Vibrational spectra of acetyl fluoride and acetyl fluoride-d3

SpectmchImJa8 Acts, Vol. 28A,pp. 1618to 1822.PergamonPrw 1072.Printedin NorthernIreland

Depax%mcr&of Chemktry, University of New Hampshire, Durham, N.H. 03824

A-t-Infrared spectra of acctyl ihzoridcand ScetyI fIuoridc&sarc reported for the gas and the solid, as arc the Ramsn spcotra of the liquids. Changes in a previous lylsignment for CH,CFO [J. Ohem. Phya. 58, 1713 (1970)] are indicated. Observed intensitiesand mdogy with COF, suggest that fundamentals at 826 and 1183 cm-1 are best described as iu@~~e and outof-phase C---C--F strctch~. The methyl rock and the in-phase C-G-F stretch am strongly m&d in CD&F0 f&39 and 778 cm-l). A product-r& cahxdatioa indicatorthat the fundamental f~ucncies of CD$?FO arc about 2 per cent bigher than expected on the basis of a strictly harmonit, potent& function for CHsCFO. A similar celculation is used to compare CE,CFO with CF,CFO.

Aasmxa fiu&de is hardfy an ex&iic molecule,but its viixa,Gm~ ~~~~~~~ is of interest for several re&sons, It is one of a series of oompounde with the general formula CH&OZ; reliable assignments have been reported for members of this series with 2; = H, Cl, s,ad Br [l-5], but not for the present case, where 2 = F. We are currently carrying out normal-coordinate analysss of Cl&CO!?3and CF&OZ compounds, and this report will present experiments evidence for the blent used in these ~l~~~tio~ [S]. The &man spectrum of liquid CR&30 w= reported by ~EEW~-~~T and KAHOVEC[7], and EVANS and BERNSTEIN used their d&a in suggesting B tentative assignment [l]. More recent infrared results have been presented by RAMSEY and LAD)I) 181. Siuoe rtoetyl fluoride is isoelectranic with acetb a&d, BEXNEY, RXNNCYNX and Lzx used it as 8 model oompound in their study of the a&d [9f; they offered a different assignment, baaed on gss-phme md m&ix-isolated infmred spectra. In the present study, ga.s-phase spectra of CH,CFO were repeated and extended, and the spotrum of the solid was acquired, The deuter&ed species CD&F0 was also synthesised and studied. On the basis of this work, several ohmges in the previous assignment [9] are indicated. * hn%

address: AFRPL, Edwards, California 83523. t Presanof&&is: Dept. of chemistry Mkuquettc Univ. DGlwa&cc, Wise. 813233. [I] J. C. %%UXB and H. J. ]BI#KNBTEIN, Can. J. C&m. S&l083 (1956). [Z] P. COSSXZand J. H. SOH~CJHTSCENEIDIB, J. Ciwm. Phya. a,97 (1966). [33 R. J. C.&!?wntz,J. Chem, 2%$!8.49,1436 (1968). [a) 3. O-m, R. A. N~QTJXST, J. C. Evans and W. J. Prrmrs,SpeMcch&~~.Acta 17, 1206 (fsSi). [6] L. C. HIALLand J. O~XRXNII,~~~~rn. AC&S%A, 2536 (XSS?). [6] A. D. Coxuixxmand C. V, B-Y, to be publish& [7] H. S&%iWLLN-&BEIbT snd L, &EOVEC, Acta Phys. At&nbca 1,352 (1948). [S] J. A. BAMSEY and J. A, Y&XI, J. C&m. Sao. B 118 (1963). 191 C. V. I%~wEY, R. L. RISIXXGTON and K. C. tiN, J. Chem. Phya. a,1713 (1970).

