Vibrational spectrum, force Field calculations, thermodynamic functions and barrier to internal rotation for benzoyl fluoride

Spectrochimica Acta, Vol. 43A, No. 7, pp. 901-909, 1987.

0584-8539/87 $3.00 + 0.00 © 1987 Pergamon Journals Ltd.

Printed in Great Britain.

Vibrational spectrum, force field calculations, thermodynamic functions and barrier to internal rotation for benzoyl fluoride R. A. YADAV,* S. RAM,? R. SHANKER~ and I. S. SINGH* *Department of Physics, Banaras Hindu University, Varanasi 221005, India; tAdvanced Centre for Materials Science, I.I.T., Kanpur, 208016, India; and ,~Department of Applied Physics, I.T., Banaras Hindu University, Varanasi 221005, India (Received 8 October 1986; in final form 8 December 1986; accepted 10 December 1986)

Abstract--The i.r. and Raman spectra of benzoyl fluoride have been studied in solid, liquid and vapour phases. All of the 36 normal modes of vibration have been assigned in the light of normal coordinate analysis. The carbonyl group stretching mode exhibits a broad band group at ~ 1800 cm-t, with three bands of comparable intensities. The appearance of the triplet is explained by the existence of strong Fermi resonance. The C=O stretch, C-F stretch and CFO wag modes have been confidently assigned at 1810, 1014 and 682 cm - t (in solid/liquid state spectra), respectively. The former two of these show an upward shift and the latter one shows a downward shift in going from solid phase to vapour phase. These results evidence the presence of strong intermolecular interactions in solid and liquid phases. Thermodynamic functions and barrier to internal rotation of the CFO group have also been computed using the fundamental frequencies. The barrier height is found to be ~ 4.91 kCal/mole.

INTRODUCTION

The Raman [1, 2] and infra-red [2, 3] spectra of benzoyi fluoride have been reported earlier. GREEN and HARRISON [2] have proposed an extensive vibrational analysis. However, out of the 36 expected normal modes 2 C - H stretch and the C F O torsion were not assigned by them. A frequency at 59.5 c m - i in microwave spectrum was assigned for the C F O torsion [4]. More recently, DURIG et al. [5], using far i.r. spectral data, have assigned the frequencies 63.36 and 61.91 cm-~ to this mode corresponding to the 0 --, 1 and I --* 2 torsional transitions, respectively. In the present investigation we have studied the i.r. spectra of benzoyl fluoride in solid phase at liquid N 2 temperature and in gaseous phase at 100°C. The Raman spectrum has also been recorded in the region 50-4000 c m - 1 and an effort has been made to observe the C F O torsional mode. Unfortunately, we could not observe this but three additional frequencies are noted at 115, 117 and 156 c m - 1. We have carried out normal coordinate analysis using the valence force field. The results, so obtained, suggest that some of the assignments of GREEN and HARRISON [2] have to be revised. In addition, the thermodynamic functions (heat capacity, enthalpy, free-energy and entropy) have been computed using the above vibrational data in rigidrotator harmonic-oscillator approximation for one mole of ideal gas at standard pressure in the temperature range 100-1500 K. The barrier height for hindered rotation of the C F O group has also been computed using harmonic approximation. EXPERIMENTAL The purified grade benzoyl fluoride purchased from Fisher Scientific Co. was distilled twice under vacuum. SA43:7A-C

The i.r. spectra were recorded in the region 4 0 0 - 4 ( 0 cm - 1 on a Perkin-Elmer 580 spectrometer using a KBr cell for the solid sample at ~ 77 K and a 20 m long folded-path cell heated to 100°C for the gaseous sample. The low temperature spectra were obtained using a Specac variable temperature accessory with liquid N2 as coolant. The Raman spectra of unannealed samples were recorded in the range 50-4000 cm - i on a Spex 1403 Ramaiog spectrometer using the 5145 A line from a Spectra Physics Ar + laser with ~ 100 mW power at the sample. The low temperature spectra were achieved by mounting the sample capillary in a Hornig type dewar. The sample was cooled by passing the liquid N2 vapour over it. The spectra o f the gaseous sample were recorded using a heating assembly. The spectra (i.r. and Raman) of the annealed sample were also recorded. The annealing was achieved by leaving the solid samples for overnight at the liquid N2 temperature prior to recording the spectra. The frequencies quoted here a r e believed to be correct within + 2 cm - t for sharp bands and + 4 cm - 1 for broad bands in both the Raman and i.r. spectra.

