3 sector, 8 X 10 inch Kodak projector slide plates of medium contrast which were developed for 10 min in D-19 developer diluted 1: 1,0.40 to 0.44 PA beam currents, exposure times of 60-70 s (long camera) and 180 s (intermediate camera), electron wavelengths of 0.05716 to 0.05718 a and nominal nozzle-to-plate distances of 750 mm (long) and 300 mm (intermediate). The ambient pressure in the diffraction chamber was 3.8 X 10” Torr (long camera) and 6.2-7.0 X 10m6 Torr (intermediate camera). Data were obtained from four or five plates at each distance in the usual manner [ll] and atomic scattering subtracted as described by Huntley et al. [ 121 using the complex scattering factors of Schafer et al. [ 131. The useful data covered the range 2.0 < s < 13.4 8-l and 6.0 < s < 33.6 8-l for the long and intermediate distances, respectively. Data reduction and refinements were carried out using programs that have been described previously [ 12, 141. The reduced total scattering intensities with final difference curves (observed-calculated intensity) are shown in Fig. 1. The scale factors and correlation parameters and the weighting points used in setting up the off-diagonal weight matrices for the refinement are listed in Table 1.
129
Fig. 1. Reduced molecular scattering intensities and final difference curves (arbitrary tensity units) for long (upper trace) and intermediate (lower trace) camera distances. TABLE
1
Weighting Camera 750 300
in-
points,
distance
s-intervals, (mm)
(long) (intermediate)
STRUCTURAL
MODEL
scale factors
and correlations
As (A-‘)
swl (A?)
sw2 (a-r)
Scale factor
Correlation
0.2 0.4
4.0 8.0
12.0 30.0
0.871(10) 1.136(14)
0.422 0.402
AND ANALYSIS
With 16 atoms, there are up to 120 distinct distances between pairs of atoms in any one conformer, though this large number is reduced if C, symmetry is assumed, as for the tram conformer expected here. Our model considers a mixture of the trans form with a fixed zero degree torsional angle about the PC bond with a second conformer whose torsional angle is free to refine, and is expected to be close to 120”, corresponding to the gauche conformer. To keep the analysis as simple as possible, most of the structural parameters are assumed to be the same for the two conformers, the exceptions being the PCC bond angle and the methyl torsional angle of the ethyl group. The proportion of the second conformer was also variable, though it
130
could not be included as a refining parameter. All the C-H and P-C bonded distances were assumed to be equal, and methyl groups were assumed to have local CSV symmetry about the C-P or C-C bonds. No rotational constants appear to be known for this molecule, and we have not included the vibrational corrections (which could be calculated from the known vibrational frequencies using a harmonic force-field analysis) needed to define an r, structure that could be used to generate a set of B, rotational constants. The results reported here are calculated on an r, basis, without any correction for shrinkage effects. REFINEMENT
AND RESULTS
The data proved to be consistent with the expected mixture of conformers. The radial distribution curve (Fig. 2) shows a clear peak at about 4.2 ,iX that must arise from the longer C ***C!, distance in an unsymmetrical conformer, but it is also necessary to include some tram conformer to fit the data well, though no discrete peak in the radial distribution curve can be ascribed solely to this conformer. The overall fit is extremely good, with Rfactors of RG = 0.053, RD = 0.037. The minimum R-factor was obtained with the proportion of the second conformer set at 0.57; we estimate the uncertainty in this as kO.15. The methyl torsional angles in the ethyl groups of the two conformers were fixed after a series of refinements had shown
Fig. 2. Radial distribution curve and final difference curve, with the most important distances contributing to P(r)/r. Before Fourier inversion the intensity data were multiplied by (s) exp[--0.002
s’/(ZP
-
fp)(.%
-
fdl.