1814

C. Y. ~ERNEY and A. D. Commsx

PIERRE and K.WSHERinvestigated the microwave spectra of eight isotopic species of a&y1 fluoride [IO], and calculated a structure with the following parameters: rcc = 1.603, rev = 1.343, rQ0 = 1.186, ruH (in-plane) = 1.082, and rcB: (out-ofplane) = 1.096A; LCCF = 110.7,“, LCCO = 127.Q0,LCCH (in-plane) = 110.4’, LCCH (out-of-plane) = 108.8’, LHCH (in-plane) = 110.8,0, LHCH (out-ofplane) = 107.2,‘. l?igure 1 shows the equilibrium couflguration of the molecule with the principal inertial axes. In the light species, the oenter of mass is only 0.16 A% away from the carbonyl carbon, and the molecule thus resembles a planar trigonal B

i Fig. I. Diagram of the structure of CH,CFO [lo]. The B axisand the angle between the C---C! bond and the A axis for CD,CFO me indicated in parentheses.

molecule such as BP,. It is in fact isoelectronic with BF,, from which it oan be created conceptually by transferring a proton from one of the fluorines to the boron to form carbonyl fluoride, OF,, and then pulling three protons from the nucleus of one of the remaining fluorines. In the course of this process (if neutron numbers and bond lengths are allowed to relax to their appropriate values) the rotational constants change from 0.3466 and 0.1724 cm-l for llBF, [II] to 0.3941,0.3920 and 0.1962 cm-l for COF, [12] and 0.3682, 0.3231 and 0.1775 om-l for CH&FO [lo]. Deuteration of the acetyl fluoride moves the center of mass (and thus the B axis) 0.084 A%closer to the methyl group, reducing the intermediate rotational constant B more than A or C, and making the molecule less like a symmetric top. The deuterated rotational constants are 0.3442, 0.2664 and 0.1689 cm-l; the asymmetry parameters p*, K [13] change from 0.600, 0.626 to 0.696, 0.160. As with other CH,COZ molecules [l-6], the vibrational representation under the point group C, is 10a’ + 6a”, with mixed A- and B-type contours expected for a’ vibrations and C-type contours for the a” modes. The formulas of SETH PAUL and [lo]L.PIEIRCX ssnd1;. C. &%~IER, J. C%em. phys.

81,

876

(1969).

W. BROWN end J. O-, Cat%.J. P?q,w.46,971 (1968). [IZJ V. W. Lmm~, D. T. PEXCE and R. 33.JACKSON,J. Ohem. Php 87,2995 (1962). 1133 W. A. SETHRNX and G. DIJKSTRA,~~~~~~~. Aota $%?A, 2861 (1967). [11-J C.

1816

Vibrational speotra of amtyl fluoride and m&y1 fluoride-d, ’

I

I

’

I

’

.I....._

*“.“r,..,\...“l

I

’

I

.,,.

_,,#.-..-

*-.,”

’

I

’

’

I

’

1

..--..--r..,r.--...-~*--

1 ’

1 ;;r;;r...L.G.J..&.-‘-.

-“r/ 1474

1’1437

1440

1378

*

a

3

\

(c)

F

ltl~l~l~lrl~lrltl~ 3160

3120

I200

L~ltltl

3090

II60

3040

II20

3000

IO60

2960

1040

2920

1000

2690

960

2640

900

1920

660

620

Wawnumbsr.

J1111*1111

I660

1640

760

BOO

640

I600

600

MOO

660

1420

620

1360

1340

1300

440

400

360

cm-1

Fig. 2. Infm-red spectrum of CH,CFO (gas). Spectra were run in a IO-am oell with CsI windows, with sample pressuresaa follows: (a) 300 torr, (b) 6 tar, (c). (d) and (e) at 10 torr (inset at 6 torr), (f) 60 torr, (g) V8pOr in equilibriumwith the liquid at 33% (~1200 torr).

’ 2242 2263 CD Co 3 ‘F

(a) I

I

‘20

II,

II

2260

2240

11 2200

I

2160

2120

t

I

I

2060

I

t

2040

1240

2000

IZCKI

1160

1120

r t

5+5

(f)

kJ

1 IWO

1040

1000

960

920

660

640

600

760

720

Wawnumbw.

620

660

cm-1

540

500

660

440

1 1 ’ L 400

l

spectrum of CD&F0 (gas). Path length = 10 an. (a) 300 torr, (b) 4 torr, (c) 4, 26 and 40 torr, (f) vapor in equilibrium with the liquid at 33OC (-1200 torr).

Fig.

3. Infresed

360

1810

C.

V.

BERNEY

Is00

1400

and A. D.

CORMIER

(b) .I

3000

2900

1900

I800

1300

1200

Wavenumber. Fig. 4.