COMPUTATIONAL DETAILS

KAKAR [4] has proposed a planar molecular structure for benzoyl fluoride from the microwave studies with the following structural parameters: r(C-C) = 1.399 A, r(C-H) = 1.088 A, r ( C - C F O ) = 1.486 A, r(C=O) = 1.180A, r(C-F) = 1.348 A, a ( C - C - C ) = a ( C - C - H ) = ``(C-C-CFO) = 120 °, ``(C-C---O) = 128022 ' and , , ( C - C - F ) = 110019 '. Thus benzoyl fluoride possesses C, symmetry and 36 normal modes of vibration of an isolated molecule can be classified as: (1) phenyl ring--21a' + 9a" and (2) C F O group Aa' +2a". To compute the fundamental frequencies, thermodynamic functions and barrier height to internal rotation we have taken the above molecular parameters for benzoyl fluoride. The vibrational problem was set up in internal coordinates. Symmetry coordinates for the phenyl ring 901

902

R.A. YADAVet

were constructed according to the suggestion of WHIFFEN [6] a n d those for the C F O g r o u p as given by HOLLENSTEIN a n d GI3NTHARD [7]. The vibrational frequencies were c o m p u t e d along with the G a n d F matrices assuming valence force field using c o m p u t e r p r o g r a m m e s of SCHACHTSCHNEIDER [8] with a subroutine added to the G matrix p r o g r a m m e to enable us to use fl-type internal coordinates for the in-plane C - H / C - C F O bending modes. The force constants were taken from the work o f ZWARICH e t al. [9] for benzaldehyde a n d were adjusted by trial a n d error to give the best possible agreement between the observed and c o m p u t e d frequencies a n d finally a n iterative procedure was followed to refine the force constants. The final set o f valence force constants is given in Table 1.

al.

The t h e r m o d y n a m i c functions were calculated using well k n o w n equations given in s t a n d a r d texts [10, 11]. The principal m o m e n t s a n d reduced m o m e n t of inertia were c o m p u t e d using the relations given by HIRSHFELDER [12] a n d PITZER a n d GWINN [13] respectively. All c o m p u t a t i o n s were carried out on a n 1CL 1904 S computer.

OBSERVED F U N D A M E N T A L S CHOSEN FOR NORMAL COORDINATE ANALYSIS

The a' f u n d a m e n t a l frequencies are almost the same as given by GREEN a n d HARRISON [2]. These

Table 1. Valence force constants for benzoyl fluoride No.

Description and notations*

Value~

Dispersion:~

Planar

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

C42 stretch (R) C-CFO stretch (RI) C-H stretch ( R e ) C-F stretch (R3) C=O stretch (R,) C-C-C angle bend (r,) C-C-F angle bend (at) C~2=O angle bend (ct2) O=C-F angle bend (~%) C ~ : - C F O in-plane bend (fl0 C-H in-plane bend (/~2)

1 2 3 4 5 6 7 8 9 10 11 12

C-CFO out-of-plane bend (71) C-H out-of-plane bend 0'2) C-F out-of-plane bend (60 C=O out-of-plane bend (62) C-C-C-C torsion (~) CFO torsion (~) (~b~b)° q~iYi= - 4~J- t~i 0q72) ° = ( ~ 2 " ~ 2 ) ° ~,7t?2)"= (Y272)m (~;D'2~= (Y2Y2)p Y16t

