Microwave spectrum and the structure of benzoyl chloride

Journal of Molecular Structure, 162 (1987) 183-189 Elsevier Science Publishers B.V., Amsterdam -Printed

MICROWAVE CHLORIDE

SPECTRUM

MASAO ONDA, MOT00 ICHIRO YAMAGUCHI

in The Netherlands

AND THE STRUCTURE

OF BENZOYL

ASAI, TOSHIYA KOHNO, YASUHIRO KIKUCHI and

Department of Chemistry, Faculty of Science and Technology, 102 (Japan)

Sophia University, Tokyo

(Received 13 May 1987)

ABSTRACT The microwave spectrum of benzoyl chloride was observed in the frequency range 12-18.6 GHz. Rotational constants have been obtained for the ground vibrational state, the first three excited torsional states of the COCl group, and one of the out-ofplane bending states. The residual inertial defect obtained from the ground and the torsional excited states indicates that the equilibrium conformation is planar. Ab initio MO calculations (STO-3G) showed the potential energy curve as a function of the COCl torsional angle to be rather flat around zero degrees.

INTRODUCTION

Microwave spectra of benzaldehyde [l] , benzoyl fluoride [2] , 4-chloro[3] and 4-fluorobenzaldehyde [3] have already been reported. In this series of molecules, the torsional frequency of the -COZ group and the barrier to internal rotation were studied from the spectra of their torsionally excited states. To our knowledge no microwave study of benzoyl chloride has yet been carried out. Recently, Durig et al. [4 ] reported the far-IR spectra of benzoyl compounds (C6H,COZ, Z = H, F, Cl, CH3) measured by FT-spectrometry. They assigned definitely the three low lying vibrational modes: COZ torsion, and in-plane and out-of-plane COZ vibrations. They estimated the height of the potential barrier for the COZ torsion from the fundamental torsional frequencies and compared the values among these molecules. They indicated that the barrier heights for benzoyl chloride and acetophenone were much lower than those for benzaldehyde and benzoyl fluoride. They suggested that one of the origins of the low barrier height for the former two molecules was their non-planar equilibrium form. In this study, we have observed the microwave spectra of benzoyl chloride for two chlorine isotopic species, and confirmed the planarity of the molecule from the inertial defect of the ground and torsionally excited states. 0022-2860/87/$03.50

o 1987 Elsevier Science Publishers B.V.

184 EXPERIMENTAL

A conventional microwave spectrometer was used with 100 kHz square wave Stark modulation. A synthesizer (HP-8672A) was used as the microwave source in the range 8-18.6 GHz. The wave guide was cooled with dry ice and the sample pressure in it was below 1 Pa. Benzoyl chloride was obtained from Tokyo Kasei Kogyo Co., Ltd. and used without further purification. Calculations were carried out at the Computer Center of the University of Tokyo by using the GSCF2 computer programmed by N. Kosugi. RESULTS

AND DISCUSSION

A series of a-type R branch transitions with high K_, values appeared with a frequency interval of 1650 MHz. The other series for the 37C1 species was less intense. The assignment was not easy because many vibrational satellite lines appeared. The assignment of the vibrational state transitions was carried out by comparison with the spectrum of benzoyl fluoride and reference to the vibrational frequencies assigned by Durig et al. [ 41 (three vibrational modes below 250 cm-l are shown in Table 1). The K_, values of the observed transitions were assigned from their Stark effect with varying field strength (several V cm-’ to 3 kV cm-‘). For doublet lines due to nuclear quadrupole coupling of chlorine, the center of the splitting was taken as the unperturbed frequency. The list of the observed transition frequencies is available from B.L.L.D. as Supplementary Publication No. 26339 (5 pages). The rotational constants were obtained by least-squares fitting of the frequencies of the a-type R-branch transitions. They are listed in Table 2. The observed inertial defect for the vibrational ground state is of rather large negative value (-0.486 ~8~). This value is similar to those of substituted benzenes having a single bond between the benzene skeleton and the substituent, except for the case of phenol. It is known that the major contribution to the inertial defect of these molecules is the torsional vibration of the substituent. The values appearing in the literature are listed in Table 1. The residual inertial defect is obtained by extrapolating the inertial defect of the torsional states to the hypothetical quantum number Utorsion = -l/2. This value should correspond approximately to that of the torsional free state. The residual inertial defect for benzoyl chloride is of rather large positive value and therefore the planarity of the molecule could not simply be inferred from it. We tried to obtain evidence for the planarity of the molecule from the linear dependence of the rotational constants on the quantum number up to Utorsion = 3. If the COCl group bends slightly out of the ring plane or twists about the C-C single bond, the variation of rotational constants with the torsional quantum number might show a zig-zag behaviour. Almost linear variation was observed for the rotational constants,

