Spectra and structure of organophosphorus compounds

Journal of Molecular Structure, 34 (1976) 9-20 OElsevier Scientific Publishing Company, Amsterdam -

Printed in The Netherlands

SPECTRA AND STRUCTURE OF ORGANOPHOSPHORUS COMPOUNDS XIII*. MICROWAVE, RAMAN AND INFRARED SPECTRA OF CH,POF,

J. R. DURIG, K. S. KALASINSKY**

and V. F. KALASINSKY

Department of Chemistry, University of South Carolina, Columbia, South Carolina 29208 (U.S.A.) (Received 16 May 1975)

ABSTRACT The microwave spectrum of methylphosphonic difluoride, CH,POF,, has been investigated in the region between 18.5 and 40.0 GHz. R-branch assignments have been made for the ground and two excited vibrational states for the a-type transitions. The rotational constants were found to be A = 4495.52 f 0.04, B = 4271.84 + 0.03 and C = 4125.93 -F0.03 MHz. The values of the dipole moment components were obtained from Stark splittings to be: I~,I = 3.4 f 0.2; 1~~1= 0.4 F 0.5; and 1~~1= 3.62 f 0.2 D. The Raman spectra of CH,POF, have been recorded in both the gas and solid phases and vibrational assignments are proposed for the fundamentals. The overtone and fundamental of the methyl torsion were observed in the Raman and IR spectra, respectively. The observed splitting from the microwave spectra of vibrationally excited states of the torsion led to the calculation of the barrier to internal rotation of 3.58 5 0.04 kcal mol”. This value is consistent with the torsional frequencies observed in the vibrational spectrum. This barrier value is compared to similar quantities in related molecules. INTRODUCI’ION

For some time we have been interested [l] in the substituent effects on the barriers to internal rotation of methyl rotors attached to atoms of the IVA and VA groups. In these studies it has been found that the chlorine atom has the most pronounced effect on the barriers of the Group IVA molecules and for the chloroethane series, the effect was nearly additive with the addition of each successive chlorine atom to one end of the molecule [Z, 33. However, for the corresponding fluoroethanes, the barriers remained nearly the same with successive fluorine addition [4]. In a recent microwave study [5] of methyldifluorophosphine, a barrier to internal rotation of 2.30 f 0.05 kcal mol-’ was calculated. This value is only slightly higher thanthe value of 1.96 f 0.01 kcal mol-’ found for methylphosphine 16) which may indicate that the fluorine addition has only a very small effect on the barriers in *For part WI, see J. Raman Spectrosc., 4 (1975) 121. **Taken in part from the thesis of K. S. Kalasinsky which will be submitted to the Department of Chemistry in partial fulfilment of-the Master of Science degree.

10

organophosphorus compounds. In order to further calculate the substituent effects on the barriers to methyl rotation in organophosphorus molecules we have investigated the microwave spectrum of CH,POF,. In this study, we hoped to evaluate the change in barrier height with the replacement of the non-bonded electron pair with a double-bonded oxygen atom. There has been no previous microwave study of this molecule. A complete vibrational assignment with the exception of the torsional mode has been presented [7] for CH3POF2 in the gaseous and liquid states. Also the far-IR spectrum of the polycrystalline material was examined at a temperature of -196 oC [S] . Only two bands at 294 and 240 cm-’ were observed between 350 and 200 cm-’ and the former one was assigned as the PF, deformational mode and the latter one to the methyl torsion. With a calculated F value of 5.61 cm-‘, the barrier to rotation of the methyl group was calculated to be 3.61 kcal mol-’ in the solid. In a series of recent papers [3,9, lo], we have shown that the torsional overtones may be observed in the Raman spectrum of the gas phase. Therefore we have also investigated the Raman spectra of gaseous and solid CHsPOF2 and the results are reported herein. EXPERIMENTAL

