Vibrational spectra and rotational isomerism of fluoroacetone

197 Journal of Molecuiar Stractzfre, 15 (I 973) i 97407 @ Ekevier Scientific Publishing Company, Amsterdam - Printed in Tbe Netherlands

VIB~TI~NAL ACETONE

SPECTRA AND ROTATIONAL

ISOMERISM OF FLUORO-

CL A. CROWDEF&AND POTlEPRUETTIANGKURA Department ofChemistry and Killgore Research Center, West Texas State Uniuersity, Canyon, Tex. 79oI5 (U.S.A.) (Received 30 August 1972)

ABSTRACT

Infrared spectra have been redetermined for fluoroaeetone in the vapor, liquid, and solid states, and Raman spectra have been obtained for the liquid. There are two rotational isomers present in the liquid, but only the more polar form is present in the crystalline solid and only the less polar form is present in the vapor. Vibrational assignments were made for the two rotamers with the aid of normal coordinate calculations that utilized a twenty-five parameter valence force field.

INTRODUCTION

Fiuoroaeetone has been shown to exist as a mixture of two rotational isomers in the neat liquid, but as a single rotamer in the vapor state at room temperature”. The solid-state infrared spectrum was obtained only for the glassy state and both rotamers were present. The infrared spectrometer used in the previous study was a low-resolution instrument, and assignments were made for onIy a few bands. In an effort to complete the vibrational assignment of fluoroacetone, infrared spectra for the vapor, liquid, and solid states have been redetermined, Raman spectra have been obtained for the fiquid, and normalcoordinate calculations have been made.

EXPERIMENi.AC

Infrared spectra were obtained with a Beckman IR-12 spectrophotometer. Raman spectra were obtained with a Jarrell-Ash 25-100 dual monochromator

198 IOC

c-

L

s

cIL 3200

I

I

2400

18OOcm-’ 1400

1000

I

I

600

Fig. 1, Liauid- and solid-state spectra for Ruoroacetone. iiquid film; c, solid at about - 196 “C.

0 3200

2400

1800 cm-l 1400

Fig. 2. Infrared spectra of fiuoroacetone

1000

600

400 a. liquid at 0.015 mm thickness; b,

400

vapor.

photon-countingspectrometerwitha cooled FW-30 PM tubeand a 50 mW He-Ne laser . The sampfeof fkxoroacetone was obtainedfrom Aldrich ChemicalCompany, and was statedto be 98 oApure.

199 RESULTS

AND

DISCUSSION

Infrared spectra for Auoroacetone are shown in Fig. 1 for the liquid and solid states and in Fig. 2 for the vapor. The liquid-state Raman spectrum is shown in

Fig. 3. Unlike the previous work, the solid whose spectrum is shown here appears

/ / 3200

2800

1800

cm-1 1400

Fig. 3. Raman spectra of fluoroacetone

1000

600

200

liquid, showing polarization

of the bands.

to be crystalline, and there is undoubtedly only one rotamer present, as evidenced by the absence of several liquid-state bands, notably those at 780, 1233, and 1422 cm-’ (this latter band was resolved in CCI, solution and is easily assigned to the less polar rotamer). From the dependence of band intensities on solvent polarity, the rotamer present in the solid was identified as the more polar one. The vaporstate spectrum shows that only the less polar rotamer is present in the vapor, as had been demonstrated previously’. The observed wavenumbers are listed in TabIe 1. Vibrationalassignments

The configurations of the two rotamers in the following discussion will be assumed to be: (1) the fluorine and oxygen atoms are eclipsed, called the 0-cis form, and (2) the plane of the C-C-F bond lies at an angle of 120” to the plane of the C-C=0 bond, called the O-gauche form. The molecular dipole moment would have its largest possible value in the 0-cis form. Mizushima et al., have shown the more polar form of chloroacetone to be the 0-cis form2, and it seems a reasonable assumption that fluoroacetone behaves the same way.