185 TABLE 1 Inertial defect, torsional frequencies, pounds Inertial defect (uA’) A0= C,H,CHOf C,H,COFg C,H,COClh

-0.128 -0.325 -0.486

and barrier to internal rotation of benzoyl

Vtorsion (cm-‘)

com-

V, (kJ mall’)

ID”

IRd

IRe

MO

77.3 55.2 36.5

110.85’ 63.36’ 44.6’

19.27’ 20.80’ 13.90’

21.62j 11.73j

A -1 I2 b

0.260 0.289 0.475

aGround state. bResidual inertial defect. ‘Using the equation in the text. dObserved frequency. eCalculated from the observed frequency. fRef. 1. gRef. 2. hThis work. ‘Ref. 4. ‘Theoretical barrier height calculated from ab initio MO (basis set POP-3G).

especially for the cases of more accurately determined B and C. We concluded that the benzoyl chloride molecule is planar, although a more detailed treatment of the molecular vibration is necessary to confirm this conclusion. Durig et al. [4] pointed out that the large value of the inertial defect for benzoyl fluoride was attributed to its heavy internal rotor and not to the noncoplanarity of the CFO moiety. The larger negative value for benzoyl chloride compared with benzoyl fluoride was presumably due to its larger internal rotor: the internal rotational constants were calculated by Durig et al. as 0.5790 cm-’ and 0.4369 cm-’ for benzoyl fluoride and chloride, respectively. For a planar molecule, the torsional frequency (v~) may be estimated from AI, -AI,

= (4h/(82))/~,

at the limiting case of H#g et al. [5] . This approximation is fairly good under a condition in which the lowest out-of-plane vibrational mode, e.g. torsional mode, is isolated from the others. The calculated torsional frequencies are shown in Table 1. The frequency obtained for benzoyl chloride is nearly equal to the IR frequency observed by Durig et al. [ 41. The r, coordinates of the Cl atom have been obtained using Kraitchman’s equation both for planar (I) and non-planar (II) molecular models as given in Table 3. The (c/ coordinate apparently indicated that the Cl atom was not located in the ring plane: it is not zero beyond the uncertainty attached (calculated from the ambiguity of the rotational constants). However, the non-planarity of the molecule could not simply be concluded from the value, which is rather small (0.119 uAZ) because of the zero point vibrational effect. Using the moments of inertia of two isotopic species, four structural parameters for the rigid planar molecular model have been derived as shown in Table 4 by non-linear least-squares fitting on the SALS system [6] . In this procedure, a benzene moiety was assumed to be regular hexagonal

TABLE 2 Rotational constant and inertial defect of benzoyl chloridea C,H,C03*C1

A (MHz)

B (MHz) C (MHz) A (LL%*)~

NC o (MHz)~

C, H, C03’C1

Ground

Ut = 1

Vt = 2

Ut = 3

ueb = 1

Ground

3139.24(15) 932.4559( 30) 719.4121(29) -0.486( 12) 45 0.099

3135.04(20) 931.8471(39) 720.6410( 37) -2.254( 16) 47 0.129

3130.59(17) 931.2415(34) 721.8205(32) -3.981(14) 45 0.112

3126.93(30) 930.5968(58) 722.9982(55) -5.687(24) 38 0.190

3136.7(14) 932.979(26) 720.474(24) -1.35(11) 15 0.520

3116.91(23) 913.5042(43) 706.9802(41) -0.530( 19) 46 0.137

aFigures in parentheses are one standard deviation. stants. dStandard deviation of the fitting.

bInertial defect. ‘Number

of transitions used to calculate the rotationa

con-

187 TABLE 3 rs Coordinates of chlorine of benzoyl chloridea

/al (a 1 Ibl (A) lcl (A)

I

II

2.3859(g) 0.7796( 28)

2.3830(9)

0 (assumed)

0.7702(28) 0.119 (18)

aUsing the Kraitchman’s equation for planar (I) and non-planar (II) models. TABLE 4 Structural parameters (bond lengths in a, angles in degrees) of benzoyl chloride and principal moments of inertia (u_&‘) observed and calculated from the parameters Benzene ring

C%OCl

Parameter

Value

Parameter

Value

C-H

1.084= 1.39ga 120a 120a

C-C c=o CXl LCCO LCCCl

1.452(6)b 1.210a 1.807(2) 123.19(35) 119.59(8)

G&J LCCH

Moment of inertia

Obs.

Calc.