Methyl phosphonic difluoride was purified by low temperature fractionation on a vacuum sublimation column [ll] . A comparison of the Raman spectrum of the liquid with that of a previous publication [ 73 revealed no detectable impurities. Raman spectra were recorded on a Cary model 82 Raman spectrometer equipped with a Coherent Radiation model 53 argon-ion laser or a Spectra Physics model 171 argon-ion laser. The 5145-8 exciting line was used throughout and the power was estimated to be from 1.5 to 2.0 W at the sample. The spectrum of the gas was obtained with the sample contained in a standard Cary multipass cell with suitable modifications to allow the cell to be sealed 1121. The spectrum of the solid was obtained using a low temperature cell in which the sample holder is a solid brass plate at an angle of 15” from the normal. Samples were sublimed onto the brass plate held at - -196 oC and then annealed until the spectra showed no change. Typical spectra are shown in Fig. 1. A Perkin-Elmer model 621 grating spectrophotometer was used to record mid-IR spectra from 3200 to 200 cm-‘. The instrument was purged with nitrogen and calibrated as described in the literature [13]. Spectra were obtained using a cold cell equipped with CsI windows. Conventional vacuum sublimation techniques were used to deposit a solid film of sample on the CsI substrate held at liquid nitrogen temperature, except during the annealing process. Typical spectra are shown in Fig. 2, ,Microwavespectra were recorded in the R-band and K-band of a HewlettPackard model 84608 MRR spectrometer with a Stark cell modulation

11

Fig. 1. Raman spectra of CH,POF, phase, SBW = 2 cm-‘.

I,II

011

3006

2006

in (A) the gas phase, SBW = 5 cm-’ and (B) the solid

I ’ 1500

I

I

,

1000

1

I

500

WAVENUMBER (CM’) Fig. 2. IR spectrum of CH,POF,

in the solid phase.

frequency of 33.33 kHz. The frequencies were generaIIy measured with the sample held slightly above Dry Ice temperature (--70 “C) and are expected to be accurate to within 0.05 MHz. Typical spectra are shown in Fig. 3. VIBRATIONAL

ASSIGNMENTS

Methylphosphonic difluoride is assumed to belong to the C, point group. The normal vibrations span the irreducible representations 11A’ + 7A”. We expect all vibrations to be allowed in the IR. and Raman effects.

f

XI.0 38.0

36.0

34.0

32.0

30.0

28.0

26.4

GHz Fig. 3. Microwave spectrum of CH,POF, from 26.5 to 40.0 GHz_ The&&he& frequency motions in CH,POF, at 301’7 and 3012 cm-’ are assigned to the antisymmetric methyl stretches, vl and u12,respectively, which are split in the IR spectrum of the solid. The symmetric stretch is the strongest line in the Raman spectra and occurs at 2948 in the gas phase and 2934 cm-’ in the solid phase. The pair of antisymmetric deformations are split in the spectra of the solid with the A" motion assigned at 1433 cm‘-’ and the A' motion, at 1417 cm-‘, on the basis of intensities in the IR and Raman. The symmetric deformation, v5 is assigned to a polarized line at 1359 cm-* in the Raman spectrum of the gas shifts to 1337 cm-’ in the solid. The in-plane CH3 rock appears as a polarized Raman line at 925 cm-‘, which shifts to 930 cm-’ in the solid. The IR spectrum shows a coincident band at 928 cm-’ with the stronger out-of-plane motion at 946 cm-‘. The final methyl motion is the torsion and will be treated in a subsequent section. The assignmentof vibrations associated with the -PF, segment is facilitated b! comparison with those in F,PO [14]. The PSFZsymmetric stretch is assigned at 857 cm-’ in the Raman spectrum of the solid with the antisymmetric motion at 387 cm-‘. The PF2 bending modes are assigned in the order Vwag ’ ‘twist ’ vscissors’ These appear at 473,407, and 291 cm-‘. The remaining vibrations are concerned with the heavy atom skeleton. The C-P stretch comes at ‘759 cm-l in the Raman solid. In the IR spectra we see two bands at 761 and 757 cm-’ which we take as evidence of there being probably two molecules per unit cell in the crystalline solid. The P-U stretch is assigned to a polarized line of medium intensity in the Raman spectrum of the gas at 1370 cm-‘. The somewhat weaker CH3 symmetric deformation is to the low frequency side at 1359 cm-‘. In the spectrum of the solid two lines also appear in this region. The more intense P=O stretch has shifted to 1291 cm-’ while the methyl defo~ation has moved to 1337 cm-’ . We must assume that some effect in the crystalline solid has caused the 79-cm-” shift in the P=O stretch. The P=O in-plane and out-of-plane bends are the remaining skeletal motions and they occur at 424 and 291 cm-‘,