(RR) ° = (RRt) ° (RR)" = (RR4)" (RR~ = (RRa~ = (RR4) p R#i

=

Rioti+

t

Rifli = - Rifli + 1

(~t~t)° cqfli+ 1 = - ~ . t 3 i _ 1

(f12f12) °

(f12f12) ~ (f12f12~' RtR3 (R2R2) °

R4~tI cqct3

6.5721 5.9156 5.0499 4.5113 9.9963 0.9342 1.2945 1.1937 1.2143 1.0029 0.9939 0.7514 -0.4613 0.4091 0.3010 - 0.2165 - 0.2642 0.1165 -0.0657 -0.0405 -0.0353 0.3940 0.0892 - 0.4866 -0.3168

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1445 0.0354 0.0024 0.0658 0.0261 0.0258 0.0370 0.5437 0.3040 0.2802 0.5219

0.2360 0.2754 0.2515 0.1509 0.1400 0.0036 0.0247 0.0274 -0.0195 0.0089 0.0045 0.0783

0.0 0.0511 0.00 0.0714 0.0531 0.0009 0.0311 0.0042 0.0072 0.0341 0.0308 0.0335

Non-planar

* o, m, p stand for ortho, m e t a and p a r a respectively. Ri, ctiand ~b~stand for R, a and ~ respectively and the subscript i denotes their position in the ring. fl (in fl~) and ~,(in 73 stand for fl~ and f12 and ~'1 and )'2 and i denotes the position of//and y in the ring. tUnits for force constants are as follows: stretch and stretch/stretch = mdyn/A, stretch/bend = mdyn and bend and bend/bend = mdyn/A. ~:The force constants for which dispersion is zero were kept fixed after an initial run out.

Vibrational spectrum, force field calculations, thermodynamic functions authors have correlated the i.r. frequencies at 1311 and 1319 c m - ~observed as a doublet to the two different a' modes. The doublet structure is more pronounced in the i.r. spectrum for the sample annealed at liquid N2 temperature. Force field calculations allow only one fundamental near 1 3 0 0 c m - L We have, therefore, taken the stronger one of the doublet i.e. 1319 c m - ~as a fundamental and the weaker one as a combination band with intensity enhancement via Fermi resonance. A perusal of Table 1 of Ref. 2 shows that the Raman frequency at 1001 cm-1 has been correlated with a strong i.r. band at 1016 c m - 1. A difference of 15 cm between the two frequencies seems surprisingly large as compared to the difference for the other frequencies. Interestingly, low temperature i.r. spectrum, in the present case, shows two strong bands at 1001 and 1014cm-~ (Figs 1 and 2) around 1000cm-~. On account of their strong intensities these two bands are taken as fundamentals. However, in the frequency region 350-500 cm - 1 two fundamentals were assigned at 488 and 396cm -m and one fundamental at 212 c m - 1 below 300 cm ~ by earlier workers [2]. In the present investigation, the computations ascertain one fundamental at 390cm-~ in the region 300-500cm - t and two fundamentals at 192 and

155 cm-~ below 300 cm =. Therefore, the frequency 488cm -~ is dropped and instead a frequency at 156cm -~ observed as a shoulder in the Raman spectrum (present work) and as a weak band in the i.r. spectrum reported by DURIG et al. I-5] is included. Under the a" species, the highest fundamental has been estimated indirectly at 988 cm - ~via combination bands [2]. In fact, because of characteristically weak intensity, this fundamental (CH out-of-plane bending) does not exhibit measurable intensity at room temperature. Obviously, it grows as an i.r. shoulder for samples annealed at liquid N2 temperature. In the light of force field calculations the a" fundamental at 402 cm-~ is replaced by 488 cm-1. The frequency 63 cm ~ assigned to the C F O torsion by DURIG et al. [5] is taken for our normal coordinate analysis as well as for the computations of thermodynamic functions and barrier height. RESULTS A N D D I S C U S S I O N

The Raman frequencies of solid (liquid N 2 temperatures) and liquid samples agree well with those reported by GREEN and HARRISON [2], except the earlier mentioned three frequencies. Also, the i.r.

