Spectra and structure of organophosphorus compounds. 32. Infrared

Spectra and structure of organophosphorus compounds. 32. Infrared and Raman spectra, conformational stability, barriers to internal rotation, vibratio...
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J . Phys. Chem. 1987,91, 2769-2778

2769

Spectra and Structure of Organophosphorus Compounds. 32.+ Infrared and Raman Spectra, Conformational Stability, Barrlers to Internal Rotation, Vibrational Assignment, and Normal Coordinate Analysis of Ethyldifluorophosphlne J. R. Durig,* J. S. Church, C. M. Whang,t R. D. Johnson,g and B. J. Streusand Department of Chemistry, University of South Carolina, Columbia, South Carolina 29208 (Received: December 8, 1986)

The infrared (3500 to 20 cm-I) and Raman (3500 to 10 cm-') spectra have been recorded for both the gaseous and solid phases of CH,CH2PF2 and CD3CD2PF2. Additionally, the Raman spectra of the liquids were recorded and qualitative depolarization values were obtained. All of the normal modes of the trans conformer except the CH, torsion have been assigned based on band contours, depolarization values, and group frequencies. In the spectra of the fluid phases, several of the PF2 modes as well as the CH, torsion have been observed for the gauche conformer, but only the trans conformer exists in the solid state. A normal coordinate calculation has been carried out by utilizing a modified valence force field to calculate the frequencies and the potential energy distribution. The barriers to methyl rotation of the gauche conformer for the -d5 molecule and for the -d3molecule were determined to be 885 cm-I (2.53 kcal/mol) and 902 cm-' (2.58 kcal/mol), respectively. From a temperature study of the Raman spectrum of the gas, an enthalpy difference of 56 f 22 cm-I (160 i 62 cal/mol) was obtained with the trans conformer being more stable. Similar studies of the liquid gave an enthalpy difference of 96 f 12 cm-I (275 f 33 cal/mol) also with the trans conformer more stable. From the observed difluorophosphino torsional data, a potential function for the trans to gauche interconversion of CH3CH2PF2is suggested. Most of the fundamentals appear as doublets in the spectrum of the solid which indicates that there are at least two molecules per primitive cell. All these results are compared to similar quantities for some corresponding molecules.

Introduction As part of a broad plan to determine the conformational stability of a series of molecules with formula CH3XP(Z)Y2where X = CHI, 0,or S; Z = BH,,0,S, or the lone pair; and Y = H, CH,, F, or C1, we recently reported the microwave spectrum of ethyldifluorophosphine.' From these microwave studies it was predicted that the gauche conformer is more stable than the trans form by at least 80 cm-' (229 cal/mol). These results are in marked contrast to those obtained for ethylphosphine where from rotational2 and vibrational3 spectra it was found that the trans conformer is more stable than the gauche form. Additionally, from the far-infrared and low-frequency Raman data of ethylphosphine, the potential function governing the asymmetric torsion was obtained from which an energy difference of 150 cm-I (429 cal/mol) was found3 between the conformers. This value agrees well with the value of 200 f 100 cm-' estimated from the microwave studies2 However, these results are at variance with both CNDO/2 predictionsz as well as qualitative arguments based on the "gauche e f f e ~ t " . ~We have recently carried out vibrational5 and electron diffraction6 studies of ethyldimethylphosphine and from both of these investigations it was concluded that the gauche conformer is the predominant form for this molecule, but the potential function governing the asymmetric torsion could not be determined. Finally, it should be noted that both NMR7 and electron diffractions studies have been carried out on the corresponding chloride, CH,CH2PC12, but the results are at variance since from the electron diffraction studies it was concluded that the gauche conformer is the predominant form whereas from the N M R studies it was concluded that the trans form is the more stable conformer. Since these differences could be the result of the investigations being carried out in two different physical states, i.e., vapor and solution, we felt a conformational investigation of ethyldifluorophosphine in both the liquid and gaseous states should be carried out by a vibrational study. Therefore, we have investigated the infrared and Raman spectra of both ethyldifluorophosphine and ethyl-d5-difluorophosphinein the gaseous, liquid, and solid states for the purpose of determining the more

stable conformation in each physical state. Additionally, it was hoped that the barriers to methyl rotation could be obtained and that the potential function governing the asymmetric torsion could be determined. The results of these studies are reported herein.

Experimental Section Ethyldifluorophosphine was prepared from ethyldichlorophosphine (Strem) and antimony trifluororide (Alfa) by the halogen exchange reaction described by Drozd et al.9 The deuteriated difluorophosphines were prepared in a similar manner from the deuteriated dichlorophasphines which were prepared from ethyl-d5 chloride or ethyl-d, chloride (Merck, Sharp and Dohme) by the reduction of the aluminum chloride-phosphorus trichloridealkyl chloride complex by finely divided antimony.I0 All compounds were purified by fractionation on a low-temperature vacuum sublimation column and all samples were stored at either liquid nitrogen or dry ice/ethanol temperatures. Raman spectra were recorded on a Cary Model 82 spectrophotometer equipped with a Spectra-Physics Model 171 argon ion laser. The 5145-A exciting line was used throughout and the laser power at the sample was varied from 2 to 0.25 W depending upon the phase under investigation. The gaseous samples were examined at their ambient vapor pressure by using the standard Cary multipass accessory and their spectra were measured with a resolution of 3 cm-I except for the low-frequency region where 1-cm-I resolution was used. The spectra of the liquids were recorded at room temperature with the samples sealed in glass capillaries. The variable temperature study of liquid ethyldifluorophosphine was carried out by inserting the glass capillary (1) Groner, P.; Church, J. S.; Li, Y.S.; Durig, J. R. J . Chem. Pbys. 1985,

82, 3894.

(2) Durig, J. R.; Cox, Jr., A. W. J. Chem. Phys. 1976, 64, 1930. (3) Durig, J. R.; Cox, Jr., A. W. J. Cbem. Pbys. 1975, 63, 2303. (4) Wolfe, S . Acc. Cbem. Res. 1972, 5 , 102. (5) Durig, J. R.; Hizer, T. J. J. Raman Spectrosc. 1986, 17, 97. (6) Durig, J. R.; Sullivan, J. F.; Cradock, S. J. Mol. Srrucr. 1986,145, 127. (7) Dutasta, J. P.; Robert, J. B. J. Chem. Soc.,Cbem. Commun. 1975,747. ( 8 ) Naumov, V. A.; Turova, L. L.; Zaripov, N. M. Zh. Strukr. Kbim. 1977, 18, 67. (9) Drozd, G. I.; Ivin, S . 2.;Sheluchenko. V. V.; Telebaum, B. I.; Luganskii, G. M.; Varshavskii, A. D. Zb. Obsh. Kbim. 1967, 37, 1343. (10) Perry, B. J.; Reesor, J. B.; Ferron, J. L. Can. J . Cbem. 1963.41, 2299.

t For part 3 1, see J. Raman Spectrosc., in press.

