J . Phys. Chem. 1986, 90, 4537-4544
4531
Spectra and Structure of Organophosphorus Compounds. 29.+ Low-Resolution Microwave, Infrared, and Raman Spectra, Conformational Stabillty, Vibrational Assignment, and Normal Coordinate Calculations for (Chloromethy1)phosphonothioic Difluorlde B. J. van der Veken, P. Coppens,$ Rijksuniuersitair Centrum Antwerpen, 171 Groenenborgerlaan, Antwerp 2020, Belgium
R. D. Johnson,$ and J. R. Durig* Department of Chemistry, University of South Carolina, Columbia, South Carolina 29208 (Received: February 5, 1986) The low-resolution microwave spectrum of (chloromethy1)phosphonothioicdifluoride (CICH2P(S)F2)has been investigated from 26.5 to 39 GHz. From the spacing of the central transitions for the near-prolate top, it is shown that the values of 2224 MHz, for B + C, for the 35Clisotope and 2167 MHz for the 37CIisotope are consistent with the conformer that has the CCI bond trans to the PS bond. The infrared (3500-40 cm-I) and Raman (3500-20 cm-I) spectra of the gas and solid have been recorded. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values have been obtained. Both the trans and gauche conformers have been identified in the vibrational spectra of the fluid phases. From a temperature study of the Raman spectrum of the liquid phase, the enthalpy difference between the trans and gauche conformers was determined to be 149 f 31 cm-' (426 f 89 cal/mol), with the trans conformer being thermodynamically preferred. Band contour simulation of the infrared gas-phase bands also shows that the trans conformer is dominant in this phase. Upon crystallization, only the trans conformer remains in the solid state. The asymmetric torsion for the trans conformer was observed as a series of closely spaced Q branches, beginning at 80.1 1 cm-I and falling to lower frequency, with the corresponding transitions for the gauche conformer beginning at 69.31 cm-I. These transitions have been used to obtain the potential constants for the asymmetric torsion and an enthalpy difference of 206 h 24 cm-' (584 f 69 cal/mol), with the trans conformer more stable. All of the normal modes have been assigned based on infrared band contours, depolarization values, and group frequencies. A normal coordinate calculation has been carried out, utilizing a modified valence force field, to calculate the frequencies and the potential energy distribution. These results are compared to the correspondingquantities of some similar compounds.
Introduction
For some time we have been investigating the conformational stability of organophosphorus molecules.'-I0 In these studies, it was shown that the "gauche effect"," which predicts that the conformer with the largest number of interactions of nonbonded electron pairs or polar bonds will be more stable, often fails to predict the correct relative conformational stability. From a recent studylo of CICH2P(0)F2 it was clear that some of the conformational results obtained in earlier studies on this and related molecules are in error. In particular, Steger and Kuntze,12 using variable-solvent studies, concluded that the gauche conformers of ClCH2P(0)F2 and CICH2P(S)F2 are the more stable conformers in the liquid phase, whereas we foundlo that for ClCH2P(0)F2 the conformation that has the CCl bond trans to the PO bond is the more stable form in the fluid phases. In order to further clarify these reported discrepancies and to obtain more conformational data on this series of organophosphorus compounds, we have recorded the microwave, infrared, and Raman spectra of ClCH2P(S)F2and compared the data among the three physical states. The results of this study are reported herein. '
spectrometer with a Stark modulation frequency of 33.33 kHz. The Raman spectra (Figure 2) from 3200 to 10 cm-' were recorded with a Cary Model 82 spectrophotometer equipped with a Spectra-Physics Model 171 argon ion laser operating on the 5145-A line. The laser power at the sample was varied from 0.1 to 2 W, depending on the physical state under investigation. The spectrum of the vapor was recorded with a standard Cary multipass accessory. The spectrum of the liquid was obtained with the sample sealed in a glass capillary. Depolarization measurements were made with the standard Cary accessories. Variable-temperature data and the spectrum of the annealed solid were collected with a sealed capillary in a CTI-Cryogenics Spectrim cryostat with a Lake Shore Cryotronics Model DTC-500 temperature controller. Measured frequencies are expected to be accurate to f2 cm-l. The mid-infrared spectrum of gaseous ClCH2P(S)F, (Figure 3) from 3200 to 400 cm-' was obtained with a Digilab Model FTS-14C Fourier transform interferometer, equipped with a Ge/KBr beam splitter, a nichrome wire source element, and a TGS detector. The spectrum was recorded at ambient vapor
Experimental Section
The sample of (chloromethy1)phosphonothioic difluoride was prepared from the corresponding dichloride (Alfa, Morton Thiokol, Inc.) by direct reaction with freshly sublimed antimony trifluoride. Sample purification was performed with a low-temperature, low-pressure fractionating column. The microwave spectrum of CICH,P(S)F, (Figure 1) was investigated in the frequency region 26.5-39 G H z by using a Hewlett-Packard Model 8460A M R R For part XXVIII, see Spectrochim. Acta, Part A 1986, 42, 123. 'Taken in part from the thesis of P. Coppens, which will be submitted to the Department of Chemistry of the Rijksuniversitair Centrum Antwerpen in artial fulfillment of the Ph.D. degree. !Taken in part from the thesis of R. D. Johnson, which will be submitted to the Department of Chemistry of the University of South Carolina in partial fulfillment of the Ph.D. degree.