E 3

.6 e u r 0 .a

m

400

750

1100

903

14~ Frequency (crffl)

I

1800

Fig. 1. l.r. spectrum of benzoyl fluoride in KBr pellet annealed to liquid N2 temperature.

904

R.A.

YADAV

et al,

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4

8

i° _J

l

¢:h

¢:h

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f.h

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Vibrational spectrum, force field calculations, thermodynamic functions and Raman spectra of benzoyl fluoride recorded in the solid, liquid and vapour phases exhibit similar bands with little change in frequencies and relative intensities. This indicates that the crystal field splitting for the sample in bulk is not very effective. However, bands with relatively sharp and better resolved structure can

905

be observed for specimens annealed at liquid N 2 temperature (Fig. 1). The bands are assigned to the different fundamental modes and combination bands and/or overtones. Assignments of the fundamental modes (Table 2) are based on the magnitudes and relative i.r. and Raman intensities, changes observed in

Table 2. Assignments of fundamental frequencies (cm -~) for benzoyl fluoride Observed frequencies* Infra-red Solid (Liquid N2 temp) Unannealed:~ Annealed:~ Vapour~ 1

2

3

Ramant

Calculated Description of the modes and frequencies assignmentsll

4

5

6

a' species

18105 (100)

3112(20) 3084 (18) 3076 (19) 3040 (9) 3010(9) 18105 (100)

3103(11) 3085(13,13.9) 3072 (10, -) 3042 (7, -) 3006(12,-) 18355(100, A)

1602 (78) 1588(22) 1491 ( 1 7 ) 1454(sh) 1452 (48) } 13175(18) 1263 (77) 1243(100) 1180(23) 1163(11) 1076 (9) 1036 (70) 1014 (82) 1001 (53)

1601 (81) 1587(20) 1492(16) 1454(46) 1451 (55) 13211(20) 1261 (78) 1240(96) 1179(15) 1164(5) 1074 (7) 1034 (77) 1014 (88) 1001 (83)

1613 (13, A) 1590(6,-) 1502(7, B) 1463(15, B) 13195(11,-) 1259 (68, A) 1244(50,-) 1182(16, A) 1158(7,-) 1075 (9, A) 1041 (42, A) 1024 (54, B) 1019 (54, A)

1319 (vw, 0.7) 1256 (m, 0.17) 1240 (s, 0.15) 1180 (m, 0.17) 1163 (m, 0.72) 1075 (vw) 1032 (w, 0.32) 1001 (vs, 0.02)

1332 1265 1238 1173 1153 1065 1030 1010 996

769 (31)

768 (31)

770 (13, B)

769 (vs, 0.03)

758

645 (60)

~ 648 (63) ~ t 645 (6O) l 616 (10)

647 (26, A)

644 (vw)

635

//(C-H) + v(C-C) (3) v(C-C) +//(C-H) (14 KcKule) //(C-H) + v(C-C) (9a) //(C-H) + v(C-C) (9b) //(C-H) + v(C-F) + v(C~2) (18b) fl(C-H) + v(C-C) (18a) v(C-F) + v(C-C) +//(C-H) v(C-C) +//(C-H) + v(C-F) (1 ringbreathing) ~t(C-C-C) + v(C-C) + v(C-F) + v(C-CFO) (12) ~t(C-C-O) + ~(O-C-F) + a(C-C-C)

610 (s-sh) 376t (s, A) 213f (m) 158 (w)

615 (s, 0.75) 376 (m, 0.15) 211 (w, 0.55) 156:~(sh) a" species

605 390 192 155

~t(C-C--C) + ct(C-C-O) + ct(O-C-F) (6b) ~t(C-C-C) + v(C-F) + v(C-CFO) (6a) a (C-C-F) + a(O-C-F) +//(C-CFO) fl(C~2FO) + a (C-C-F) + a (O-C-F) (15)

