Spectra and structure of organophosphorus compounds. 44

A complete assignment of the vibrational fundamentals observed from the Raman .... Figure 1. Mid-infrared spectra of CH3CH2P(0)F2 in the (A) gas phase...
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J. Phys. Chem. 1992,96, 541-548

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Spectra and Structure of Organophosphorus Compounds. 44.+ Conformational Stability, Barriers to Internal Rotation, Vibrational Assignment, and ab Initio Calculations for Ethyiphosphonic Difiuoride J. R. Durig,* T. J. Hizer,t and R. J. Harlan5 Department of Chemistry, University of South Carolina, Columbia, South Carolina 29208 (Received: October 6, 1991; In Final Form: August 22, 1991)

The far-infrared spectrum of ethylphosphonic difluoride, CH3CH2P(0)F2,has been recorded at a resolution of 0.10 cm-' from 350 to 50 cm-I. The fundamental asymmetric torsional modes of the more stable trans (CCPO dihedral = 180°) and the higher energy gauche (CCPO dihedral = 57O) conformers have been observed at 75.7 and 67.5 cm-I, respectively, with excited states for each mode falling to lower frequencies. From these data, the asymmetric torsional potential function governing internal rotation has been determined. This potential function indicates that the trans conformer is more stable than the gauche form by 120 cm-l (343 cal/mol). From studies of the Raman spectra at various temperatures, the conformational enthalpy differences have been determined to be 126 f 53 cm-' (359 f 151 cal/mol) and 167 f 20 cm-' (478 f 56 cal/mol) for the gas and liquid, respectively. A complete assignment of the vibrational fundamentals observed from the Raman (3200-10 cm-I) and the infrared (3200-50 cm-I) spectra of the solid, liquid, and gaseous states is proposed. All of these data are compared to the corresponding quantities obtained from ab initio Hartree-Fock gradient calculations employing both the 3-21G* and 6-31G* basis sets. Additionally, complete ab initio equilibrium geometries have been determined for both rotamers. These results are discussed and compared with the corresponding quantities for some similar molecules.

Introduction We have previously investigated the structures and conformational equilibria of a number of organophosphorus compounds'-'6 with the general formula CH3CH2P(Y)X2,where Y = BH,, S,or a nonbonded electron lone pair and X = C1, F, CH,, or H. From these studies, we have found that these ethyl-substituted phosphines have a trans/gauche conformational equilibrium (methyl group to Y) with significant structural differences between the two conformers. Because of these results, we were interested in determining the conformational equilibrium in ethylphosphonic difluoride, CH3CH2P(0)F2,to see what effect the substitution of the electron-withdrawing oxygen and fluorine atoms would produce. N o previous conformational or structural studies on ethylphosphonic difluoride have been reported. Therefore, we have not only determined the conformational equilibrium of this molecule from temperature studies of the Raman spectra, but we have also determined the potential function for the asymmetric rotation from the far-infrared spectrum of the gas and proposed a complete vibrational assignment. We have recently reported an ab initio study of ethylphosphine" in which we calculated the structures, vibrational frequencies, potential energy distribution, and the conformational equilibrium of both the trans and gauche conformers with the 3-21G* and 6-31G* basis sets. Excellent agreement between these results and those obtained from experimental data was found. Therefore, we felt that the complete ab initio study of ethylphosphonic difluoride would be worthwhile so that we may compare our experimental results with those determined theoretically. The results of this study are reported herein. Experimental Section The sample of ethylphosphonic difluoride, CH3CH2P(0)F2,was prepared by the fluorination of ethylphosphonic dichloride (Alfa) with antimony trifluoride (Alfa) by the halogen exchange reaction described by Drozd et a1.I8 The product and volatile impurities were removed under vacuum, and the sample was purified by fractionation on a low-temperature vacuum-sublimation column. The mid-infrared spectra of gaseous and solid ethylphosphonic difluoride (Figure 1) were recorded on a Digilab Model FTS-14C For part XLIII, see J. Mol. Srrucr. 1989, 200, 41. Person to whom correspondence should be addressed. *Presentaddress: Department of Chemistry and Physics, Armstrong State College, Savannah, GA 31419. 'Taken in part from the thesis of R. J. Harlan which was submitted to the Department of Chemistry in partial fulfillment of the Ph.D. degree.