I.

(a)

IO00

900

BOO

700

600

500

400

cm-l

Raman spectrum of CR&F0 (liquid). Gain reduced by a factor of 3.16 above 2000 cm-l. (b) Infrared spectrum of CH&FO (solid). 48 pmole deposited.

I ’ I 2140

,

1100

I

’

I

’

I’1

II

I,

I’

I,

I,

I,

I,

I

(a)

2300

2200

2100

2000

1900

1800

1700

1200

Wavenumber.

II00

1000

900

600

700

a?Q

500

400

cm-l

Fig. 6. (a) Reman spectrum of CD&F0 (liquid). Gain reduced by a factor of 2 above 2000 cm-l, and increasedby a factor of 12 between 1100 and 1000 cm-l for the polarization study. (b) Infrared spectrum of CD&F0 (solid). ~400 rmole deposited for region 24002000 cm-l, 32 ,umolefor region 2000-300 cm-l.

Vibrational spectraof acetyl fluoride and acetyl fluoride+,

1817

DIJKSTRL~ [13] predict the P-R branch separation to be 24 cm-l for A and B-type bands in CHsCFO (22 cm-l in CD,CFO) and 36.6 (33.3) cm-l for C-type bands. The && separation for B-type bands, estimated from the figures given by UEDA and SHIMANO~CHI [14], is 6 (6) cm-l. Gas-phase band contours for CHsCFO are given in Fig. 2, and for CD&F0 in Fig. 3. Figures 4 and 6 contrast the Raman spectrum of the liquid with the i.r. spectrum of the solid for CH,CFO and CDsCFO, respectively. VIBRATIONALASSIGNMENT G-H,

C-D

&ret&s

The two highest-frequency stretching vibrations of CH,CFO, y1 and vX1,are readily identified from the gas phase spectrum (Fig. 2a), and their symmetries determined from the band contours. The symmetric stretch, ~z, is more elusive, and in fact was misidentified in our earlier work [Q] as the band at 2867 cm-i. Spectra of the condensed phases (Fig. 4) show Y, more clearly-it is the strongest peak in the Raman spectrum, at 2963 cm-l. Identification of y1and yil in CDsCFO is complicated by interaction with a number of combination bands (see Table 2); however, usis out in the open. We note that it is slightly higher in the gas phase than in the condensed phases. By analogy, then, we expect va in CH,CFO to be only slightly higher in the gas phase, and attribute a small bump at 2966 cm-l (Fig. 2a) to the & branch of this band. Methyl defcnmahma

The a’ and a” asymmetric deformations in CH,CFO (v~ and yls) are nearly degenerate at 1440 cm-l, and y5 (the umbrella mode) is prominent at 1378 cm-l. In the condensed phases vg and vi2 are distinct, and the greater intensity of the higherfrequency component in the Raman spectrum Fig. 4(a)] suggests that it is the a’ vibration. This suggestion is reinforced by the i.r. spectrum of the solid pig. 4(b)], in which the upper component is broadened and split, as are two undoubted a’ modes, Ye(1185-1166 cm-l) and yli(822-811 cm-l). In CD,CFO, we assign the umbrella mode to a weak peak at 1149 cm-i in the gas, and the asymmetric deformations to the absorption at 106Q-1020 cm-l. Again, these vibrations are resolved in the condensed phases, but now the evidence regarding their symmetry is conflicting. Raman intensity and polarization considerations Fig. 6(a)] dictate choosing the upper peak as the a’ mode, while the shape of the absorption in the i.r. (Fig. 6b) indicates that the lower one is totally symmetric. The latter choice seems more reasonable in that the pattern of fundamentals between 1160 and 900 cm-l is then a’, a”, a’, a” rather than a’, a’, a”, a” (i.e. the well-known tendency of energy levels of the same symmetry to “repel” each other makes an alternating pattern more likely than a bunched one). The contradiction in the experimental evidence is puzzling, however, and we are probably dealing with unusually anharmonic motions, or a display of nonrigid behavior [16], or both. [14] T. UEDA end T. SEIMANOUCHI, J. Mol. &e&y. a8, 630 (1968). [16] C. V. BEBNEY,Speetwchim. Acta WA, 663 (1971).