993

y(C-H) + ~b(C-C~2-C) (5) ~,(C-H) + ~(C-C-C-C) (17a)

615 (12)

982 (sh) 946 (sh) 942 (10) j 954 (7) 806 (sh) ~ 798 (16) J 699 (93) 683 (sh) ~ 681 (19)3

991 (sh) 983 (13) 978 (13) 940 (8) 941 (9) 938 (9) 852 (6) 802 (sh) 796 (10) 794 (5) 698 (98) 683 (18) 680(31) 491 (5) 405 (4) 408(lO) 409 (4)

3079 (s, 0.16) 3065 (sh) 3042 (sh) 1809(vs, 0.25) 1601 (vs, 0.50) 1493 (w, 0.4) 1454 (w, 0 . 5 )

3110 3083 3067 3050 3023 1807 1625 1593 1512 1442

v(C-H) (20a) v(C-H) (2) v(C-H) (20b) v(C-H) (7b) v(C-H) (7a) v(C=O)+a(OCF)+ a(CCO) + v(C-C) v(C~C) +//(C-H) (8a) v(C-C) +//(C-H) (8b) v(C~) +fl(C-H)+ v(C-CFO) (19a) v(C-C)+//(C-H) (19b)

v(C-CFO)+//(C-H)+v(C-C)+v(C-F)(13)

972 (vw)

984

939 (7, -)

940 (vw)

922

3,(C-H) + q~(C-C-C--C) (17b)

849 (6, -)

848 (vw, 0.57)

853

)'(C-H) + q5(C-C~C-C) (lOa)

795 (8, -)

797 (w, 0.76)

825

)'(C-H) + ~b(C-C-C-C) (ll)

706 (86, 6) 670 (9, -)

705 (w, 0.7) 683 (vw)

707 641

~b(C-C~2-C) + y (C-H) (4) co(CFO) +)'(C-H)

480 (I lb) 406 (37, -)

490 (w, 0.5) 403 (vw)

470 432

~(C-C-C~C) + ),(C-H) (16b) ~ (C--C~2~C) + co(CFO) + )'(C-CFO) (16a)

167 (vs, 0.78)

143 64

)'(C-CFO) + co(CFO) + ~ (C-C-C-C) (10b)

63§(vw)

~(CFO)

*Where the notations under intensity columns have their usual meanings. tFrom Ref. 2; :~present work; §from Ref. 5; v: stretch;//: in-plane;a: angle bend; )': out-of-plane bend; ~: ring torsion; co:wag; 3: torsion; 5corrected for Fermi resonance.

906

R.A. YADAVet al.