0022-3654/92/2096-541$03.00/0

Fourier transform interferometer equipped with a Ge/KBr beamsplitter and a TGS detector. For the gas, a 10-cm cell fitted with CsI 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 far-infrared spectrum of ethylphosphonic difluoride from which the torsional transitions were obtained was collected with the gas contained in a 1-m cell fitted with polyethylene windows. These data were collected on a Nicolet Model 200 SVX interferometer equipped with a vacuum bench and a liquid helium cooled Ge bolometer containing a wedged sapphire filter and a polyethylene window. Spectra were obtained utilizing 6.25-, 1 2 5 , and 25.0-pm Mylar beam splitters, and the data were collected a t a resolution of 0.10 cm-'. The far-infrared spectrum of the annealed solid was obtained with a Digilab Model FTS-15B Fourier transform interferometer equipped with a 6.25-pm Mylar beam splitter and a TGS detector. The sample was deposited onto a silicon plate cooled by boiling liquid nitrogen and housed in a (1) Groner, P.; Johnson, R. D.; Durig, J. R. J. Chem. Phys. 1988.88.3456. (ZrDurig, J. R.; Cox, A. W. J. Chem. Phys. 1975,63, 2303. (3) Durig, J. R.; Cox, A. W. J . Chem. Phys. 1976,64, 1930. (4) Durig, J. R.; Johnson, R. D.; Groner, P. J. Mol. Srrucr. 1986,142, 363. (5) Durig, J. R.; Church, J. S.; Whang, C. M.; Johnson, R. D.; Streusand, B. J. J . Phys. Chem. 1987, 91, 2769. (6) Durig, J. R.; Hizer, T. J. J . Raman Spectrosc. 1986, 17, 97. (7) Durig, J. R.; James, C. J.; Stanley, A. E.; Hizer, T. J.; Cradock, S. Spectrochim. Acra 1988, 44A, 91 1 . (8) Durig, J. R.; Hizer, T. J. J . Raman Specrrosc. 1987, 18, 415. (9) Durig, J. R.; Hizer, T. J. J . Mol. Srrucr. 1986, 145, 15. (10) Groner, P.; Church, J. S.; Li, Y. S.;Dung, J. R. J. Chem. Phys. 1985, 82, 3894. (1 1) Odom, J. D.; Hizer, T. J.; Stanley, A. E.; Tonker, T. L.; Durig, J. R. Spectrochim. Acta 1988, 44A, 631. (12) Durig, J. R.; Hizer, T. J.; Odom, J. D. J . Mol. Srrucr. 1987, 159, 85. ( 1 3) Durig, J. R.; Rizzolo, J. J.; Sullivan, J. F.; Cheng, Mei-Shiow;Hizer, T. J.; Odom, J. D. J . Mol. Szrucr. 1987, 156, 267. (14) Odom, J. D.; Brletic, P. A.; Johnson, S. A.; Durig, J. R. J . Mol. Srrucr. 1983, 96, 247. (15) Dung, J. R.; Brletic, P. A.; Li, Y. S.; Johnson, S. A.; Odom, J. D. J . Chem. Phys. 1981, 75, 1644. (16) Dung, J. R.; Johnson, R. D.; Nanaie, H.; Him, T. J. J . Chem. Phys. 1988,88,7317. (17) Durig, J. R.; Lee, M. S.;Harlan, R. J.; Little, T. S. J . Mol. Srrucr. 1989, 200,41. (18) Drozd, G . I.; Sheluchenko, V. V.; Telebaum, B. I.; Luganskii, G. M.; Varshavskii, A. D. Zh. Obshch. Khim. 1967, 37, 1343.

0 1992 American Chemical Society

Durig et al.

542 The Journal of Physical Chemistry, Vol. 96, No. 2, 1992

!

A

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

3000

’

I

I

I

1

1

1

2000 I I I llooo WAVE NUMBER (cm-l) I

1

Figure 1. Mid-infraredspectra of CHSCH2P(0)F2 in the (A) gas phase

and (B) solid phase. I’

I

C

Wavenumber (cm-1)

Figure 3. Raman spectra of CH3CH2P(0)F, in the (A) gas phase, (B) liquid phase, and (C) solid phase.

I

I

I

1

I

300

I

I

I

I

I

I

I

200 WAVE NuMBER (cm-1)

I

I

1

I

1

100

1

1

I

Figure 2. Far-infrared spectra of CH,CH,P(O)F, in the (A) gas phase with a water trace on top and (B) solid phase.

cell fitted with polyethylene windows. After several cycles of warming and cooling, the spectrum was recorded. Typical spectra are shown in Figure 2. The Raman spectra (Figure 3) were recorded on a Cary Model 82 spectrophotometer equipped with a Spectra-Physics Modd 171 argon ion laser operating on the 5145-A line. The Raman spectrum of the gas and the variable-temperature studies of the vapor were recorded by using the standard Cary multipass accessories. The spectrum of the liquid was obtained with the sample sealed in a glass capillary. The variable-temperature studies of the liquid were carried out by inserting the capillary into a cell similar to the one described by Miller and Harney.I9 Depolarization measurements were obtained using the standard Cary accessory. The spectrum of the annealed solid was obtained by depositing the sample onto a blackened brass block cooled by boiling liquid nitrogen and housed in a cell fitted with quartz windows. All measured Raman frequencies are expected to be accurate to f 2 . 0 cm-I. Vibrational Assignment

The trans conformer of ethylphosphonic difluoride has C, symmetry and should exhibit 27 fundamental vibrations spanning the irreducible representations 16A’ and 1IA”. The A’ vibrations are expected to give rise to A/B hybrid infrared gas-phase band (19) Miller, F. A,; Harney,

B. M. Appl. Spectrosc. 1970, 24, 291

contours and polarized Raman lines. The A” vibrations are expected to exhibit C-type infrared band contours and depolarized Raman lines. The gauche d o r m e r belongs to the trivial CI point group and is expected to yield A/B/C hybrid infrared-band contours and polarized Raman lines. Assignments are based on infrared-band contours and relative intensities, Raman depolarization ratios, and group frequencies. The observed infrared and Raman frequencies and vibrational assignment are given in Table I. The assignment of the carbon-hydrogen modes follows closely the assignment for these modes in other ethylphosphines. Of major interest is the CH2 twist, which is observed as a doublet in the Raman spectrum of the liquid, with a polarized band of medium intensity at 1239 cm-I and a depolarized shoulder at 1248 cm-I. Upon solidification and annealing, only a 1250-cm-’ band remains, which is assigned to this mode for the trans conformer. The PO stretch is observed as a weak polarized line in the Raman spectrum of the liquid at 1392 cm-l, which has a corresponding very strong band in the infrared spectrum of the gas a t 1360 cm-I. The CC stretch is assigned to the polarized Raman line at 1041 cm-’ in the spectrum of the liquid. The PC stretch is observed as a doublet at 721 and 715 cm-’ in the Raman spectrum of the liquid, and these are assigned to this mode for the gauche and trans conformers, respectively. The PF2symmetric and antisymmetric stretches for the trans conformer are assigned to the very strong bands at 875 and 895 cm-I, respectively, in the infrared spectrum of the solid. The CPO bend is assigned to the conformer pair observed at 338 and 330 cm-I for the trans and gauche conformers in the Raman spectrum of the liquid, respectively. The CCP bend is also observed for both the trans and gauche forms in the far-infrared spectrum of the gas in the 180-cm-I region. There are numerous Q-branches associated with this fundamental which presumably are hot bands of the asymmetric torsion. The PF2 wag, deformation, rock, and twist for the trans conformer are assigned to the lines at 489,427,419, and 292 cm-l in the Raman

