J . Phys. Chem. 1989, 93, 3492-3503
3492
Spectra and Structure of Organophosphorus Compounds. 40.+ Microwave, Infrared, and Raman Spectra, Dipole Moments, r Structure, Conformational Stability, and Vibrational Asslgnment of Isopropyldif luorophosphine
,,
J. R. Durig,* Mei-Shiow Cheng,f Y. S. Li,g P. Groner, Department of Chemistry, University of South Carolina, Columbia, South Carolina 29208
and A. E. Stanley Research Directorate, Research Development and Engineering Center, US.Army Missile Command, Redstone Arsenal, Alabama 35898-5248 (Received: August 3, 1988)
The microwave spectra of two conformers of isopropyldifluorophosphine, (CH3)2CHPF2,and its isotopic species, CH,(CD3)CHPF2and (CD3)2CDPF2,have been measured and assigned. Molecular electric dipole moments of 2.21 and 2.23 D were obtained from the Stark effects for the gauche and the trans conformers, respectively. The observed rotational constants have been used to derive ro structures for both conformers. The principal structural differences between the conformers are LCCP = 110.9 (3)' (trans), 107.6 (1)O and 113.6 (1)' (gauche);LPCH = 106.1 ( 1 2 ) O (trans), 103.2 (1)' (gauche); LCPF = 97.9 (8)O (trans),98.5 ( 1 ) O and 100.7 (2)O (gauche). For all three isotopic species, the infrared and Raman spectra (40-3200 cm-') have been recorded for the gaseous and solid states as well as the Raman spectra of the liquids. Both trans and gauche conformers are present in the fluid phases but only the gauche form is stable in the solid. The gauche conformer has been found to be more stable by 33 & 7 cm-' from relative intensity measurements in the Raman spectra of the liquids as a function of temperature. An almost complete vibrational assignment is proposed for all three isotopic species of both conformers.
Introduction
Many organic amines and phosphines have been shown to exist as a mixture of conformers at ambient pressure and temperature. This has been demonstrated for a number of ethyl compounds, CH3CH2X, where X = NH2,*v2PH 29 PF2,536and PSF2' by rotational and vibrational spectroscopy. The symmetric conformer of each of these compounds in which the lone pair electrons or the sulfur atom are trans with respect to the methyl group (trans conformer) has been found to be more stable in the fluid phases than the asymmetric gauche form, although only by 60-200 cm-I. Similar to the ethyl compounds, molecules containing the isopropyl group, (CH3)2CH-, attached to an asymmetric group are also expected to exhibit rotational isomerism. Vibrational spectroscopy of isopropylamine, (CH3)2CHNH2,8and -phosphine, (CH3)*CHPH2? has indeed demonstrated the existence of two conformers. Both have been reported to be a little more stable in the symmetric trans form (lone pair frans to methine hydrogen). However, only one conformer of these molecules has been assigned by microwave spectroscopy, namely the trans form of (CH3),CHNH2I0and the asymmetric gauche form of (CH3)2CHPH2." Structural differences between conformers have been well established for many ethyl compounds, CH3CH2X,where X = PH2? SH,I2-l4 SeH,'* and SiH2F.I6 Similar in-depth studies of molecules with the isopropyl group are rather scarce. Only for isobutyraldehyde, (CH3)2CHCH0,17has a sufficient number of isotopic species been investigated to allow for a reliable assessment of structural differences between the conformers. Many ethyl compounds experience bond angle changes of several degrees. As an example, a difference of 4.3' between the CCP angles of the conformers of ethyldifluorophosphine, CH,CH2PF2,6 has been proposed on the basis of a very limited set of rotational constants. In the present paper, we report the results of our investigation of the microwave, infrared, and Raman spectra of isopropyldifluorophosphine, (CH,),CHPF,, and its isotopic species, CH,(CD3)CHPF2and (CD,)2CDPF,. This study was initiated to 334
' For part 39, see Inorg. Chem. 1989, 28, 298
*Taken in part from the thesis of M. S . Cheng which was submitted to the De artment of Chemistry in partial fulfillment of the Ph.D degree, 1987 !Present address: Department of Chemistry, Memphis State University, Memphis, TY 38152
0022-3654/89/2093-3492.$01.50/0
determine the more stable conformer of another molecule containing an isopropyl group and to establish, if possible, structural differences between conformers of such a molecule. At the same time, we wanted to continue our investigationsof the characteristic vibrations of the difluorophosphino group, PF2, which began with studies of the vibrational spectra of methyl-'* and ethyldifluorop h ~ s p h i n e .We ~ also hoped to learn more about the barriers to internal rotation of coupled methyl rotors,19 although we failed in this last task. Experimental Section
Isopropyldifluorophosphine was prepared by the reaction of isopropyldichlorophosphine (Strem Chemicals) with antimony ( I ) Tsuboi, M.; Tamagake, K.; Hirakawa, A. Y.; Yamaguchi, J.; Nakagawa, H.; Manocha, A. S.;Tuazon, E. C.; Fateley, W. G. J. Chem. Phys. 1975,63, 5177.
(2) Fischer, E.; Botskor, I . J . Mol. Spectrosc. 1982, 91, 116; 1984, 104. 226. (3) Durig, J. R.; Cox, A. W. J. Chem. Phys. 1975, 63, 2303. (4) Groner, P.; Johnson, R. D.; Durig, J. R. J. Chem. Phys. 1988,88, 3456. (5) Durig, J. R.; Church, J. S.; Whang, C. M.; Johnson, R. D.; Streusand, B. J. J. Phys. Chem. 1987, 91, 2769. ( 6 ) Groner, P.; Church, J . S.; Li, Y. S.; Durig, J. R. J . Chem. Phys. 1985, 82, 3894. (7) Durig, J. R.; Johnson, R. D.; Nanaie, H.; Hizer, T. J. J. Chem. Phys. 1988,88, 7317. (8) Durig, J. R.; Guirgis, G. A,; Compton, D. A. C. J. Phys. Chem. 1979, 83, 1313. (9) Durie. J. R.: Cox. A. W. J . Phvs. Chem. 1976. 80. 2493. {IO) MeKrotra, S . C.: Griffin, L. i.;Britt, C. O.;'Boggs, J. E. J. Mol. Spectrosc. 1977, 64, 244. (11) Durig, J. R.; Li, Y. S. J . Mol. Spectrosc. 1978, 70, 27. (12) Hayashi, M.; Imaishi, H.; Kuwada, K. Bull. Chem. SOC.Jpn. 1974, 47, 2382. (13) Schmidt, R. E.; Quade, C. R. J . Chem. Phys. 1975, 62, 3864. (14) Nakagawa, J.; Kuwada, K.; Hayashi, M. Bull. Chem. SOC.Jpn. 1976, 49, 3420. (15) Nakagawa, J.; Okutani, H.; Hayashi, M. J . Mol. SpecfTosc. 1982,94, 410. (16) Hayashi, M.; Imachi, M.; Oyamada, M. J. Mol. Sfruct. 1981, 7 4 , 9 7 . (17) Stiefvater, 0. L. Z . Naturforsch. A 1986, 4 1 , 641. (18) Durig, J. R.; Stanley, A. E.; Jalilian, M. R. J . Raman Specfrosc. 1981, 10, 44. (19) Groner, P.; Sullivan, J. F.; Durig, J. R. In Vibrational Spectra and Structure; Durig, J. R., Ed.; Elsevier: Amsterdam, 1981; Vol. 9.