going from the solid to vapour phase spectra and potential energy distributions (PEDs)* for the corresponding frequencies computed from the normal coordinate analysis. The present assignments have almost proved to be compatible with the data of the earlier work [2]. In the following section we shall discuss briefly a few assignments which either do not confirm the earlier [2] assignments or require some justification. In mono-substituted benzenes the ring breathing mode and the trigonal C - C - C angle bending mode interact normally with the substituent stretching mode and give rise to two characteristic frequencies in the regions 750-825 c m - 1 and 990-1050 c m - 1. One of these always retains the ring breathing or the trigonal C-C~C bending character and the other is assigned to either the remaining mode or the substituent stretching mode. GREEN and HARRISON [2] have assigned the frequencies 1001 and 644 c m - 1 of benzoyl fluoride to the ring breathing and the trigonal C-C--C angle bending modes, respectively. The assignment of the former is confirmed by the normal coordinate analysis, however, the frequency corresponding to the latter seems too low. A band at 769 era- 1, assigned earlier to the FCO angle bending mode [2] appears with strong and medium intensities in the Raman and i.r. spectra, respectively. In addition to having an appreciable contribution from the C - C - C angle bending, the PED for this frequency also contains contributions from several force constants, namely, C-C, C - C F O , C - F and C=O stretchings. We, therefore, assigned this frequency to the trigonal C - C - C angle bending. The C - C - O and C - C - F angle bendings have shown a negligibly small mixing with this mode. The aldehyde C - H stretching and the C - C H O stretching modes in benzaldehyde are well separated and observed at 2860 and 1260 cm -1 respectively [14-16]. However, in benzoyl fluoride the corresponding C - F and C - C F O stretching modes would interact with each other on account of their comparable magnitudes and same symmetry. Consequently, the two modes are expected to be well separated in frequency, contrary to the assignment by GREEN and HARRISON [2] for the frequencies 1258 and 1243 cm -~. Also neither of these two frequencies possesses C - F and/or C - C F O stretching character (as evident from PEDs). A suitable frequency with characteristics of C - C F O stretching mode is 1315 c m - 1. In addition to contributions from v(C-C), v(C-F), ct(C-C-C) and fl(C-H) modes, the PED of 1315 c m - 1 contains a significant contribution from C - C F O stretching mode. The C - F stretching mode usually appears with strong i.r. intensity. On intensity and PED considerations either of the i.r. frequencies 1001 and 1014 c m - 1 (in the C - F stretching region 1000-1400 cm-1) could be assigned to the C - F stret*The details of the potential energy distributions (PEDs), symmetry coordinates and internal coordinates can be obtained from the authors.

ching mode. As discussed earlier, the frequency 1001 c m - ~has been assigned to the breathing mode on account of its inherently stronger Raman intensity. Therefore, the frequency 1014cm -1, is the only suitable candidate for the C - F stretching mode. The corresponding vapour phase i.r. frequency has been observed at 1024 c m - 1. The normal coordinate analysis places four fundamentals below 300 cm - 1, two under each of the species a' and a". The two modes likely to belong to a' species are the fl (C-F) and fl (C-CFO) modes. The corresponding calculated frequencies at 192 and 155 c m - 1 are in good agreement with those observed at 212 and 157 c m - 1 respectively. Similarly the two a" modes below 300 c m - 1 involve ~(C-CFO) and z(CFO) modes. The computed frequencies for the respective modes at 143 and 64 c m - ~ are correlated to a depolarized Raman line at 167 c m - ~ [2] and a weak i.r. band at 63 c m - 1 [5]. The Raman spectrum in the neighbourhood of 64 cmdoes not show any frequency. The depolarized Raman frequencies at 115 and l l 7 c m -1 are perhaps the overtone of the CFO torsion. However, a possibility of their origin due to librational modes is not ruled out here. The PEDs suggest the frequency 167 cm-1 to arise due to a coupling of y (C-CFO) mode with the ring torsion and CFO wagging modes. The frequency at 63 cm- 1, however, arises due to T(CFO) mode alone suggesting the localization of this mode within the CFO group. It is interesting to note that both the modes, ~,(C-CFO) and z(CFO), are good candidates for mutual vibrational dephasing [17]. The characteristic i.r. spectrum of annealed solid in the carbonyl stretching mode shows a complex structure in the vicinity of 1800 cm-1 with three almost equally intense bands at 1790, 1804 and 1810 c m (Fig. 3). The band features are not affected much with a variation of temperature or a change of physical state (solid to liquid to vapour). However, their total halfbandwidth, Av 1/2, varies markedly showing a decrease for the solid (at liquid N2 temperature) and gaseous samples. A maximum Avl/2 value (of ~ 35 cm-1) is noted for the solid sample at room temperature. The triplet structure is not observed in the spectra of benzaldehyde [9] and substituted benzaldehydes [ 18]. The PED corresponding to the carbonyl stretch evidences a mixed character of v(C=O) with the or(C-C-F) and or(O-C-F) modes. However, the C=O stretching character, in the present case, is more dominant as compared to that in benzaldehyde [9] and isomeric trifluoromethyi benzaldehydes [19]. This may be responsible for the highest magnitude of the v(C=O) mode for benzoyl fluoride among all of the benzene derivatives with CXO group(s), where X = F, CI, Br and I. Three possibilities for the triplet structure are: (1) the factor group splitting of the v(C=O) mode, (2) the Fermi resonance of the v(C=O) mode with a suitable combination or overtone band and (3)the occurrence of more than one molecular species in the