200 A number of the bands can be readily assigned to the appropriate vibrational mode. The C-O stretch bands of the two forms are not resolved, but that for the O-cis form is undoubtedly the band at 1740 cm-‘*_ The C=O stretch of the O-gauche form appears as a weaker shoulder on the low-frequency side of the 1740 cm-’ band, at about 1725 cm-‘. These two bands show the expected change in relative intensities with change in solvent polarity. There are three bands in the liquid-state spectrum between 1450 and 1300 cm-‘, but a fourth band is resolved, at 1422 cm-r, in CCI, solution. Only three

TABLE

I

INFRAFtED

AND

RAMAN

SPECTRAL

DATA

Infrared

Vapor 3033 3024 3014 1 2977 2958I 2943 2878 2127 1757 1751 -1745 1 1542 1450 1435

1374 1363I -1275 sh 1236 1226I

1111

FOR FLTJOROACETONE

Roman Solid

Liquid

Liquid

3012

3015 d

2971=

2979 d? 2945

294s p 2861 p

1739

1737 p

1438

I436 d

1390

1389 d

1363

1363

1364 p

1273 -1250 sh

1257

1256 d

1193 1170 1125 -1090 1067

1191 d

2939 2860 2143 1740 -1725

sh

1540 -1450 1437 1422= 1388

1233 1189

1091” 1066

1067 p

* All wavenumbers referredto are for the liquid state, since the C&is form is absentin thevapor, and liquid-vapor frequencyshiftsfor the O-gaucheform wouId changethe reIativepositionsOf some of the frequ&cies of the two forms if the vapor-statewavenaatberswere used for the O-soudreform-

201 Table 1 (continued) infrared Vupr 1075 1066 -1059 I 1020

Liquid

Solid

Raman Liquid

sh

-1050

sh 968 890 vvw

-1020

970

972 p

874

-

860 875 845I

850

-

826

850 b 827

795 779I

780

766 608

607

604

495

f

825 p 782 p 732 612 wp 575

543 495 4881 483 -

-

474 d

475 sh

No

430 sh bd

399

400

250b

260

365

130b 81b a Observed in Ccl,

b From ref. 1.

498 p

data

404P

364 267 d

c

solution.

bands are present in the solid-state spectrum, and oniy two for the vapor. The CHs bend, the CH, symmetric deformation, the two CH, asymmetric deformations, and the CH, wag should appear in this region- The 1435 cm-’ band is apparently due to both asymmetric deformations and the 1365 cm-” band is due to the symmetric deformation_ Both these bands are present in the solid and vapor-state spectra, and are not expected to be conformation dependent. The CHt wag of the 0-cis form can be assigned to the band at 1388 cm-r, which is present in the solid-state spectrum, and for the O-gauche form, it can be assigned to the band at 1422 cm-’ present in the spectrum of the Ccl, solution. The CH2 wag of CH,FCNwas observed at 1389 cm” [refs. 3, 4].TheCH2wagandCHs asymmetric deformations of the O-gauche form are apparently unresolved in the vapor-state spectrum, giving rise to only two bands in this region.

202 There is a medium intensity liquid-state band at 1233 cm-’ that is absent for the solid, and a liquid-state band of about the same intensity, at 1189 cm-‘, that is absent for the vapor. These bands are assigned to the CHI twist of the O-gauche and 0-cis rotamers, respectively. A solid-state band appears at 1257 cm-’ that corresponds to the medium intensity Raman band at 1256 cm-‘, that is assigned to the C-C-C asymmetric stretch of the 0-cis form. This band is absent in the vapor-state spectrum, but a weak shoulder is present at about 1275 cm-’ that can be assigned to the same vibration for the O-gauche form. The very strong liquid-state infrared band at 1066 cm-’ is undoubtedly due to the C-F stretch. There is a strong solid-state band at 1067 cm-’ and a strong vapor-state band at 1066 cm-‘. It may seem therefore that the C-F stretching vibration has the same frequency in both rotamers. However, we expect a liquidvapor frequency shift for this vibration, and since the vapor-state frequency of the the liquid-state frequency should be several O-gauche rotamer is 1066 cm-i, wavenumbers lower than for the 0-cis form, and is assigned to the shoulder observed on the low-frequency side of the liquid-state band at about 1050 cm-‘. In the previous study, the C-F stretch of the 0-cis form was assigned’ to a band at 1082 cm-‘, but the presence of the 1067 cm-l band in the spectrum of the crystals shows this assignment to be incorrect. The liquid-state band at 968 cm-’ is absent in the vapor-state spectrum, but present in the solid-state spectrum, and is therefore due to the 0-cis form. It can only be due to a methyl rock or the CH2 rock. It can be assigned to the CH, rock by comparison with the spectra of symmetrical difluoroacetone’, which has a band at 988 cm-’ that shows the same behavior as this one. There does not seem to be a corresponding band for the O-gauche form. The weak liquid-state infrared bands at 778 and 825 cm-’ can be assigned to the C-C-C symmetric stretches of the O-gauche and O-&s forms, respectively, from the high Raman intensities expected for this vibration. The 825 cm-’ band is absent in the vapor-state spectrum, but present for the solid, and the 778 cm-’ band shows the opposite behavior. Only two other vibrations can be readily assigned at this time. The liquidstate band at 607 cm- ’ is absent in the vapor-state spectrum, but present for the solid, and can be assigned to the C-O in-plane rock of the 0-cis form. The vaporstate band at 488 cm-’ appears to be its counterpart for the O-gauche form. The lowest-frequency band observed, at 8 1 cm- I, can be assigned to the CH2F torsion. Normal-coordinate calculatiom The calculations were done with an IBM 360/40 computer and utilized programs written by Schachtschneider 6*7 for calculation of the G matrix, solution of the vibrational secular equation, and for the least-squares refinement of the force constants to fit the observed frequencies. The programs were modified slightly to run on our computer.