0 1989 American Chemical Society
Spectra and Structure of Isopropyldifluorophosphine
I
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3Ooo
XKK)
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The Journal of Physical Chemistry, Vol. 93, No. 9, 1989 3493
I1
1
lDm
0
C
WAVENUMBER (CM')
Figure 1. Raman spectra of isopropyldifluorophosphine-do:(A) gas, (B) liquid, and (C) annealed solid.
d 2000
1500
WOO
500
WAVENUMBER t c d )
A
Figure 3. Raman spectra of isopropyldifluorophosphine-d,: (A) gas, (B) liquid, and (C) annealed solid.
I
I! 1 1
1
r;,l
3000
2500
2000 1500 1000 Wavenumber (cm-1)
500
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Figure 2. Raman spectra of isopropyldifluorophosphine-d3:(A) gas, (B) liquid, and (C) annealed solid.
trifluoride.m The isopropyldichlorophophine-d7used to synthesize isopropyldifluorophosphine-d7was prepared by the reaction of isopropyl-d7chloride (Cambridge Isotope) with aluminum chloride and phosphorus trichloride.2' The sample of isopropyldifluorophosphine-l,l,l-ct, was prepared by starting with isopropanoll,l,l-d3 (Merck, Sharp and Dohme) which was converted to isopropyl-l,l ,1-d3 chloride with phosphorus trichloride. All preparative work and sample handling was carried out in a conventional vacuum system employing greaseless stopcocks. The samples were purified by using a low-temperature vacuum fractionation column. The identity of the product was verified by comparing the mid-infrared spectrum with the previously reported band frequencies for isopropyldifluorophosphine.22 Microwave spectra were recorded with a Hewlett-Packard Model 8460A M R R spectrometer with a Stark modulation fre(20) Drozd, G.I.; Ivin, S. Z.; Sheluchenko, V. V.; Tetel'baum, B. I.; Luganskii, G. M.; Varshavskii, A. D. J . Gen. Cbem. USSR (Engl. Trans[.) 1967, 37, 1269. (21) Perry, B. J.; Reesor, J. B.; Ferron, J. L. Can. J . Cbem. 1963,41, 2299. (22) Lines, E. L.; Centofanti, L. F. Inorg. Cbem. 1974, 13, 2796.
L , ; ' 1 ' ' 3000
1500 1000 WAVENUMBER (cm-1)
500
Figure 4. Mid-infrared spectra of isopropyldifluorophosphine-do: (A) gas, (B) annealed solid.
quency of 33.3 kHz. All frequencies were measured with the sample held near dry ice temperature and are expected to be accurate to within 0.05 MHz. Raman spectra of isopropyldifluorophosphine-do, -d,, and -d7 are shown in Figures 1,2, and 3, respectively. They were recorded on a Cary Model 82 Raman spectrophotometer equipped with a Spectra Physics Model 171 argon ion laser operating on the 514.5-nm line. The typical laser power used for obtaining the spectra of the liquids or solids was 0.25-0.5 W at the sample, whereas for the gases a maximum power of 1.5 W at the sample was used. The instrument was calibrated with lines of a mercury arc or the plasma lines of the laser. Spectra of the gaseous samples were obtained by using the standard Cary multipass accessory and an approximate sample pressure of 300 Torr. Spectra of the liquid samples were obtained with the sample sealed in a glass capillary held in a standard Cary accessory. A Cryogenic Technology cryostat, cooled with a closed cycle helium refrigerator, was used to study relative intensity changes in the spectra of the liquids as a function of temperature. The spectra of the solids were obtained either by condensing the sample onto a blackened copper block in a typical cold cellz3cooled by liquid nitrogen or by cooling the capillary held in the Cryogenic Technology cryostat.
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The Journal of Physical Chemistry, Vol. 93, No. 9, 1989
Durig et al.
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‘..i ‘ 1 1 3000
2500
2000 1500 1000 WAV ENUMBER (crn-1)
5bO
I
Figure 5. Mid-infrared spectra of isopropyldifluorophosphine-d3:(A) gas, (B) annealed solid.
I
500
’
300
100
WAVE NUMBE R Cc nil) Figure 7. Far-infrared spectra of isopropyldifluorophosphine-do: (A) gas, (B) annealed solid.
I
I
I
2000
1500
1000
500
WAVENUMBER (cni‘1)
Figure 6. Mid-infrared spectra of isopropyldifluorophosphine-d,: (A) gas, (B) annealed solid.
Solid samples were annealed until no further changes in the spectra were observed. Polarization measurements were made with the standard Cary accessories. The frequencies of the Raman lines should be accurate to f 2 cm-] for sharp resolvable lines. Typical mid-infrared spectra are shown in Figures 4, 5, and 6. They were recorded from 3200 to 400 cm-’ on a Digilab Model FTS- 14C Fourier transform infrared interferometer. A Ge/KBr beamsplitter, globar light source, and TGS detector were used. The instrument was continuously purged with dry nitrogen and calibrated against frequencies of atmospheric water and standard gases.24 The spectra of the gases were obtained using a 10-cm cell with CsI windows and approximately 40 Torr of sample pressure. The spectra of the solids were obtained by using a typical cold cellz3with outer windows and sample substrate fashioned from CsI. For both the samples and the reference, 100 interferograms were averaged and then transformed by using a boxcar truncation function. The nominal’ resolution was 0.5 or 2 cm-]. The far-infrared spectra are shown in Figures 7, 8, and 9. Those of the gaseous samples were recorded at resolutions of 1.O or 0.12 cm-’ on a Nicolet Model 2OOSXV vacuum Fourier transform interferometer, equipped with a high-pressure mercury arc source, a Mylar beamsplitter, and a liquid helium cooled germanium bolometer. The gases were contained in a 1-m cell (23) Durig, J. R.; Bush, S. F.; Baglin, F. G. J . Chem. Phys. 1968,49, 2106. (24) Cole, A. R. H., IUPAC: Tables of Wauenumberkfor Calibration of Infrared Spectrophotometers: Pergamon Press: Oxford, U.K.,1977.
I I
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I
400
300
200
100
WAVENUMBER
I
I
I
500
(ern-')
Figure 8. Far-infrared spectra of isopropyldifluorophosphine-d3:(A) gas, 0.5 cm-’ resolution, (B) gas, 0.12 cm-’ resolution, ( C ) unannealed solid, (D) annealed solid.
with polyethylene windows. The spectra of the solid samples were recorded on a Digilab FTS-15B Fourier transform interferometer which was purged with dry nitrogen gas to remove atmospheric moisture. The samples were condensed onto a liquid nitrogen cooled silicon plate in a cold cell equipped with polyethylene windows. These spectra were recorded at a resolution of 2.0 cm-I.