131
which values gave the minimum R-factor. All other parameters were free to refine in the final stage, except for the methylene HCH angle, which remained fixed at 108.8” throughout. The final parameters are shown in Table 2, and the interatomic distances contributing to the radial distribution curve are listed in Table 3, with the amplitudes of vibration (Zi) refined or assumed. Amplitudes for all bonded distances and for all non-bonded distances not involving hydrogen atoms were allowed to refine. Amplitudes for a set of 2-bond P* - H distances near 2.45 a were finally fixed after refining, and the same was done for a set of 3-bond Pa* *H and Cm* -H distances in the range 2.7-3.8 8, each set being assumed to have a common 1value. Other distances involving hydrogen atoms were assigned fixed amplitudes. The most striking feature of the results, apart from the confirmation of the conformational composition deduced from the vibrational spectrum, is the large change in the PCC bond angle as the PC torsional angle changes from (tram) towards 120” (gauche). The direction of the change suggests that steric forces on the ethyl group are greater in the gauche conformer, for which the methyl group is closer to the phosphorus lone pair. The reverse effect has been suggested for ethyldifluorophosphine, where the limited microwave data [7] have shown a greater PCC bond angle in the tram form. It appears that an electron diffraction study of the fluoro-compound would be useful. Alternatively, microwave data of more isotopic species could be used to determine the structure of the fluoro-compound more precisely. The C-C bond appears longer than expected at 1.56 8; the corresponding peak in the radial distribution curve is weak and overlapped by the much stronger C-P bonded distance peak, but the correlation between the two refining distance parameters is low (0.21) and we have no reason to question the correctness of the C-C distance. It is known that the C-C single bond distance is significantly shorter in ethyl fluoride [ 151 (l-513(4)) and chloride [ 161 (1.520(3)) than in ethane [ 171 (1.534(2)) and other alkanes (1.540 a). The refined amplitude of 0.060(5) a is somewhat larger than expected from other C-C single bonds (0.050-0.054 .& seems to be a typical range) but the discrepancy is only 1.5 e.s.d. from the mid-point of the expected range. It l
TABLE
2
Model parameters
and estimated
r( P-C)
1.848(2) A 1.559(5) a 1.097(3) a 99.6( 20)” 101.5(35)” 112.3(35)” 107.6(27)”
tic-C) r( C-H) LC,PC, LC,PC, LPCC, LPCC,
*e, Ethyl;
standard
m, methyl;
g, gauche;
deviations
(in parentheses)a
LPCH, LCCH, CH,P, or PCbX(,) PC,,,(,) CH,Ct.w,) CH,Cw,) Proportion
t, trans; tor, torsional
angle.
gauche
110.5(9)” 107.5(21)” 29.8( 50)” 114.1(80)” 0 fixed -6” fixed 30” fixed 0.57 c 0.15
132 TABLE
3
Interatomic (a) Distances
distances
and amplitudes
common
to both
of vibration
conformers
Atoms
r
1
C-P
1.8485 1.5592 1.0970 2.863 2.824 2.178 2.159 2.458 2.436 2.936 3.045 3.780 2.731 2.752 2.755 3.294 3.502 3.686
0.064( 1) (I,) 0.060(5) 0.080(S) (I,) 0.085(7) (I,) 0.085 (I,) 0.12 f* 0.12 fc 0.132 f 0.132 f
C-C C-H c.*c! c**c C.-H C..H P..H P..H H*..P
H...C
(b) Distances Atoms
C*.P C.-C C..H
unique
(A)’
0.243
f
0.243
f
to conformers
gauche
tram
r
1
r
1
2.834 3.342 4.249 2.776 2.837 2.956 3.104 3.124 3.214 3.556 3.807 4.236 4.306 4.328 4.464 4.586 4.620 4.976 5.126
0.085 (I,) 0.083(20) 0.071(15) 0.20 f* 0.15 f* 0.15 f* 0.15 f* 0.18 f* 0.18 f* 0.20 f* 0.15 f* 0.20 f* 0.18 f* 0.18 f* 0.20 f* 0.20 f* 0.18 fc 0.20 f* 0.18 f*
2.756 3.016
0.085 (1,) O.OSS( 33) (I,)
2.402 2.838 2.864 3.069 3.706 3.801 3.811 4.110
0.18 0.20 0.20 0.15 0.18 0.18 0.15 0.20
f* f* f* f* f* f* f* f*
aThe symbol f indicates that an amplitude iKas fixed after refining to this value, f* that it remained fixed at this initial value. Non-bonded C*mH distances are listed in increasing order. Non-bonded H.-H distances are not listed; a copy of a list of these distances can be obtained from one of the authors (SC.).
