Spectra and structure of organophosphorus compounds. LVII. Raman and infrared spectra, conformational stability, and ab initio calculations for methoxydifluorophosphine

Journal of

MOLECULAR STRUCTURE ELSEVIER

Journal of Molecular Structure 375 (1996) 53-66

Spectra and structure of organophosphorus compounds. LVII. Raman and infrared spectra, conformational stability, and ab initio calculations for methoxydifluorophosphine J.R. Durig*, J.B. Robb, II 1 Department of Chemistry, University of Missouri-Kansas City, Kansas City, MO 64110-2499, USA

Received 1 May 1995; accepted in final form 15 June 1995

Abstract The Raman (3500 to 20 cm -I) and infrared spectra (3500 to 40 cm -l) of gaseous and solid methoxydifluorophosphine, CH30PF2, have been recorded. Additionally, the Raman spectrum of the liquid has been recorded from 3500 to 50 cm-i and qualitative depolarization values have been obtained. These data have been interpreted on the basis that only the trans conformer (methyl group trans to the phosphorous lone pair) is present in all three physical states and a complete vibrational assignment is given for the normal and d3 isotopomer. Ab initio calculations have been carried out with the RHF/3-21G* and RHF/6-31G* basis sets, as well as with electron correlation at the MP2 level with the 6-31G* basis set to obtain the structural parameters, relative conformational stabilities, fundamental frequencies, and infrared and Raman intensities. The fundamental vibrational frequencies and barriers to internal rotation which have been obtained experimentally are compared to those obtained with MP2/6-31 G* basis set. From the ab initio calculations, the gauche conformer is estimated to be more than 4.6 kcal mol -l less stable than the trans conformer. All of these results are discussed and compared with the corresponding quantities obtained for some similar molecules.

1. Introduction We recently carried out a conformational study of methoxydichlorophosphine utilizing infrared and R a m a n spectroscopy [I]. In the initial vibrational study [2] there was no mention of the presence of conformers in the fluid phases at

~" For part LVI, see Chem. Phys., (1995) in press. * Corresponding author. t Taken in part from the thesis of James B. Robb, II which will be submitted to the Department of Chemistryand Biochemistry, University of South Carolina, Columbia, SC, in partial fulfillment of the Ph.D. degree.

ambient temperature but from a more recent vibrational study by Remizov et al. [3], it was concluded that the number of bands observed in the liquid and solution was larger than expected for a single conformer. Our infrared study [1] of CH3OPCI2 dissolved in xenon as a function of temperature as well as a temperature study of the R a m a n spectra of all phases definitely showed that only the trans conformer with the methyl group trans to the phosphorus lone pair is present in all phases. This result is in marked contrast to the conformational stability of methoxydimethylphosphine where in the vapor state at ambient temperature CH3OP(CH3) 2 exists almost entirely

0022-2860/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0022-2860(95)08979-9

54

J.R. Durig, J.B. Robb, ll/Journal of Molecular Structure 375 (1996) 53-66

as the cis or near-cis conformer (methyl group oriented about 20 ° from eclipsing the phosphorus atom lone pair) but in the liquid state there is a significant amount of the trans conformer present. In order to obtain a better understanding of the differences in the conformational stabilities of these two molecules we have initiated a spectroscopic and theoretical study of the conformational stability of methoxydifluorophosphine, CH3OPF2, which is isoelectronic with methoxydimethylphosphine. In the initial structural study of CHaOPF2, Codding et al. [4] reported that the assigned microwave spectra of four isotopomers was consistent with the methyl group trans to the lone pair on the phosphorus atom although these authors referred to this form as the cis conformer. We shall refer to this rotamer as the trans conformer. These investigations [4] pointed out that the species for which the microwave spectrum was assigned must have a plane of symmetry because of the near equality of the values of Pbb = (Ia + Ic-Ib)/2 for the parent, naC and 180 species. However, they [4] did point out that there were many unassigned transitions which included some with resolvable Stark effects. A subsequent theoretical conformational analysis within the CNDO/2 approximation by Ro'binet et al. [5] gave the gauche conformer as being more stable than either the trans or cis forms by 2.0 kcal mol -n . These authors [5] suggest that the unassigned microwave lines could be due to the gauche conformer. However, in our [6] initial vibrational study of CH3OPF2 we found no evidence for a second conformer but we had some problems with impurities. Finally, it should be noted that an electron diffraction study has been recently reported [7] and the data could be interpreted on the basis of a single conformer. Nevertheless, we believed a temperature study of the infrared spectrum of CH3OPF2 dissolved in liquefied xenon could be useful for detecting a second conformer if there is one present. Additionally, we have carried out ab initio calculations utilizing both the RHF/6-31G* and MP2/6-31G* basis sets to obtain structural parameters and conformational stabilities. The results of this study are reported herein.