The Journal of Physical Chemistry, Vol. 93, No. 9, 1989 3495
Spectra and Structure of Isopropyldifluorophosphine
TABLE I: Rotational Transition Frequencies (MHz) of gaucbe -1sopropyldifluorophosphine in the Ground State (CH312CHPF2
transition
obsd
obsd-calcd
20 817.01 21 964.16 24 788.04 24 592.59 26218.01 25537.13 26438.95 25 968.36
0.037 0.000 -0.001 -0.077 -0.005 0.049 -0.025 -0.037
30 382.60 29 707.65 30919.71 30 133.1 1 30432.39 32708.38 32 632.88
-0.012 0.037 -0.014 0.036 -0.006 0.000 0.050
33 844.10 35 343.10
-0.080 0.085
34 960.14
-0.007
36 678.36 36635.60 38 438.24 37 947.32 39689.10 38 688.67 39 530.33
-0.044 0.037 -0.018 0.015 -0.033 0.020 -0.008
(CD3)CH,CHPF2"
obsd
CH3(CD3)CHPF,b
obsd-calcd
28 078.77 27 428.22 28 478.69 27 784.45
-0.005 0.005 -0.024 -0.001
30 244.17 30 155.12 31 866.97 31 257.52 32 576.70 31 747.13
-0.046 0.027 -0.006 0.015 0.027 -0.006
33 909.01 33 855.98 35 578.00 35057.79 36614.71 35691.06 36 356.29 37 580.34 37 549.90 39 234.83 38 830.60
-0.014 0.030 -0.007 -0.004 0.019 0.037 -0.024 0.030 -0.024 0.001 -0.020
39 610.02
-0.010
obsd
obsd-calcd
27 138.00 27 05 1.47 28 790.62 28 181.09 29 524.86
-0.027 0.035 0.017 0.004 0.001
30 88 1.78 30 834.67 32 578.49 32 077.95 33 693.45 32 751.40 33 474.32 32 962.91 34633.81 34 609.44 36 301.82 35 938.43 37 759.28 36 782.86 37 856.00 38391.33 38 379.26
-0.030 0.02 1 -0.026 0.010 0.012 -0.01 1 -0.030 0.0 16 -0.038 0.039 -0.012 -0.0 12 0.028 0.006 -0.009 -0.081 0.089
(CD3)2CDPF2
obsd
obsd-calcd
22935.96 22 33 1.99 23 210.80
0.000 0.025 0.003
25 056.69 24 963.70 26 546.46 25 967.31 27 129.08 26 390.00 26718.51 28 5 1 1.46 28 458.65 30065.78 29 569.33 30983.97
-0.041 0.039 0.009 0.017 0.015 -0.018 -0.053 -0.034 0.028 0.000 -0.010 -0.007
30 714.13 30 289.69 31 973.76 31 945.28 33 518.56 33 139.57 34 756.92 33 860.84 34 736.18 35 441.60 35426.83 36942.33 36681.85 38 432.04 37 549.04 38 738.98
0.007 0.024 -0.035 0.023 0.001 -0.013 0.003 0.009 0.003 -0.036 0.053 -0.008 0.005 0.008 -0.009 -0.005
'The C H 3 group is trans to lone pair electrons. *The CD3 group is trans to lone pair electrons.
c
propyldifluorophosphine is expected to occur in two different conformations. The trans conformer has a plane of symmetry whereas the gauche conformer possesses only the trivial C, element of symmetry. Initial rotational constants were calculated on the basis of reasonably assumed molecular structural parameters taken from the isopropyl moietyz5and methyldifluorophosphine.26 Based on these assumptions, calculations for the trans conformer predicted the b principal inertial axis to be perpendicular to the plane of symmetry and a value of -0.34 for the asymmetry parameter K . Therefore, a- and C-type transitions were expected for the trans conformer. For the gauche conformer, a value of -0.57 was predicted for K with a-, b-, and C-type transitions active. In the low-resolution spectrum of the normal species, three regions of pile-ups of transitions showed up at 30.0, 34.3, and 38.6 GHz in the R-band region, typical for certain near prolate rotor molecules. They were eventually assigned to the pile-ups of the higher K , components of the a-type transitions J + 1 J, J = 6, 7, 8, of the gauche conformer. Subsequently, individual asymmetric rotor transitions up to K, = 3 were identified based on asymmetric rotor predictions and qualitative Stark effects. Although a good number of strong transitions had been identified in this manner, many strong lines distributed over the whole spectral range were left unassigned. After the components of the electric dipole moment had been determined by the measurement of Stark effects, it was concluded that the remaining strong lines could not be explained as b- or C-type transitions of the same molecule. They must therefore originate from a second conformer. Subsequently, we were able to assign many a-type R transitions of the trans form of (CH3)$HPF2, based on asymmetric rotor
-
r
!I 400
300
200
100
WAVENUMBER (cm-1)
Figure 9. Far-infrared spectra of isopropyldifluorophosphine-d7: (A) gas, 0.5 cm-' resolution; (B) gas, 0.12 em-' resolution, (C) unannealed solid, (D) annealed solid. Microwave Spectrum
As stated previously, depending on the orientation of the difluorophosphino group relative to the isopropyl moiety, iso-
( 2 5 ) Tobiason, F. L.; Schwendeman, R. H. J . Chem. Phys. 1964,40, 1014. ( 2 6 ) Codding, E. G.;Creswell, R. A.; Schwendeman, R. H. Inorg. Chem. 1974, 13, 856.
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The Journal of Physical Chemistry, Vol. 93, No. 9, 1989
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TABLE 11: Rotational Transition Frequencies (MHz) of trans -1sopropyldifluoropbopbine (CH3)2CHPF2 (CDj)CH$HPF2
transition
(CD3)2CDPF2
obsd
obsd-calcd
obsd
obsd-calcd
obsd
18288.87 18 152.08 21 207.24 20 183.34 22 506.11 21 625.65 21 570.44 24 607.63 23 870.72 26 832.99 25 200.87 26 665.42 24 986.38 24965.92 27 854.42 27 436.68 30 778.78 29 186.74 31 514.95 28 358.66 28 351.57 31 113.01 30 914.56 34 305.61 33016.23 36 100.82 31 735.71 31 733.32 34423.34 34339.16 37 541.52 36 694.40
0.053 0.022 -0.016 0.026 0.029 -0.040 -0.039 -0.031 0.005 0.022 0.003 -0.058 0.015 -0.020 -0.012 0.000 0.006 0.018 0.003 -0.014 0.045 -0.019 -0.003 0.002 0.022 0.009 -0.029 -0.0 12 0.068 0.010 -0.041 -0.003
obsd-calcd
28 746.47 27 251.58 29 377.74 26 524.34 26517.10 29095.72 28 899.94 32 066.28 30 837.88 33 673.57 29 683.35 29 680.92 32 189.97 32 105.75 35 106.77 34 284.64
0.033 -0.015 -0.006 0.033 -0.002 -0.002 0.033 -0.016 -0.004 0.002 -0.037 -0.009 -0.003 0.005 -0.018 0.001
26 842.43
0.007
27 539.44
-0.007
27 195.93 27041.02 29907.91 28 850.20 31 516.19 27 830.97 27 829.23 30 108.50
0.023 0.003 0.007 -0.006 0.004 -0.03 1 0.019 0.002
32 738.81 32 062.36 35 141.05
-0.019 0.004 0.002
38 155.62
-0.021
35 114.60 35 113.82 37 769.51 37 736.42
-0.012 -0.004 0.006 0.021
37 613.21 32 844.25 32 843.45 35318.71 35 285.09 38 071.81 37 6 15.40
-0.001 -0.014 0.001 0.007 -0.005 -0.001 -0.010
38 494.06O 38 494.06"
-0.090 0.163
36 005.67" 36 005.67" 38 465.24 38 452.51
-0.144 0.121 0.017 0.015
39 167.57 39 167.57
-0.043 0.041
30 798.39 30797.81 33 050.35 33 025.54 35 528.78 35 169.94 38 360.07 33766.17" 33766.17" 36006.54 35 997.48 38 373.22 38 206.65 36734.35 36 734.35 38 969.09 38 965.88
0.001 -0.004 0.004 -0.009 0.002 -0.005 -0.005 -0.124 0.057 -0.007 0.022 -0.013 -0.005 -0.037 0.019 0.016 0.013
"Not used in the least-squares fit.