13

respectively, in the Raman spectrum of the solid. The observed vibrational frequencies and their assignments are listed in Table 1. MICROWAVE

RESUIaTS

The microwave spectra of CH,POFz were recorded in the frequency range 18-40 GHz. A portion of the spectrum showing the J = 3-t 4 transitions is shown in Fig. 3. The Spectra were assigned on the basis of calculated Stark shifts [15] and an initial assumed structure. The assignments of observed absorption lines are listed in Table 2 &long with rotational constants calculated by a least-squares fitting of these transitions. Additionally, there are, in Table 2, assignments and rotational constants for the u = 1 and u = 2 excited states of the torsion. Structure Although it is not possible to determine a complete structure for Cl&POF,, we have found a reasonable structure. The methyl top is assumed to have tetrahedral geometry with rfC--H) = 1.09 a. The C-P distance is taken to be 1.81 S, from the similar quantity in (CM,),l?O [163 and CH,PH,BH, 1171. The P=O and P-F distances are assumed to be 1.46 A 116, IS] and 1.52 a [18], respectively. The knowledge of three angles, LCPO, LCPF, and LFPF, fixes the structure of CH,POF, and these have been calculated to be 105.8 + l-O”, 101.4 + O-5”, and 115.2 + O-3”, respectively. These data indicate that the CPF and FPF angles open as the lone pair in CH,PF, is bonded to the oxygen [S] . The structural parameters reproduce the observed rotational constants to within 2 MHz. Dipole

moment

The dipole moment of CH,POF, has been determined from observed Stark shifts and calculated Stark coefficients [ 151. Field strengths were calibrated with either OCS or CH,CCH. For high field strengths the /Ml = 2 Stark component of the 3+2 transition of OCS (0.7152 D) 1191 at 36488.82 MHz was used. Lower values of the field strength were calibrated using the first-order Stark effect of the 24 1 transition of C!H&CH at 34183.37 MHz. The value of the dipole moment of CH&CN [ZO] was corrected (0.7840 D) for the currently accepted value of the OCS dipole moment [19]. The dipole moment components for CH3POF, have been calculated to bepl, = 3.44 + 0.20 D, cc, = 0.41+ 0.50 D, with p, = 3.62 + 0.20 D. BARRIER

TO INTERNAL

ROTATION

The barrier to internal rotation has been previously determined in the solid state to be 3.61 kcal mol-’ [S] . Thiswas calculated from a band

VW 1419

M

w

1430 -

1370 p

1359 p

1306

1333

VW 1419

2819

1430 -

W

W

p

p

dp

dp

-

dp dp p

W

w

W

W

1291

1337

s

M

M

M

1426 1417

M

W

VS VS vs

1433

VW 2823

M M s

3007 3007 2934

1335 1320 1310

1428 1381 1372 1359 1353 1347

1511 1501 1484 1436 1430 1428 1424 1414

3020 3020 2950

VW

W W W

Int.

1499

3018 3018 2941

cm-’

Infrared (liquid)b

Q

P

R

P

Center

Q

P,R

R

M 1309

M

S

s 1334

Center W 1417

P

Q

Center W 1417

Q

R

P

Q

R

3005 3005 2933

W vs

W

3024 3024 2948 2880 2830

dp dp p p p

Cm-’

ht.