2. Experimental The sample of methoxydifluorophosphine was prepared by the reaction of methoxydichlorophosphine (Aldrich Chemical Co.) with antimony trifluoride (Aldrich). All sample handling and preparative work was carried out in a vacuum system equipped with greaseless stopcocks. The sample of methoxydichlorophosphine was condensed onto an excess of freshly sublimed SbF3 and allowed to warm to room temperature while constantly being stirred with a magnetic stirrer. The mixture was roughly purified through a -78°C trap and was further purified by using a low-pressure, lowtemperature fractionation column. The pure sample was then stored under vacuum at liquid nitrogen temperature.

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Fig. 1. Raman spectra of methoxydifluorophosphine: (A) gas; (B) liquid; and (C) annealed solid. The bands marked with an asterisk (*) are due to the PF3 symmetric deformation of the P(O)F 3 impurity.

J.R. Durig, J.B. Robb, II/Journal of Molecular Structure 375 (1996) 53-66

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The R a m a n spectra (Fig. 1) were recorded on a Cary model 82 spectrophotometer equipped with a Spectra-Physics model 171 argon ion laser operating on the 5145 A line. The spectrum of the gas was recorded by using the standard Cary multipass accessory with the laser power at the sample 2 W. The R a m a n spectrum of liquid methoxydifluoroo phosphine was obtained at r o o m temperature with the sample sealed in a Pyrex capillary and held in a standard Cary accessory. The R a m a n spectrum of the annealed solid was recorded by depositing the sample onto a blackened brass block which was contained in a cell fitted with "quartz windows and cooled by boiling liquid nitrogen. The reported wavenumbers are expected to be accurate to + 2 c m - I with all of the observed lines listed in Table 1. The F T - R a m a n spectrum of the liquid (Fig. 2) was recorded from the sample sealed in a Pyrex capillary from 3500 to 100 cm -1 using a Bruker model IFS-66 interferometer equipped with a Bruker model FRA-106 R a m a n accessory and a liquid nitrogen cooled G e r m a n i u m detector. A 30 minute time flame was allowed for detector cooling, laser warming and stabilization to occur. The spectrum was collected, processed, and displayed on an Oceanic monitor of the Bruker

I

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WAVENUMBER(cm-1) Fig. 2. Comparison of theoretical and experimental Raman spectra of methoxydifluorophosphine: (A) pure trans (theoretical) and (B) FT-Raman of the liquid phase. The band marked with an asterisk (*) is due to the PF3 symmetric deformation of the P(O)F3 impurity.

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WAVENUMBER(cm-1) Fig. 3. Far infrared spectrum of gaseous methoxydifluorophosphinewith the top spectrum that of water.

J.R. Durig, J.B. Robb, II[Journal of Molecular Structure 375 (1996) 53-66

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J.R. Durig, J.B. Robb, ll[Journal of Molecular Structure 375 (1996) 53-66

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J,R. Durig, J.B. Robb, H/Journal of Molecular Structure 375 (1996) 53-66

58

Aspect-3000 computer using standard programs to give the calculated Raman frequency shifts, intensities and half-widths. The far infrared spectrum of gaseous methoxydifluorophosphine (Fig. 3), from which the torsional transition was measured, was recorded using a Nicolet model SXV 200 Fourier transform interferometer with the sample contained in a 1 m cell fitted with polyethylene windows. This instrument is equipped with a vacuum bench and a liquid helium cooled germanium bolometer. The spectra were recorded with a 12.5/zm Mylar beamsplitter at a resolution of 0.10 cm -~. Mid-infrared spectra of the gas and annealed solid (Fig. 4) from 3200 to 400 cm -1 were recorded on a Digilab model FTS-14C Fourier transform interferometer equipped with a Ge/KBr beamsplitter and a TGS detector. For the gaseous sample, a 12 cm cell equipped with KBr windows was used. The spectrum of the annealed solid was obtained by depositing the sample onto a CsI plate cooled by boiling liquid nitrogen and housed in a cell fitted with CsI windows. The mid-infrared spectra of the sample dissolved