-
predictions for an assumed structure. Because of the K-value of -0.34, each set of J 1 J transitions was spread over a very 7 transitions). large region (about 8 GHz for the 8 No attempts were made to assign any of the numerous vibrational satellites in the vicinity of the ground-state lines. The spectra of the isotopic molecules were very similar to the spectra of the normal species and therefore easily assigned. As expected, two different spectra were observed and assigned for the gauche conformer of CH,(CD,)CHPF2 depending on the position of the heavy methyl group, but only one spectrum for the trans form. The measured frequencies of the rotational transitions of guucheand truns-isopropyldifluorophosphine-do, -d3,and -d7 in the vibrational ground state are listed in Tables I and 11, respectively. The rotational and centrifugal distortion constants (P representation A reductionz7) determined by least-squares fitting of the observed frequencies are listed in Tables 111 and IV.
+
-
TABLE III: Rotational Constants (MHz) and Centrifugal Distortion Constants (kHz)OTgaucbe -1sopropyldifluoropbophine" (CH3)ICHPFa 3954.577 (99) B 2288.7821 (65) C 1994.0207 (53) A, 0.882 (30) AJK -1.35 (37) Ax O.Od 6, 0.264 (19) hK 4.41 (76) 8 0.045 d 25
A
CH3CHPF2b 3790.159 (55) 2107.9602 (53) 1842.1102 (39) 0.732 (18) -0.75 (21)
CHPF2' 3623.088 (62) 2191.5630 (89) 1882.5141 (46) 0.849 (32) -1.65 (22)
(CD3)2CDPF2 3393.711 (35) 2011.2113 (31) 1737.9773 (26) 0.640 ( 1 1 ) -0.91 (13)
0.Od
O.Od
O.Od
0.188 (13) 0.228 (20) 0.1768 (78) 3.15 (46) 2.55 (24) 4.21 (40) 0.026 0.040 0.026 22 22 31 "Standard error in parentheses in units of last digit. bThe CH3 group is trans to lone pair electrons. OThe CD, group is trans to lone pair electrons. dKept constant during fit. e u = standard deviation (MHz). f n = number of fitted transitions.
The Journal of Physical Chemistry, Vol. 93, No. 9, 1989 3491
Spectra and Structure of Isopropyldifluorophosphine TABLE IV: Rotational Constants (MHz) and Centrifugal Distortion Constants (kHz)of trans-Isopropyldifluorophosphine"
A
(CHS)2CHPF2
CH,(CD,)CHPF2
(CD,),CDPF,
3971.100 (15) 2441.9650 (36) 1690.1877 (19) 0.380 (21) -0.79 (18)
3720.921 (14) 2276.171 l'(38) 1581.1646 (15) 0.340 (1 8) -0.88 (15)
3401.035 (10) 2129.2988'(30) 1484.2627 (13) 0.263 (13) -0.578 (88)
O.Ob
O.Ob
O.Ob
0.112 (12)
0.1126 (96)
0.0823 (77)
r(CH6) r(cHmethyl)
O.Ob
O.Ob
O.Ob
0.029 37
0.022 27
0.016 28
ff(CPF4) 4CPFs) ff(PCH6) ff(PCC7) ff(PCC8) a(CCHmcthyl) '/,r(PFCF) s(H,CPd)d 7(C7"CPdj d r(CBCPd)d
"Standard error in parentheses in units of last digit. bKept constant during the least-squares fit. 'Standard deviation (MHz). dNumber of fitted transitions. TABLE V: Dipole Moments" (debye) and Stark Coefficientsb of Isopropyldifluorophosphine, (CH3)*CHPF2
gauche
-
transition 726
827
826
-
625
726
725
(MI 4 3 2 1 6 5 4 3 2 7 6 5 4 3 2
lM 4 3 2 1 4 3 2
+
-
2.118 (12)
IpJ
= 0.285 (41) Ip,J = 0.679 (94)
1
5 4 3 2 1
lo6 -8.65 -5.04 -2.46 -0.92 24.04 13.37 5.75 1.18 -3.57 -2.47 -1.61 -1.00 -0.64
''O'O 110.0 110.0 49.7 120.0
113'55 (11)] 107.62 (5) 110.48 (2) 48.79 (8) 117.85 (14)
of24,0 :;,6eod(4)
95.78 (19)
)
trans final 1.842 (30) 1.585 (18)
fixed fixed fixed 97.94 (80) 106.1 (12)
110.0
108.73 (18)
110.0 49.7 0 f120.0
110.93 (26) 50.06 (53) f119.52 (28)
97.99
98.8 (13)
fixed
" The initial diagnostic least-squares method became a regular nonlinear least-squares refinement during the later stages of the iterative procedure. Standard error in parentheses in units of last digit. bConversionconstant 505379.05 MHz.UA2. CDistancesr in A, angles a and dihedral angles 7 in '. d d represents a dummy atom halfway between the F nuclei. 'Dependent parameter.
= 2.205 (6)
a/MHz = 0.19
u/MHz = 0.19
Figure 10. Numbering of atoms used to define the molecular structure of trans (left) and gauche (right) isopropyldifluorophosphine.Number 3 (not shown) refers to a dummy halfway between the fluorine atoms. Methyl hydrogen atoms attached to C7 and C8 are not shown for clarity.
lpbl IPCI
"Standard error in parentheses in units of last digit. bMHz/(V/ cm)2. CAssumed,see text. Dipole Moment. The electric dipole moment components were determined for both conformers by using the quadratic Stark effect. The results are given in Table V. For the gauche conformer lpal = 2.12 D, Ipbl = 0.29 D, Ipcl = 0.68 D, and wt = 2.21 D were obtained from a direct fit of their squares to the observed 625, 827 726, and 826 Stark shifts6 of the transitions 726 725. Following the same method for the trans conformer, lpal = 1.97 D and IpcI = 1.53 D were obtained originally from the Stark 523 and 7,, 634 under the asshifts of the transitions 624 sumption of C, symmetry (Pb = 0). However, the correlation coefficient between :p and :p in the least-squares fit became -0.99 and the total dipole moment was relatively large in comparison with the values for other alkyldifluorophosphines. We also failed to locate any of the predicted C-type transitions. The inclusion 624 transition during a of the Stark effect data of the 7,, reanalysis improved the correlation coefficient to -0.63 and reduced the total dipole moment to & = 2.23 D. Because w> became now a small negative quantity, it had to be fixed at the zero value during the final analysis. The final results for the trans conformer were p b = 0 (symmetry), wc = 0 (assumption), and IwCLal= pLt= 2.23 D. The electric field was calibrated with the Stark components of OCS, using 0.71619 D for its dipole moment.2s
-
1.83 1.59 1.535 1.093 1,093 98.3 98.3 110.0
final initial 1.8279 (40) 1.83 1.5984 (20) 1.59 fixed 1.535 fixed 1.093 fixed 1.093 98.52 (12) 100.72 (16)) 98'3 103.22 (10) 110.0
= 2.231 (4)
= = 0' gt = 2.231 (4)
p,
initial
Av/E2 X
Au/E2 X lo6 transition 12.82 624 523 7.12 3.05 0.61 12.15 735 634 8.38 5.31 2.91 1.20 725 624 -9.29 -6.88 -4.84 -3.17 -1.88 -0.95 -0.39
Ipbl
gauche parametersc OC) r(PF,) = r(PF,) r(CC7) = r(CC8)
97.99
trans
1 Ipal =
TABLE VI: Molecular Structure of gaucbe- and tram -1sopropyldifluorophosphine from Least-Squares Fitting of Rotational Constants"*b
- -
-
-
-
(27) Watson, J. K. G . In VibrafionalSpectraand Structure; Durig, J. R.. Ed.; Elsevier: Amsterdam, 1977; Vol. 6. (28) Lovas, F. J. J . Phys. Chem. ReJ Doto 1978, 7, 1445.