0022-3654/86/2090-4537$01.50/0
Durig, J. R.; Cox, A. W., Jr. J . Chem. Phys. 1975, 63, 2303. Durig, J. R.; Cox, A. W . , Jr. J . Chem. Phys. 1976, 64, 1930. Durig, J. R.; Cox, A. W., Jr. J . Phys. Chem. 1976, 80, 2493. Durig, J. R.; Li, Y. S. J . Mol. Spectrosc. 1978, 70, 27. Durig, J. R.; Streusand, B. J. Appl, Spectrosc. 1980, 34, 65. (6) Odeurs, R. L.; van der Veken, B. J.; Herman, M. A,; Durig, J. R. J . Mol. Struct. 1984, 117, 235. (7) van der Veken, B. J.; Little, T. S.; Li, Y. S.; Harris, M. E.; Durig, J. R. Spectrochim. Acta, Part A 1986, 42, 123. (8) van der Veken, B. J.; Odeurs, R. L.; Herman, M. A.; Durig, J. R. Spectrochim. Acta, Par? A 1984, 40, 563. (9) Groner, P.; Church, J. S.; Li, Y. S.; Durig, J. R. J . Chem. Phys. 1985, 82, 3894. (10) van der Veken, B. J.; Coppens, P.; Johnson, R. D.; Durig. J. R. J . Chem. Phys. 1985,83, 1517. (11) Wolfe, S. Acc. Chem. Res. 1972, 5 , 102 (12) Steger, E.; Kuntze, M. Spectrochim. Acta, Part A 1967, 23A, 2189. (1) (2) (3) (4) (5)
0 1986 American Chemical Society
4538
The Journal of Physical Chemistry, Vol. 90, No. 19. 1986
r
I
I
,
I
36.0
34.0
I
38.0
van der Veken et al.
1
I
1
32.0
30.0
28.0
I
FREQUENCY (GHzl
Figure 1. Microwave spectrum of (chloromethyl)phosphonothioic difluoride from 26.5 to 39.0 GHz. , Y
,
A
3000
500
1000
1500
WAVENUMBER (cm-1)
I
Figure 3. Mid-infrared spectra of gaseous (A), amorphous solid (B), and annealed solid (C) (chloromethyl)phosphonothioic difluoride.
I
3600
1500
lo00 WAVENUMBER (CMS)
Figure 2. Raman spectra of gaseous (A), liquid (B), and solid (C) (chloromethyl)phosphonothioic difluoride.
pressure in a 10-cm cell fitted with CsI windows. The mid-infrared spectrum of the solid (Figure 3) was obtained with the same instrument, by condensing the sample onto a CsI plate held at 77 K, and annealed until no further change in the spectrum was observed. Far-infrared spectra (Figure 4) of gaseous and solid CICH2P(S)F, were recorded with a Digilab Fourier transform interferometer Model FTS-1 SB,purged with dry nitrogen, and equipped with a 6.25-pm Mylar beam splitter and a high-pressure Hg arc lamp source. The spectrum of the gas was recorded at ambient vapor pressure in a IO-cm cell fitted with polyethylene windows. Interferograms were recorded 2000 times with an effective resolution of 0.25 cm-', averaged, and transformed with a boxcar truncation function. The far-infrared spectrum of the vapor, from which the torsional transitions were obtained (Figure 5 ) , was collected on a Nicolet Model 200 SXV Fourier transform interferometer equipped with a vacuum bench, a liquid helium cooled Ge bolometer with a wedged sapphire filter and a polyethylene window, a high-pressure Hg arc source, and a 12.5-km Mylar beam splitter. The sample was contained in a Perkin-Elmer 1-m gas cell at ambient vapor pressure. Interferograms were recorded
500
400
300
200
100
WAVENUMBER bm-?
Figure 4. r-infrared spectra of gaseous (A) and annded solid (B) (chloromethyl)phosphonothioic difluoride. 256 times with an effective resolution of 0.12 cm-' and treated
as described earlier for the spectrum obtained from the Digilab interferometer. Microwave Spectrum
The low-resolution microwave spectrum of ClCH,P(S)F, in the 26.5-39-GHz region is shown in Figure 1. Over this region
The Journal of Physical Chemistry, Vol. 90, No. 19, 1986 4539
(Chloromethy1)phosphonothioic Difluoride
TABLE II: Initially Assumed Structural Parameters for Trans and Gauche CICHzP(S)Fz and a Comparison between the Calculated and Observed Rotational Constants Parameters
bond
r(C-CI) r(C-P) r(P=S)
r(C-H) r(P-F)
length. 8, 1.77 1.82 1.88 1.09 1.55
I
80 75 WAVENUMBER (cm’)
85
70
Figure 5. Far-infrared spectrum of gaseous (chloromethy1)phosphono-
rotational constants 3SCICH2P(S)F2
A
thioic difluoride.