Spectra and Structure of Organophosphorus Compounds

The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 543

TABLE I: ObservedaInfrared and R a m n Frequencies (cm-I) and Vibrational Assignments for CH3CH2P(0)F2

Raman infrared re1 int sol m 2997 m 299 1

gasb 3002 R 2997 Q, C 2969 R 2963 Q,A/B 2956 max

w, bd

2940 Q

W

2912 R 2905 Q, C 2898 P 1469Q,C 1463 1415Q,C 1372 R 1366Q, A 1360 P 1320 Q 1293 R 1289 Q

w

w

m sh, m m

re1 int depol

liq

~

sol

re1 int

vi

assignment approx description

w

m

2999

m, p

2974 2953 2944

s

2962

s,

2893

m m m

1466 1460 1418 1384

m sh, w m

vs

1342

vs

vw

1329 1300

s

2995

s,

dp

2992

vs

~ 1 7

CH3antisym str

vI

vs, p vs, p

2975 2953 2944 2907

vs

2957 2936

s

vI8

vs m

v2

CH3 antisym str CH2 antisym str CH2 sym str

1277 1270

m

1247

m

1046 1043 1035 1032

m m

2939 2906

s, s,

v3

p p p

m

1046 Q

m

1035 R 1031 Q 1024 Q, C 985 max 953 Q

m m mw, bd m

897 max 888 Q 752 max 721 Q 713 Q 501 R 492Q,A, B 487 P

ws ws

m m m s

m m,bd S S

2898

s, p

2893

s

2808 2763 1466

w, p w, p m, dp

1411

m, dp

2821 2755 1469 1460 1425 1386

m m sh, m ms m

vw

2v4

CH3 sym str 2v5 2v6

v5

CH3 antisymdef CH3 antisym def CHI def

vI9 v4

w

1392

w, p

1334

m

v6

P o str

1331

m, P

1327

sh, m

v,

CH3 sym def

1286

sh, m, p

vgl

CH2 wag

1273

m, p

1271

s

v8

CHI wag

1248 1239

sh, w, dp m, p

1250

m

v20 v20/

CH2 twist CH2 twist

m,p

1041

m, P

1049

m

ug

CC str

1033

m

m, p

mw, p dp

vZI vZI'

980

1027 987

988

ms

vl0

CH3rock CHI rock CH3 rock

902 879 742 721 713

w,dp m, p m, p s, p s,p

906 882 752 721 715

896 884 756

m m

Y~~

w

"23

715 496 489

ws

v12

PF2 antisym str PF2 sym str CH2rock PC str PC str

m m

"13

PF2

435 427 419 340

s

s

q5 vls)

PF2 def PF2 rock CPO bend CPO bend

1366

s, p

w

m

vw, bd vw, bd

291 Q, C 284 P 192 Q, C 185 Q 180 Q 75.7 Q 67.5 Q

gas

W

1287 Q 1280 P 1242 max 1232 max

426 Q, C 416 max 333 Q 324 Q 322 Q 317 P

re1 int

re1 int depol

s

1275

1041

w, p

s s

992 946 926 895 875 750

m m m vs vs m

713

m

492

s

468 428 425 411 338 (330)

w

491

w, p

493

sh, w, dp

m, p mw,p vs, p vs, p m, p

427 415

m, p sh, w,dp

m

332

298 292

m m

187

m

S

wag

468 vw

s

sh, s

vI1

m, p

430 414 338 330

m, p sh, m,dp m, p m, p

289

w, dp

292

w, bd, dp

292

w

v25

PF2 twist

w, bd,p

184

w.p

201 187

w, bd w, bd

"26

187

126

m

CHI torsion CCP bend CCPbend asym torsion asym torsion

S

m m

~ 1 4

"24

S

W

m m m

v16/

130 m

~ 2 7

v2,'

98 82 56

m m

102

w

74 74 61

m

42

s

lattice modes

m

m

m

"Abbreviations used: w, weak; s, strong; m, medium; v, very; p, polarized, dp, depolarized; sh, shoulder; bd, broad; A, B, C refer to infrared-band envelopes; P, Q, and R refer to the rotational-vibrational branches; uj refers to the more stable trans conformer, vi, refers to the gauche conformer. Parentheses indicate bands observed in the spectra of the unannealed solid. bBands with no indicated contour are broad and featureless, and the frequency of the maximum absorption is reported. spectrum of the solid, respectively. For most of these modes, a second peak for the gauche conformer has not been observed in

the fluid phases and it is assumed that they are nearly degenerate with the corresponding modes for the trans conformer.

Durig et al.

544 The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 TABLE II: Temperature and Intewity Ratios ( K = 1721/I,1p (Vapor) and K = 1721/171s (Liquid)) for the Conformrtid Stability Study of

gas

25.0 36.0 47.0 56.0 64.0

1.50 1.53 1.55 1.57 1.62

3.35 3.23 3.12 3.04 2.97

0.405 0.425 0.438 0.451 0.482

AH = 126 f 53 cm-I (359 f 151 cal/mol) liquid

-22.0 3.98 -14.0 3.86 -3.0 3.70 3.58 6.5 3.49 13.0 3.38 23.0 AH = 167 f 20 cm-I (478 f

TABLE LII: Observed .sd Calculated Froqacncies (cm-') for the Asymmetric Torsion in Etbylphospboaic Mfluoride transition obs obs - c a l e Trans l+O 75.72 -0.04 2-1 73.62 -0.12 3+2 7 1.64 0.00 4+3 69.56 0.13 1' 2'+ 3*

+

0.937 -0.0651 0.973 -0.0274 0.999 -0.0013 1.04 0.0344 0.0488 1.05 0.0871 1.09 56 cal/mol)

+

Of 1' 2'