Vibrational spectrum, force field calculations, thermodynamic functions

t

(I) Sotid - o - - o - (2) Annealed sotid - - - - - - (3) Vapour

907

A

, /i IrA I

IItl I

Ire

A -lJ C

I I I I I I

J~ k. O

I

!

O C a

JD ,<

I

I

'3

1720

1760

1840

1800

1880

Frequency (cm"I)

Fig. 3. I.r. spectrum of benzoyl fluoride in C=O stretching region.

system. The first possibility is automatically ruled out as similar vibrational structure is also observed for spectra in the liquid and vapour phases. The remaining two factors may jointly be responsible for the triplet structure. However, in the absence of temperature dependent studies of intensities of the i.r. bands near 1800 c m - ] and accurate measurements of depolarization ratio of the Raman lines, presence of the rotamerism cannot be established. Therefore, the appearance of the triplet is explained as a result of Fermi resonance. The band of the highest magnitude (1810 cm - ~) and intensity is assigned to the carbonyl stretching mode. The middle frequency (1804cm -~) of the triplet is attributed to a combination band 1035+769 = 1804 c m - ~, the intensity of which is enhanced via strong Fermi resonance with the C=O stretching mode at 1810 c m - ~. According to WILSON et al. [20] this type of resonance is manifested in vibration transition of substituted benzenes. To approach the phenomenon more closely, we have calculated the unperturbed positions of the resonating bands. With the assumption that the two photon band has zero intensity, the unperturbed separation (6) between the Fermi doublet may be estimated from the perturbed (observed) separation (A) and the observed intensity ratio of the bands using the following equation [21]. A-6 A+ 6

11 (resonating two photon band) 12 (parent one photon band)

The unperturbed (if there was no Fermi resonance) frequencies of the resonating bands may then be computed assuming that the levels are equally deviated

on Fermi resonance. This secured the unperturbed v(C=O) mode at 1810 c m - 1 and the combination band at 1800 cm-1. The frequency of the latter matches completely with the experimental value of 1800 c m - 1. The result is also consistent with the liquid and vapour phase data. Some asymmetric structure of the bands on lower frequency side observed for the solid samples and a weak band at ~ 1790 cm-1 for the vapour sample may be accounted for intramolecular bonding

//9

between the O and F atoms in the C ~ 1~ group. The traces of the broad band structures (absent in the vapour phase)at still lower frequencies ( ~ 1760 cm - ~) could be due to the intermolecular association [22, 23]. The in-phase and out-of-phase coupling of the ~,(C=O) and ),(C-F) modes give rise to the wagging and torsional motion of the CFO group. The PED for the frequency 682cm --1 contains appreciable contributions from the ~,(C=O) and ~,(C-F) force constants. Therefore, the ~o(CFO) mode is associated with the frequency 682 c m - 1 rather than to that at 702 c m - 1. A frequency shift of ~ 12 c m - ' is observed for this mode to lower side in going from the solid/liquid to the vapour phase. The two CFO group in-plane bending modes, i.e. fl(C=O) and fl(C-F) arise as a result of changes in three valence angles ct(C-C=O), ct(O=C-F) and ~ (C-C-F). We have already discussed earlier the assignment of the fl(C-F) mode at 157 cm-1 and that of the fl(C=O) is secured for a frequency at 644 c m in the light of normal coordinate analysis and in accordance with the work of ZWARICH et al. [9] and YADAVand SINGH [19]. The PED for the frequency at

908

R.A. YADAVet

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180

160 =T~.lZ,0 0

J 120 Z Z

IO0

o F.(,3 Z LI.