Molecular Structure. A complete determination of the molecular structure was impossible because the number of available rotational constants was not sufficient to determine all structural parameters (33 for gauche, 18 for trans). From the outset, the methyl groups were assumed to be identical symmetric internal rotors with one common C H distance and one common C C H angle. Extensive calculations using the diagnostic least-squares m e t h ~ dwere ~ ~ used ~ ~ to identify ill-determined parameters. Subsequently, the methyl and methine C H distances were fixed at the value 1.093 A. For the gauche form, equal P F and CC distances were assumed. Amon the remaining parameters, the CC distance was fixed at 1.535 . After these assumptions were made, eight independent parameters were left to be determined from nine constants for the trans conformer. For the gauche conformer-12 parameters were left, but a very strong correlation existed between the P F and P C distances and the dihedral angle relating the methyl carbon Cs to the plane containing the P and C2 atoms and bisecting the FPF angle. (See Figure 10 for the labeling of the nuclei.) Exploring calculations with a constant value between -120 and -123' for this dihedral angle resulted in rather large P F distances (> 1.60 A). If this angle is changed to -1 25 or -1 26O, the PC distance became rather short compared to the value for the trans conformer or to the P C distances in ethylphosphine (1.85 A).6 Therefore, this dihedral angle was fixed, somewhat arbitrarily, at -124'. This assumption left 11 parameters to be determined from 12 rotational constants for the gauche
1
(29) Curl, R. F. J . Comput. Phys. 1970, 6, 367.
Durig et al.
3498 The Journal of Physical Chemistry, Vol. 93, No. 9, 1989 TABLE VII: Observed" Frequencies (cm-') and Vibrational Assignment of Isopropyldifluoropbosphine-d, infrared Raman re1 int re1 re1 re1 re1 int gas int solid int gas and depol liquid and depol solid int 2982 R 2974 m 2968 S 2976 s 2976 Q 2975 m, dp 2969 2966 m, p W 2940 w 2944 m 294 1 2953 2949 s, p 2943 s, p 2927 m 2927 w, p W 2909 vw 2902 2898 s 2913 2910 s, p 2890 R 2885 Q 2878 P 1483 Q 1477 R 1470 Q
1393 Q 1384 P 1373 Q 1300 R 1293 Q 1286 P 1271 1241 1168 1100 R 1096 Q 1088 P 1028 Q 1020 Q 1015 P 934 884 R 878 Q 870 P 826 818 714 693 R 688 Q 677 P 640 R 633 Q 627 P 512 R 505 Q 499 P 462 R 450 Q 441 P 420 313
m
2900
vw
2876
s
q , u2 u I , u2 y3
gauche CH3 antisym str trans CH3 antisym str
CH str 2V8
u4, u5
CH, antisym str
u6, ut
CH, sym str
2882
s, p
2874
2871
m
2175 2734
w, p w, p
2768 2722
2756 2717
w, bd w
1418 1464
w
1465
m
us, u9
CH, antisym def
1450 1428 1388
w vw vw
ulo, vI1
CH3 antisym def
Y12
CH3 sym def
"13
CH, sym def CH def
2Ul2
w, sh 1470 1460 1456 1438 1390
vw w w vw vw
w, sh
1362 1326
w vw
W
1292
vw
1292
vw
1293
1293
w
"14
vw vw vw
1277 1254 1168
vw vw vw
1267 1244
w, p
1270 1244 1167
1278 1254 1170
w w w
"I5 "I6
CH def C-C antisym str
W
1094 1086
w vw
1085
vw, p
1090 1082
CH3 rock
1022 1006 968 938 926
w vw vw vw w
w, p vw, p
w w w
VI1
1029 1018
1093 1087 1027
VI8
CHI rock
879
w
877
814
s
826
W
W
W
W
w, bd W
1458
m, dp
1450
1392
vw
1390
m, p
1020
2v24
968
966
w
"19
CH, rock
928
926
w
y20
CH, rock
w.p
878
877
m
y2 I
C-C sym str
w, p
813
812 802
vw
v22
PF2 sym str
sh S
m
790 173
m
w, sh
s
817
w, bd,dp
788
782
m
u23
PF2 antisym str
m
690
w
688
w, p
689
690
vw
u24
trans PC str
m
634
s
632
m, p
636
636
s
y24
gauche PC str
W
502
m
503
503
w
y25
PF2 wag
y26
trans CC2 wag
h 6
gauche CC2 wag
"21
trans PF2 def
W
vw
419
R
366 Q 353 P 345 Q 338 P 308 Q 292 Q 278 R 212 Q 264 P 232 Q 218 210 199.5 Q 187.6 Q
"i
assignment approx descripn
m
452
m,p
453
420
w, p
420
419
w
W
346
348
w
y21
gauche PF2 def
w, p vw
310 293
315
m
Y28
gauche PF2 twist
211
vw, p
277
y29
trans CCC def
w
228
vvw
228
246
w
u29
gauche CCC def
w
212
ww, p
212
21s
vw
u30
CC2 twist
185
vvw, p
187
p31
CH, torsion
m
345
m
vw vw
310 285
vvw
308 292
216
vvw
238 212
W
m
W
W
vw vvw
Spectra and Structure of Isopropyldifluorophosphine
The Journal of Physical Chemistry, Vol, 93, No. 9, 1989 3499
TABLE VI1 (Continued)
infrared re1
Raman re1
gas
int
solid
160 ctr 74 ctr
vvw
187
int vvw
gas
re1 int and depol
w
65 62
58
re1 int
liquid
re1 int and depol
solid
w, P
192 78
w
15
38 28
m
m
m m m
vi vq2 . . vj3
assignment approx descripn CH, torsion PF,-torsion lattice modes
m
“Abbreviations used: s, strong; m, medium; w, weak; sh, shoulder; bd, broad; ctr, center of b-type band; p, polarized; dp, depolarized; P, Q,and R refer to vibrational-rotational branches; sym = symmetric; antisym = antisymmetric; def = deformation; str = stretch. conformer. The diagnostic least-squares process changed to an ordinary nonlinear least-squares refinement during the later stages of the iterative procedure. For all calculations, the rotational constants were given weights inversely proportional to the square of their standard errors. The results are presented in Table VI. The standard errors quoted are those obtained if the values of the fixed parameters are assumed to be correct. Since we are not assured of this the “true” standard errors are underestimated by an unknown factor. One must also keep in mind that some values depended strongly on the assumptions made. This was particularly true in the gauche form for the PC and P F distances and all parameters involving methyl carbon cg. To a lesser extent, it also held for the C P F and FPF angles. On the other hand, other parameters were much less affected by the assumptions, e.g., the angles a(PCH6) and a(PCC7). Also the difference between the C P F angles was rather insensitive to the assumptions. For the tram conformer, one other assumption was rather critical. On changing all C H distances from 1.093 to 1.090 A, the P C distance jumped to 1.923 8, and the P F distance dropped to 1.54 8,. The results for the gauche form were affected much less by a change in the C H distance. Regardless of the arbitrariness of the assumptions, the main structural differences were as follows. The PCC angles changed dramatically from one form to the other. In the trans conformer they were 110.9 (3)O, whereas in the gauche form, one was reduced to 107.6 (1)O and the other one (for the methyl group gauche to both F nuclei) increased to 113.6 (1)’. The PCH6 angle changed in a similar manner. It increased from 103.2 (1)’ in the gauche form to 106.1 (12)’ in the trans conformer where H6 is gauche with respect to both F nuclei. Also the C P F angles changed significantly. From 97.9 (8)’ in the trans form, one increased to 98.5 (1)’ and the other one, involving the F atom gauche to both methyl groups, changed to 100.7 (2)’. All these angle changes can be explained by nonbonded repulsive interactions between the F atoms and the methyl groups or the methine hydrogen atom. Vibrational Assignment