203 Calculations were first made for the 04s rotamer. initial values for the force constants used were taken from acetone’ for the CH3C=0 group (except for the CCH angle, which was taken from a hydrocarbon force fieldg) and from fluoroacetonitrile’” for the CHzF group. Most of the resulting frequencies seemed reasonable_ However, three calculated values differed by as much as 70 cm-l from what seemed to be the correct assignment. Several other calculations were made after manually adjusting some of the force constants until better agreement between observed and calculated frequencies was obtained. Finally, the observed frequencies were used in a least-square force constant refinement procedure that provided the best fit of calculated with observed frequencies with the set of force constants that were refined. The two methyl rocking frequencies were not used in the refinement procedure. The observed and calculated frequencies and approximate vibrational assignments for the 0-ci.s form are given in Table 2. A number of the normal modes are highly mixed, and only the most important contributions are given in Table 2. The final values of the force constants, some of which were held fixed at the transferred values, are listed in Table 3. TABLE

2

OBSERVED FOR

THE

AND o-CiS

CALCULATED ROTAMER

WAVENUMBERS OF

(Cm-

‘)

AND

APPROXIMATE

Observed

Calculated

Approximate assignment’

3012 3012 2979 2939 2939 1740 1450 1435 1435 1388 1368 1256 1189 1066

3013 a’ 3011 a” 2979 aIt 2944 a’ 2934 a’ 1740 a’ 1459 a’ 1437 (1” 1435 a’ 1394 a’ 1358 a’ 1258 a’ 1189 a” 1062 a’ 1061 a” 1007 a’ 966 a” 828 a’ 604 a’ 406 a’ 364 a” 266 a’

CH3 CH3 CH2 CH2 CHJ c-o CH2 CH3

825 607 404 364 267

VIBRATIONAL

ASSIGNhlENTS

FLUOROACETONE

CH3

as as as ss ss s 6, C-F s

a6 a6

CH2 w, CH2 6 CHs sS, CH2 w C-C-C as, CH, sa CH2 tw C-F s CH3 r CH3 r CHZ r C-C-C ss C-O r, C-C s C-C-C 6 c-o w C-C-F 6, CCC 6

D Abbreviations used: a = asymmetric; s = symmetric or stretch; 6 = deformation; w = wag; tw = twist; r = rock.

204 TABLE

3

FORCE CONSTANTS FOR FLUOROACETONE

Force constant

Coordimtes involved

Vaiu@

A. Stretch Kt & KK &’ C KS

C-H (CHs) C-H (CH,) C-CH, C-CHZF c-o C-F

4.865 4.775 4_071b 4.071b 9.644 6.213

(4.882) (4.907) (4_071) (4.07 I) (9.717) (55,268)

0.047

(0.073)

0.059

(0.059)

0.566 0.73Ob

(0) (0.730)

0.514 0.449b 0.77Ob 0.670b 1.013 0.753 1.560 0.210

(0.521) (0.449) (0.645) (0.770) (0.670) (1.375) (I ,647) (1.006) (0.217)