Gauche 67.48 65.18 63.20

-0.14 -0.17 0.27

"Calculated using the potential constants from Table IV and Fo = 0.885996, F1 = 0.827682 X lov2, F2 = 0.388941 X 1W2, F, = 0.107995 x F4 = 0.658553 X F5 = 0.435084 X lod, F6 = 0.159585 X lo-', F, = 0.113570 X lo-*, Fs = 0.532523 X 1O-Io. TABLE I V Ewrgy Parameters (cm-') for Ethylphosphoaic Dinuoride as Determid by Infrared Spectroscopy and ab Initio calcUlrtiw4 ab initio 3-21GS 6-31G' potential constants infrared 3-21GS opt' 6-31GS opt" VI -35 f 6 V2 203 f 4 v, 661 f 1 AH 120f 1 1 121 86 50 -29b trans to gauche 808 804 751 859 801 barrier gauche to gauche 506 649 620 677 670 barrier gauche to trans 688 684 665 809 830 barrier

,

80

I

I

I

I

I

L

I

t

70

L

I

I

k

60

WAVENUMBER (cm-')

Figure 4. Far-infrared spectrum of the asymmetric torsion in CH3CH2P(0)F2in the gas phase with a water trace on top.

Conformational Stability Since bands due to both conformers are observed in the spectra of the fluid phases, a study of the relative intensities of a conformer pair as a function of temperature was undertaken in order to determine the enthalpy difference between the trans and gauche forms. The bands observed at 721 and 715 cm-I in the Raman spectrum of the liquid (721 and 713 cm-' for the gas), which have been assigned to the C-P stretching modes of the trans and gauche conformers, respectively, appear to be good candidates for such a study since these bands are fairly well resolved, symmetric, and fairly intense. Also, there is no interference in this region due to other fundamentals. Measurements of these bands were made at five temperatures (25.0-64.0"C)for the gas phase and at six temperatures (-22.0to +23.0 'C) for the liquid phase (Table 11). It has been shown by Hartman et al.,O that the ratio of the band areas is related to AH by

In ( A * / A ) = (-AH/RT) and the AHcan be obtained by plotting In (A*/A) against (l/R7'). Since both bands utilized in the calculation were rather sharp and highly symmetric, as well as very similar in contour, the ratio of their intensities was used instead of the ratio of their areas. The AH values obtained from the slope of the least-squares fit to the observed data points were 126 f 53 cm-' (359 f 151 cal/mol) for the gas and 167 f 20 cm-I (478 f 56 cal/mol) for the liquid. The error limit at 95% confidence is estimated from the results of the least-squares fit and consideration of the quality of the observed data. (20) Hartman, K. 0.;Carlson, G. L.; Witskowski, R. E.; Fateley, W.G. Spectrochim. Acta 1968, 24A, 157.

"Opt refers to optimization of structural parameters at energy minima and maxima. b A negative value indicates that the gauche form was calculated to be more stable.

Torsional Potential Function The asymmetric torsional fundamentals for both the trans and gauche conformers of ethylphosphonic difluoride are assigned to the Q-branch transitions observed at 75.72 and 67.48 cm-', respectively, in the far-infrared spectrum of the gas (Figure 4). In addition to the torsional fundamentals, a number of torsional excited-state transitions falling to lower frequencies from the fundamental for both conformers have been observed, and these data are tabulated in Table 111. Using optimized structural parameters for both conformers determined from the 6-3lG* basis set, the internal rotation constant was calculated as a function of the internal rotation angle 4 with structural relaxation according to the following expression F(4) = Fo + CFi cos i4 i

where 4 is defined as zero for the trans conformer. Utilizing the assignment given in Table 111, we can represent the torsional potential as a cosine-based function in the dihedral angle 4 having the form

V(4) = !l2CV(l -cos i4) i

Initially, only the ground- and first excited-state torsional transitions of each conformer along with the experimental enthalpy difference for the gas were used in the calculations to obtain the potential coefficients V,, V,, and V,. Additional torsional transitions were subsequently added to further refine the fit. The V4, V,, and V, terms were then tested,and it was found that they were not necessary and were set to zero for the final calculation. The values of all coefficients, their dispersions, and AH are listed in Table IV,and the resultant potential function is shown in Figure 5. This potential function shows minima at q5 = 0 and 122.65' for the trans and gauche wells, respectively. The trans to gauche and the gauche to trans barrier heights are 808 cm-* (2.31

The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 545

Spectra and Structure of Organophosphorus Compounds

TABLE V Ab Initio Structural Parameters of Ethylphosphonic DifluorideO

trans

gauche

3-21G'

160

reo

0 DIHEDRAL ANGLE,

CP

Figure 5. Potential function of the asymmetric torsion in CH3CH2P(O)F, determined from the far-infrared torsional frequencies.

kcal/mol) and 688 cm-I (1.97 kcal/mol), respectively, whereas the gauche to gauche barrier height is 506 cm-' (1.45 kcal/mol). The value of 120 f 11 cm-' for the enthalpy difference agrees well with the value of 126 f 53 cm-' obtained experimentally from the temperature dependence of the Raman spectrum of the gas.