*Study carried out while on sabbatical leave from the Department of Ceramic Engineering, Inha University, Incheon, Korea. 'Current address: Department of Chemistry and Physics, Armstrong State College, Savannah, GA 31419.

0022-3654/87/2091-2769%01.50/0 0 1987 American Chemical Societv , , I

-

2770 The Journal of Physical Chemistry, Vol. 91, No. 11, 1987 into a cell similar to the one described by Miller and Harney.” The temperature of the sample was monitored with an ironconstantan thermocouple referenced to 0 O C . Depolarization measurements were obtained using the standard Cary accessory. The Raman spectra of the crystalline samples were recorded at 20 K in a Cryogenic Technology Inc. Spectrim cryostat equipped with a Lake Shore Cryotronics Model DTL 500 temperature controller. The samples were annealed until no further changes in the spectra were observed. Frequencies measured for sharp, resolvable Raman lines are expected to be accurate to at least k 2 cm-I. The far-infrared spectra were recorded on a Digilab Model FTS- 15B Fourier transform interferometer equipped with a high-pressure H g arc lamp source, 6.25- and 12.5-pm Mylar beamsplitters and a TGS detector. The spectra of the gases were recorded by holding the samples at their ambient vapor pressures in 10-cm cells equipped with polyethylene windows. Interferograms for both the sample and reference cells were taken 2500 times, averaged, and then transformed with a boxcar truncation function. The effective resolution was 0.25 cm-l and no spectral enhancement was performed. The far-infrared spectra from which the torsional data were obtained were recorded with a Nicolet 2OSXV Fourier transform interferometer with 0.IO-cm-l resolution. The sample was contained in a 1-m optical path length cell at room temperature vapor pressure. The spectra of the solids were recorded at a resolution of 2 cm-’ by condensing the samples onto a silicon plate held in the cryostat described above. The samples were annealed until no further changes were observed in the spectra. Mid-infrared spectra were recorded on either a Perkin-Elmer Model 621 spectrophotometer or a Digilab Model FTS-14C Fourier transform interferometer at a resolution of 1 cm-I. The interferometer was equipped with a high-intensity Globar source, Ge/KBr beamsplitter, and TGS detector. Spectra of the gaseous samples were recorded with a 12-cm cell equipped with CsI windows. Spectra of the solids were recorded by condensing the samples onto a CsI plate held at 20 K by using the cryostat described above. Standard deposition and annealing techniques were employed. Atmospheric water vapor was removed from the infrared spectrometer housing by purging with dry nitrogen. Measured frequencies of the gaseous samples are expected to be accurate to better than k0.2 cm-’ in the far-infrared region and all other frequencies measured in the infrared spectra are accurate to fl cm-’.

Vibrational Assignment Since trans-ethyldifluorophosphineis of C,molecular symmetry, the 24 normal modes belong to the following irreducible representations: 14A’ 10A”. The A’ modes are expected to yield polarized Raman lines and infrared bands of A-, C-, or A/Chybrid band contours. The A” modes are out-of-plane vibrations and should give rise to depolarized Raman lines and B-type infrared bands. For a structure of CI symmetry, all 24 normal modes belong to the A’ symmetry species and are expected to yield polarized Raman lines with corresponding infrared bands of A-, B-, C-, or hybrid-type contours. The various band contours and their P-R separations were calculated from the previously reportedi rotational constants. Very little difference was found between the band contours predicted for the two conformers. The A- and C-type bands were both characterized by strong central Q-branches. The separations of the P- and R-branches were determined to be 16 and 20 cm-l for the A- and C-type contours, respectively. The B-type bands were predicted to be void of Q-branches and to have a P-R separation of 12 cm-I. The bands observed in the infrared spectra of the gaseous compounds were found to have contours that were very consistent with those predicted. It is interesting to note that several bands that were assigned to A” vibrations possess central Q-branches. These Q-branches arise from bands due,to the gauche conformer that are nearly degenerate in frequency with the corresponding modes

+

( 1 1) Miller,

F. A,; Harney, B. M. Appl. Spectrow. 1970, 24, 291.

Durig et al.

l

D

3Ooo

2000

1ooo

l 0

WAVENUMBER &Ma) Figure 1. Raman spectra of ethyl-do-difluorophosphine:(A) gas, (B) liquid, (C) unannealed solid, (D) annealed solid.

of the trans conformer. The observed vibrational frequencies of ethyldifluorophosphine and ethyl-d5-difluorophosphineare listed in Tables I and 11, respectively, and are numbered according to the trans conformer since this is the isomer that remains in the solid. Bands which are only due to the gauche conformer are so indicated. The Raman spectra of ethyl-do-difluorophosphineand the -d5 species are shown in Figures 1 and 2, respectively. The mid-infrared spectra of the two isotopic species are shown in Figures 3 and 4 and the far-infrared spectra are shown in Figures 5 and 6. Numerous unsuccessful attempts to obtain an ordered molecular crystal of ethyl-d5-difluorophosphine were made, but continued efforts toward this goal were abandoned since it was found that the lack of these spectral data did not create any difficulties in the assignment of the normal modes. The assignments presented herein were made on the basis of group frequencies, depolarization ratios, and infrared band contours. All frequencies mentioned in the following discussion of the proposed vibrational assignment refer to bands measured from gaseous samples unless otherwise noted. Carbon-Hydrogen Modes. The normal modes of the CH, group, exclusive of the torsion, can be described as three stretching and five bending motions whereas for the C H 2 group there are two stretching modes and four bending modes. The CH3 antisymmetric stretching mode, vlr has been assigned to the polarized line of medium intensity observed at 2980 cm-I; on deuteriation this mode shifts to 2232 cm-I. The CH, symmetric stretching mode, v3, has been assigned to the very strong, polarized line observed at 2896 cm-I with the corresponding mode in the deuteriated compound a t 2084 cm-I. The A” CH, antisymmetric stretching fundamental, v15, appears as a depolarized Raman line of medium intensity centered at 2988 cm-’ with the corresponding infrared band having a B-type band contour. The CH, antisymmetric and symmetric stretching modes are assigned at 2955 and 2905 cm-’, respectively, with the antisymmetric mode shifting with deuteriation to 2217 cm-’ which is a depolarized Raman line. The relatively strong intensity of the 2955-cm-I Raman line is probably

The Journal of Physical Chemistry, Vol. 91, No. 11, 1987 2771

Spectra and Structure of Ethyldifluorophosphine

I

A

1

I

I

2500

I

I

1500

500

WAVE NUMBER (cm-1) Figure 4. Mid-infrared spectrum of gaseous ethyl-d5-difluorophosphine.

I

I

I

I

2000

1

I

1000

0

WAVENUMBER (cm-l) Figure 2. Raman spectra of ethyl-d,-difluorophosphine: (A) gas, (B)

liquid.