B
TABLE I: Rotational Transitions (MHz) of 3sCICHzP(S)Fz and 3’CICH2P(S)F2 35C1isotope 37C1isotope J’- J” u(obsd) B + C J’+ J” u(obsd) B + d
K
-
1213 1415 1617-
-
11 12 13 14 15 16
26690 28910 31 150 33350 35590 37800
C
2224 2224 2225 2223 2224 2224
-
I3 1415 1617-
B+C
37C1CH2P(S)F2
C K
12 13 14 15 16
28180 30340 32510 34670 36850
B+C
2168 2167 2167 2167 2168
“ B + C (average) = 2224 MHz. b B + C (average) = 2167 MHz.
two series of equally spaced transitions are found, with the first series approximately 3 times as intense as the second. The position of the major transitions and their separation, for both series, are listed in Table I. It is believed that these major transitions are due to a-type transitions for an approximately prolate rotor, where the spacing between the rotational bands is expected to be B C. Keeping in mind the relative intensities of both series of transitions, one must conclude that the most intense series is due to the rotation of 35ClCH2P(S)F2,while the second series arises from 37ClCH2P(S)F,. In order to identify the conformers from these major transitions, a complete set of structural parameters was assumed for both the trans and gauche conformers (Table 11). The structural parameters for the ClCH2P moiety were obtainedi0 from ClCH2P(O)F,, while the P(S)F2 parameters were takenI3 from CH3P(S)F2. With this assumed structure, the rotational constants for both isotopic species and both conformers were calculated (Table 11). It is clear, from a comparison of the calculated B + Cvalues with the observed values obtained from the position of the main bands, that the trans conformer must give rise to the equally spaced a-type transitions. Therefore, the low-resolution microwave spectrum indicates that the trans conformer is dominant in the gas phase. Attempts to assign the individual Ki = 0 and 1 lines were unsuccessful, apparently because of the large number of excited-state transitions associated with the asymmetric torsion as well as possible transitions arising from the gauche conformer. For each central transition there were several candidates, but no unique set could be found that gave a satisfactory frequency fit to the rigid rotor. Therefore, further attempts to assign the individual lines were abandoned.
+
Vibrational Assignment
Although there has been a previous vibrational assignment’, of ClCH,P(S)F, the infrared data were collected at prism resolution above 400 cm-’ and only data for the liquid phase were reported. The infrared spectrum of the gas has not been previously reported, neither have the Raman spectra of the gas and solid. (13) Durig, J. R.; Meadows, J. A.; Li, Y.S.; Stanley, A. E. Inorg. Ckem. 1983, 22, 41 34.
A B
anale fClCP LCPS
LHCCl f CPF LHCH LFPF LClCPS
anale. dee 113.0 116.0 110.0 101.0 110.0 99.0 0 trans 120 gauche
trans,
gauche,
obsd,
MHz 3866 1138 1085 -0.962 2223 3971 1113 1066 -0.967 2179
MHz 261 2 1391 1136 -0.653 2527 2659 1366 1112 -0.671 2478
MHz
2224
2167
Therefore, with more complete vibrational data available, a more definitive assignment should be possible. The gauche conformer, having trivial C1symmetry, will give rise to A / B / C hybrid infrared gas-phase band contours and polarized Raman lines. For the trans conformer, which has C,symmetry, the 11 A’ fundamentals are expected to show A / B hybrid band contours and polarized Raman lines, whereas the 7 A” modes give rise to C-type infrared band contours and depolarized Raman lines. Except for the symmetric CH, stretch, which appears as a doublet, the assignment of the CH, modes is straightforward. The results are listed in Table 111. The fact that the CH, symmetric stretch is a doublet in all three phases, as well as in both the Raman and infrared spectra, leaves, as the only possible explanation, a Fermi interaction of the CH2 symmetric stretch with a ternary combination of fundamentals (the highest overtone possible is calculated at 2 X 1400 cm-I). The combination of the C H I deformation, the PC stretch, and the CCl stretch leads to a calculated frequency of 2967 cm-’, while the Fermi doublet is found at 2970 and 2957 cm-’. The assignment of the PF2 stretching modes is rather difficult even if one considers the Raman spectrum of the gas, where the sharp Q branch at 938 cm-I can be assigned to the symmetric PF2stretch. In the infrared spectrum of the vapor phase this fundamental is found as a very strong broad absorption also centered at 938 cm-’. Although it is probable that this broad contour contains the absorption of the other conformer as well, at a slightly shifted wavenumber, no conformer bands for this vibration could be confidently assigned. The antisymmetric PF2 stretch is tentatively assigned to the broad band at 924 cm-’ in the Raman spectrum of the gas, whereas in the infrared spectrum two C-type Q branches at 916 and 898 cm-’ were observed, probably due to the absorption of both conformers. It is obvious that there is a significant wavenumber shift with condensation and, therefore, it is not clear which one of the two bands remains in the solid. Therefore, the band at 916 cm-I has been rather arbitrarily chosen as the one that remains in the solid. Although strong vibrational coupling is anticipated between the PS, PC, and CCI stretching vibrations, because of the similarity of the C1, S and P masses, absorptions at characteristic frequencies for each of these normal modes are found. The strong Raman conformational doublet at 662 and 625 cm-’ is confidently assigned to the PS stretching mode, but some uncertainty exists about the PC and CCI stretches. In CICH2P(0)F2,an 803-cm-’ band was assignedI0 as the CCl stretch and a 706-cm-’ band as the PC stretch, based on group frequencies, Raman line intensities, conformational splitting (the PC stretch is expected to show the largest conformational shift), and normal coordinate calculations.
4540 The Journal of Physical Chemistry, Vol. 90, No. 19, I986
van der Veken et al.