in liquefied xenon as a function of temperature (Fig. 5) were recorded on a Bruker model IFS-66 Fourier transform interferometer equipped with a Globar source, a Ge/KBr beamsplitter, and a TGS detector. The temperature studies, ranging from - 5 0 to -100°C were performed in a specially designed cryostat cell which consisted of a 4 cm pathlength copper cell with wedged silicon windows sealed to the cell with indium gaskets and is attached to a pressure manifold to allow for the filling and evacuation of the cell. The temperature was monitored with two platinum thermoresistors and cooled with boiling liquid nitrogen. Once the cell is cooled to the desired temperature, a small amount of sample is condensed into the cell. The cell is then pressurized with xenon, allowing the compound to dissolve. For each temperature investigated 200 interferograms were collected at 0.5 cm -] resolution, averaged, and transformed with a boxcar truncation function.

3. Conformational stability

Since Robinet et al. [5] suggested that the A

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Fig. 5. Temperature dependence of the mid-infrared spectrum of methoxydifluorophosphine dissolved in liquid xenon. The band marked with an asterisk (*) is due to an impurity.

J.R. Durig, J.B. Robb, ll/Journal of Molecular Structure 375 (1996) 53-66

unassigned transitions in the microwave spectrum could be due to the presence of the gauche conformer, an investigation of the temperature dependence of the infrared spectrum of CH3OPF 2 dissolved in liquefied xenon was carried out. Because xenon is expected to have only minimal interaction with the sample, the sharpness of the lines attained at reduced temperatures and at relatively high resolution would make it easy to observe bands belonging to a second conformer if they are present. Six sets of spectral data ranging in temperatures from - 5 0 to -100°C were recorded for the CH3OPF 2 molecule dissolved in liquefied xenon and are shown in Fig. 5. These bands have halfwidths at half-heights of ,,~ 10 cm -I and there is no indication of any extraneous bands that could be attributed to a second conformer as previously suggested [5]. Although the bands became sharper as the temperature was lowered, no change was

59

observed in their relative intensities. Based on these data, we concluded that there was only one conformer preserh in the temperature range investigated and it is assigned as the trans rotamer. The Raman spectra of gaseous, liquid and solid methoxydifluorophosphine as well as the FTRaman spectrum of the liquid are shown in Figs. 1 and 2, respectively. Examination of the spectrum of the gas shows several lines that exhibit broad nondescript features while other lines have sharp well-defined Q-branches. These spectral data are typical for a molecule having Cs symmetry where the A' modes should give rise to sharp polarized bands while the A" modes yield broad depolarized Raman lines. Thus, the broad lines in the spectrum are attributed to the A" modes of the trans conformer. Also, a comparison of the Raman spectrum of the vapor to that of the liquid does not reveal any significant changes in the relative intensities of these modes.

Table 2 Structural parameters a, dipole moments, and total energies for methoxydifluorophosphine obtained from ab initio calculations Parameter

RHF/3-21G*

RHF/6-31G*

MP2/6-31 G*

r(P-O) r(C-O) r(P-F) r(C-H t) r(C-H2) = r(C-H3) •~(P-O-C) '~(O-P-F) •~ ( O - C - H t ) '~(O-C-H2) = "~O-C-H3 •~(HI-C-H2) = "~HI - C - H l "~(H2-C-H3) "~(FI-P-F2) q~(H2-C-O-H 0 ~b(F2-P-O-C) A B C

1.580 1.464 1.580 1.075 1.079 125.1 99.1 106.3 109.4 110.7 110.1 94.8 119.6 48.2 6017 3659 3158 1.709

1.584 1.424 1.584 1.078 1.080 124.8 100.0 106.5 110.2 109.9 110.1 94.9 119.2 48.4 6027 3670 3147 1.606