Acm”

ACM-’

Int.

hem-’

Int.

Infrared (gas)b

Raman (solid)

Raman (liquid) b

“Raman (gas)

Vibrational spectra of methylphosphonic difluoridea

TABLE 1

S

s

W

W

w

w W w

Int.

1303

1328 1319 1314

1340 1335

1428 1425 1421 1415

1433

1519 1505

3017 3012 2939

cm-’

Infrared (solid)

S

S

S

S

S

S

W W W W

M

W W

M M M

Int.

+

VI0

’

“IS

U$P=O stretch

VI$

?

?

VI.’ + VI0

f

VI)

v, CH, symmetric deformation

vJ CH, antisymmetric deformation

“9 + VI4

v, 3 CH, antisymmetric deformation

2x vg

U, CH, antisymmetric stretch u, 1 CH, antisymmetric stretch u2 CH, symmetric stretch 2x lJ4 2 x VI1

Assignment

z

VW

S

W

W

W

858 -

750 p

470 p

457 p

413 dp

W

VW

p

880 -

925

413

486

684

700

751

856

884

923

dp

P

W

M

VVW

vvw

-

__

S

VW

VW

VW

P

P

dp

P

-

M

w

413

M

S

z

VW

vs

M

W

W

424

465

473

672 550 506

759

857

887

933

423 412 408 404 394

480 469 458

S

R Q Center M Q P

R Q P

W W

M M

M

R Q P R Q Q P

860 858 850 762 752 750 739 684 678

935 925 915 887 878 870

R Q M P, R Q M P R Q M P

W VVW VVW VVW

960 953

1269 1227 1205 1025

415

468

752

855

879

923

940

M

S

S

S

+

106

420

478

761 757

859

S

S

M w

S

M

880

stretch

rock

v,,

PF, deformation

u,, I- 122

u, o CPO bend (in-plane)

2 x 11,~or

vg PF, wag

v,o -I 122 v,~ t 106

VI0 -t VI1 ? ? ?

vI C-P stretch

v, PF, symmetric stretch

VIIt 122

v, 5 PF, antisymmetric

S

rock

v, ,, CH, antisymmetric

v7

?

lr6 -I- VII

895

S

VI0

us CH, symmetric

s

+

VI) + v9

VI

s

927

945

S

964

s

s

W

1282

W

VW -

287 dp

230 dp

287

413

dp

dp

M

W

ht.

291

407

M

W

ACM-’ ht.

Raman (solid)

-

cm”’

Infrared (gas)b Int,

M w W w

283

Ink

407 399 295

cm-’

Infrared (liquid)b

240 122= 106e

295

411 346 312

cm”’

Infrared (solid)

yl, CPO bend (gut-of-planed and (z+I PF, deformation ?)

Y1b PF, twist 11~~ + 106 3 x 106

VW v, d torsion Lattice mode W W Lattice mode

M

W W

8

Xnt,

Assignment

“Abbreviations used: S, M, W, V, p and dp denote strong, medium, weak, veryt polarized and depolarized, respectively~ P, Q and R refer to the branches of an individual band, bPrevious work, ref. 7; Cprevious work, ref. 8,

W

413 dp

Acm”’

hrt.

Raman (ii~uid)b

Acm-’

Raman (gas)

TABLE 1 (continued)

G -

24946.34

25018.87 25193.03 25367.40 25372.02 33222.76 33259.00 33549.72 33667.76 33733.77 33739.86 33879.92 35670.20 35681.46 4495.52 f 0,04 4271.84 f 0,03 4125.93 f 0.03 112.421 118.308 122.491 -0.211