U J

I

I

I

I

I

I

I

I

I

I

I

I

4J

I

A

1

I

I

I

I

I

2000 1000 WAVENUMBER (cm-1) Figure 3. Mid-infrared spectra of ethyl-do-difluorophosphine: (A) gas, (B) unannealed solid, ( C ) annealed solid. 3000

the result of overtones or combination bands of the deformations which are probably in Fermi resonance with the symmetric stretching modes. The five methyl bending motions can be characterized as three deformations and two rocking modes. The two A' deformations, one antisymmetric, v4, and one symmetric, v6, have been assigned to bands observed in the infrared spectrum at 1472 and 1386 cm-I, respectively, whereas the corresponding A" mode is assigned at

I

I

I

I

200 loo WAVENUMBE R (CM") Figure 5. Far-infrared spectra of ethyl-do-difluorophosphine: (A) gas, (B) unannealed solid, (C) annealed solid.

400

300

1432 cm-I. Both the A' modes exhibit A/C-type band contours and are of moderate intensity. In the -dScompound these modes are found at 1132, 1054, and 1075 cm-I, respectively. The two CH, rocking modes are assigned at 988 and 975 cm-' and shift to 750 and 743 cm-I, respectively, in the -dScompound with the lower frequency band being depolarized in each case and assigned to the A" motion. Two of the four CH, bending modes are A' motions, the CH2 deformation and the CH, wag, and these modes are generally observed at higher frequencies than the two A" modes which are

2772 The Journal of Physical Chemistry, Vol. 91, No. 11, 1987 TABLE I: Observed" Infrared and Raman Frequencies infrared re1 re1 re1 gas int solid int gas int liquid 2996 R 2996 w 2989Q, B m 2984 m 2988 m 2981 P 2978 2980 m 2950 2955 s 2955 m 2958 w 2941 w, sh 2948 s 2944 2916 vs 2905 m 2910 w 2903 vs 2907 2896 vs 2900 2889 w 2886 ~

1481 1472 Q, A/C m 1463 w 1432 Q 1415 1408 Q, A/c 1399

1386 Q, A/C w 1380 P w 1260

1375 w 1263 w

1252 Q, A/C w 1246 P vw 1232 Q

993 R 988 Q, A/C m 980 P

vs, p vs, p vs, p vs, p

1036

S

987 w, sh

m m s m

2990 ul CH3 antisymmetric stretch (100%) 2972 u 1 6 CH2 antisymmetric stretch (97%) 2908

u2

CH2 symmetric stretch (98%)

s m m

2886

~3

CH, symmetric stretch (98%)

1471

u4

s

CH3 antisymmetric deformation (67%); CH2 deformation (20%) 7 CHI antisymmetric deformation (93%)

m

1400 w, dp

1464 1455 1429 1399 1395

w 1466 w wv vw, sh m 1452

1385 wv, sh, p

1375

w

1395

u6

1260 W, sh

1261

1263

VW,sh 1247

~7

1260

w

1252 m

1251 m,p

1236

~ 7 '

CH2 wag (gauche)

1232 vw, br

1231 vw,sh,dp 1239 vw 1232 w

1240

~ 1 8

CHI twist (97%)

1040 m

1037

1024

W

1021

988

W

987

1408 vw

975

W,

sh, p

w, sh 1036 1031 m m

987 982

vw, sh

~ 1

u5

CH, deformation (43%); CH3 antisymmetric deformation (26%); CH, symmetric deformation (25%) CH, symmetric deformation (56%); CH, deformation (34%); CH2 wag (6%) CH2 wag (74%); CH3 symmetric deformation (15%)

1048

C-C stretch (67%); CH2 twist (1 5%)

1013

C-C stretch (gauche)

965

CH3 rock (75%)

W

W

8 3 0 Q , A / C vs

831 m

974 819

819 Q

782

819 w, sh

796

S

2975 2958 2941 2926 2911 2892 2885 2881 1471

967 w, br 809 S

vs

m, p

1455 m

1395 w

1047 R 1038 Q, A/C s 1029 P

(cm-I), Assignment, and Potential Energy Distributionbfor CH3CH2PF2 Raman assignment and PED re1 int, re1 & depol solid int calcd uI approximate description 2997 s 2985 s 2994 CH, antisymmetric stretch (97%)

1462 m , d p

1472 vw

m

Durig et al.

967 809 794 782

m vvw m m

983 834

u19

818

y20

PF2 antisymmetric stretch (78%); CH2 rock ( 10%)

737

y21

CHI rock (66%); PF, antisymmetric stretch (20%); CHI twist (13%)

693

Yll'

P-C stretch (gauche)

649

VII

P-C stretch (63%); CCP bend (8%); CH, wag (8%); C-C stretch (6%)

VI0

CH3 rock (71%); CH, rock (19%) PF, symmetric stretch (84%)

778 755 R 750Q, A 744 P 688 R 680 Q 673 P 660 R 651 Q, A/C 642 P 510 R 501 Q, A / C 495 P 408 Q, A/C 358 R 349 Q, A/C 341 P 323 R 314Q,A 307 P

s

750 m

S

m m

658 m 499

S

W

750 vw, dp

680

S

682

65 1

S

655 s, P

501 vw 408 m

m m

750

353 m

m

223 218

W W

W

352

314

W

316 286 vw, dp

208

W

W

659 655

S

500 495

m

361 354

W

S

216 w, P

296 29 1 234 223 213

407

PF, wag (53%) CC stretch (19%); PF, antisymmetric stretch (18%) PF, wag (gauche)

351

PF, deformation (94%)

298

PF, twist (gauche)

28 1

PF, twist (95%)

209

CCP bend (54%); PF2 wag (29%); PC stretch (9%)

501

W

407

350

290 vw 217 R 208 Q , A / C m 200 P

499

757

vw

w, sh W

m W W

The Journal of Physical Chemistry, Vol. 91, No. 11, 1987 2113

Spectra and Structure of Ethyldifluorophosphine

TABLE I (Continued) infrared re1 gas

int

188 Q,C 90

w

-

solid

re1 int

gas 185 -90

W

88 78

re1 int

liquid

Raman re1 int, & depol

w

192

vw, p

w

s

s

solid 192 130 105 96 88 78 m, bd 61 56 51 44

re1 int W W W

calcd 239

assignment and PED v, approximate description ul( CCP bend (gauche) u24 PF, torsion