1.613 1.448 1.614 1.087 1.090 121 .I 100.4 106.0 110.0 110.3 110.2 94.9 119.2 48.5 5760 3624 3112 1.527

I~al I~I

o.o

o.o

o.o

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0.432 1.763 0.870354

0.345 1.642 4.150826

0.283 1.553 4.934301

[#,l - ( E + 650)(Hartrees)

Microwave b 1.560 4- 0.015 1.446 4- 0.005 1.591 + 0.006 1.090 4- 0.010 1.090 4- 0.010 123.7 4- 0.5 102.2 4- 1.0

110.5 4- 1.0 110.5 4- 1.0 94.8 4- 0.6

5991 3642 3128

a Bond distances in ,~, bond angles in degrees, rotational constants in MHz, and dipole moments in debye. b Ref. [4]. c Ref. [7].

E.D. c 1.574 4- 0.004 1.446 -4-0.002 1.595 + 0.004 1.089 + 0.003 1.089 + 0.003 123.9+0.1 101.65:0.1

94.8 + 0.1

J.R. Durig, J.B. Robb, ll[Journal o f Molecular Structure 375 (1996) 53-66

60

4. Ab initio calculations

H

The L C A O - M O - S C F restricted Hartree-Fock calculations were carried out with the GAUSSIAN-90 program [8] using Gaussian-type basis functions. The energy minima with respect to the nuclear coordinates were obtained by the simultaneous relaxation of all of the geometric parameters using the gradient method of Pulay [9]. The calculated structural parameters determined with the RHF/3-21G*, RHF/6-31G* and MP2/6-31G* basis sets are listed in Table 2. The final optimized geometry for the trans conformer was utilized to obtain a potential surface scan in which only the FPOC dihedral angle was allowed to vary from 0 to 180°. From the potential surface scan with the RHF/3-21G* basis set, another two equivalent minima were predicted at the gauche position. The structural parameters corresponding to the trans, as well as those at the gauche configuration, were optimized by the relaxation of all of the geometric parameters. From the optimized geometries obtained with the RHF/3-21G* and RHF/6-31G* basis sets, the total energies for the trans and gauche conformers are -650.870354 and

~l

z2 R / O ~ Q

-k "~.," r2 //J32

-650.859865 Hartree (! Hartree = 219474 cm -l) -654.150826 and -654.143878 Hartree, respectively. Therefore,. the trans conformer is more stable than the gauche form by 2302 cm -l (6.58 kcal mol -I) with the RHF/3-21G* basis set and 1525 cm -I (4.36 kcal mol -I) with the RHF/6-31G* basis set. A similar calculation with the MP2/6-31G* basis set gives the trans conformer (E =-654.934301 H) more stable than the gauche form (E=-654.926956 H) by 1612cm -t (4.61 kcal mol-l). Therefore, with such a large difference in the energy, the gauche conformer is not expected to be detectable by vibrational spectroscopy at ambient temperature.

and

Species

Description

Symmetry Coordinate ~

A~

C H 3 a n t i s y m m e t r i c stretch C H 3 s y m m e t r i c stretch CH3 a n t i s y m m e t r i c d e f o r m a t i o n CH 3 symmetric deformation CH3 r o c k C - O stretch PF2 s y m m e t r i c stretch P - O stretch PF2 r o c k PF2 s y m m e t r i c d e f o r m a t i o n P O C bend Redundancy

St = 2rl - r2 - r3 $2 = rl + r2 + r3 $3 = 2 a l - t~2 - a3 $4 = fll + f12 + f13 - a l - a2 - a3 $5 = 2~t - / 3 2 - / 3 3 $6 = R $7 = sl + s2 Ss = Q S9 = 71 + 72 Sto = 6 Sit = A

C H 3 a n t i s y m m e t r i c stretch CH3 a n t i s y m m e t r i c d e f o r m a t i o n CH~ r o c k PF2 a n t i s y m m e t r i c stretch P F 2 twist Methoxy torsion M e t h y l torsion

SL2 Si3 $14 Sis Sl6 S~7 Sis

a N o t normalized.

'2"~.j ~ S ,

Fig. 6. Molecular diagram showing internal coordinates for the trans conformer of methoxydifluorophosphine.