0.31 0.05 -0.27 -0.38 0.27 -0.06 -0.02 0,04 0.27 -0.41 0.09 0,40 -0.71 0.67

Avb

4505.11 f 2.53 4270.10 -+0.20 4124.06 f 0.17 112.182 118.356 122.547 -0.230

4512.86 ?: 2.41 4267.80 + 0.15 4122.55 + 0.13 111.989 118.420 122.592 -0.257

-0.35

33841.39

33859.37

-0.08 -0,52 0.82 -0.16

33198.44 33240.24 33523,85 33633.60

0.32 -0.67 0,25 0.05

33210.22 33248.55 33536.89 33651.26

0,20

0.24

Avb

25171.24

24927.06

vAobsd

0.61 -0.48

Avb

26012.69 25181.97

* obsd

aConversion factor: 505, 391 MHz a.m.u, A’. bAv = Vobsd - V&,d . Calculated from the rotational constants given in this Table.

K

:bC

3,,+2,, 3,,+-2,, 3,,+-2,, 3,,+2,, 3,,+2,, 4,,+-31, 4&-30, 4,1+3,, 4,,+-33, 4,,+3,0 4,,+3,, 4&-32, 4,0+-330 4,,+3,, A B C 4

‘obsd

obsd

-1.81 0.70 13.89

AveA

calcd 0,31 -0.60 4.30 -4.19 +I,20 1,32 -1.82 0.73 13.23 -13.03 -0.37 -0,71

A vE_,,

Rotational transitions (MHz), rotational constants (MHz), moments of inertiaa (a.m.u. a’), and torsional splittings (MHz) in the ground and Y,8 excited states of CH,POF, Transition u=0 vls (u = 2) Splitting VI8(0 = 2) VII(b = 1)

TABLE 2

18

observed at 240 cm-’ in the IR. It has been found that Au = 2 transitions of methyl torsions appear in the Raman spectra of gaseous samples in a number of cases [3,9, lo] _ In CH,POF, two Q-branches are observed between 450 and 500 cm-‘, a region where only one fundamental had been assigned. Both lines are polarized and the higher frequency, 470 cm-‘, has been assigned to the PF, wag (A’). If the other line, at 456 cm-‘, is assigned to 2v**, the overtone of the torsion, a barrier of 3.60 kcal mol-’ can be calculated. The computer program used [21, 223 in the calculation uses a ‘IO-term sine-cosine free rotor basis set to calculate the eigenvalues and eigenvectors within the threefold torsional potential. For this barrier, the v = l-+0 transition is calculated to be at 235 cm-‘. We see, in fact, a depolarized line at 230 cm-’ in the gas phase_ The fact that only the 2+-O transition was observed is contrary to previous observations, but in this case, we feel that its intensity has been selectively enhanced by its proximity to a much stronger A’ fundamental. In the Raman spectra of the solid there is a resolvable shoulder on the low frequency side of the 473 cm- ’ line assigned to the PF2 wag. When 465 cm-’ is assigned to the v = 2+0 of the torsion, we calculate a barrier of 3.65 kcal mol-’ with VI8 at 240 cm-‘. This is consistent with the previously reported data [ES]. In order to verify the frequency of the fundamental in the solid state, the IR spectra were recorded to 200 cm-‘. A weak band appeared at 240 cm-‘. To insure that this was not an instrumental artifact, the sample thickness was increased and the 240 cm-’ band was seen to increase in intensity. To check the validity of the assignment of the torsion in the vibrational spectra, a study was carried out on the vibrational satellites in the microwave spectrum. Preliminary calculations using the program of Laurie and Lau [23] indicated that measurable A-E splittings should appear in the second excited state of the torsion. Splittings were observed in some of the vibrational satellites attributable to the v = 2 state of the torsion. These are listed in Table 2 and compared with splittings calculated [23] for a barrier height of 3.58 kcal mol-’ with F = 5.65 cm-‘_ The consistency between the vibrational and microwave measurements of the barrier to internal rotation is indeed good. We have determined that the value of this barrier is 3.58 r 0.05 kcal mol-’ in the gas phase. As expected [l] the barrier in the solid state is slightly higher, 3.65 kcal mol-‘. DISCUSSION