W

w, bd

lattice modes

m W W

W

4Abbreviationsused: s, strong; v, very; w, weak; m, medium; dp, depolarized; sh, shoulder, bd, broad; P, Q,and R refer to band contours in the gas-phase infrared spectrum. bContributionsof less than 6% are not reported. both the trans and gauche conformers is found from the assignment of the PF2 modes since bands for many of these vibrations are observed for both conformers. However, upon annealing, only bands due to the trans conformer were observed in the spectrum of solid CH3CHzPF2. One expects the skeletal bending modes to be the most sensitive vibrations to the molecular conformation and this certainly true experimentally for ethyldifluorophosphine. The PF2 symmetric stretching mode, vIo, is assigned to the A/C-type band at 830 cm-', whereas the corresponding antisymmetric mode is observed a t 819 cm-'. Both of these modes are very strong in the infrared spectrum, but rather weak in the Raman effect. In accord with our previous studies on PF2 containing molec~les,'~J~ the PF2 symmetric stretching mode has been assigned to the band at higher frequency and the observed polarization data for CH3CH2PF2support this assignment. No separate bands were observed that could be attributed to the PF2 stretching modes of the gauche conformer but both vIo and v20 were found to possess central Q-branches in the infrared spectrum, and it is likely that the trans and gauche PF2 stretching fundamentals are very nearly degenerate in frequency. In the crystalline phase the PF2 stretching fundamentals appear at 809 and 782 cm-' and this large shift in frequency upon crystallization was also observed for the corresponding modes of CH3PF2I3and C H 3 0 -

PF2.I4 A comparison of the low-frequency vibrational spectrum of methyldifl~orophosphine'~ with that for ethyldifluorophosphine

I 450

I

I 250

I

I

5

WAVENUMBE R (cm-' 1 Figure 6. Far-infrared spectra of ethyl-d,-difluorophosphine: (A) gas, (B) unannealed solid.

the CH2twist and the CH2 rock. The CHI deformation, u5, has been assigned to an A/C-type band of medium intensity observed a t 1408 cm-' with a polarized Raman counterpart and the CH2 wagging fundamental has been assigned to the weak, A/C-type band observed at 1252 cm-I. These bands shift to 1063 and 892 cm-I in the spectrum of CD3CD2PF2.The CH2 twisting mode is assigned to a weak infrared band at 1232 cm-' and the corresponding Raman mode is observed as a broad, depolarized feature. The CH2 rock is assigned to the B-type band of strong intensity centered at 750 cm-'; the CD2 bends are observed at 833 and 565 cm-' in the spectrum of CD3CD2PF2. The assignments of these carbon-hydrogen modes are very similar to the corresponding modes of ethyl iodide.'* Skeletal Modes. The vibrations of the PF2 moiety, exclusive of the asymmetric torsion, can be described as two P-F stretching motions, one symmetric and one antisymmetric, and three bending motions. Evidence for the existence at ambient temperature of (12) Durig, J. R.; Thompson, J. W.; Thyagesan, V. W.; Witt, J. D. J. Mol. Struct. 1975, 24, 41.

simplifies the assignment of these modes. The PF2 wagging fundamental, u I 2 , has the highest frequency of the three bending modes and is observed at 501 cm-I for frans-CH3CH2PF2.The corresponding mode of the gauche conformer is observed at 408 cm-l. In the spectrum of the deuterium compound v12 is observed at 476 cm-I, and the corresponding mode in the gauche conformer is observed at 392 cm-'. The PFz deformation is assigned to a weak, polarized Raman line at 349 cm-' but the corresponding mode for the gauche conformer is not observed; however, in the spectrum of CD3CD2PFz,the PF2 deformations for both the trans and gauche conformers are observed at 342 and 319 cm-I, respectively. The PF2twisting mode is observed as a trans-gauche pair with the band at 314 cm-I assigned to this mode for the gauche conformer and the depolarized line at 286 cm-' in the Raman spectrum of the liquid to this mode for the trans conformer. The three remaining skeletal modes to be assigned are the C-C stretch, the P-C stretch, and the CCP bend. The C-C stretching fundamental, us, is anticipated to have the highest frequency of all the skeletal modes and it is generally observed in the spectral region between 950 and 1050 cm-' for the ethyl moiety. It is observed as an A/C-type band of strong intensity centered at 1038 cm-I with the Raman counterpart being polarized and of medium intensity. In the spectrum of the deuterium compound, this mode is observed at 1046 cm-I. Carbon-phosphorus stretching fundamentals are characteristically strong in the Raman effect and (13) Durig, J. R.; Stanley, A. E.; Jalilian, M. R. J . Raman Spectrosc. 1981, 10, 44. (14) Durig, J. R.; Streusand, B. J. Appl. Specrrosc. 1980, 34, 6 5 .

Durig et al.

2774 The Journal of Physical Chemistry, Vol. 91, No. 1I , I987 TABLE 11: Observed Vibrational Frequencies (cm-I), Assignment, and Potential Energy Distribution for Gaseous and Liquid

Ethyl-d,-difluoropbosphinea~b Raman infrared gas re1 int 2244 R 2239 Q, B 2232 P

m

2217 2197

w w, sh

2134

m

gas

re1 int liquid

2234 v I s

2232 m 2217 w

2235 2215 2160 2143 2124 2111 2091 2077

m, p m,dp w,sh vs m,p m,p m, p s, p

2232 u1 221 1 u I 6 CD2 antisymmetric stretch (85%); CD, antisymmetric stretch (14%)

vs m

1132 Q

W

2069 s 1132 w

1075 1063 1054

m

1046

m

846 R 840Q, A/C 834 P 827 757 R 750Q, A/C

Y,

m,dp

m

897 R 891 Q, A/C 885 P

calcd

2240

2089 R 2084 Q, A/C 2079 P

m

re1 int, depol

2239 m

2153 2135 2119 2096 2084

m

assignment and PED approximate description

~

1075 1063 1054 1050 1046 1043 958

w

w s

m

m m m, sh m, sh m, sh vw, bd

2040 1135 1126 1071 1063 1054

2132

CD, antisymmetric stretch (84%); CD, antisymmetric stretch (13%); CD, antisymmetric stretch (97%)

u2

CD2 symmetric stretch (94%)

2085 v,

CD, symmetric stretch (95%)

1122

CD3 symmetric deformation (62%); C-C stretch (24%)

u4

1078 v5 CD2 deformation (51%); CD2 wag (17%); PC stretch (15%) 1056 v I 7 CD3 antisymmetric deformation (95%) 1052 u6 CD, antisymmetric deformation (85%); CD, deformation (9%) 1015 u8 976 vs’

1048 1038 958

927

C-C stretch (47%); CD2 wag (22%); CD, deformation (16%) C-C stretch (gauche)