Ab Initio Calculations We have camed out LCAO-MO-SCF restricted HartreeFock calculations with the program2I Gaussian-86 for ethylphosphonic difluoride. Similar calculations carried out for ethylphosphine" gave results in very good agreement with those obtained experimentally. The energy minima with respect to the internal coordinates were determined by simultaneous relaxation of all geometric parameters using the gradient method of Pulay.22 Structural Parameters. Structural optimizations were carried out for both the trans and gauche conformers utilizing both the 3-21G* and 6-31G* basis sets, and the calculated structures for these two conformers are given in Table V. One notable difference between the two conformers of ethylphosphonic difluoride is the CCP bond angle which closes by approximately 2.5' when rotating the P(0)F2 moiety from the trans form to the gauche form. At the 3-21G* basis set level, the CP bond distance is calculated to be approximately 1.770 A. This value is somewhat shorter than the microwave value of 1.795 f .019 A for this bond in methylphosphonic diflu0ride2~and significantly shorter than the value of 1.821 f 0.017 A for this bond in (chloromethy1)phosphonic d i f l ~ o r i d e . ~Similarly, ~ a short PC bond length of 1.753 A was calculated with a similar basis set in a theoretical study of methylphosphonic difluoride.2s The PC distance from the 6-31G* basis (1.791 A) is in mare reasonable accord with the expected value. The CC bond length is similarly calculated too large with the 3-21G* basis set, which gives a distance of 1.55 A. From microwave spectroscopy, we have determined the CC bond length to be approximately 1.53 A for ethyldifluorophosphine,loethylphosphonothioic difluoride,16and ethylphos hine.' At the 6-3 1G* basis set level, however, a value of 1.53 is calculated, which is consistent with the value of this distance from the microwave results. The remaining bond distances should be in error only by small systematic amounts, and the angular parameters are expected to be determined with relatively high accuracy. The rotational constants, as well as the dipole moments for both conformers of ethylphosphonic difluoride, determined with the

8:

(21) Frisch, M. J.; Binkley, J. S.;Schlegel, H. B.; Raghavachari, K.;

Melius, C. F.; Martin, R. L.; Stewart, J. J. P.; Bobrowicz, F. W.; Rohlfing,

C. M.; Kahn, L. R.; Defrces, D. J.; Seeger, R.; Whitwide, R. A.; Fox, D. J.; Fleuder, E. M.; Pople, J. A. Guussiun-86;Carnegie-Mellon Quantum Chemistry Publishing Unit: Pittsburgh, PA, 1984. (22) Pulay, P. Mol. Phys. 1969, 17, 197. (23) Durig, J. R.; Stanley, A. E.; Li, Y.S.J . Mol. Srrucr. 1982, 78, 247. (24) van der Veken, B. J.; Coppens, P.; Johnson, R. D.; Durig, J. R. J . Chem. Phys. 1985.83, 1517. (25) von Carlowitz, S.;Zeil, W.; Pulay, P.; Boggs, J. E. J . Mol. Srrucr. 1982, 87, 113.

r(PO) r(PC) r(CC) r(PF,,) ~(PFII) a(CCP) a(CP0) a(CPF10) a(CPFd ~(FIoPFII) r(Fl0PCO) r(FI1PCO) s(C~C~PO)

6-31G* 3-21G' Frame and P(0)F2 Group 1.4382 1.4421 1.4415 1.7915 1.7681 1.7699 1.5334 1.5532 1.5512 1.5503 1.548 1 1.5482 1.5470 1.5482 1.5503 115.06 111.27 113.04 118.14 119.26 119.86 104.08 103.31 103.32 103.95 103.32 104.08 98.94 98.56 98.60 128.59 128.05 128.85 -128.59 -129.05 -128.85 180.00 300.02 180.00

r(CH7) GHa) 43337) 4CCH8) a(H7CH8) a(PCH7) dPCH8) r(H+22C3Hd) r(HgC2C3H4)

1.0853 1.0853 110.07 110.07 106.57 108.43 108.43 58.59 -58.59

CH2 Group 1.0853 1.0853 11 1.07 11 1.07 106.45 106.34 106.34 59.14 -59.14

CH3 Group 1.0832 1.0812 1.0832 4%) 1.0821 1.0832 r(CH6) 1.0821 109.62 a(CCH4) 109.73 111.31 4CCHS) 110.49 111.31 a(CCH6) 110.49 108.33 a(H5CH.J 108.64 108.08 (Y(HqCH6) 108.73 r(PCCH4) 180.00 180.00 119.52 T(HSC~C~H~)119.87 -1 19.52 f(HgC3C2H4) -1 19.87

r(CH4)

6-31G' 1.4387 1.7909 1.5349 1.5505 1.5493 112.49 118.58 103.92 103.68 98.75 128.48 -128.72 302.65

1.0849 1.0847 109.55 110.03 107.34 108.74 109.82 59.09 -58.69

1.0851 1.0846 110.59 111.22 107.34 106.76 108.21 59.49 -59.68

1.0813 1.0830 1.0822 109.67 110.51 110.42 108.73 108.88 179.36 119.68 -1 19.99

1.0832 1.0839 1.0829 109.71 11 1.24 111.00 108.37 108.42 178.75 119.43 -119.81

Distances in angstroms, and angles in degrees.

TABLE VI: Ab Initio R o t P t i ~ Constants ~l (MHz), Dipole Moments (D), and Energy (hartrees) of Ethylphosphonic Difluoride

trans A

B

C pa pb Pc p,

-E

3-21G' 4391.1 2366.9 2320.2 4.23 0.50

0.0 4.26 689.7277318

6-31G' 4329.2 2336.2 2277.1 4.29 0.14 0.0 4.29 693.2178397

gauche 3-21G' 4296.6 2306.6 2263.3 3.06 2.73 0.55 4.14 689.7273401

6-31G' 4279.4 2276.0 2236.1 2.95 2.90 0.33 4.15 693.2179701

3-21G* and 6-31G* basis sets, are given in Table VI. At the 6-31G* basis set level, the rotational constants are consistent with

a near prolate top for both the trans and gauche conformers with K values of -0.94 and -0.96, respectively. While the total dipole moments determined by ab initio calculations are roughly the same (approximately 4.2 D) for both conformers, the individual dipole moment components are quite different. For the trans conformer, pais the predominant component with a value of 4.29 D at the 6-31G* level, whereas the component pb is very small with a value of 0.14 D and pc = 0 by symmetry. On the other hand, p, and p b of the gauche conformer are almost equivalent with values of 2.95 and 2.90 D, respectively, at the 6-31G* basis set level. The component pc of the gauche conformer is also quite small with a value of 0.33 D. Therefore, from these ab initio results, one would expect the microwave spectrum of ethylphosphonic difluoride to have relatively intense A-type rotational transitions for the trans conformer and equally intense A- and B-type transitions for the gauche conformer.