The crystal structure of CH,POF, is not known. Only two lattice modes have been observed [S] and these appeared in the far-IR spectrum. Our mid-IR spectrum of the solid indicate that there are at least two molecules per unit cell. We observe doubling of the C-P stretching mode at 761 and 757 cm-’ and the symmetric methyl deformation at 1340 and 1335 cm-‘. The sample used for recording the Raman spectrum of the solid was probably

19 not as well annealed as the sample used for the IR spectrum and, therefore,

showed no doubling. The frequencies of the Raman lines lie at the average of the two IR frequencies in each case. The only striking feature of our structure of CHXPOF2is the magnitude of the F-P-F angle. In comparison with other molecules, we see that the F-P-F angle has increased from 98.4 + 0.5” in CH3PF2 [5] and 99.8 + 2.0” in CH3PF2BH3 [24] to 115.2 -t-0.3” in CHXPOF2. C-P-F angle, however, shows a less drastic change and is found to be 97.8 + 0.5” [5], 101.9 + 20” [24], and X01.4 + 0.0” in these respective compounds. The opening of the F-P-F angle in CHsPFzBHs and CH3POF2 can be understood in terms of a reduction of the lone pair/bond pair interaction that is present in CH3PF2. This reduction is apparently more pronounced in CH3POF2 because of the multiple band character in the phosphorus-oxygen bond. A similar effect was not observed in PF3 and F3P0 where LF=P-F was 100.0 f 2.0” 1253 and 102.5 * 2.0” [18], respectively. Part of the reason for the large F-P-F angle may be that the P-F distance (1.52 a) is shorter than the corresponding quantity in CH,PF, (1.582 A)” and CH3PFzBH3 (1.552 A) [24]. We feel that the coordination of the lone pair contributes to both of these phenomena. The dipole moment components of CHaPOF have been calculated to be 3.4 +- 0.2 and 0.4 f 0.5 D for pa and pc, respectively. Part of the difficulty in calculating these quantities is the fact that EC,is very small. The inability to determine II, any better than to +0.5 D causes the large error in ~1,. The barriers in CH,PF, 153, CH,PH, 163, and CH3PH2BH3 1261 have all been determined by the microwave splitting method. A comparison of CH3PH2 and CH3PH2BH3 shows an increase in the methyl barrier from 1.96 kcal mol-’ [6] to 2.49 kcal mol-’ [26]. The reasons for this increase have been discussed [26] in terms of the theory of the origin of rotational barriers proposed by Fink and Allen [ 271. This model offers a balance of the sum of a nuclear-electronic potential ( Vne),a nuclear-nuclear potential (V,,)and a kinetic term (T) with the electron-electron repulsive potential (V,,)as the factor determining the barrier. The barrier in CH3PF2 has been found to be 2.30 kcal mol-’ 151, and we have found a value of 3.58 kcal mol-’ in CH3POF2. The V,, term can be expected to contribute to a lower barrier in those molecules with a lone pair of electrons as in the case of CHJPHZcompared to CHJPH~BHS. By bonding the lone pair electrons to some atom or group, the barrier to internal rotation rises and the amount of this increase is determined by all four terms. We can see that the V,, term could change drastically as the electrons of the lone pair are more tightly bound upon coordination. If the V,, term is important in these molecules, then we can infer that the bond to oxygen is more localized than is the bond to a -BHs group, as we might expect. The increase in the barrier to internal rotation about a C-P bond when the phosphorus lone pair is bonded to an oxygen atom is also observed in the solid phase. The barriers in CH3PC12and CH3POClz are 3.4 and 4.4 kcal mol-’ , wncnaA-i*rnlxr

rQ1

20

Because of the proximity of the A i and E deformations in F3P0 [ 141, it is possible that the lines in the Raman spectra of gaseous and solid CHaPOF which we have assigned to the overtone of the torsion actually arise from the PF* scissors. This reassignment would have no effect on our discussion of the barrier since this quantity has been determined quite independently by the microwave splitting technique. The data obtained from the rotational and vibrational spectra have allowed us to determine structural parameters and the barrier to internal rotation for CHsPOFz. These results can assess the role of the phosphorus lone pair in the bonding of phosphorus compounds. The lone pairs are known to affect molecular structures and stabilities of rotational isomers [28, 291. ACKNOWLEDGEMENT