892 m

889

m, p

883 m, sh

886

m, sh, dp

CD, symmetric deformation (42%); CD2 wag (25%); CD2 deformation (1 3%); CD, rock (8%) 898 v l 8 CD2 twist (83%) CH, rock (10%)

vs

840 m

818

m,p

840 uI0 PF2 symmetric stretch (82%)

vs

827 m

797

m, dp

827 v20

PF2 antisymmetric stretch (65%); CD2 rock (1 5%); CD, rock (1 5%)

m

750 m 743 w, sh

747 736

m, p w, dp

732 753

CD3 rock (61%); CD2 wag (17%); PF, symmetric stretch (8%) CD3 rock (42%); PF, antisymmetric stretch (28%); CD2 twist (15%); CD, rock (13%)

w

u,

ug u19

733 w, sh 639 632 605 598

R Q R Q, A/C

568 R 563 Q, B 558 P 390 Q 350 R 342Q, A/C 335 P 319 Q 317 P 197.5 R 190.5 Q, A/C 182.5 P 169 Q 137.5 -85

m

632 s

633

s, p

629 v 1 I’ P-C stretch (gauche)

m

597 vs

601

vs, p

578 v l l

P-C stretch (48%); CD2 wag (17%); C-C stretch (12%); CD3 rock (10%); PF, wag (8%)

W

565 m

565

m,dp

563

CD2 rock (61%); CD, rock (31%)

c

476 vw, bd 392 m

474 391

vw, bd m,p

472 u 1 2 PF, wag (50%); CCP bend (17%); C-C stretch (13%); CD, rock (9%) 374 uI2’ PF2 wag (gauche)

344

w, sh

350 u I 3 PF, deformation (93%)

320 m

322

m, p

357

298 w 260 vw, bd

299 262

w, p vw, dp

281 u 2 i PF, twist (gauche) 261 vZ2 PF, twist (91%); CD2 rock (7%)

s

190 w

198

w, p

192 ui4

m

170 vw

176

vw, p

221 u , i CCP bend (gauche) 1 0 CD, torsion (gauche)

s S

vw W

-85

w

u21

~13’

wZ4

PF, deformation (gauche)

CCP bend (56%); PF2 wag (26%); P-C stretch (8%)

-

PF, torsion

“*bSeeTable I . this mode is assigned to a strong polarized Raman line at 65 1 cm-I for the trans conformer. T h e C-P stretching fundamental of the gauche conformer is observed a t 680 cm-I. These modes shift to 597 and 632 cm-’ for the trans a n d gauche conformers, respectively, in t h e deuteriated molecule. T h e skeletal mode of lowest frequency (excluding the torsional fundamentals) is expected to be t h e C-C-P bending mode which is observed a t 208 cm-I for the trans conformer and 185 cm-’ for the gauche form. In t h e spectrum of ethyl-d5-difluorophosphine these modes shift to 190 a n d 170 cm-’ for t h e trans and gauche conformers, respectively. Torsional Modes. T h e methyl and difluorophosphino torsional fundamentals of trans-ethyldifluorophosphineare both A” modes

and, thus, give rise to B-type infrared bands. Since the B-type bands were predicted to have no Q-branches, there is little hope of observing any series of torsional transitions of A” symmetry in the infrared spectrum. In the Raman effect the A” modes give rise to depolarized lines, and these are anticipated to be very weak, broad, and unobservable in the R a m a n spectrum of the gas. However, for gauche-ethyldifluorophosphine,polarized R a m a n lines a r e expected for these two modes. The methyl torsion is anticipated to have the higher frequency of the two torsional modes. Examination of the far-infrared spectra of ethyl-&difluorophosphine (Figure 7B) reveals a series of Q-branches starting a t 137.5 cm-I and progressing to lower

Spectta and Structure of Ethyldifluorophosphine

The Journal of Physical Chemistry, Vol. 91, No. 11, 1987 2775

B WAVENUMBER (cm‘)

Figure 7. Far-infraredspectra of (A) ethyl-d3-difluorophosphine and (B) ethyl-d5-difluorophosphineshowing the CD, torsional transitions. frequency. This series has been confidently assigned to the CD3 torsion of the gauche conformer. In the spectrum of ethyldodifluorophosphine, this series is obscured by the “hot band” series of the C-C-P bending fundamental. However, the similar series has been observed in the far-infrared spectrum of CD3CH2PF2 (Figure 7A). The difluorophosphino torsional fundamental is anticipated to have the lowest frequency of all the normal vibrations of ethyldifluorophosphine. The infrared and Raman spectra of ethyldifluorophosphine between 150 and 50 cm-’ are shown in Figure 8 (traces A and B, respectively). It is evident from these spectra that considerable torsional information is present in this spectral region; however, the interpretation of these data is not at all straightforward. The series of transitions observed in the Raman spectrum starting at about 90 cm-’ are polarized and sharp Qbranches are observed for the corresponding bands in the infrared spectrum; therefore, the assignment of these transitions to the PF2 torsion of the gauche conformer is certain. The fact that the spectral features assigned to the torsional transitions are rather strong in intensity compared to the strongest Raman lines of the rotational spectrum of air (lines marked with an asterisk in Figure 7) also supports this assignment. The assignment of a torsional fundamental for the trans conformer is hampered by the fact that the mode is of A” symmetry. The 1 0 transition of the torsional fundamental of the trans conformer has been assigned to the center of the apparent B-type minimum observed at 92.6 cm-’ in the far-infrared spectrum of the gas. The separation of this band is consistent with that expected for a B-type band; however, since considerable fine structure is observed on the P-branch, the accurate determination of the minimum is impossible. Attempts to obtain torsional data for the difluorophosphino group of the deuteriated compound were fruitless since no Q-branches were observed on the torsional fundamental in the far-infrared spectrum. In the Raman spectrum, the resolution of torsional fine structure was hampered by the interference from the exciting line. Teller-Redlich product rule calculations were carried out in an attempt to support the proposed assignment. Generally, an agreement of 3% between the calculated and observed ratios is sufficient to support the assignment. The calculated product rule values for the A’ and A’’ blocks are 14.01 and 9.55, respectively. Ideally, frequencies from the spectra of the gas phase should be used for this calculation since intermolecular interactions are at a minimum in this phase. The calculations for the A’ block were carried out using the frequencies observed for the gaseous compounds and a value of 13.68 was obtained, which results in a 2.4%

-

I 150

I

I

100

50

WAVENUM B ER (cm-l) Figure 8. Far-infrared spectrum (A) and Raman spectrum (B) of gaseous ethyl-do-difluorophosphinebetween 150 and 50 cm-’.

error. The product rule calculations for the A” block could not be carried out without making several assumptions. The frequency for the PF2 twisting fundamental of ethyl-do-difluorophosphine had to be taken from the Raman spectrum of the liquid. Since no methyl torsional transitions were observed for the trans conformer of either ethyldifluorophosphine or ethyl-d5-difluorophosphine, a shift factor had to be assumed. In general, torsional motions are found to be relatively pure, and therefore the theoretical shift factor of 1.414 was utilized. The exact value of the shift factor for the difluorophosphino torsion is also in some doubt since no Q-branches were observed on this fundamental for the deuterium compound; thus, only an approximate frequency was obtained. With the above-mentioned uncertainties, a value of 9.33 for the product rule ratio was obtained. This value results in an error of 2.3% which is within the acceptable limits and supports the proposed assignment. The assignments of the fundamentals for CH3CH2PF2and CD3CD2PF2are listed in Tables I and 11, respectively.