I, (“‘Cl) &, (“‘Cl) I, (=Cl) I, (37c1) Ih (3’Ci) z, (“?Cl)

160.987 541.984 702.485 162.140 553.228 714.838

160.553 541.970 702.523 161.691 553.202 714.893

aAssumed parameters. bNumbers in parentheses are the dependent range of the parameter on the variation of rc=o from 1.20 to 1.22 a.

was fixed to 1.210 A. The parameters of the CCOCl group for and r,, several molecules are summarised in Table 5. The C-C length is apparently somewhat smaller than that of acetyl chloride. This shortening corresponds to a larger conjugation effect between COCl and the benzene ring. The difference of 0.038 A was larger than those of 0.107 a and 0.024 A for benzoyl fluoride and benzaldehyde, respectively. An ab initio MO calculation without any structure optimization (POP3G basis set) was made on the total energy of benzoyl halide for a rigidly rotating model of the COZ group with a regular hexagonal skeleton. The variation of the energy as a function of torsional angle of C-COZ is shown in Fig. 1. For benzoyl fluoride, the minimum energy appeared at the planar conformation. The total energy of benzoyl chloride has a rather wide flat part near a torsional angle of zero degrees with a very small hump (0.2 kJ mol-‘). This hump was again obtained by the calculation using the 4-31G basis set. The rotational constants obtained for the ground and excited states, however, varied linearly with the torsional quantum number up to ut = 3, showing no apparent anharmonicity of the torsional motion. The energy difference between the planar and orthogonal configuration of the molecule was taken as the theoretical barrier height. The calculated

188 TABLE 5 Structural parameters of the CCOCl group (in a and degrees)

CH,COCla s-trans H,CCHCOClb C,H,COCIC HCCCOCld

C-C

C=O

C-Cl

LCC=O

LCCCl

1.499(10) 1.476 1.452 1.424(16)

1.192(10) 1.192e 1.210e 1.209( 14)

1.789(5) 1.816 1.807 1.763(2)

127.08(17) 127.2e 123.2 125.9(6)

112.65(50) 116.3 119.6 113.1(6)

aRef. 8. bRef. 7. ‘This work. dRef. 9. eAssumed.

C6H5COF

C6H5COCI

0

30

60

90 120

degree

Fig. 1. Potential energy curve obtained from the STO-3G ab initio calculation. abscissa is the torsional angle of COZ from the ring plane.

The

results are given in Table 1. The theoretical values are nearly equal to those reported by Durig et al. [4]. Doublets were observed for some of the transitions due to nuclear quadrupole coupling of the chlorine nucleus. The doublet structures of most of the lines were, however, obscured by excited state lines. Tentative coupling constants were obtained from the separations in the apparent doublets and are listed in Table 6. The difference between the observed and calculated separations are rather large and, therefore, the values of the x-tensor have a large uncertainty. Assuming that the principal z-axis of the x-tensor lies on the C-Cl bond, the values of xz and xY are obtained as given in Table 6. The xz value for benzoyl chloride (-23 + 10 MHz) was smaller than that for s-trans-acryloyl chloride (-50.3 MHz) [7] and acetyl chloride (-59.2 MHz) [8].

189

TABLE 6 Splitting separation (MHz) of the doublet due to the chlorine nucleus and tentative values of the x-tensor (MHz) Transition 8 0 817-7 909-808 10 0 10 1 10 7 10 8 10 9 11 2 11 8 11 10 &a Xbb XCC

8-7

10 9 3 2 1 10 3 1

0 17 9 9 9 9 9 10 -10 10

-6.5+2 -19.0 f 6 25.5

0 1 7 8 9 2 8 10 x2 XY

XX

7

9 8 2 1 0 9 2 0

Obs.

Calc.

1.03 0.45 0.38 0.54 0.77 0.39 0.62 0.48 0.31 0.53 0.55

0.78 0.57 0.60 0.45 0.82 0.32 0.43 0.56 0.41 0.32 0.53

-23 + 10’ -2 f 5 25

aA coordinate system was taken so that the z-axis coincides with the C-Cl bond and the x-axis is perpendicular to the molecular plane.

REFERENCES 1 R. K. Kakar, E. A. Rinehart, C. R. Quade and T. Kojima, J. Chem. Phys., 52 (1970) 3803. 2 R. K. Kakar, J. Chem. Phys., 56 (1972) 1189. 3 R. K. Kakar, Ph.D. Thesis, University of Wyoming, Laramie, WY, 1970. 4 J. R. Durig, H. D. Bist, K. Furic, J. Qiu and T. S. Little, J. Mol. Struct., 129 (1985) 45. 5 J. H. HQg, L. Nygaard and G. 0. SQrensen, J. Mol. Struct., 7 (1971) 111. 6 T. Nakagawa and Y. Oyanagi, in K. Matusita (Ed.), Program system SALS for nonlinear least-squares fitting in experimental sciences. Recent Developments in Statistical Inference Data Analysis, North Holland, Amsterdam, 1980, pp. 221-225. 7 R. Kewley, D. C. Hemphill and R. F. Curl, Jr., J. Mol. Spectrosc., 44 (1972) 443. 8 K. M. Sinnott, J. Chem. Phys., 34 (1961) 851. 9 R. W. Davis, M. C. L. Gerry, S. Visaisouk and W. J. Balfour, Chem. Phys. Lett., 26 (1974) 561.