133
may be that the greater apparent amplitude here reflects slightly different bond lengths in the two conformers, but the difference must be very small, of the order of 1 e.s.d. (0.005 A). We must point out that our assumption of a zero torsional angle for the tram conformer and our neglect of shrinkage corrections associated with the torsion about the PC bond may well be responsible for some distortion of the apparent CC bond length and PCC bond angles. In the gauche conformer, some of the shrinkage effects may be taken up by the deviation of the torsional angle from 120”, though the large e.s.d. on this refining parameter and its high correlations with other parameters and amplitudes (Table 4) mean that the discrepancy is not statistically significant. Fixing the PC torsional angle in the gauche form at 120” led to a rather poorer fit to the data (Ro = 0.058), but to very little change in the other refining parameters which were all less than 1 e.s.d. in every case. The P-C bonded distance, which is very well-determined at 1.848(2) A, is of course a weighted mean of the presumably slightly different P-Cmethyl The apparent amplitude and P-Cethyi bond distances in the two conformers. of 0.064(l) ,4 is rather higher than expected. For example the spectroscopically calculated value [X3] is 0.052 A in trimethyl phosphine and the refined values [ 191 were found to be 0.054(3) and 0.045(3) A in trimethylphosphine sulfide, respectively. The apparent excess amplitude may indicate some spread of the true bonded distances about the mean, which could be as much as 0.02 A, much larger than the statistical uncertainty of the mean itself. The apparent C-H bonded distance is again a weighted mean of up to 11 potentially-different actual C-H bond distances in each conformer; our data are clearly unable to allow us to investigate small differences such as those found spectroscopically [ 201 in similar but simpler molecules. Inspection of the radial distribution curve shows that the geometry of the gauche conformer can be specified more precisely than that of the tram form because of the distinct peak at 4.2 A referred to earlier, whereas there is no distinct peak due to long non-bonded distances in the tram form, all of which appear in the congested region near 3.0 A. TABLE
4
Least squares correlation matrix X 100 Only off-diagonal elements whose absolute
L C,PC,
IC,PC, LPCC, LPCH, 1, 1, 1, 1,
LPCC,
LPCC,
-64 -54 100 53
-94
83
70 52
LCCH,
value exceeds tCH,P
tpc,
-54 96 -59
50 are given. I,
I,
-78
-62 69
Scale 1
Scale 2
53
77 54
83
52
83
100 63
100
134 The proportion of the gauche and tram conformers found here is indistinguishable from the 2:l ratio expected for two forms of equal energy on statistical grounds. The discrepancy between the present results and those deduced from the vibrational spectrum [lo] must indicate that the assumption made in the earlier study that the intrinsic intensities of the corresponding normal modes for the two forms are the same is not a very good approximation. Our present results lead to an energy difference between the two forms of 70 t 100 cm-’ (200 f 300 cal mol-‘) which is considerably lower than the value obtained spectroscopically from the liquid. Nevertheless, the vibrational study provided an invaluable basis for the present study, as it showed that two conformers are present in the gas phase, and gave an indication as to which they are and of their relative abundances, allowing us to formulate the molecular model in a suitable fashion. ACKNOWLEDGEMENTS
We are happy to thank Professor K. Hedberg of Oregon State University for his help in obtaining the data, and Dr. D. W. H. Rankin of the University of Edinburgh for the use of his data reduction and refinement programs. This work was supported by grants from the National Science Foundation (Grant CHE-83-11279 to the University of South Carolina and CHE-84-11165 to the Oregon State University) and the NATO Scientific Affairs Division through their collaborative research program under Grant No. 140/82. REFERENCES 1 J. R. Durig and A. W. Cox, Jr., J. Chem. Phys., 64 (1976) 1930. 2 J. R. Durig and A. W. Cox, Jr., J. Phys. Chem., 80 (1976) 2493. 3 J. D. Odom, P. A. Brletic, S. A. Johnston and J. R. Durig, J. Mol. Struct., 96 (1983) 247. 4 R. L. Odeurs, B. J. van der Veken, M. A. Herman and J. R. Durig, J. Mol. Struct., 117 (1984) 235. 5 J. R. Durig and B. J. Streusand, Appl. Spectrosc., 34 (1980) 65. 6 J. R. Durig and J. S. DiYorio, J. Chem. Phys., 48 (1968) 4154. 7 P. Groner, J. S. Church, Y. S. Li and J. R. Durig, J. Chem. Phys., 82 (1985) 3894. 8 J. S. Church, Ph.D. Thesis, University of South Carolina, 1980. 9 A. E. Stanley, Ph.D. Thesis, University of South Carolina, 1982. 10 J. R. Durig and T. J. Hizer, J. Raman Spectrosc., 17 (1986) 97. 11 K. Hagen and K. Hedberg, J. Am. Chem. Sot., 95 (1973) 1003. 12 C. M. Huntley, G. S. Laurenson and D. W. H. Rankin, J. Chem. Sot., Dalton Trans., (1980) 954. 13 L. Schafer, A. C. Yates and R. A. Bonham, J. Chem. Phys., 55 (1971) 3055. 14 S. Cradock, J. Koprowski and D. W. H. Rankin, J. Mol. Struct., 77 (1981) 113. 15 P. Nosberger, A. Bauder and Hs. H. Gunthard, Chem. Phys., 1 (1973) 418. 16 R. H. Schwendeman and G. D. Jacobs, J. Chem. Phys., 36 (1962) 1245. 17 T. Iijima, Bull. Chem. Sot. Jpn., 46 (1973) 2311. 18 B. Beagley and A. R. Medwid, J. Mol. Struct., 38 (1977) 229. 19 C. J. Wilkins, K. Hagen, L. Hedberg, Q. Shen and K. Hedberg, J. Am. Chem. Sot., 97 (1975) 6352. 20 D. C. McKean, J. Mol. Struct., 113 (1984) 251.