Normal Coordinate Analysis In order to ascertain the amount of mixing among the normal modes and to obtain a more accurate description of the fundamental vibrations, a normal coordinate analysis was undertaken. It was also felt that the results of these calculations would make a significant contribution to our continuing effort to characterize both the force fields of the ethy112+’5-’8 and the difluoroph~sphinol~ groups. The calculations were carried out by the Wilson FG matrix methodIg with a computer program written by Scha-

(15) Durig, J. R.; Lopata, A. D.; Groner, P. J . Chem. Phys. 1977.66, 1888. (16) Wurrey, C. J.; Bucy, W. E.; Durig, J. R. J . Phys. Chem. 1976, 80, 1129. (17) Durig, J. R.; Heusel, H. L.; Sullivan, J. F.; Cradock, S.Spectrochim. Acta, Part A 1984, 40A, 739. (18) Durig, J. R.; Li, Y. S.;Sullivan, J. F.; Church, J. S.; Bradley, C. B. J . Chem. Phys. 1983, 78, 1046. (19) Wilson, Jr., E. B.; Decius, J. C.; Cross, P. C. Molecular Vibrations; McGraw-Hill: New York, 1955.

2776 The Journal of Physical Chemistry, Vol. 91, No. 11, 1987 TABLE III: Symmetry Coordinates'vb for CH3CHzPF2 A'

CH3 antisymmetric stretch CH2 symmetric stretch CH3 symmetric stretch CH3 antisymmetric deformation CH2 scissors

Species SI = 2r1 - r2 - r, S2 = d2 d, S, = rl r2 r, S4 = 2al - a2 - a,

+ + + s5= ( 4 6 + 2)6 - (46- 2)8 - 6 2 - - €2 S6 = al + a2 + a3 - PI - P2 - (3, s, = - € 1 + -

61

€,

CH3 symmetric deformation CH2 wag C-C stretch CH, in-plane rock PF2 symmetric stretch C-P stretch PF2 wag PF2 scissors C-C-P bend

62

61

€2

Sg = R s9 = 261 - 82 - P3

si, = XI + x2 SII= Q SI2 = (1 + E 2 =7 SI, = ( 4

6 - 2)6 - ( 4 6 + 2)O + + 62 + + SI, = ai + a2 + a3 + PI + + P, SI6 = + 62 + +

61

€1

CH3 bending redundancy CH, bending redundancy

€2

P2

€1

61

€2

A" Species SI, = d2 - d3 SI8 - r2 - r3 SIP= a2 - a3 Szo= 61 - €1 - 6 2 + €2 S2I = P2 - P 3 s 2 2 = XI - x2 S,, = 61 €1 - 6 2 - €2

CH2 antisymmetric stretch CH3 antisymmetric stretch CH3 antisymmetric deformation CH2 twist CHI out-of-planerock PF2 antisymmetric stretch CH2 out-of-plane rock PF, twist

+

s24

=

(1

- t2

'Not normalized. bThe symbols utilized are defined as follows: LHCH(CH3) = CY; LHCH(CH2) = 6; LCCP = 0; LHCC(CH2) 6; LHCP = c; LHCC(CH,) = P; K P F = 5; other symbols are self-explanatory. TABLE IV: Internal Force Constants for CH3CH2PF2and CDqCD*PFT coordinates involved value," mdyn/A force constant C-H (CH,) 4.77 1 K. C-H (CH;) 4.713 4.467 P-F 4.466 c-c 2.740 c-P 0.560 H-C-H (CH,) 0.579 H-C-H (CH3) 0.528 H-C-H (CH2) 0.673 H-C-C (CH2) 0.662 H-C-P 1.488 C-P-F 1.254 F-P-F 0.969 c-c-P 0.259 P-F, P-F 0.428 CPF, CPF -0.155 CCP, CPF C-C, HCP 0.655 0.329 C-C, HCC (CH3) HCC (CH,), HCP 0.040

"All bending coordinates are weighted by 1 8, chtschneider.20 The assumed structural parameters were taken from the microwave study.I The nonnormalized symmetry coordinates are given in Table 111. All of the vibrational frequencies used in the calculation were measured from the gaseous phase with the exception of the PF, twisting mode which was measured from the liquid phase. Also, the methyl and phosphino torsional modes were omitted from the calculations. The observed and calculated frequencies along with their approximate potential energy distributions are reported for ethyldifluorophosphine and ethyl-d5-difluorophasphinein Tables I and 11, respectively. With a force field of 19 force constants ( 1 3 main diagonal and 6 interaction terms) given in Table IV, an average error of 6 . 7 5 cm-' or 0.77% was obtained for ethyldifluoro(20) Schachtschneider, J. H. "Vibrational Analysis of Polyatomic Molecules", V and VI; Technical Reports No. 231-64 and 57-65; Shell Development Co., Houston, TX.

Durig et al. phosphine. By employing the same force field, we obtained an average error of 10.29 cm-' or 1.58% for ethyl-d5-difluorophosphine. Addition of more interaction force constants would probably improve the frequency fit, but it was felt that further pursuit of these calculations would not significantly increase the information already acquired. All of the A" modes were found to be relatively pure with the exception of the CH3 and CH2 rocking fundamentals. The mixing of the normal modes of the A' block is more extensive than that found for the A" modes. As expected, considerable mixing was found among the C-C-P bend, C-C stretch, and C-P stretch. Some mixing was also found among the CH3 symmetric deformation, CH, scissors, and CH, wag. In general, the mixing of the normal modes of the ethyl group was found to be very similar to the mixing reported by Durig et al.'29'5-'8 for the corresponding modes of several other ethyl compounds. Of particular interest is the PED of the C-C stretching fundamental. In ethyl-do-difluorophosphine this mode is found to be fairly pure (91%); however, in the deuteriated compound, this mode becomes severely mixed with the CD3 and CD2 bending motions. Mixing of this nature was also reported for the deuteriated species of ethyl iodide,I2 ethyl cyanide,I6 and eth~1germane.I~ This increase in mixing can be attributed to the shift of the CH3 and CH2 bending modes to lower frequency upon deuteriation. The value obtained for the C-C stretching force constant, K,, of ethyldifluorophosphine is very consistent with the values reported for the corresponding force constants of ethyl iodide], and eth~1germane.I~ Values on the order of 4.5mdyn/A were reported for these molecules, but KR of ethyl cyanideI6 was determined to be 3.97 mdyn/A. It is interesting to note that two of the six interaction force constants utilized in the calculation involve the C-C stretching coordinate and either a CHI or CH3 bending coordinate. Interaction force constants of this type were found to be very significant for the force fields of the other ethyl comp o u n d ~ . ~ By ~ ~comparing ~ ~ - ~ * the force fields for the PF2groups and ethyldifluorophosphine, sigof methyldifluor~phosphine'~ nificant differences are evident. The P-F stretching force constants reported for the two molecules are in good agreement, but the bending force constants differ considerably. This indicates that the substitution of a CH3 group for a hydrogen atom of methyldifluorophosphine has significantly altered the mixing of the PF, bends which in turn has affected the PF2 force field. The sensitivity of the normal modes of the PF2 group toward the spacial configuration of the ethyl group is also reflected by the large frequency differences found between the corresponding PF, modes assigned to the trans and gauche conformers. Significant difference is also found when comparing the values of the C-P stretching force constants of methyl- and ethyldifluorophosphine. Durig et al.I3 reported a value of 3.04 mdyn/A for this force constant in methyldifluorophosphine which is significantly larger than the 2.74 mdyn/k value reported herein for ethyldifluorophosphine. AH Calculation Since there exist bands due to both conformers in the spectra of the fluid phases, a study of the relative intensities of a conformer pair as a function of temperature was undertaken in order to determine the energy difference between the trans and gauche forms. The bands observed at 655 and 682 cm-I in the Raman spectrum of liquid ethyldifluorophosphine, which have been assigned to the C-P stretching modes of the trans and gauche conformers, respectively, seemed to be good candidates for such a study since these bands are well resolved, symmetric, and fairly intense. Also, there is no interference in this region due to other fundamentals. Measurements of these bands were made at 5 temperatures (61.5 to 21.5 "C) for the gas phase, and at 15 temperatures (24.0 to -143.0 "C) for the liquid phase (Table V). It has been shown by Hartman et al.,' that the ratio of the band areas is related to AH by (21) Hartman, K. 0.; Carlson, G. L.; Witskowski, R. E.; Fateley, W. G. Specfrochim. Acta, Part A 1968, 24A, 157.