TABLE HI: Observed"VbInfrared and Raman Freauencies (cm-'A Assienment, and Potential Energy DistributionCfor CICH,P(S)F, infrared re1 int
gas 3014 R 3009Q, C 3005 P 2970 2960 Q 2954 Q 2950 Q
w
Raman re1 int
solid 2996 s
gas
re1 int
liq
re1 int
re1 int and depol solid
assignt and PED calcd
3010 bd, w
2995 m, bd, dp
2995 m
3032 u 1 2 CHI antisymm str (100%)
2970 m
2957 w, sh, p
2951 m
Fermi doublet
2957 s
2940 s, p
2935 s
2955 u1
CHI symm str (100%)
1387 s 1383 m
1400 bd, vw
1393 w, dp
1386 m 1383 w
1417 u2
CHI deformn (89%), CH, wag (6%)
1225 w 1220 m
1224 vw
1227 w, p
1222 m
1221 u3
CH, wag (61%), PC str (16%), CH, deformn (12%), CCI str (9%)
1125 m 1121 w
1124 vw
1127 w, dp(?)
1125 w 1120 w
1146 u I 3 CH, twist (99%)
925 s
938 w
914 vs, bd 895 s
924 vw, bd -894
m, sh 2954 m
m m m
1406 R 1403 Q S 1401 Q S 1398 P 1230 R l226Q, A / C m 1224 P 1221 1132 R 1130 Q 1128 Q, C 1126 P 942 sh 938 Q s, bd 932 max 916 Q
S
898 Q
S
2936 2930 2921 2853 2848
s w, sh
w w w
925 vw, p
948 w
S
VW,dp(?)
912 vw
PF, symm str (5S%), PS str (2S%), PF2 deformn (6%), CPS bend (6%) PF, symm str (gauche) 927 uI4 PF, antisymm str (88%), CH2 rock (5%)
933 u4
PF2 antisymm str (gauche) UT uIO 900
+
887 w 863 vw, bd 838
808 R 804 Q 802 Q 797 P 793 Q 754 730 R 726 Q, A 722 P 670 R 665 Q, A 660 P 626 C 420 R 416 Q, A 412 P 390 R 386 Q, A / C 381 P 374 R 371 Q, A / C 369 P 319Q, C 317 Q, C 297 Q, C
840 m, sh, p 831 s, p
CCI str (gauche) CCI str (16%), PC str (36%), P F symm str (24%), PF, wag (lo%), PS str (10%)
S
(844) m 831 m 826 vs
w
(803) vw
801 vw, p
(763)
s
792 w (747) m
793 vw 792 vw, dp 747 vw, dp(?) (743) m
759 ul5 CH2 rock (91%), PF2 antisymm str (7%) (743) PC str (gauche)
S
716 vs
835 s
833 vs 824 vs
831 us
CHI rock (gauche)
W
W
m
659 s 655 m w. bd (623) w
719 w, p 662 vs
660 vs, p
625 s
624 vs, p
410 s 407 s
412 m
411 w, p
s
384 s
385 m
s
(368) m
m
(319) w
vs
318 w
385 m, p
718 w 660 vs 656 w, sh (624) vs 412 vw 405 m 388 s 382 w
728
u6
655 u7
PC str (14%), CCI str (SO%), PS str (20%), CPS bend (7%)
P=S str (27%), PF, symm str (40%), PC str (21%) P=S str (gauche)
(635) 41 1 us
PF, deformn (68%), PF2 wag (20%)
383 u9
PF, wag (45%), PF, deformn (27%), PS str (18%)
368 w, p
(370)
(376)
PF, wag (gauche)
318 w, p
(322)
(303)
PF, twist (gauche)
m
m
300 w
298 w, dp
300 w 296 m
299 uI6 PF, twist (57%), PFI rock (36%)
258 w, p
262 w 257 w
247
240 235 151 141
240 u I o CPS bend (71%), PF, wag (9%)
296 w
256 R 250 Q, A / C m 240 P 238 R 234Q, A I C m 230 P 131 B,ctr w
265 m 256 m 242 238 155 146
w
230 s
234 s, p
w m
130 vw
141 w, p
w
123 vw 85 R
approx descripn
ui
s w, sh w w
u,,
PF, rock (54%), PF2 twist (44%)
133 u I 1 ClCP bend (83%). CPS bend (7%) 2uI8 (gauche)
The Journal of Physical Chemistry, Vol. 90, No. 19, 1986 4541
(Chloromethy1)phosphonothioicDifluoride TABLE 111 (Continued)
gas 80Q 69 Q
infrared re1 int solid m 117
re1 int
69
w
liq
Raman re1 int and depol
solid
re1 int
calcd
s
w 70
gas
re1 int
vi v18
w
76
w
69 66 60 48 38 32 24
m m m
assignt and PED approx descripn asymm torsion asymm torsion (gauche)
lattice modes
s
m s
m
"Abbreviations used: s, strong; m, medium; w, weak; v, very; sh, shoulder; bd, broad; ctr, center; P, Q and R refer to infrared gas-phase band contours. *Frequenciesin parentheses indicate bands that disappeared on annealing, and the corresponding calculated frequencies for the gauche conformer were obtained with the determined force constants for the trans conformer but with the appropriate G matrix elements for the gauche conformer. cContributionsof less than 5% are not included. In ClCH2P(S)F2 a unique assignment for the PC and CC1 stretches is more difficult. The Raman line intensity and normal coordinate calculations indicate that the band at 835 cm-' should be assigned as the PC stretch, while the conformational splitting and group frequencies indicate that the PC stretch should be assigned at 726 cm-'. It is believed that the potential energy distribution (PED) resulting from the normal coordinate calculation, and listed in Table 111, gives a reasonable description of the normal modes. With the same order of PF2 deformational modes13 as in CH3P(S)F2, these modes are assigned as listed in Table 111. The corresponding bands for the gauche conformer were observed for both the PF2 wag and PF2 twist. Additionally, the weak line observed at 258 cm-' in the Raman spectrum of the liquid, which remains in the solid, has been assigned to the PF2 rock but its polarized nature indicates that it must arise from both the trans and gauche conformers. The CPS bend found13 at 234 cm-l in CH3P(S)F2is assigned to the band a t 234 cm-' in the infrared spectrum of gaseous ClCH2P(S)F2,and the ClCP bend, foundlo at 145 cm-' in ClCH2P(0)F2,is assigned to the band at 131 cm-'. The only remaining fundamental is the asymmetric torsion, which is confidently assigned to the moderately strong infrared absorption in the vapor at 80 cm-'. The complete vibrational assignment is given in Table 111, and it is clear from both the infrared and Raman data that two conformers are present in the fluid phases a t ambient temperature. Therefore, in order to determine the relative stabilities of the two conformers, a variable-temperature study of the Raman spectrum was carried out, but it was first necessary to identify which bands belong to the trans and gauche conformers.