Durig et al.

546 The Journal of Physical Chemistry, Vol. 96, No. 2, 1992

TABLE W: Symmetry Used io the Normal-Coordinate Calculations of Ethylphosphonic Difluoride species symmetry coordinates description CH, antisym str A CH2 sym str CH3 sym str CH, antisym def CHI def PO str CH, sym def CH2 wag CC str CHI rock PF2 sym str PC str PF2 wag PF2 def

Figure 6. Numbering and internal coordinates of CH,CH2P(0)F2.

Conformational Stabilities. The optimized structures (Table V) were also utilized to obtain a potential surface scan in which only the CCPO torsional dihedral angle was allowed to vary from the more stable trans position (180') in 10' increments to the saddle point a t 360' or the cis position. Both the 3-21G* and 6-31G* basis sets were employed in an identical manner. The potential surface obtained in this manner predicted the only other stable conformer to be approximately 120' away or the gauche conformation, with an energy difference of 121 cm-I (346 cal/mol) at the 3-21G* level and 50 cm-' (143 cal/mol) at the 6-31G* level. However, upon optimization of the structural parameters for the gauche form, this energy difference decreases to 86 cm-I (246 cal/mol) with the 3-21G* basis set, and at the 6-31G* basis set level, the gauche conformer is calculated to be the more stable conformer by 29 cm-I (83 cal/mol). It is not uncommon to find theoretical studies in which the geometries are optimized only at those points on a potential surface which occur as minima. If the presently obtained results are handled in this manner, a trans/gauche potential surface is obtained having a trans to gauche barrier of 804 cm-' (2.30 kcal/mol) with the 3-21G* basis set which increases to 859 cm-' (2.46 kcal/mol) with the 6-31G* basis set and a gauche to gauche barrier of 649 cm-] (1.86 kcal/mol) and 677 cm-' (1.97 kcal/mol) with the two basis sets, respectively. If geometry optimization is carried out at these maxima, then the resulting trans to gauche barrier decreases to 751 cm-' (2.15 kcal/mol) and 801 cm-l (2.29 kcal/mol) for the 3-21G* and 6-31G* basis sets,respectively. The gauche to gauche barrier decreases to 620 cm-' (1.77 kcal/mol) and 670 cm-l (1 -92 kcal/mol) with the two basis sets, respectively. These theoretical results are compared to those determined experimentally in Table IV. Normal Coordinates. The optimized structural parameters which were obtained from the 3-21G* basis set for both conformations were subsequently used to calculate the force fields and the vibrational frequencies of ethylphosphonic difluoride. From previous studies, it has been found that the values of the force constants and frequencies are not significantly different when the smaller, as opposed to the larger, basis set is used.26 In order to obtain a more complete description of the molecular motions involved in the normal modes of ethylphosphonic difluoride, we carried out a normal-coordinate analysis. The following procedure was used to transform a b initio results, which are in terms of Cartesian coordinates, into the form required for our iterative normal-coordinate program. The Cartesian coordinates obtained for the optimized structures were input into the Gmatrix program together with the complete set of 30 internal coordinates (Figure 6). This complete set of internal coordinates was used to form the symmetry coordinates with three zero-frequency redundancies, and they are listed in Table VII. The output of this G-matrix program consists of the Bmatrix and the unsymmetrized Gmatrix. The B-matrix was used to convert the ab initio force fields in (26) Forgarasi, G.;Pulay, P. In Vibrational Spectra and Structure; Durig, J . R., Ed.; Elsevier: Amsterdam, 1985: Vol. 14.

CPO bend CCP bend

A"

redundancy redundancy redundancy CH, antisym str CH2 antisym str CH, antisym def CHI twist CHI rock PF2 antisym str CH2 rock PF2 rock PF2 twist CHI torsion asym torsion

'Not normalized. bThe symbols utilized are defined as follows: LHCH(CH,) = LHCH(CH,) = 6 ; LCCP = e; LHCC(CH,) = u; LHCP = C; LHCC(CH3) = 8; LCPF = E; LFPF = 7;LCPO I ) ; LFPO = 4; other symbols are self-explanatory.

Cartesian coordinates to force fields in the desired internal coordinates which are provided in Supplementary Material Tables S1 and S2. All diagonal elements of the obtained force fields in internal coordinates were assigned scaling factors. These force fields were then used as input, along with the unsymmetrized Gmatrix and scaling factors, in the perturbation program written by Schacht~chneider.~~ Initially, all scaling factors were maintained a t a value of 1.O to produce the pure a b initio calculated vibrational frequencies. Subsequently, scaling factors of 0.9 for stretching coordinates, 0.8 for bending coordinates, and 1.0 for torsional coordinates and the geometric average of scaling factors for interaction force constants were used to obtain the "fmed-scale" force fields and resultant frequencies along with the potential energy distributions (PED) which are given in Tables VI11 and IX.

Discussion The Raman and infrared spectra of ethylphosphonic difluoride indicate that two conformers are present in the gas and liquid, whereas only one conformer remains in the annealed solid. The CH2 twist in the Raman spectrum of the liquid was observed as a doublet consisting of a depolarized line corresponding to the trans conformer and a polarized line corresponding to the gauche conformer. Because the polarized line disappears in the spectrum of the annealed solid, we have concluded that the trans form is the more stable conformer of ethylphosphonic difluoride in the solid state. We have also observed conformer bands for the CP stretch in ethylphosphonic difluoride. Similar conformer pairs were observed in the vibrational spectra of many of these (27) Schachtschneider,J. H. Vibrational Analysis of Polyatomic Molecules (Parts V and VI). Technical Report Nos.231 and 57; Shell Dcvelopment Co.: Emeryville, CA, 1964 and 1965.