The authors gratefully acknowledge the financial support of this work by the National Science Foundation through grant number GP-20723. REFERENCES 1 J. R. Durig, S. M. Craven and W. C. Harris, in J. R. Durig (Ed.), Vibrational Spectra and Structure, Vol. 1, Marcel Dekker, New York, 1972. 2 J. R. Durig, A. E. Sloan and J. D. Witt, J. Phys. Chem., 76 (1972) 3591. 3 J. R. Durig, W. E. Bucy and C!. J. Wurrey, J. Chem. Phys., 60 (1974) 3293. 4 J. R. Durig. W. E. Bucy and C. J. Wurrey, J. Chem. Phys., 63 (1975) 5498. 5 E. G. Godding, R. A. Creswell and R. H. Schwendeman, Inorg. Chem., 13 (1974) 856. 6 T_ Kojima, E. L. Breig and C. C. Lin, J. Chem. Phys., 13 (1961) 856. 7 J. R. Durig, B. R. Mitchell, J. S. DiYorio and F. Block, J. Phys. Chem., 70 (1960) 3190. 8 J. R. Durig and J. M. Casper, J. Phys. Chem., 75 (1971) 1956. 9 J. R. Durig, W- E. Bucy, L_ A. Carreira and C. J. Wurrey, J. Chem. Phys., 60 (1974) 1754. 10 J. R. Durig, W. E. Bucy, L. A. Carreira and C. J. Wurrey, J. Phys. Chem., 79 (1975) 988. 11 J. Dobson and R. Schaeffer, Inorg. Chem., 9 (1970) 2183. 12 L A. Carreira, R. 0. Carter and J. R. Durig, J. Chem. Phys., 59 (1973) 812. 13 R. N. Jones and A. Nadeau, Spectrochim. Acta, 20 (1964) 1175. 14 K. Nakamoto, Infrared Spectra of Inorganic and Coordination Compounds, WileyInterscience, New York, 1970. 15 R A Beaudet, Ph. D_ Thesis, Harvard Univ., 1962. 16 H K Wong, Acta Chem. Stand., 19 (1965) 879. 17 P. S. Bryan and R. L. Kuczkowski, Inorg. Chem., 11 (1972) 553. 18 Q. Williams, J. Sheridan and W. Gordy, J. Chem. Phys., 20 (1952) 164. 19 J. S. Muenter, J. Chem. Phys., 48 (1968) 4544. 20 J. S. Muenter and V. W. Laurie, J. Chem. Phys., 45 (1966) 855. 21 L. A. Carreira, J. Chem. Phys., 62 (1975) 3851. 22 J. D. Lewis, T. B. Malloy, Jr., T. H Chao and J. Laane, J. Mol. Struct., 12 (1972) 426. 23 V. W. Laurie and K. K. Lau, private communication. 24 R. A_ Creswell, R. A. Elzaro and R. H. Schwendeman, Inorg. Chem., 14 (1975) 2256. 25 Y_ Morino, K. Kuchitsu and T. Moritani, Inorg. Chem., 8 (1969) 867. 26 J. R. Durig, V. F. Kalasinsky, Y. S. Li and J. D. Odom, J. Phys. Chem., 79 (1975) 468. 27 W. H. Fink and L. C. Allen, J. Chem. Phys., 46 (1967) 2261. 28 J. R, Durig, B. M. Gimarc and J. D. Odom, in J. R. Durig (Ed.), Vibrational Spectra and Structure, Vol. II, Marcel Dekker, New York, 1975. 29 J. R_ Durig and A W. Cox, J. Chem. Phys., 79 (1975) 2303.