Table 3 S y m m e t r y c o o r d i n a t e s for m e t h o x y d i f l u o r o p h o s p h i n e

A"

~;1

Sn = 3t + 132 + 133 + al + a2 + c~3 = = = = = = =

r2 a2 f12 sl "71 r2 rt

r3 a3 f13 s2 72

R

sI

s2

rl

r2

r3

A

7J

TI 72

~2 Ot3

~3

~2

7.161 0.311 0.429 0.429 -0.047 -0.081 -0.081 0.485 0.235 5.391 -0.075 -0.075 0.108 0.144 0.144 0.495 -0.152 6.673 0.327 0.001 0.008 0.030 0.002 0.362 $1 6.673 0.001 0.030 0.006 0.002 -0.048 $2 6.112 0.056 0.056 0.000 0.001 rl 5.975 0.061 -0.048 0.006 r2 5.975 -0.048 -0.064 r3 0.743 -0.066 A 1.875 71 72 6

Q R

Q

Table 4 Ab initio harmonic force constants for trans methoxydifluorophosphine

0.235 -0.152 -0.048 0.362 0.001 -0.064 0.006 -0.066 0.361 1.875

72

-0.125 -0.012 0.304 0.304 0.001 -0.022 -0.022 -0.040 0.264 0.264 1.841

~

f12

0.054 -0.102 0.311 0.284 0.001 0.008 0.0(31 0.053 0.021 -0.058 -0.055 0.011 -0.055 -0.057 0.024 0.002 -0.014 0.006 -0.014 -0.016 -0.007 -0.006 0.801 -0.187 0.799

fll

-0.102 0.284 0.053 0.008 -0.058 -0.057 0.011 0.002 -0.016 0.006 -0.006 -0.187 -0.175 0.799

B3

0.041 -0.276 -0.012 -0.012 -0.093 0.098 0.098 -0.004 0.004 0.004 -0.007 -0.172 -0.125 -0.125 0.596

oq

0.052 -0.284 -0.033 -0.015 0.094 -0.095 0.092 -0.011 0.021 -0.002 0.012 -0.115 -0.164 -0.135 -0.103 0.601

ot 2

0.052 -0.284 -0.015 -0.033 0.094 0.092 -0.095 0.011 -0.002 0.021 0.012 -0.115 -0.135 -0.164 -0.031 -0.092 0.601

o~3

0.000 0.000 -0.009 0.009 0.000 0.012 -0.012 0.000 0.024 -0.024 0.000 0.000 0.002 -0.002 0.000 -0.004 0.004 0.024

7"I

o.ooi

0.000 0.000 0.005 -0.005 0.000 -0.001 0.001 0.000 0.002 -0.002 0.000 0.000 -0.003 0.003 0.000 0.000 "0.000 -0.001

7"2

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62

J.R. Durig, J.B. Robb, II/Journal of Molecular Structure 375 (1996) 53-66

The results from previous structural studies by Codding et al. [4] (microwave) and Davis et al. [7] (electron diffraction) are given along with the calculated structural parameters in Table 2. Since there are experimental data on the structural parameters of the trans conformer from the microwave [4] and electron diffraction [7] investigations, the reasonableness of the structural parameters from the ab initio calculations for this rotamer can be ascertained. When comparing all three basis sets, the structural parameters obtained with the MP2/ 6-31G* basis set are closest in agreement with the values obtained experimentally. However, the P-O and P - F bond distances calculated with the MP2/ 6-31G* basis set are slightly longer than the experimental values even with the stated uncertainties. Additionally, the P - O - C and O - P - F bond angles are calculated smaller than the microwave and electron diffraction values by as much as 2.60/2.8 ° for the P - O - C angle and 1.8°/1.2 ° for the OPF bond angle. A normal coordinate analysis has been carried out for methoxydifluorophosphine to support the assignment of the fundamental vibrational frequencies and to obtain the potential energy distributions (P.E.D.) among a set of symmetry coordinates for the normal modes. These calculations were performed utilizing the Wilson FG matrix method with the computer programs written by Schachtschneider [10]. The force field in Cartesian coordinates was calculated by the GAUSSIAN-90 program [8] with the RHF/3-21G* basis set. To convert these force fields in Cartesian coordinates into the required internal coordinates, the following procedures were utilized. The Cartesian coordinates obtained for the optimized trans structure were input into the G-matrix program together with the complete set of 19 internal coordinates (see Fig. 6). This complete set of internal coordinates was used to form 18 symmetry coordinates along with one redundancy which are listed in Table 3. The output of the G-matrix program gives the B matrix which was used to convert the ab initio force field in Cartesian coordinates into the desired internal coordinates by utilizing a program developed in our laboratory. The resulting force field in internal coordinates is given in Table 4 for the trans conformer since it is probable