Spectra and Structure of Organophosphorus Compounds TABLE WI:Observed and Calculated Frequencies (cm-I) for the Trans Conformer of Ethylphospboaic Difluoride vi

u9

description ab initioa CH3 antisym 3296 3217 CH2 sym str 3223 CH3 sym str 1680 CH3 antisym def 1624 CHI def PO str 1525 CH, sym def 1582 CH2 wag 1458 CC str

obs 2963 2940 2905 1463 1415 1366 1320 1287

946

1046

Y~~

vI0

CH3rock

1160

u22 PF2 antisym str

1078

wI1 u2, uI2

PF2 sym str CH2rock PC str

1041 855 808

3294 3252 1671

u13

PF2

wag

536

1403

~ 1 4

ul0

CH3rock

1165

1047

985

wII

PF2 sym str

1050

993

888

uI2

PC str

798

750

713

u13

PF2

uI4

wag PF2 def

539 461

uI5

CPO bend

362

uI6 CCPbend

99

A" uI7 CH, antisym str u I 8 CH2 antisym str uI9 CH3 antisym def w~~

CH2 twist

PED' 99s1 98S2 99S3 9OS4, 8Slo 76S5, 16s6 69s6, 2os5 96S, 69S,, 8Sl0, 7S12 42S9,41Sll, 8SlO 58Sl0, 18SR, 8S9 37SIl, 38S9, 18S1, 61S12,'iOSIl,

TABLE IX: Observed" and Calculated Frequencies (cm-I) for the Gauche Conformer of Ethylpbosphonic Difluoride

description ab initiob scaled' obs PEDd CH, antisvm str 3287 3119 2997 68S1,. 19S,. 12S,* CH; anti4m str 3295 CH2 antisym str 3258 CH2 sym str 3220 3217 CHI sym str CH3 antisym def 1669 CH3 antisym def 1678 1626 CH2 def PO stretch 1516 CH3 sym def 1581 1471 CH2 wag 1400 CH2 twist 1006 CC str 1167 CH3rock

1003

scaledb 3127 3052 3058 1503 1457 1434 1414 1315

The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 547

ui Y,,

vi' uI8 u2

u3 uI9 u4

us Y6

u1

us u20 u9

PF2 def PF2 rock 5 CPO bend

440 468 321

PF2 twist CH3 torsion CCP bend

351 217 178

u21 asym torsion

63

~ 2 4

u~~ CH3rock

1171

u22 PF2 antisym str

1066

u23 u~~

CH2rock PF2 rock

842 453

~ 2 5

PF2 twist

308

Y26

CH, torsion

224

u27

asym torsion

80

~ 1

~ 2 5

"Ab initio frequencies determined from the 3-21G* basis set. bSealing factors: 0.9 for stretching, 0.8 for bending, and 1.0 for torsional coordinates. For a description of the symmetry coordinates (S,) see Table VII. ethyl-substituted phosphines. We have further found that the CP stretch corresponding to the gauche conformer falls a t a higher frequency than that of the trans conformer in these molecules. This is also observed in ethylphosphonic difluoride. From our variable-temperature studies of these bands in the Raman spectra of the vapor and liquid, we determined that the trans conformer is the more stable form by 126 f 53 and 167 f 20 cm-I, respectively. These results were further supported by the asymmetric torsional potential function of ethylphosphonic difluoride where the enthalpy difference was determined to be 120 f 11 cm-I. It should be noted that no reasonable fit could be obtained when attempting to fit the torsional transitions to a potential function in which the gauche conformer is of lower energy. While the ab initio results from the 3-21G* basis set are in agreement with these findings, the results from the 6-31G* basis set are contradictory. It is possible that the experimentally obtained energy differences could be more accurately reproduced by adding electron correlation up to MP4, but the added cost probably does not warrant such a calculation unless one is trying to minimize the values between calculated and experimental parameters. The potential energy distribution for both the trans and gauche conformers of ethylphosphonic difluoride show substantial mixing of the normal modes, especially for the low-frequency vibrations. The error in the frequencies calculated for the normal modes was found to be 13% for both the trans and gauche conformers;

Y26

Normal vibrations are numbered according to the trans conformer. bAb initio frequencies determined from the 3-21G* basis set. CSealing

factors: 0.9 for stretching, 0.8 for bending, and 1.O for torsional coordinates. dFor a description of the symmetry coordinates (S,) see Table VII. however, upon scaling, the average error of the frequencies drops to 5% for the two conformers. For the trans conformer, a b initio results indicate that the CPO bending motion is nearly equally distributed among the 426, 333-, and 185-cm-' bands. It is not too surprising that the PF2deformation, CPO bend, and CCP bend are heavily mixed, considering the similar masses of the oxygen and fluorine atoms. Several of the heavy atom vibrations are calculated to have significantly different frequencies for the two conformers, which agrees with our experimental assignments for the most part. The ab initio structure determined with the 6-31G* basis set are consistent with those determined from microwave spectroscopy for many of the ethyl-substituted phosphines. It is interesting to note that the CP and PF bonds exhibit a shortening in distance within the series CH3CH2PF2,5CH3CH2P(S)F2,16and CH3CH,P(0)F2 with values of 1.835, 1.814, and 1.791 A, respectively, for the C P bond distance and 1S86, 1S63, and 1.550 A for the PF bond distance. This shortening of the PC and PF bonds can be attributed to the withdrawal of electron density from the phosphorus atom by the sulfur atom in CH3CH2P(S)F2and the resulting changes in the nature of the bonding on going from a P(II1) to a P(V) arrangement, which is further intensified by the more electronegative oxygen atom in CH3CH2P(0)F2. The significant structural differences in the CCP bond angle predicted by ab initio calculations is also evident in the microwave structures of several ethyl-substituted phosphines.'s'0s'6 It is interesting that conformer bands are observed for the CCP bending fundamental in all of these compounds. It is also interesting to note that the PS bond length in CH3CH2P(S)F2was found to