TABLE I V Directional Cosines for a p / a Q in the Principal Axis System of ClCH2P(S)F2
v,(CH,)
gauche trans
cos ff
cos 6
cos y
0.126 0.000
0.806 0.000
0.578
1.000
Conformational Analysis and Stability
Band Contour Analysis. The utility of infrared band contours for conformational analysis has been clearly demonstrated previously.68 As stated in our earlier studylo on ClCH2P(0)F2,the fundamentals used in the simulation must be relatively pure and the calculated contours must differ sufficiently to allow for conclusive conformational results. For the ClCH,P(0)F2 band contour simulation welo used the antisymmetric CH2 stretch and the PO stretch. As can be seen from Table 111, the PS stretching mode is strongly coupled, and therefore only the CH2 antisymmetric stretch can be used in the simulation for the thio compound. The gas-phase pure type A , B, and C infrared contours of both conformers were calculated with the rotational constants listed in Table 11. All pure type contours were calculated to a maximum J level of 120, in the rigid-rotor approximation with a Gaussian slit width of 1 cm-'. With the assumption that the direction of &.t/13Q for the v,,(CH2) fundamental is in the CHI plane perpendicular to the bisector of the CH2angle, the directional cosines for in the principal axis system were calculated for the trans and gauche conformers (Table IV). These directional cosines were used to obtain the hybrid contours for the corresponding fundamentals as described1° earlier. Additional convolutions, with
Figure 6. Comparison of experimental with simulated profiles for vas(CH,) stretch: (A) simulation using the trans conformer profile, convolved with a Gaussian slit of 2.1 cm-'; (B) experimental profile; (C)
simulation using the gauche conformer profile, convolved with a Gaussian slit of 1.3 cm-'. Gaussian functions, of the theoretical hybrids were necessary to account for thermal effects. The reproduction of the full width at half-height of the Q branches was used as the criterion for the slit width to tye applied in these additional convolutions. The trans u,(CH2) fundamental is expected to be nearly pure C type, while the v,(CH,) hybrid for the gauche conformer shows an appreciable amount of B type character (Table IV). First, the experimental contour was simulated with the calculated trans or gauche contour. The results are shown in Figure 6, and as can be seen, the trans
4542
The Journal of Physical Chemistry, Vol. 90, No. 19, 1986
TABLE V Temperature and Intensity Ratios (660/624 cm-' Lines) for the Conformational Study of Liquid (Chloromethvl)phosDhonotbioicDifluoride T, "C 1000(1/T) K-l K = IJIE In K 21.0 2.0 -13.0 -33.0 -43.0 -58.0 -73.0 -78.0 -83.0 -88.0 -90.5 -93.0 -98.0 -100.5 -103.0 -108.0 -113.0 -1 18.0
3.40 3.63 3.84 4.16 4.35 4.65 5.00 5.12 5.26 5.40 5.48 5.55 5.71 5.79 5.88 6.06 6.24 6.45
1.72 1.75 1.88 2.02 2.13 2.26 2.44 2.45 2.60 2.70 2.75 2.65 2.79 2.80 2.96 2.93 3.10 3.38
0.542 0.560 0.63 1 0.703 0.756 0.8 15 0.892 0.896 0.956 0.993 0.101 0.975 0.103 0.103 0.109 0.108 0.113 0.122
hybrid gives a much better fit than the gauche. Then, with the realization that the v,,(CH2) fundamental for both conformers are absorbing at the same frequency and that there is a fair amount of the gauche conformer present in the gas phase (compare the relative intensities of the conformer doublets), optimization of the relative amounts of the trans and gauche conformers was attempted. The best fit (Figure 7) is obtained for an 80% trans and 20% gauche mixture. Although these percentages do not give the exact abundance of both conformers because the extinction coefficients for the antisymmetric CH2 stretch for both conformers are expected to be slightly different, they should give a good estimate of the relative amount of each conformer. Further support for this is found in the fact that the 80/20 trans/gauche ratio compares well to the relative intensities of the conformational doublets found in the infrared spectrum of the gas. Taking into account the statistical dominance of the gauche conformer and the experimental result that the trans is considerably more dominant than the gauche conformer, one must conclude that the trans form is the more stable conformer in the gas phase. AH of the Liquid. In order to determine the enthalpy difference between the trans and gauche conformers, a variable-temperature Raman study of the liquid phase was undertaken with an experimental technique similar to that described by Miller and Harney.14 As can be seen from the Raman spectrum (Figure 2), the lines assigned to the PS stretch of the trans (660 cm-]) and gauche (624 cm-I) conformers show significant change upon solidification. Since these lines are very strong and well separated from each other and from other lines, this set of lines was chosen for the temperature study. Eighteen sets of spectral data were collected at eighteen temperatures ranging from +21 to -1 18 OC (Table V). With the equation In K = -(AH/RT) + (AS/R), the enthalpy change, AH, was evaluated by plotting In K vs. 1/ T, which gives A H I R as the slope of the line. For this equation K is the ratio Itram/Zgauche, where Z is the intensity of the corresponding trans or gauche Raman line, respectively, and it is assumed that AH is not a function of temperature. A value of 149 f 31 cm-' (426 f 89 cal/mol) was obtained for AH, based on the slope of the line. This result clearly indicates that the trans conformer is the more stable form in the liquid state. Normal Coordinate Calculations. With the structural parameters listed in Table 11, a normal coordinate calculation was carried out on the more stable trans conformer. The resulting force field for the trans conformers was then used to predict the frequencies for the gauche conformer. Twenty internal coordinates were used to construct the symmetry coordinates listed in Table VI. The perturbation program developed by Schachts~hneider'~ was used to adjust the force constants, thereby obtaining the best (14) Miller, F. A,; Harney, B. M. Appl. Spectrosc. 1970, 24, 291. (15) Schachtschneider, J. H. Technical Report No. 231 and 57. Shell Development Co., Emeryville, CA, 1964 and 1965.