that it could be transferred to other substituted methoxyphosphine molecules. Initially, all scaling factors were kept fixed at a value of 1.0 to produce the pure ab initio calculated frequencies. Scaling factors of 0.9 for stretches, 0.8 for bends and 1.0 for the torsion and the geometric average of scaling factors for the interaction terms were input into the same program to obtain the fixed scaled force fields and resultant wavenumbers along with the potential energy distributions. All of these results are listed for the trans conformer in Table 5. The theoretical Raman (Fig. 2) spectrum for CHaOPF 2 was calculated using the frequencies, scattering activities, and intensities that were calculated from the RHF/3-21G* basis set. The GAUSSIAN-90program [8] with the option of calculating the polarizability derivatives analytically was used. The Raman scattering cross sections, 0crj/0fl, which are proportional to the Raman intensities, can be calculated from the scattering activities and the predicted frequencies for each normal mode using the relationship [11]:

Off

k,"~--,]

1 - exp [--~-~]

where u0 is the exciting frequency, uj is the vibrational frequency of the jth normal mode and Sj is the corresponding Raman scattering activity. To obtain the polarized Raman scattering cross sections, the polarizabilities are incorporated into Sj by Sj[(1 - pj)/(1 + pj)] where pj is the depolarization ration of t h e j th normal mode. The Raman scattering cross sections and calculated frequencies are used together with a Lorentzian function to obtain the calculated spectrum. Since the calculated frequencies are approximately 10% higher than those observed, the frequency axis of the theoretical spectrum was compressed by a factor of 0.9.

5. Vibrational assignment The observed vibrational frequencies of CHaOPF2 are listed in Table 1 and the vibrational assignments and potential energy distributions

vI v2 v3 1/4 t:5 t,6 v7 v8 v9 vl0 vii

v12 ~'13 v14 L,15 t'16 t,17 ~'18

A'

A"

CH3/CD 3 antisymmetric stretch CH3/CD 3 antisymmetric deformation CH3/CD 3 rock PF 2 antisymmetric stretch PF2 twist methoxy torsion CH3/CD 3 torsion

CH3/CD 3 antisymmetric stretch CH3/CD 3 symmetric stretch CH3/CD3 antisymmetric deformation CH3/CD 3 symmetric deformation CH3/CD 3 rock C - O stretch PF2 symmetric stretch P - O stretch PF~ rock PF2 deformation POC bend

Fundamental

3240 1563 1203 848 366 150 56

3261 3138 1569 1520 1224 1097 870 804 536 355 231 18.1 8.1 1.5 162.4 20.6 7.0 1.6

15.5 19.9 7.8 0.1 53.2 403.1 140.4 42.8 45.2 7.7 15.8 40 21 6 2 0.3 0.2 0.1

71 87 17 4 6 4 2 18 0.7 0.2 0.5 3023 1458 1154 797 370 122 81

3048 2965 1470 1438 1181 1044 829 786 536 351 216

Obs.d

100Si2 95SI3 95S14 99S15 82S16,16St7 54S17,29SI8, 16St6 69St8, 29SI7

97S I 97S2 95S3 99S4 59S s, 20S8, 15S6 39S6,33S5, 23S8 95S7 53S8, 49S6 6 3 S 9 ,19SH, 13Sio 86S10,11S9 79Sit, 22S9

P.E.D.

2463 1213 974 .964 420 115 52

2848 2321 1229 1222 1187 993 978 815 577 401 200

Ab lnitio a

Raman Act.c

Ab Initioa

IR Int. b

d3

do

a Calculated using the MP2/6-31G* basis set. b Infrared intensities in units of km mol -I . c Raman activities in units of ,~4/amu. d Frequencies are taken from the infrared spectra of the gas and others are those from the Raman.

Vibration No.