548

J. Phys. Chem. 1992,96, 548-554

decrease from 1.880 A in the gauche conformer to 1.861 A in the trans conformer and that conformer bands were observed for the PS stretch in the Raman spectra. No significant change in the PO bond distance was evident in the ab initio structures of CH,CH2P(0)Fz, and similarly, no conformer bands for the PO stretch in the vibrational spectra were observed. This study represents another one in our investigation of a series of ethyl-substituted phosphines. The ab initio results, although not in total agreement with the experimental findings, are reasonable, given the size and the addition of heavy atoms in CH3CH2P(0)F2as opposed to CH3CH2PH2. A study of the corresponding compounds where the lone pair is used for bonding to the borane group would be of interest since the change in

bonding for these molecules would not be as drastic as those for the corresponding oxygen- or sulfur-containing molecules. Acknowledgment. We gratefully acknowledge the partial financial support of this study by National Science Foundation Grant CHE-83-11279. Also we thank Dr. C. G. James for recording the initial infrared spectrum. Registry NO. CH,CH,P(O)F,, 753-98-0. Supplementary Material Available: Listings of a b initio predicted force constants for trans-ethylphosphonic difluoride and gauche-ethylphosphonic difluoride (2 pages). Ordering information is given on any current masthead page.

Vibrational Studies, Normal-Coordinate Analysis, and Infrared VCD of Alanylalanine in the Amide I11 Spectral Region M. Diem,* 0. Lee, and G.M. Robertst Department of Chemistry, City University of New York, Hunter College, 695 Park Avenue, New York, New York 10021 (Received: May 20, 1991; In Final Form: September 17, 1991)

Detailed vibrational assignments and normal coordinate calculations for L-alanyl-L-alanine and L-alanyl-palanine and several deuteriated isotopomers in the spectral region between 1200 and 1700 cm-' are reported. In this spectral region, the peptide amide I, 11, and I11 vibrations occur. Large infrared vibrational circular dichroism (VCD) intensities are reported for the amide I11 vibration. These studies were undertaken to obtain a description correlating the atomic displacements to strong VCD intensities. We demonstrate that large VCD signals occur in either delocalized, coupled C-H/N-H deformations, or in C*-H deformations which are similar to those which produce large VCD in alanine itself.

Introduction Vibrational spectroscopy has been used for the determination of peptide solution and solid phase conformation. In particular, observed frequencies of the amide I11 vibration have been utilized as a qualitative probe for the solution conformation of peptides and proteins in Raman and infrared spectroscopies.' This is possible since the amide 111 vibration exhibits frequency shifts which depend on the secondary structure. Lordl proposed a quantitative correlation between the conformational angle 9 and the frequency of the amide TI1 vibration; however, for most purposes the correlation between the amide I11 frequencies and the secondary structure remained purely qualitative. The qualitative character of this correlation is partially due to the poor understanding of the nature of the amide I11 vibration. We have found over the past years that this vibration is a complex, delocalized motion of C-H and N-H deformation coordinates, quite different from the description developed originally by Miyazawa et aL2 A reevaluation of this vibration via detailed normal coordinate calculations will be a major part of this publication. The vibrational assignment which underlies the normal-mode calculations is based on our previous Raman ~ t u d i e s . ~ In addition, new VCD features in the amide 111vibration will be presented. Previously, amide I11 VCD had been reported for L-Ala-L-Ala in H 2 0by us4 and in D 2 0 by Freedman et aL5 In order to discuss the origin of the amide 111VCD, vibrational data in the 1200-1400-~m-~region of a number of small peptides will be presented and interpreted. The results presented here suggest that large VCD signals are observed in delocalized vibrations composed of N-H and C-H deformation coordinates, or in C-H deformation vibrations which resemble those modes in alanine which produce large VCD effects6 'Present address: Biophysical Research Division and Department of Physics, The University of Michigan, Ann Arbor, MI 48109.

0022-3654/92/2096-548$03.00/0

Experimental Methods All VCD results discussed were obtained on the Hunter College dispersive VCD instrument, which has been operative since 1987. Its design and performance were described in detail.' Infrared data were obtained simultaneously with the VCD data via the VCD spectrometer. Undeuteriated peptide samples were obtained commercially (Sigma Chemical Co., Chemical Dynamics or Research Plus). Peptides were checked for purity via N M R and Raman spectroscopies. All alanyl peptides, which are deuteriated at the alanine 2-carbon, were synthesized and purified in-house via solid-phase peptide synthetic method^.^ Deuteriation of any labile protons was achieved by lyophilization of the samples from DzO. Samples, dissolved in either H20or D20, were contained between CaFz plates separated by 15- or 25-pm spacers. Concentrations were 0.5 M. Due to difficulties in reproducing the sample path length exactly, results are given in units of absorbance AU, rather than molar extinction coefficients. Computational Procedures The purpose of the normal-coordinate calculations presented here is to derive a force field and atomic displacement vectors ( 1 ) Lord, R. C. Appl. Spectrosc. 1977, 31, 187. (2) Miyazawa, T.; Shimanouchi,T.; Mizushima, S.J. Chem. Phys. 1958, 29, 611. ( 3 ) Oboodi, M. R.; Aha, C.; Diem, M. J . Phys. Chem. 1984, 88, 501. (4) Roberts, G. M.; Lee, 0.; Callienni, J.; Diem, M. J . Am. Chem. Soc. 1988, 110, 1749. ( 5 ) Freedman, T. B.; Chernovitz, A. C.; Zuk, W. M.; Paterlini, M. G.; Nafie, L. A. J. Am. Chem. SOC.1988, 110,6970. (6) Diem, M. J . Am. Chem. Soc. 1988, 110,6967. (7) Diem, M.; Roberts,G. M.; Lee, 0.;Barlow, A. Appl. Specrrosc. 1988, 42, 20.

0 1992 American Chemical Society