van der Veken et al. TABLE VI: Symmetry Coordinates Used in the Normal Coordinate Analysis of ClCH2P(S)F2 species (CJ
symmetry coord'sb
descripn
A'
CH, symm str CH2 deformn CH, wag PF, symm str C-P str C-CI str PS str PF2 deformn PF, wag CPS bend ClCP bend CH, redund PF, redund CH, antisymm str CH, twist PF2 antisymm str CH, rock PF, twist PF2 rock asymm torsion
A"
'Not normalized. bThe utilized symbols are defined as follows: LHCH = y;LClCP = A; LHCCI = 6; LHCP = f ; LFPF = n; LCPS = 6; LCPF = 7; LFPS = u; other symbols are self-explanatory.
TABLE VII: Valence Force Constants for CICH2P(S)F2 force const descripn value," mdyn/k KR C-C1 str 3.30 C-P str 3.63 KQ P=S str 5.05 Ks KT C-H str 4.89 Kx P-F str 5.18 HA
Hd H7
Ht H6
H, H, H, FQS
Fxs Fxx
FQt F€t
ClCP bend CPS bend HCH bend HCP bend HCCl bend FPF bend CPF bend FPS bend PC str/PS str PF str/PS str P F str/PF str PC str/HCP bend HCP bend/HCP bend CPF bend/CPF bend
0.53 1.30 0.5 1 0.58 0.60 1.57 1.42 0.66 0.47 0.34 -0.17 -0.03 -0.1 1 0.32
Fnn "The bending coordinates are weighed by 1 A.
TABLE VIII: Observed Frequencies (cm-I) and Proposed Assignments for the Asymmetric Torsional Transitions of Gaseous CICH,P(S)F, and the Potential Constants (cm-I) for the Asymmetric Torsion ~~~~
obsd, cm-I 80.11 78.29 76.62 74.58 69.31 66.77
re1 int" soecies transn vs trans 1 0 s trans 2 1 m trans 3 2 w trans 4 3 w gauche l r t - O f vw gauche 2f lrt
---
~
~
~~
calcd,b calcd A cm-' re1 int obsd - calcd 80.17 0.19 -0.06 78.34 0.19 -0.05 76.49 0.19 0.13 74.62 0.17 -0.03 69.15 0.15 0.16 66.92 0.21 -0.15
nAbbreviations used: w, weak; m, medium; s, strong; v, bCalculated with the potential constants from Table IX and 0.556 121, Fl = -0,024317, F2 0.025833, F3 = -0.002100, 0.001 703, F, = -0.000160, F6 = 0.000113, F7 = -0.000012, 0.000007.
very. FO = F4 = Fg =
fit for the frequencies and the potential energy distribution. The calculated vibrational frequencies of the trans conformer are listed
The Journal of Physical Chemistry, Vol. 90, No. 19, 1986 4543
(Chloromethy1)phosphonothioic Difluoride
TABLE I X Potential Constants (cm-') for the Asymmetric Torsion of (Chloromethy1)phosphonothioic Difluoride
coeff
value, cm-I
trans/gauche barrier gauche/gauche barrier
-167 479 947 49 206 1280 571
VI v2 v3
v6
AH
I SOM
I
I
M30
SO10
I 5010
PIS"-'
Figure 7. Comparison of experimental with simulated profiles for v,(CH,) using different trans-gauche ratios; (A) 100%trans, 0% gauche; (B) 90% trans, 10% gauche; (C) 80% trans, 20% gauche; (D) 60% trans, 40% gauche; (E) 40% trans, 60% gauche; (F) 0% trans, 100% gauche.
The calculated curves are the smooth ones. in Table 111, along with the associated potential energy distribution. The force field containing 13 diagonal and 6 interaction force constants is listed in Table VII. The observed frequencies were reproduced with an average error of less than 1%. The initial force field was taken1°J3 from ClCH2P(0)F2 and CH3P(S)F2. In the final force field, the force constants are in reasonable agreement with those found for the above-mentioned molecules. The potential energy distribution indicates that extensive mixing exists between the PS stretch and symmetric PF2 stretch, while the PF stretch mixes with the CC1 stretch, the PS stretch, and the CPS bend. The CCl stretch mixes with the PC stretch, the PF2 symmetric stretch, the PF2 wag, and the PS stretch. All other modes are found to be reasonably pure. The force field determined for the trans conformer was then used, together with the G matrix for the gauche conformer, to calculate the gauche frequencies. In all cases where a conformer pair was observed, the direction of the frequency shift from trans to gauche is correctly predicted.