Species

Table 5 Observed and calculated frequencies (cm -j ) and potential energy distributions for methoxydifluorophosphine-doand -d3

100SI2 99S13 98S15 97S14 82S16,14Si7 65S17,19S1g,°16S16 80S18,19S17

99Si 99S2 53Ss ' 35S6 69S4,22S3, 9S6 77S3, 16S4 93S7 78S5, 13S6 44S8,39S6, 12S5 61S9, 15Si0, 15Sll 84S10,13S9 80Sit, 20S9

2281 2084 1064 1098 917 1079 827 746 526 469 202 2156 1064 890 796 368 116 65

P.E.D.

Obs. d

t.,o I O~ Ox

~, -~ ,o ~-~

~.~

ca-

64

J.R. Durig, J.B. Robb, ll[Journal of Molecular Structure 375 (1996) 53-66

(P.E.D.) for the normal and deuterated species are listed in Table 5. Assignments of the normal modes were based on the Raman depolarization data, infrared band contours for the gas phase, group frequencies, and the calculated wavenumbers and P.E.D.'s obtained from the MP2/6-3 IG* basis sets. The assignment of the fundamental vibrations listed in Table 1 are in close agreement with those given in the earlier study [6] except for a few exceptions. In the carbon-hydrogen stretching region the A' CH 3 antisymmetric stretch, ul, is assigned to the strong Raman line at 3048 cm -l which was previously assigned [6] as the A" mode. The A" CH3 antisymmetric stretch is now assigned [6] to the line at 3023 cm -1 The CH3 symmetric stretch which was previously assigned to the band at 2851 cm -1 is now assigned to the strong polarized Raman line at 2965 cm -j and the band at 2851 cm -I is assigned to the overtone of the symmetric methyl deformation. The large intensity of the 2851 cm -l is due to Fermi resonance with the CH 3 symmetric stretch. This assignment of the fundamentals in the carbon-hydrogen stretching region is in agreement with those given for the corresponding fundamentals of other methoxyphosphine compounds [1,12]. The band observed at 474 cm -1 in the infrared and Raman spectra which was previously assigned to the PF2 bend is due to the PF 3 symmetric deformation of the P(O)F 3 impurity. From the MP2/6-31G* normal coordinate calculation, the frequencies for the PF2 deformation and the PF2 twisting vibrations are calculated to be 355 and 366cm -I, respectively. The Raman spectra of all three phases show only one band at 370 cm -l to be present in this region. However, in the far infrared spectrum of the gas there is an indication of a second band in this frequency region, and in the far infrared spectrum of the solid there are two bands at 372 and 351 cm -I. Thus the band at 370 cm -l is assigned to the PF2 twist and the PF 2 deformation is assigned to the band at 351 cm -I. In the earlier study [6] it was not possible to determine whether the A ~ or the A" mode of the antisymmetric deformation had the higher frequency. However, in the current study it is clear that the A' mode has the higher frequency

of 1470 cm -l, whereas the A" mode is observed at 1458 cm -I. The remaining vibrations are assigned as suggested in the earlier study [6].

6. Discussion

The vibrations are relatively pure with only a few exceptions. The band at 786 cm -I has nearly equal contribution from the P - O and C - O stretches, whereas the band at 1044 cm -1 has nearly equal contribution (33%) from the C - O stretch and CH 3 rock with a significant contribution of 23% from the P - O stretch. The mixing in the d3-isotopomer is similar for the lower wavenumber band at 746 cm-l which is still made up of nearly equal contributions from the P - O and C - O stretches, but now there is also a 12% contribution from the CD 3 rock. Of course, the CH 3 rock has significant contribution from the C - O and P - O stretches. The PF2 rock has significant contribution from the PF2 deformation and the POC bend. Finally, it should be noted that the methoxy torsion has significant contribution from the methyl torsion and the PF2 twist. In the initial microwave study [4], it was concluded that the most stable conformation exists as the trans rotamer. However, from a theoretical conformational analysis at the CNDO/2 level Robinet et al. [5] predicted the gauche conformer to be more stable than any other form by 2.0 kcal mo1-1. The initial vibrational analysis [6] and a subsequent electron diffraction investigation [7] found no evidence of a second conformer and both studies concluded that the trans rotamer was the most stable form. In this present study, we investigated the possibility of an additional conformer by carrying out a variable temperature study of the infrared spectrum of the sample dissolved in liquefied xenon gas. We diligently searched the spectrum for a band that could be assigned to a second conformer but we could not find any candidates. Thus, we believe these data are definitive and conclude that there is a singly trans conformer present in all three physical states. In both the infrared and Raman spectra of the solid, most of the fundamentals appear as multiplets with many of the infrared bands as triplets. In