Isopropyldifluorophosphine contains 13 atoms resulting in 33 normal modes. The tram conformers of the do and d7 compounds have a plane of symmetry. For these molecules, the normal vibrations separate into 18 A’ and 15 A” modes. The A” vibrations are expected to cause depolarized lines in the Raman spectra of the fluid phases and to have B-type band contours in the infrared spectra of the gases while the A’ modes should produce polarized Raman bands and A / C hybrid contours in the infrared spectra. All isotopic modifications of the gauche conformer and the trans form of isopropyldifluorophosphine-d3possess only the trivial symmetry element, the identity. All vibrational transitions of these molecules should be polarized in the Raman spectra of the fluid phases and should have, in general, A/B/C hybrid band contours in the infrared spectra of the gaseous phase. In the spectra of the solid samples, a number of bands were observed to decrease in intensity or to disappear completely if the solids were annealed. Usually, only one conformer is present in a well annealed solid. However, despite repeated efforts, it was impossible to anneal the solids completely because some bands did not vanish entirely. Sometimes it is possible to identify
conformers by comparing the gas-phase band contours of a pair of conformer bands where one component disappears in the solid state. This was impossible in the case of isopropyldifluorophosphine because the conformer bands had similar band contours or because their band contours could not be identified unambiguously due to interference by other bands. The question of the relative stability of the conformers could be answered only with the help of the spectra of the d3 compound. In this case, one can expect up to three conformer bands, one from the trans conformer and two from the inequivalent gauche forms. While in some cases three conformer bands were present in the spectra of the fluid phases, only one of them was absent from the spectrum of the annealed solid state. The best examples are the PCstretching modes, where the 606- and 600-cm-I bands remained while the 636-cm-l feature disappeared in the solid, and the FPF deformation (band at 355 cm-’ disappearing, bands at 338 and 333 cm-l remaining). Therefore, it was concluded that the trans conformer is less stable in the solid state and that all decreasing or vanishing bands in the solid-state spectra must be assigned to this conformation. The vibrational assignments for isopropyldifluorophosphine-do, -d7,and -d3 presented in Tables VII-IX are based on the group frequencies of the isopropyl moiety30and of the C-PF2 g r o ~ p . ~ , ’ ~ Comparisons with the spectra recorded recently for the normal and perdeuterated species of isopropyldichlorophosphine, (CH3)2CHPC12and (CD3)2CDPC12?1were useful for the assignment of the spectra of the do and d7 species. The assignments of the spectra of CH3(CD3)CHPF2were based on comparisons with those made for the other two isotopic modifications. Because the vibrational assignments for a molecule of the size of isopropyldifluorophosphine are necessarily somewhat speculative, only a few of them are now discussed in some detail. In the CH-stretching region, the methine C H stretch was reassigned because of the spectra observed for the d3 and d7 species. The CD stretching region of the d3 compound contains three fundamentals at 2226, 2137, and 2082 cm-l. The Raman spectrum of gaseous isopropyldifluorophosphine-d7shows three bands at the same positions with an additional strong band at 2162 cm-’ without counterpart in the spectrum of the d3 compound. Therefore, this band was assigned to the methine CD stretch. Correspondingly, the methine C H stretch was attributed to the second highest frequency in the CH-stretching region near 2945 cm-I. Of particular interest in this study is the assignment of the fundamentals of the C-PF2 moiety. The symmetric and antisymmetric PF2 stretching vibrations were assigned to the strong band complex near 800 cm-I in the infrared spectra. They appeared in the same frequency region in the spectra of methyl-I8 and ethyldifluor~phosphine.~As in the methyl and ethyl analogues, the band complex shifted to lower frequencies upon solidification. The spectra of the solid samples were even more complicated, although only one conformer should be present. The additional features may be due to dimer formation in the solid. The Raman spectra in this region were not of much help because the corresponding bands were weak and depolarization measurements difficult due to overlap. Therefore, specific assignments (30) Klaboe, P.Spectrochirn. Acta A 1970, 26, 87. (31) Durig, J. R.;Cheng, M. S.; Harris, M. E.;Hizer, T.J. J . Mol. Strucr. 1989, 192, 47.
3500 The Journal of Physical Chemistry, Vol. 93, No. 9, 1989 TABLE VIII: Observed' Frequencies (cm-') and Vibrational Assignment of Isopropyldifluorophosphine-d, infrared Raman re1 re1 re1 int re1 re1 gas int solid int gas int liquid and depol solid int W vw 2322 2312 2320 2297 ww vvw 2260 W 2259 2260 2226 vs 2224 S 2224 S 2230 2227 vs 2164 2157 2158 2162 W vs S 2156 sh W 2138 2140 m 2130 2127 m 2130 m m 2135 S 2126 21 18 2125 21 17 sh sh W 2075 m vs 208 1 S 2087 208 1 2067 2079 m 2046 vvw 2057 S 205 1 2047 m 1170 S 1171 R W ww W 1160 1160 1161 1167 Q m 1172 W 1162 W m ww W 1132 1131 1130 1135 1128 m 1072 ms 1079 m 1077 W 1080 1078 Q 1073 sh 1063 1071 R 1052 ms S 1060 m 1056 m 1066 Q 1057 1060 P W 1031 sh 1028 S 1030 1028 W 998 W m W W 976 980 982 979 980 vw 964 W vvw 920 935 vw 904 W vw W 894 rn 894 894 896 894 vw 868 W sh 838 839 Q W W 830 818 vvs vs 822 814 815 W 820 vs 800 801 794 m 784 R vs 771 W 771 W 774 781 Q S 772 W 762 776 P W 750 m 753 W 739 R 741 736 729 S mw 735 Q rns 730 730 732 m 728 Q ww 687 S 685 614 R W S 610 m 610 612 609 Q 584 R S S 583 vs 577 578 Q S 580 583 sh 578 W vw 468 ms S 468 470 469 468 Q m m 420 420 416 Q m W m 384 W 384 384 385 386 348 R W m 343 344 342 Q 336 R vw vw m 332 m 328 329 327 Q 326 321 P W vvw vw 28 1 274 278 278 W 256 259 256 R 250 Q m 245 P W vw 208 210 W W 210 rn 20 1 208 214 vw 202 Q 179 ctr vvw WR ww vvw 182 184 188 178 160 154 vvw vvw vvw 148 Q 68 ctr 79 W 67 61 vw vw 52 vw vw 56 39 W 26 W For abbreviations used see Table VII.