Asymmetric Torsion Unfortunately, as in our studylo of ClCH2P(0)F2,the Raman spectrum of gaseous ClCH2P(S)F2exhibited no lines due to the asymmetric torsional transitions. Therefore, all of the torsional data were obtained from the far-infrared spectrum (Figure 5) recorded at 0.12-cm-' resolution between 50 and 85 cm-'. Six Q branches are readily apparent in the far-infrared spectrum of the gas. The most pronounced Q branch is found at 80.1 1 cm-I, which is assigned to the 1 0 transition of the trans conformer. The torsional fundamental for the gauche conformer is assigned a t 69.31 cm-I. This assignment was made because the spacing between the fourth and fifth Q branch is significantly larger than that between the first four Q branches. The potential function for the asymmetric torsion was calculated with the torsional transitions listed in Table VIII. With the structural parameters from Table 11, the reduced rotational constant was calculated as a function of the internal rotation angle
-
a F(a) = Fo + CFi cos ia i
where a is defined as zero for the trans conformer. The potential function used to fit the observed frequencies varied in a as 1
V ( a )= - z K ( l -cos ia)
-
dispersion IO 4 10 3 24
Q branch was assigned to the 1 0 transition (80.1 1 cm-I for the trans and 69.31 cm-I for the gauche conformer). This calculation then resulted in reasonable VI, V,, and V3terms. At this point, the three additional transitions for the trans and the second transition for the gauche conformer were fit with the initial VI, V2,and V, and additional V,, V,, and V, terms, while the AH was allowed to vary, until the best fit was obtained. For the final calculations, it was found that the V, and V, terms were insignificant, and the value of the other four coefficients and their relative dispersions are listed in Table IX. This potential function shows minima at a = Oo and a = f126O for the trans and gauche potential wells, respectively. A barrier of 1280 cm-I separates the trans from the gauche conformer, while the two equivalent gauche wells are separated by a 571 cm-' barrier. The final AH between the gauche and trans conformers was calculated to be 206 f 24 cm-' (589 f 69 cal/mol). This value of AH is consistent with the value obtained from the Raman spectrum of the liquid since the relative intensities of the P=S stretch indicate a greater relative proportion of the gauche conformer in the liquid state compared to the gas, which suggests a smaller value for the AH value in the liquid compared to the value in the gas.
Discussion The microwave spectrum indicates a predominance of one of the conformers, and the observed value for B + C is more consistent with the trans conformer than the gauche conformer. The Raman and infrared spectra also indicate the predominance of one conformer in the gas phase and; from an analysis of the infrared band contours and the asymmetric potential function, it is concluded that the trans conformer is more stable in this physical state. The value of AH, obtained from the temperature study of the Raman spectrum of the liquid, indicates that the trans conformer is also the more stable form in this phase, and the trans conformer is the only form present in the solid state. Additional support for the stability of the trans conformation was provided from the normal coordinate analysis. Therefore, from these results, we have concluded that the earlier study,12 in which it was determined that the gauche conformer is more stable, is in error. With utilization of C N D 0 / 2 calculations, the total dipole for the gauche conformer was found to have a value of 3.3 D, whereas for the trans conformer the value is 2.2 D. The earlier assessmentI2 of the conformer stability was supposedly based on variable-solvent studies and, based on the calculated dipole moments, this method should have been adequate for distinguishing the conformer stability. However, the description of the experiments conducted in the earlier study12 is so sparse that it is not clear which bands were used for the analysis or whether their relative intensities were compared among the various solvents. Nevertheless, it is clear from the comparison of the Raman spectra of the gas and liquid that the amount of the gauche conformer increases with liquification, which is consistent with the larger dipole moment for this conformer. There have been electron diffraction inve~tigations'~J' of both (chloromethy1)phosphonic dichloride (ClCH2P(0)C12) and (chloromethy1)phosphonothioic dichloride (ClCH2P(S)C12),and in each study it was concluded that the trans and gauche con-
2 i
For the initial calculations, the energy difference between the lowest trans and gauche energy levels was taken as 149 cm-' from the temperature study of the Raman liquid, and the most intense
(16) Khaikin, L. S.;Vilkov, L. V.; Vasil'ev, A. F.; Mel'nikov, N. N.; Tulyakova, T. F.; Anashkin, M. G. Dokl. Akad. Nauk. SSSR 1972,203, 1090. (17) Vajda, E.; Kolonits, M.; Hargittai, I.; Szoke, S. J . Mol. Szruct. 1976, 35,235.