J.R. Durig, J.B. Robb, ll/Journal of Molecular Structure 375 (1996) 53-66

some cases, the wavenumbers of the components of the infrared bands are significantly different from those in the Raman spectrum. Since several lattice modes are observed in the Raman spectrum of the solid, it is believed that the sample had good crystallinity. Thus, the number of components for the fundamentals indicates that there are several molecules in the primitive cell. The calculated Raman spectrum of the trans conformer is shown with the FT-Raman spectrum of CHHOPF2 in the liquid phase in Fig. 2. The intensities of several of the lines in the calculated spectrum are much greater than those obtained experimentally. Since the experimental spectrum of the liquid phase is similar to that of the gas phase spectrum with broad and poorly defined bands, a comparison is rather difficult. Nevertheless, similarities between the two spectra are significant. The breadth of the Raman lines in the spectrum of the liquid is believed to be due to the low barriers to rotation of the two internal rotors. In fact, the calculated spectrum more closely resembles that of the Raman spectrum of the solid phase and allows for a better comparison. The notable differences are the separation of the three lines in the 1100 cm -I region of the theoretical spectrum which are observed at 1178, 1154, and 1019 cm -1 in the spectrum of the solid. The bands in the 700 to 800 cm -1 region of the experimental spectrum show extensive splitting. These data clearly demonstrate the utility of the calculated intensities of the Raman lines for conformer identification or for analytical purposes although there may be significant differences in the calculated versus experimental intensities of a few of the fundamentals. From the initial microwave investigation of methoxydifluorophosphine, the authors [4] were able to obtain reasonably accurate structural parameters except for the P-O and C - H bond distances which had large uncertainties of0.015 and 0.010 A, respectively. It was shown, however, that the P-O distance was strongly affected by the type of data analysis performed. Also, the C - H distances were expected to be poorly determined because relatively small changes in the PF2 geometry were magnified in their effect on the CH 3 geometry [4]. On the other hand, the authors [7] from the

65

electron diffraction study combined the analysis of electron diffraction data and existing rotational constants to give much more accurate and precise structural parameters. A comparison of the experimental structural parameters to those obtained with the MP2/6-31G* basis set shows the calculated values of the P-O and P - F bond distances to be significantly longer by 0.035 and 0.025 ,~, respectively. These ab initio values are expected to be too long based on the results [1] for CH3OPC!2 where the P-CI distance was too long by 0.014 A and the P-O distance by 0.019 ~.. Additionally, the POC and OPF bond angles are predicted smaller than the experimental values. The remaining structural parameters calculated with the MP2/6-31G* basis set are probably good to 0.005 ]k and 0.5 ° and they all fall within the error limits of the experimental reported values. These results show the value of the ab initio calculations in predicting structural parameters and conformational stabilities as well as providing guidance for vibrational assignments for these organophosphorous molecules.

Acknowledgement The authors acknowledge partial financial support for this study from the Department of Energy by Grant DEFG 0291 ER 75663.

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Robb, J.S. Binkley, C. Gonzalez, D.J. DeFrees, D.J. Fox, R.A. Whiteside, R. Seeger, C.F. Melius, J. Baker, R.L. Martin, L.R. Kahn, J.J.P. Stewart, S. Topiol and J.A. Pople, Gaussian, Inc., Pittsburgh, PA, 1990. [9] P. Pulay, Mol. Phys., 17 (1969) 197. [10] J.H. Schachtschneider, Vibrational Analysis of Polyatomic

Molecules, Parts V and VI, Technical Report Nos. 231 and 57, Shell Development Co., Houston, TX, 1964 and 1965. [11] G.W. Chantry, in A. Anderson (Ed.), The Raman Effect, Vol. 1, Chap. 2, Marcel Dekker, New York, 1971. [12] G.H. Pieters, B.J. van der Veken, T.S. Little, W. Zhao and J.R. Durig, Spectrochim. Acta, 43A (1987) 657.