0.417 0.281b -0.08gb

(0.311) (0.28 1) (-0.089)

- 0.022~

(-0.022) (0.095) (- 0.052)

B. Stretch-stretch Fit

C-H, C-H,

C-H C-H

FR

c-c,

c-c

&i

C-C,

C-F

HCH HCH HCC HCC HCF CCF ccc cc0 C-O

(CH3) (CH,) (CH,) (CH2)

Ff

(CH$ (CHz) (C common)

C. Bend

H, Hb HP HIT HY HB HID HE: HP

0.692

out-of-plane

D. Stretch-bend fiS fi, %

C-C, CCH C-C, CCH C-F, CCF

(CH,) KHz)

E. Bend-bend HCC, HCC,

HCC (CH3) HCF (C-H common)

HCC,

CCC

(C)-C

gcu&e

C-(H)

0.095b - 0.060’

interaction constants are in units of p Stretch constants are in units of mdyne A- ‘; stretch-bend mdyne rad-I; bending constants are in units of mdyne A rad -z. The transferred values are given in parentheses. b These constants were held fixed at the transferred value. e This constant was used only for the gauclre rotamer.

There was one difference in the force field used in the present work for the CH&=O group from that of Cossee and Schachtschneider’. They used different values for the C-C-H force constant for the in-plane an& out-of-plane hydrogen& We used the same force constant for all three CCH angles of the methyl group and

205 obtained a least-squares value of 0.692 mdyne A rad-‘. The CH, symmetrical deformation frequency could not be reproduced exactly this way, as Table 2 shows, with the error being 10 cm-‘. Assignment of the C-H stretches are not certain because only four bands were observed in the 2800-3100 cm-’ region, and only three of these are fundamentals. The 2860 cm-’ band is assigned as a combination band rather than the symmetric CHs stretch, which should have a frequency above 2900 cm-’ for this compound. This band is also present in the spectrum of sym-difluoroacetone, and cannot be due to a fundamental for that compound. There is a large difference in the liquid and vapor-state intensities of the 2939 and 2968 cm-t bands, which seem to indicate a conformation-dependent C-H stretching frequency. However, the calculated values for the two rotamers differ by only I cm-’ for one vibration and are the same for the others. It certainly does not seem that the CH, frequencies can be assigned to the bands at 3012 and 2939 cm-’ because with these assignments the calculation requires F, to be negative, and there are no compounds for which calculations have been made for which this is the case. In the refinement of the C-H force constants for the cis rotamer, the CHs asymmetric stretch was assigned to the liquid-state infrared band at 3012 cm-’ and the CH, asymmetric stretch was assigned to the band observed at 2979 cm-’ in the lower Raman spectrum” shown in Fig. 3. This band appears only as a shoulder on the much more intense symmetric stretch band on the upper spectrum’ in Fig. 3. Both the 3015 and 2979 cm -r bands appear to be depolarized. Both the CH, and CH, symmetric stretches are assigned to the liquid-state infrared band at 2929 cm- ’ _ The Raman band is very intense and highly polarized. The a” methyl rock was calculated to be 1061 cm- ’ and may overlap the C-F stretch band, but there does not seem to be a band assignable to the a’ rock, based on the calculated frequency. However, H, contributes about 75 % to the methyl rocks, and there would probably be difficulty reproducing the methyl rocking frequencies, just as there was for the symmetrical deformation. The force constants obtained for the 0-h rotamer, with the exception of f ,“,, were used in a zero-order calculation of the vibrational frequencies of the O-gauche rotamer. The resulting CH2 twisting frequency was caiculated to be 1185 cm-‘, which is essentially the same as for the 04s form. However, the assignment of the 1189 and 1236 cm-’ bands to the CH2 twist of the two rotamers is fairly certain, and after trying nonzero values forf,‘, ,f,“, , and F’. in both rotamers, it was determined that the only way to increase the calculated value of the CH, twist in the O-gauche form relative to the 0-cis form was to includef,g, only in the force field for the O-gauche form. This force constant had the effect of increasing the CH, twist (from 1185 to 1290 cm -‘) and decreasing the C-F stretch (from 1051 to 1042 cm-r) and CHs symmetric deformation (from 1379 to 1372 cm-‘). The calculated and observed frequencies and approximate band assignments for the O-gauche rotamer are listed in Table 4.