C. V. BIORNEYand A. D. CO~MISR

1818

Table 1. Infrared and Raman spectra of CH,CFO, 4000-200 om-l Liquid (Reman) G-fa (i.r.) v (om-l) 3716 3630 3694 3618 3460 3372 3307 3243 3169 3110 3043 3004 2966 2920 2867

Av (em-l)

Intensity

1766 1612 1649 1474 1440

VW

VW VW VW

VW

3043 3004 2962

3.4 2.8 100

3048 3006 2963

w w mw

w

2853

2736

2

2

w

VW

2836 2800 2780 2720 2660 2641 26331 2470 2439 2416 2376 2344 2276 22671 218 “) 2164 2096 1997 1976

w

w

21

VW

VW

38

VI +

V?

VW

v4 +

vs

VW

vs +

%a

VW

2%

w w

va +

V8

v5 +

VI

v4 +

VI

VW VW

w VW

w VW

w VW

w w

1064

mw

100 993

m

2

w w VW

VI -I-VI v6 +

v8

%s VI +

VW

%a

+

1861 18461 1807 18001 1727 17111 1620

v2

VW

w

-1440

6

-1420 1379 ~1178

2 3 1

1003

822 602 673 428

6

32 7 2 2.6

1482 1438 1431 rh 1 1417 1373 1186 11661 1063 1049> 1003 8991 923 906 862 6%2 6111 604 669 428

w

v14 v9

% V? f

v.3

VI +

Vlb

Vld + v7 +

VW

VB

V2

VI)+

1847

VW

m

711

w

VW VII

%o

VI

va +

VW

w

698 667 420

v1+ VlO 2v, I- 97 % + VI1 % + vs Vl + 3s va+ vs +

VW

w w

2% Vl f VI vs + vi1 VP+ VP

VW

1437 1378 1188

826

Assignment w

VW

w w

1974 197 1863

Intensity

VW

2694 2661

1998

3663

w

VW

2260 2186

p-k height

v (cm-l)

VW

2808

2426

Solid (ix.)

Relative

VI0 +

m

V4

m *

Vll V5

V8

V6

8

VI* VI

VW VW

VW

%o

B

%

s

VI

m

Vl&

w

VlO

VI4 %4 VlS

%

1819

Vibrational spectra of acetyl fluoride and acetyl fluoride& Table 2. Infrared and Ram&n spectra of CD,CFO, 4000-200 cm-l Liquid (Reman) Gas (ix.) Y (cm-~) 3714 3434 3321 3283 3139 31221 3069 2983 2916 2800 2706 2663

Au (cm-‘) Intensity

Relntive peek height

v (on-‘) 3680 36691

lnw

VW

3321 3266 3174 3133

w

w VW

3064 30401

VW

Intensity Assignment

2% v1+ % v1+ v12

vs+ v2

v4 + VlO

++%a

v11+

vs+

Vl8

VI

VI + VT

VW

w VW

2684

w VW

2380

2401 2286 22631 2242

w

2144 2094

w VW

2041 20371 1969 187 “) 1868 -1824 sh

w

1734 1683 1620 1666 1412

VW VW VW mw mw

1360 1294

mw w

1204

“S

1149

w

1067

m

1030

B

916

m

839 778

mw 8

846 774

18 21

m mw VW

678 496 378

7 2 2

676 491 -396

Solid (i.r.)

nlw

~2272

6

w

~2260

7

2137 2098 2069

62 2 2

mw vs

1849

14

w

1811

4

2286 2266 2240 2223 2206 2137 2102 2068 2029 1963 ~1860 18471 1824 1792 1779 1688

-1196

1063 Ml030

1.3

3.6 2.3

1633 1426 1402 1374 1349 1289 1262 1223 ~1206 11901 1169 1142 1066 1042 ~1018 10101 982 947 916 899 866 314 N772 776> 681 491 379