Durig et al.
vi
assignment approx descripn
V I , v2 u3
CD, antisym str C D sym str
u4r y5
CD, antisym str
u 6 ~yl
CD, sym str
y8
C-C antisym str
y9 VIO, V I 1
CD, sym def CD, antisym def
Y12r v13
CD, antisym def
y14
CD, sym def
y 1 5 > u16
C D def, CD, rock impurity
y17
C D def
y18 y19
PF2 sym str PF2 antisym str
y20
C-C sym str
y21
CD, rock
v22
CD, rock
y23
CD, rock
y24
trans PC str
y24
gauche P C str
v25 "26 y26
PF2 wag trans CC2 wag gauche CD2 wag
y21
lrans PF2 def
v27
gauche PF2 def
y28 y28
gauche PF2 twist trans PF2 twist
y29
trans C C C def
v29 "30 yPl yP2
gauche C C C def CC2 twist CD, torsion gauche CD3 torsion
u33
PF2 torsion lattice modes
Spectra and Structure of Isopropyldifluorophosphine
The Journal of Physical Chemistry, Vol. 93, No. 9, 1989 3501
TABLE I X Observed" Frequencies (cm-I) and Vibrational Assignment of Isopropyldiiluorophosphine-d3
infrared re1 int solid
gas 2981 R 2974 Q 2970 R 2966 Q 294 1 2912 Q 2884 R 2878 Q
2976
S
sh
s
m
2965 2948 2942
s
2934
2938
S
CH str
m
2975
gauche CH, antisym str
vs
trans CH, antisym str
2962
sh 2904
2899
vs
CH3 antisym str
S
2877
m
2883 2877
vs
2874
2875
S
CH3 sym str
2743
m 2730 2724
W
2228 2222
S S
gauche CD3 antisym str gauche' CD3 antisym str
2168 2147 2136
W
y14
2116 2080 2076 2064 2056
w,
1458
m
1285 1280
sh
VI 1
W
VI 1
1256 1246
W
W
gauche CH in-plane bending gauche' CH in-plane bending trans CH in-plane bending
C-C antisym str
S
sh
m
ms
m
766 R 760 Q 637 Q 606 Q 600 Q 493 Q 486
sh
s
1383 Q 1287 Q 1282 Q 1258 R 1252 Q
820
2980 2973
2911
S
999 Q 990 P 812 Q 868 Q 865 P
assignment approx descripn
Y,
m
1462 Q
1062 Q 1058 Q 1050 P 1040 Q
re1 int
solid
sh
vw
1120 R 1113 Q
re1 int
liquid
sh
2146
2082
gas
2940 2907 2900
sh
2137
re1 int
S
S
2842 Q
2232 R 2226 Q 2215 Q
Raman re1 int
W W
W
m
2825
W
2732 2726 2268 2238 2230 2224 2222 2168
vw vw
2138 21 18 2082 2077
sh
2735
m
sh
W
sh S S
m
2228 2216
sh
2172
w
2234
m
sh W
W
2136 2128
2167
m
W
2132
m
2117
sh
sh
2055 1871 1852 1549 1481 1474 1455 1450 1377 1375 1286 1281
W
1258 1247
m
1135 1132
m
1112
m
2083 2071 2062
s
1462
w,
1388 1286
vw
m
2077 2068
S
1458
W
1382
ww
sh
w
m
W
m
bd
w
1282
W
W
1250 1240 1226
w w
1248 1236 1222
1134
vw
1130
W
1131
W
1108 1102
sh
1109 1102
sh
vw
w
W
W
sh
1036
m
1040
vw
1038
W
1036 1030
W
1008 992
ms ms
874 870
S
825 814 809 780
sh sh
CH3 sym def CH out-of-plane bending gauche' CH out-of-plane bending
Y14
CH, rock
VI5
Y16
CD, sym def CD3 antisym def
V17
CD, antisym def
W
1056
ms
CH, antisym def CH, antisym def VI0
W
ms
2y17
W
1058
S
CD, sym str
W
vw
S
y7
W
1060
759 640 61 1 604 492
2y16
sh sh
m m, bd
S
bd
sh
1063 1052
ms, sh
CD, antisym str
W
ms ms
ws
+
sh
W
m
sh
W
W
m S
vs vs
s, sh W
S S S
CH, rock
992
ww
992
vw
993
W
870
vw
868
W
870 867
m sh
m
830 800
W
821 812 794
vw
833 816 804
777
m
y22
757
W
??3
760 636 m 605 599 492
w
w,
bd
W
mw
VI9 y19
y20 V2I
gauche' C-C sym str gauche C-C sym str
PF2 sym str . PF2 antisym str
mw w
sh
ms ww
759 635 604 599
W S
sh
ms
?!4
608 602 490
S
y24
S
k?4
vw
V25
CD3 rock CD, rock trans PC str gauche' PC str gauche PC str gauche PF2 wag
3502
The Journal of Physical Chemistry, Vol. 93, No. 9, 1989
Durig et al.
TABLE IX (Continued)
infrared re1
Raman re1
re1
gas
int
solid
int
gas
479 Q 435 Q 402
ms m
477 434 405 399
S
478 435 400
340 334 303 290
S
355 Q 338 Q 333 Q
270 R 263 Q 256 P 222.9 Q
W
ms ms ms
W
m
int vv w
m w, bd
re1 liquid 480 434 400
solid
vw, bd
476
vw
402 397
W
m W
m 355 S
W
W
vvw, bd
335 301 288 275
vw vw
354 340
vw, bd sh
303 288 275
W
sh
W
re1
int
W
int
ui v2s u26 *26
sh
Y28
trans PF2 def gauche' PF2 def gauche PF2 def gauche' PF2 twist gauche PF2 twist trans PF2 twist
v29
trans CCC def
u27
340 334 302 29 1
assignment approx descripn gauche' PF2 wag trans CC2 wag gauche CC2 wag
W
u27
sh
u27
W
u2a
W
Y28
W
ms
266
vw
265
w, sh
W
m m
222
W
222
ww
233
vw
u29
gauche CCC def
m
195 183
vw vw
207
sh
207
vvw
y30
CC2 twist
VW
189
vvw
Y31
Y32
CH3 torsion CD3 torsion
"33
PF2 torsion
196 Q
W
232 228 210
162 ctr 146.2 Q 141 Q 71 ctr
vw
185
vw vw vw
72 62 58 54 46
sh m sh sh sh
82 76
W
36 24
m, bd
W
lattice modes m
For abbreviations used see Table VII. to the symmetric or antisymmetric mode or to the individual conformers were not attempted. The PC stretching mode was easily identified in both infrared and Raman spectra in the 700-600-~m-~region. It was assigned to the bands at 632 cm-' for the gauche conformer and at 686 cm-l for the trans form. These modes shifted to 518 cm-I (gauche) and 609 cm-' (tram) for the d7 compound while 606 and 600 cm-I (gauche) and 637 cm-l (trans) were observed for the d, modification. For comparison, the corresponding mode was observed near 7 15,680, and 65 1 cm-' for methyl-I8 and gauche and trans ethyldifluoroph~sphine,~ respectively. Below 600 cm-I, the frequencies of nine normal vibrations were expected. They included two methyl torsions and the PF2 torsion, and the CCC and FPF deformations as well as the symmetric ("wag") and antisymmetric ("twist") combinations of both the CCP and C P F angle deformations. Among these, the PF2 torsional mode was the easiest to assign because it had by far the lowest frequency (74,7 1, and 68 cm-' for the do, 4,and d7 species, respectively). For ethyldifluorophosphine,the corresponding mode was observed between 85 and 90 ~ m - ' . Because ~ of the b-type band contours of this mode, no conformational splittings could be observed. The other low-frequency vibrations were more difficult to assign. For all three isotopic species, bands with characteristic C-type contours were observed in the infrared spectra of the gases at 450, 435, and 416 cm-I. Each of these bands disappeared completely in the spectra of the annealed solids. Therefore, they belonged to the trans conformers. They were assigned to the CC2 wagging modes (symmetric combination of the CCP deformations) because an almost pure C-type character could be expected only for such a motion. The corresponding modes for the gauche conformers were assigned to the bands at 420, 402, and 384 cm-' for the three isotopic species. These assignments contradict those made by Klaboe for the isopropyl halides.30 However, Klaboe did not mention any evidence for the CCC bending mode to have a higher frequency than the CC2 wagging vibration. The other assignments of the heavy atom modes listed in Tables VII-IX were made in analogy with spectra observed for methyl-I8 and ethyldifluorophosphineS (for the PF2 modes) and for the isopropyl halides30 (for the remaining modes of the CCC skeleton). In the spectra of the do and d3 species,