J. Phys. Chem. 1986, 90, 4544-4545
4544
formers are in about equal abundance at ambient temperature. Because of the larger statistical weight for the gauche conformer it was concluded that the trans conformer is the more stable form for both of these molecules. Therefore, on the basis of our earlier studyl o of (chloromethy1)phosphonic difluoride (C1CH2P(0)F2), along with the current study of C1CH2P(S)F,it can be concluded that all four of these molecules have the trans conformer as the stable form in the fluid phases. Utilizing the “gauche effect”” and CNDO/2 calculations, which show the P=S bond to be more polar than the P-F bonds, one would predict the gauche conformation to be more stable than the trans conformation since the former has more interactions of polar bonds. Therefore, it appears that the “gauche effect” cannot be utilized to predict the conformer stability of these alkylphosphorus(V) molecules. It is not possible to rationalize the stability of the trans conformer of ClCH2P(S)F2on the basis of steric hindrance as was done for ethyldimethylphosphine’8 compared to ethylphosphine] since the nonbonded distances for the chlorine to fluorine atoms is estimated from the assumed structural parameters to be 3.12 A, which is slightly less than the sum of the van der Waal’s radii19 of 3.15 A for these two atoms. For the gauche conformer the chlorine to fluorine nonbonded distance is 3.33 A and the chlorine to sulfur atom distance is 3.73 A based on the estimated structural parameters, and both are larger than the sum of the van der Waal’s radii. Alternatively, the weak attractive potential between the halogen atoms with distances less than the sum of the van der Waal’s radii may be stronger than the electrostatic repulsions resulting from the negative charges on these atoms, thereby stabilizing the trans conformer. At this time it does not appear possible to predict the more stable conformer of these alkylphosphorus(V) molecules with current chemical first principles, but further studies on similar compounds with different substituents may provide some trends from which reliable predictions may be made. Although the dearth of potential function data available for asymmetric torsions of organophosphorus compounds makes the assessment of the reasonableness of the potential parameters and barriers difficult, they are expected to be similar to those foundlo for the CICH,P(O)F, compound, due to its similar structure and correspondingly similar far-infrared spectrum in the torsional region. The V3term, 947 f 10 cm-l, in ClCH2P(S)F, is expected to have a value similar to the bamer foundL3for the methyl torsion of CH3P(S)F,, 1340 f 10 cm-’. It is possible this difference lies with the experimental difficulties m e n t i ~ n e d in ’ ~ the barrier de(18) Durig, J. R.; Hizer, T. J. J . Raman Spectrosc. 1986, 17, 97. (19) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell University: Ithaca, IVY, 1960.
termination for the CH,P(S)F, molecule. The threefold barrier for CH,P(S)F, was based upon the observation of one very weak and broad transition at 243 cm-’ in the Raman spectrum of the gas phase, but two additional transitions were observed at 231 and 217 cm-I and it was pointed out that either of these bands could be the methyl torsion or “hot bands“ of the CPS bend at 236 cm-l. Therefore, we do not view the difference as indicating that the determined asymmetric potential function has unreasonable values. In fact, the potential terms have values very similar to the corresponding values1° for the asymmetric potential function for ClCH,P(0)F2. The assignment of the symmetric and antisymmetric P-F stretching modes reverses the order of these normal modes compared to the corresponding motions in (ch1oromethyl)phosphonic difluoride.I0 For this latter molecule the depolarization values from the Raman spectrum provided the necessary data for making the choice, whereas for the sulfur compound the choice is not as straightforward since the broad band at 894 cm-I is not clearly depolarized. Also the shift in frequency of the Raman lines between the liquid and solid phase makes it difficult to confidently follow the modes among the three phases. Therefore, the assignments in this spectral region must be considered as tentative. Additionally, it should be noted that almost every infrared absorption or Raman line is found to be a doublet in the crystalline solid. This fact, along with the numerous lattice modes observed in the far-infrared and Raman spectra of the solid, indicates that there are at least two molecules per primitive cell. Interestingly, the C N D 0 / 2 calculations predict the trans conformer to be more stable than the gauche conformer by 149 cm-’ (426 cal/mol), as experimentally found in the temperature study of the Raman spectrum of the liquid. It should be noted that, while C N D 0 / 2 calculations are reported to be poor for determining energy differences between conformations, we have recently foundI0J8*20 that, for the alkylphosphorus( V) compounds, CNDO/2 calculations not only predict the experimentally determined more stable conformer but also the approximate experimentally determined energy differences! Acknowledgment. We gratefully acknowledge the financial support of this study by the NATO Scientific Affairs Division through Grant No. RG 569/82. Also, we acknowledge the support of the National Science Foundation through Grant No. CHE83-1 1279. Registry NO. CICH2P(S)F, 1426-00-2;’’ClCH,P(S)F2, 103591-08-8; 37CICHzP(S)F,, 103591-09-9. (20) Johnson, R. D. Ph.D. Dissertation, University of South Carolina, Columbia, SC, 1986.
Approximate Rotational Band Shifts P. C. Engelking+ Joint Institute for Laboratory Astrophysics, National Bureau of Standards and University of Colorado, Boulder, Colorado 80309-0440 (Received: March 14, 1986)
Interpretation of spectroscopic experiments in which the rotational lines are not resolved often requires an expression for the shift of the center of the rotational band. Previous expressions are corrected and extended to cases of linear, symmetric, spherical, and asymmetric rotors, with typical accuracy of a fraction of the rotational B constant.
Introduction Rotational structure often is not resolved in spectroscopic experiments. From other information available, the rotational + JILA Visiting Fellow, 1985-1986. Permanent address: Department of Chemistry, University of Oregon, Eugene, OR 97403.
0022-3654/86/2090-4544$01.50/0
constants of the initial and final states may be known. Alternatively, the rotational constants may be believed to be within a range Of values. The experimenter often tries to use this information to estimate, in the first case, the magnitude of the shift of the measured band center from the origin and, in the second case, the possible range of the shift from the origin. Typical 0 1986 American Chemical Society