The Journal of Physical Chemistry, Vol. 91, No. 11, 1987 2771

Spectra and Structure of Ethyldifluorophosphine TABLE V: Temperature and Intensity Ratios (655/682 cm-I Lines) for the Conformational Stability Study of Gaseous and Liquid Ethyldifluorophosphine

T , OC

IOOO(l/T) K

61.5 44.0 35.0 30.0 21.5

24.0 2.0 -1 5.0 -28.0 -43.0 -48.0 -53.0 -58.0 -68.0 -78.0 -93.0 -1 13.0 -123.0 -133.0 -143.0

3.00 3.15 3.25 3.30 3.39 AH = 160 f 62 3.37 3.63 3.87 4.08 4.35 4.44 4.54 4.65 4.87 5.12 5.55 6.24 6.66 7.14 7.68 AH = 275 f 33

K = l,/lg

In k

0.932 0.941 0.955 0.958 0.960 (56 f 22

-0.705 -0.573 -0.498 -0.455 -0.379

gas

cal/mol Liquid

cal/mol

obsd CDjCDzPF2

1-0 2-1 3-2 4-3

0.199 0.131 0.157 0.247 0.239 0.262 0.270 0.278 0.351 0.385 0.470 0.536 0.658 0.621 0.693

F, cm-l V3, em-' v6

calcd

137.5 132.3 126.6 119.0

137.4 132.4 126.6 119.2 2.861 1 885 f 1 -25 f 1 2.53

e?-'

barrier, kcal/mol CD3CH2PF2 l+O

138.4 133.5 127.9 120.6

2 6 1 3-2 4-3

F,em-' V,, em-' V6, em-' barrier, kcal/mol

obsd

1 6 0

calcd"

obsd - calcd

92.6

92.7

-0.1

89.7 85.6 80.8

89.8 85.5 81.0

-0.1 0.1 -0.2

gauche 1 4 0 2 6 1 3+-2

-23 f 44 100 f 40 752 f 18 63 f 3 58 823 67 1

VI

TABLE VI: Observed Methyl Torsional Frequencies, Assignments, F numbers, and Internal Rotation Barriers for gauche-Ethyl-d,-difluorophosphineand gauche -Etbyl- d 3-difluorophosphine

transition

Ethyl-do-difluorophosphine transition trans

potential constants, em-'

cm-l)

1.22 1.14 1.17 1.28 1.27 1.30 1.31 1.32 1.42 1.47 1.60 1.71 1.93 1.86 2.00 (96 f 12 em-')

TABLE VII: Observed DifluorophosphineTorsional Frequencies, Assignments, Potential Constants, and Barriers (cm-1) for

138.4 133.6 127.9 120.9 2.8750 901 f 2 -29 f 1 2.58

In ( A * / A ) = (-AH/RT) and that AH can be obtained by plotting In ( A * / A ) against (l/RT). Since both bands utilized in the calculation were rather sharp and highly symmetric, as well as very similar in contour, the ratio of their intensities was used instead of the ratio of their areas. The AH value obtained from the slope of the least-squares line fit to the observed data points was 275 f 33 cal/mol (96 f 12 cm-I) for the liquid and 160 f 62 cal/mol (56 f 22 cm-I) for the gas phase. The error limit is estimated from the results of the least-squares fit and consideration of the quality of the observed data. It should be noted that since the Raman data indicate that the trans is the more stable conformer, in contrast to the microwave study,' an attempt was made to determine the more stable conformer by an infrared variable-temperature study. While no quantitative data could be obtained, the infrared data also indicate that the trans conformer is more stable in the gas phase. Barriers to Internal Rotation

In order to determine the barrier to internal rotation of the CD3

v2 v3 v6

AH

trans/gauche barrier, em-l gauche/gauche barrier, cm-I F3

"Calculated using Fo = 0.996600, Fl = 0.019709, F2 = -0.028020, = -0.000612, F4 = 0.000974, Fs = 0.000014, F6 = -0.000031, F7 =

-0.000000, FB

0.000000.

group, the observed torsional frequencies were fit to a potential function varying in torsional angle, a,by the equation V(a) =

'/zCV,(I- cos ia) i

with the retention of only the V, and V, terms. The internal rotation constant, F = h2/8r21r (where I , is the reduced moment of intertia for the internal rotation), the frequencies, assignments, and calculated potential constants are given in Table VI. The values obtained for the barrier heights support the assignment of the torsional series to the gauche conformer. Upon comparison of these barrier values to the corresponding values reported3 for ethylphosphine, one finds that the barriers obtained for the difluorophosphine molecule are considerably smaller. This deviation in the torsional barrier of the methyl group must be rationalized in terms of the effect of substitution of a PF2 group for the PH2 group in these ethylphosphines. Presumably the PF, group has an electron withdrawal effect on the CH, group which reduces the methyl barrier. The potential function for the asymmetric torsion was calculated by using the torsional transitions listed in Table VI1 and the kinetic constant or F "series" which was calculated as a function of the internal rotation angle a. From the structural parameters from the microwave study by Groner et al.,' the F number was calculated at various values of a and the results curve fit to a Fourier series via a least-squares procedure, so that F(a) = Fo