two fairly strong bands were observed just above 200 cm-l whereas only one is observed for the d7 compound. The methyl torsional modes could not be assigned in a satisfactory manner. The weakness of the bands due to these modes and interference by the much stronger skeletal vibrations near or above 200 cm-l made it virtually impossible to disentangle the expectedly complicated region. For isopropyldifluorophosphine-do, four bands due to two different conformers were expected in this region of the gas-phase spectra. In the spectra of the solid compound, very weak bands near 188 cm-' were assigned to one of the C H 3 torsions of the gauche conformer. We tentatively attributed a weak b-type feature in the infrared spectrum of the gas near 160 cm-' with this same mode anticipating a large frequency shift upon solidification. However, this band could also be assigned to the a" mode of the trans conformer for which a pure b-type character was expected. We also tentatively assigned two very weak but sharp Q branches at 199.5 and 187.6 cm-' to a CH3 torsion without speculating which mode of which conformer might be responsible for it. The spectra of the solid state of the d3 modification contained only one weak band near 187 cm-' in the region of interest. As in the case of the undeuterated compound, it was assigned to the CH, torsions of one of the inequivalent gauche forms. A weak b-type feature near 160 cm-' in the gas-phase infrared spectrum was associated with this mode. The weak Q branches near 146 and 141 cm-' were assigned to the CD, torsion of an unidentified conformer. In the infrared spectrum of the gaseous perdeuterated compound, the absorptions near 150 cm-I seemed to be a little more intense but except for a Q branch at 148 cm-l showed no characteristic features. This region was assigned to unspecified CD3 torsions. A weak b-type band was observed near 179 cm-I, also due to a CD3 torsion. The very weak band at 188 cm-l in the Raman spectrum of the solid was associated with it. Because of the difficulties in assigning the methyl torsional modes correctly, we did not attempt to determine barriers to internal rotation. Conformational Energy Difference
As already described above, the gauche conformer is the only one present in the well-annealed solid of isopropyldifluorophosphine. The presence of bands due to both the trans and
Spectra and Structure of Isopropyldifluorophosphine
gauche conformers in the Raman spectra of the liquids allowed for the determination of the enthalpy difference between the conformers by studying relative intensities as a function of temperature. The pairs of bands assigned to the PC-stretching vibration of the trans and gauche conformers (689 and 636 cm-', respectively, for the do species; 610 and 578 cm-', respectively, for the d7 species) were clearly the best choices for this study because they were well separated, sufficiently intense, and symmetric. N o other bands interfered with relative intensity measurements. Relative peak heights were measured at various temperatures between 120 and 280 K. The intensity ratio [ * / I of the conformer bands is related to the enthalpy difference AH by the relation In I * / I = A H / R T
+ AS/R
AH values of 32.4 f 7.9 and 31.4 f 6.3 cm-' (93 f 23 and 90 f 18 cal/mol) in favor of the more stable gauche conformer were obtained for isopropyldifluorophosphine-doand -d7,respectively, from the slopes of the least-squares fitted straight lines in plots of the natural logarithms of the intensity ratios versus 1/T. No attempts were made to identify the more stable conformer in the gaseous phase.
Discussion The assignment of the microwave spectrum of isopropyldifluorophosphine and its isotopes posed no major problem. A-type R transitions have been analyzed for both conformers in the vibrational ground state. No splittings due to internal rotation effects have been observed with these transitions. N o attempts were made to identify b- or C-type transitions for the gauche conformer or to assign the spectra of the numerous vibrational satellites. The total electric dipole moments of gauche- and trans-isopropyldifluorophosphine(2.21 and 2.23 D, respectively) compare well with those reported for the other alkyldifluorophosphines (2.06 D for CH3PF2,262.08 and 2.17 D for trans- and guuche-CH3CHzPFz6). As explained in the subsection Molecular Structure, some critical assumptions were necessary in order to derive reasonable structures for both conformers of isopropyldifluorophosphine. Therefore, only those structural differences between the conformers are discussed and compared which are not very sensitive to the assumptions made. CCP angles of 110.9 (3)' were obtained for the trans conformer. In the gauche form, the CCP angle gauche with respect to both fluorine atoms increased to 1 13.6 (1)' whereas the other one decreased to 107.6 (1)'. A change of similar magnitude was found for the P C H angle (106' and 103.2' for
The Journal of Physical Chemistry, Vol. 93, No. 9, 1989 3503 the trans and gauche conformers, respectively). The differences in the C P F angles were not quite as large but nevertheless fairly well established. Whereas one of them increased very little from 97.9 (8)' in the trans conformer to 98.5 (1)' in the gauche form, the other one (gauche with respect to both methyl groups) increased by 2.8O. Not many molecules which contain an isopropyl group and occur in more than one conformation have been studied by microwave spectroscopy. Only one conformer has been assigned so far for isopropylamine10and -phosphine. Two conformers have been identified for 2-propan01'~and 2-propanethi01.~~Limited structural information has been derived for isopropanol where one CCO angle in the gauche conformation was estimated to be 4.3' smaller than the same angle in the trans form.32 The best structural information about conformers of an isopropyl compound is available for isobutyraldehyde (2-methylpropanal).I7 For this molecule, conformational differences have been obtained for the HCX angle (X = CHO) (4.2') and for the CCX angles (2.6'). However, a direct comparison between these results and our isopropyldifluorophosphinedata is not possible because of the very different geometrical arrangements of the -HC=O and -PF2 groups. However, StiefvaterI7 ascribed the structural differences between the conformers of isobutyraldehyde to repulsive forces between the C=O bond and the C-C or C-H bonds. The same argument explains most structural differences between the conformers of ethylph~sphine~ and many other ethyl compounds.I2-l6 Due to the impossibility of isotopic substitutions, structural changes within the PF2 group are much more difficult to prove. The results obtained for methyl-26and ethyldifluorophosphine6are unsuitable for comparison because only one isotopic species has been studied in detail. However, the P F distances in isopropyldifluorophosphine are longer by at least 0.02 A than the distances in PF3.34 This fact agrees with the expectation that the substitution of one fluorine atom in the PF3 group by a much more electropositive alkyl group releases electron density into the remaining P F bonds, which as a consequence become longer.
Acknowledgment. We gratefully acknowledge the financial support of this work by the National Science Foundation by grant CHE-83-11279. Registry No. D1,7782-39-0; (CH&2HPF2, 52760-78-8. (32) Abdurakhmanov, A. A.; Elchiev, M. N.; Imanov, L. M. J . Struct. Chem. 1974, 15, 37. (33) Griffiths, J. H.; Boggs, J. E.J . Mol. Spectrosc. 1975, 56, 257. (34) Kawashima, Y.;Cox, A. P. J . Mol. Spectrosc. 1977, 65, 319.