VII + VP+

%a Vll

%

%o

vs+ %

+

VI

1820

C=O,

C. V. BERNEY and A. D. CORXIBZ

C-F,

C-C stretches

The fact that acetyl fluoride and carbonyl fluoride are isoelectronic suggests that there will be resemblances between some of the normal modes. The stretching vibrations in O==CF, are at 1928 (or, a,), 1249 (v~,b,) end 965 cm-l (v,, a,) [16]; they can be approximately described as the C=O stretch, the out-of-phase F-C-F stretch, and the in-phase F-C-F stretch [17]. The three most intense i.r. bands in CH&FO are at 1870, 1188 and 862 cm-l, and it seems natural to describe them as the C=O stretch, the out-of-phase C-C-F stretch, and the in-phase C-C-F stretch. This description is in agreement with the recent observations of TUA~ON et al. [18]. The corresponding vibrations in CD&F0 are at 1869,1204, and 778 cm-l. The Raman spectrum shows thst the latter vibration is strongly mixed with the mode at 846 cm-l (839 cm-1 in the gas phase), nominally the CD, rock. Assignment of the remaining fundamentals is straightforward, and is summarized in Table 3. The positions of the COF scissors deformation (Q, a’) and the COF wag (Q, a”) have been exchanged with respect to the previous assignment [9] after further consideration of the gas-phase band contours. Table 3. Fundamental frequencies (cm-l) of acetyl fluoride and aoatyl fluoride-d, Approximate desoription

CH,CFO

CD&F0

8’ species Vl VI % VI VS V* VT VS V@ VlO 8” ape&a Vll

V11 VXS V11 V16

esymmetrio methyl stretch aymmetrio methyl stretch C=O stretch esymmetrio methyl deformation symmetrio methyl deformation out-of-phese C-C-F atretoh methyl rook (ant&r) in-phsse C-C-F stretah COF soiasors deformation COF rook (gear)

3043 2866 1870 1440 1378 1188 1000 826 698 420

aaymmetrio methyl stretch asymmetric methyl deformation methyl wag (antipeer) COF wag (gear) torsion

3004 1437 1064 667 v231t

2242 1067 916 491

cw t

Meanvalue

of resonating bands et 2286 and 2283 cm-‘. 7 Values oelouk~ti using the Milathieuequation end B btier l

(2274)* 2144 1869 1030 1149 1204 839 778 676 396

of 364 cm-’ [lo].

The torsional vibrrttions were not observed in this study. CsJculated frequencies obtained from the microwave barrier [lo] and tabulated solutions for the Ms,thieu equation are 123 cm -1 (CH,CFO) and 93 cm-l (CD,CFO). A number of bands (e.g. vr and v14)in both isotopic species exhibit sharp satellite & branches. Their intensities are temperature-dependent, and they sre probably upper-stage transitions from torsionally excited states. Further study of these satellites may provide information about torsional barriers in vibrationally excited states. A. H. NIELSEN, T. G. BURKE, P. J. H. WOLTZ and E. A. Jo-s, J. Ohem. Php. 20, 696 (1962). [17] M. J. HOPPEB, J. W. RUSSELL and J. OVIEBEND, J. CJum. Phya. 48,3766 (1968). [lS] E. C. TUAZON, W. G. FATELEY and F. F. BINTIZY, Appl. Spctt-y. 26,374 (1971). [IS]

Vibrational sp&ra of metyl fluoride and m&y1 fluoride-&

1821

Product rule ratios are 0.2341 (obs), 0.1963 (talc) for the a’ vibrations, and 0.3120 (obs), 0.2921 (talc) for the a” modes. The a’ block is off by 19 per cent. There are not many places where the assignment can be changed to produce better agreement. Switching the assignments of the asymmetric CD, deformations (v, and YJ increases the discrepancy in the a’ block to 23 per cent while reducing the disagreement in the a” block from 7 to 4 per cent; it robs from the poor to give to the rich, and thus, on the whole, seems unwarranted. The position of the band center for v10for CD&E’0 in the gas phase Fig. 3(f)] was chosen by analogy with the light species. It is conceivably as low as 370 cm-l. This change reduces the a’ discrepancy to 12 per cent, which is still large, though not unprecedented [19]. In either event, we conclude that several of the a’ vibrations in acetyl fluoride are significantly anharmonic. DISCUSSION Our motive in undertaking the normal-coordinate calculations presently in progress [6] is to determine the effect of fluorine substitution on the potential functions of a representative series of compounds. It is also interesting to see if valid comparisons can be made without a full-scale calculation. Comparing CH,CFO (CD&FO) with CF,CFO [15], we see that yo_o moves up from 1870 (1869) to i899 cm-l, and door, the scissors deformation, goes from 598 (667) to 761 cm-l. We thus get the impression that substituting CF,- for CH,- tightens up the -CFO group. It is diflccult to draw conclusions about the other vibrations, since they either involve comparing CH, vibrations with CF, vibrations or are mixed in different ways. If CH&FO and CF&!FO had identical geometries and identical potential functions, then fluorination of the methyl group would simply be an exaggerated form of isotopic substitution. This suggests that we can use a quasi-product-rule calculation to compare the potential functions of the two molecules independently of normal-coordinate calculations. The parameters involved in this calculation are defined below : P x = n[ 1O-s - Y,(CX~CFO) Pobs(X*/X) = px*/px