206 TABLE 4 OBSERVED

O-gauche

AND

CALCULATED

ROTAMER

Obserued

I435 1422 1368 1273 1236

~1050 shb 1020 - 980 sh 779 608 488 399 250

(cm-

‘)

AND

Calculated

Approximate assignmentc

3013 3011

CH, CHS CHJ CH2

2979 2945 ~1725 sh= 1450 1435

WAVENUMBERS

APPROX1hbk.I-E

ASSIGNMENT

FOR

TXE

OF FLUOROACETONE

2934

1723 1450 1438

as as as ss

CH, ss c-o s CH2 S, CF s CH3 a6

1437

CHI aa

1424 I372 I284

Crr, w CHs .sa C-C-C as CH2 tw CHs r C-F s CH3 r CHz r c-c-c ss C-0 i-p r, C-C C-O i-p, CCF c-o o-p, ccc C-O o-p, CCF

1209

1065 1042 1017 976 799 595 486 396 240

s 6 6 i,

s Liquid-state frequency-vapor-state value is 1751 cm- *. b Liquid-state frequency-vapor-state value is 1066 cm- I_ Liquid-state values are given so the relative order of calculated and observed frequencies will be the same. c See footnote, Table 2 for abbreviations.

As for the O-c& rotamer, the high-frequency methy rocking band probably overlaps the C-F stretch, but in this case, there is a band that can be assigned to fhe low-frequency rock, There is a weak shoulder in the vapor-state spectrum at

about 970-980 cm-’ that is assigned to the CI& rock. This band is much weaker for the 0-gu&ze rotamer than for the O-c&. It is not possibie to compare frequencies of corresponding

vibrations of the

two rotamers in the low-frequency region because of a larger amount of mixing of normal modes in the O-gauche rotamer permitted by the lower molecular symmetry. For example, the lowest-frequency band listed in Table 2 for the 0-cis form has calculated major contributions from the CCF and CCC bending modes, but for the O-gauche form the GO out-of-plane wag and CCF bend mix to give rise to the lowest-frequency band. Bella*y and Williams” suggested that the carbonyl stretching frequency of the cis rotamer of chforoacetone is higher than for the gauche rotamer

becausa

207 the near approach of a negatively charged halogen atom to the oxygen results in a decrease of polarity of the carbonyl bond. Such an effect would increase the C=O force constant_ However, the present calculations show that the difference in carbonyl frequencies of the two rotamers can result simply from the difference in molecular configuration, which affects the three vibrational kinetic energy matrix elements involving the C-O bond with each of the two CCH and CCF angles of the CH,F group. Preliminary calculations for symmetrical difluoroacetone show the same results for the carbonyl stretching frequencies, i.e., v(cis-cis) > v(ci.rgauche) > v(gauche-garrche)‘2.

ACKNOWLEDGEMENTS

This work was supported by The Robert A. Welch Foundation, Houston, Texas. The authors are grateful to Dr Paul Devlin for obtaining the Raman spectra.

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4 5 6 7

N.

SHIDO,

J. Cfit?ZZZ.P/ZyS., 21 (1953)

NAKAGAWA

81%

R. G. JONES AND W. J. ORVILLE-THOMAS, J. Chem. .%c_, (1965) 4635. J. R. DURIG AND D. W. WERTZ, Spectrocizitzz.AC&Z,24A (t968) 21. G. A. CROWDER AND B. R. COOK, J. Mol. Spectrosc., 25 (1968) 133. J. H. SCHACHTSCHNEIDERAND R. G. SNYDER, Spectroclzizzz. Acta, 29 (1963) 117. J. H. SCHACHTSCHNEIDER, SheN Development Co. Tedz. Repts., Nos. 231-64 (1964) and 57-65

(1965). 8 P. COSSEE

AND J. H. SCHACHTSCHNEIDER, J. Cizetzz.Phys., 44 (1966) 97. SNYDER AND J. H. SCHACHT~CHNEIDER, SpectrocJzitn. Acta, 21 (1965) 10 G. A. CROWDER, Mol. Phys., 23 (1972) 707. 11 L. J. BELLAMY AND R. L. WILLIAMS, J. Chetzz. Sot., (1957) 4294. 12 G. A. CROWDER, unpublished results. 9 R. G.

169.