+ CFi cos ia I

where a is defined as zero for the trans conformer. The Fourier coefficients of this series are listed in Table VII. For the initial calculations, the minimum at 92.6 cm-' and the Q-branch at 89.7 cm-l were taken as the 1 0 transitions of the asymmetric torsion for the trans and gauche conformers, respectively, and from the gas-phase Raman temperature study, the value of 56 cm-' was taken as the energy difference between the lowest trans and lowest gauche energy levels. This initial calculation resulted in reasonable values for the VI, V,, and V3terms. At this point, the additional transitions of the gauche conformer were fit by using the initial VI, V2.and V3terms and, additionally, the V, term, while AH was allowed to vary. The V, and V, terms were found to be ineffective and were removed from the final calculations. The values of the four coefficients and their dispersions are listed in Table VII. +-

Discussion

The conformational stability determined for ethyldifluorophosphine is consistent with that determined for ethylphosphine where, from rotational2 and vibrational3 spectra, it was determined that the trans conformation is the more stable conformer in the gas, liquid, and solid. The stability of the trans conformer of ethyldifluorophosphine is indicated by the variable-temperature

2778 The Journal of Physical Chemistry, Vol. 91, No. 11, 1987 studies using both the Raman and infrared spectroscopic techniques. If one assumes that the polarizability change for the P-C stretches in the two conformers is about the same, then this conclusion is also supported by the Raman spectrum of gaseous ethyldifluorophosphine where the gauche PC stretch (680 cm-') is only slightly more intense than the trans PC stretch (651 cm-I). Taking into account the two-to-one statistical advantage of the gauche conformer one can infer that the trans is the more stable conformation. One finds similar support with the relative intensities of the C-C stretches for the two conformers, but the skeletal bending modes show much greater variability in intensities. However, the skeletal bending modes show much greater mixing of the internal coordinates between the conformers so the comparison of relative intensities of the skeletal bending modes between the conformers is not expected to be meaningful. Considering the results from the earlier microwave study,' in which it was concluded that the gauche conformer is more stable in the gas phase, it should be noted that variable-temperature studies in the microwave are notoriously difficult and that measurements of line intensities were taken at only two temperatures. Therefore, we feel confident with the assignment of the more stable conformer to the trans form. Unfortunately, only the barrier for the CD, torsion of the gauche conformer of ethyl-d3- and ethyl-d5-difluorophosphine could be determined. The lack of any observed methyl torsional transitions for the trans conformer of any of the species precluded the determination of the methyl barrier for this conformer. This lack of observed data is not surprising since the methyl torsion of the trans conformer is expected to give rise to a B-type infrared band contour and a depolarized Raman line. Utilizing the barrier for the methyl group from the deuteriated molecules made it possible to identify the weak 191-cm-' Q-branch along with the very weak Q-branch at 180 cm-I as the fundamental and first "hot band" of the methyl torsion of the gauche conformer for the "light" molecule, respectively. These two bands along with the appropriate internal rotation constant give values of 902 cm-I (2.58 kcal/mol) for V, and -25 cm-' for V6. These values are very consistent with the values of these parameters from the CD3CH2PF2and CD3CD2PF2molecules. The uncertainties in the value of the barrier to internal rotation for the gauche conformer should be relatively low even though the structural parameters for ethyldifluorophosphine have not been well determined. Small changes in the heavy atom distances and angles should not significantly alter the value for the internal rotation constant. Also, the coupling of the methyl torsion with the asymmetric torsion is not expected to be significant so the average barrier value of 895 cm-l (2.56 kcal/mol) for the gauche conformer should be accurate to at least 5%. The barrier for the internal rotation of the methyl group in gauche-ethylphoshine has been determined from the torsional transitions observed in the Raman spectrum of the gas which, in fact, requires that they arise from the gauche and not the trans ~ o n f o r m e r . ~Again, the structural parameters have not been accurately determined for ethylphosphine but the internal rotation constant should not be altered by more than a few percent which indicates that the methyl barriers -differ by about 500 cal/mol between guuche-ethylphosphine and the corresponding fluoride. Thus, the PF, group has had a pronounced effect on the electronic environment around

Durig et al. the methylene group which significantly reduced the value of the barrier to internal rotation of the methyl group. The determination of the asymmetric potential function is hampered by the fact that the torsional transitions for the trans conformer are expected to give rise to B-type infrared bands. The closeness of the torsional excited states will cause the P- and R-branches of these transitions to overlap thereby obscuring the minima. Also, since the energy difference between conformers is small, the trans and gauche transitions may overlap. However, we feel the data are sufficient for a correct assignment and determination of the potential function. The choice of the minimum 0 transition of the trans conformer is at 92.6 cm-' for the 1 consistent with the observation of a weak spectral feature at 181 cm-' which we have assigned as the overtone (2 0) of the trans asymmetric torsion. While it is possible that the gauche transitions are distorted by the presence of the trans excited states, this is not expected to have a significant effect on the potential function. The V3term of the potential function for ethyldifluorophosphine is expected to be similar to the threefold barrier of the methyl torsion in methyldifluorophosphine. In the vibrational study of methyldifluorophosphine, the methyl barrier was determinedI3 to be 609 cm-' (1.74 kcal/mol) and in the microwave study the authors concluded22that the barrier is 80.5 cm-' (2.3 kcal/mol). Therefore, the value determined in this study for V3(752 cm-I) is consistent with the barrier value determined for methyldifluorophosphine. The value of the normal coordinate calculations for identifying the correct conformer should be noted. By utilizing force constants for the trans conformer and the appropriate G-matrix for the gauche conformer, all of the bands which were assigned to the gauche conformer were predicted. Although the exact difference in frequency for a particular normal mode for the two conformers was not predicted the correct direction for the shift in frequency was predicted. For example, the C-C stretch for the gauche conformer was predicted to have a lower frequency than the corresponding mode for the trans conformer whereas the P-C stretch was calculated to have a higher frequency for the gauche conformer than for the trans conformer. These trends were found in the spectra and the predicted trends for the skeletal bends were also found. Thus, normal coordinate calculations can be very valuable for identifying the particular bands to the correct conformer if a reasonable vibrational assignment can be made for one of the conformers. Finally, it should be noted that many of the fundamentals of the trans conformer appear as doublets in the spectrum of the solid. Such doubling of the normal modes in the spectrum of the crystal is consistent with at least two molecules per primitive cell. This conclusion is also supported by the number of Raman lines assigned as lattice modes. +-

-

Acknowledgment. The authors gratefully acknowledge the financial support of this study by the National Science Foundation by Grant CHE-83-11279. Registry No. CH,CH2PF2, 430-78-4; CD3CD2PF2,107474-43-1; CD3CHZPF2, 107474-44-2. (22) Codding, E. G.; Creswell, R. A,; Schwendeman, R. H. Inorg. Chem. 1974, 13, 8 5 6 .