The effect of geometrical differences between CH,CFO and CF,CFO was removed by calculating moments of inertia for a hypothetical C lsHsCFO molecule (102.7, 176.9 and 218.3 amu-A*). In addition, we can effectively factor the torsion out of the a” block by using a torsional frequency for CF,CFO (45.2 cm-l) calculated from the CH,CFO barrier of 364 cm-‘- [lo]. The results of these calculations are given in Table 4. [19] C. V. BERNEY,&mctmchim. Acta

%A, 793 (1969).

1822

C. V. BERNEY and A. D. Commcx Teble 4. Quasi-product-rulecalcuirttionfor CX,CFO (X = H, D, F)

a’ an

1.192 1.068

6.197 0.873

4.360 0.816

1.02 1.02

1.18 0.97

1.16 0.96

If the ~~ntial factions for these molecules were identical and perfectly would be unity. Values greater than unity indicate that the harmonio, po&~, observed frequencies for the starred ape&a are higher than expected. Thus we see that the effeot of anharmonicity in CH&FO is to tightin up the deuterated species, both in the a’ and a” blocks. An indication of the extent of this tightening is given by the quantity labeled p0 in Table 4; it is the tenth root of p~~~~~~ (u’) and the fourth root of p~~~~p~~ (a”), that is, it represents the average change in each fundamental frequency (excepting the torsion) required to reproduce pob,Jpdc for the indicated symmetry block. The value of 1.02 for j? (D/H) suggests that anharmonioity affects the frequencies on deuterating acetyl fluoride to the extent of about 2 per cent. If the deuterated molecule is then fluorinated, the a’ vibrations are tightened up by a much larger amount (16 per cent on the average), while the nontorsional a” vibrations are looser by about 6 per cent. These changes are undoubtedly due to the interplay between two factors-the bulky nature of the fluorine atom compared to hydrogen, and its higher eleotronegativity-and further work will be necessary to disentangle the two effects.

CH,CFO was commercially obtained and used without special puri&ation. CD&F0 was synthesized by heating CDsCOOD with a 10 per cent excess of benzoyl fluoride at 70°C and distilling the product [20]. Infrared spectra were obtained on Beckman IR-9 and IR-12 instruments, with a resolution and frequency accuracy of 1 cm-1 or better. Solid samples were deposited from the vapor onto a CsI window in a squid-nitrogen cold cell of conventions design. It was generally possible to reduce SiF, contamination to negligible levels by flushing and careful handling; however, the spectra run at ~1200 torr [see Fig. 2(g)] still show traces of this impurity. The Reman spectra were run on a Gary 81 laser-equipped spectrophotometer, using 4880ipexcitation. Ackrsm&dg~m~t cm very grateful to Dr. M~IS BUZA for his help with the syntheaieof CD,CFO, to CHRISTINE COBB, who cmried out preliminarym&ions, and to Dm. G. L. CARL~ON and D. F. PEJNSENSTADLEB of the Mellon Institute for runningthe Ramm qmtra. We also thank the Central University Research Fuud of UNH for support of thirdwork under &-ant No. 441, Professor JOHN O~EREN~ for OOmnuI&8tiIIg unpublished rem&a, and Profeeear R. L. REDINGTON for hospita&ty in Lubbock, Texas. [20] C. A, OLAH, S. J. KUHN, W. S. TOLGYHI (1982).

and E. B.

BAKER,

J.

Am. G%na.Sm. 84,27X3