80

(.3

z <

Z :>. Q 0

60

~E t~J "zI.-

20

I 100

i

I 300

i

I 500

,

I 700

I

I 900

TEMPERATURE

i

I I I , .I 1100 1300 1500

IN°K

D

Fig. 4. Thermodynamic functions for benzoyl fluoride. Table 3. Thermodynamic function (cal./mole/K) Temperature K 100 200 298.15 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500

Heat capacity C~,

Enthaipy (H°-E~)/T

Free-energy

13.9366 20.8150 29.0791 29.2376 37.3534 44.1432 49.5733 53.9114 57.4227 60.3058 62.7012 64.7099 66.4067 67.8494 69.0829 70.1432

10.6128 13.9154 17.5290 17.6007 21.5467 25.4112 29.0029 32.2627 35.1957 37.8306 40.2015 42.3408 44.2776 46.0367 47.6400 49.1057

54.1825 62.5442 68.7598 68.8685 74.4705 79.6975 84.6529 89.3727 93.8758 98.1761 102.2869 106.2206 109.9891 113.6038 117.0751 120.4126

769 c m - 1 assigned to the u ( O = C - F ) m o d e earlier [2] does not involve O = C - F , O = C - C a n d F - C - C angle bending force constants at all a n d here, as discussed earlier, is assigned to the trigonal C - C - C angle bending.

- (F°-E~)/T

Entropy S° 64.7954 76.4596 86.2888 86.4692 96.0173 105.1087 113.6558 121.6355 129.0716 136.0067 142.4884 148.5615 154.2667 159.6405 164.7151 169.5183

T H E R M O D Y N A M I C F U N C T I O N S AND BARRIER T O INTERNAL ROTATION

In determining the t h e r m o d y n a m i c functions of benzene derivatives with substituent(s) as group(s) o f

Vibrational spectrum, force field calculations, thermodynamic functions atoms, the torsional motion(s) of the substituted group(s) relative to the phenyl ring imposes some limitations. If the barrier against free rotation is relatively high (> 2.5 kCal/mole), the torsional mode may be approximated by a vibrational mode [24]. In such a case, the contribution to the thermodynamic functions due to the torsional mode may easily be computed as for the vibrational mode [10, 11]. On the other hand, if the barrier height is small ( < 2.5 kCal/mole), the approximation of the torsional mode by vibrational mode may not be valid and one has to drop the corresponding frequency from the vibrational contributions and instead its contribution has to be included into rotational part. DURIG et al. [5] have calculated the barrier to internal free rotation to be 4.97 kCal/mole for benzoyl fluoride. This is sufficiently high to treat the torsional mode as a vibrational mode. Therefore, in the present work the torsional frequency at 63 cm -~ together with the other 35 fundamental frequencies (Table 2) were used to evaluate the thermodynamic functions. The results are presented in Table 3. Figure 4 shows their variation as a function of temperature in the range 100-1500 K. The principal moments of inertia computed from the structural parameter [4] as suggested by HIRSHFELDER [12] came out to be 95.05 x 10 -39, 73.33x 10 -39 and 21.73 x 10-39gm-cm 2 and well agree with the experimental values reported in the literature [4]. The reduced moment of inertia calculated following PITZEI~and GWlNN [13] is found to be 4.87 x 10 -39 gm-cm 2. The translational, rotational and vibrational contributions to thermodynamic functions were computed separately but in the Table 3 only the total sum is given. The separate evaluations show that the translational and rotational contributions to the enthalpy and heat capacity are temperature independent while the vibrational contribution increases gradually with temperature. Moreover, the translational and rotational contributions to the free energy and entropy dominate over those of the vibrational contribution at lower temperatures and at higher temperatures, the three contributions become comparable for free energy and the vibrational contribution dominates over the other two contributions for entropy. The barrier to internal rotation was also computed under harmonic approximation using the frequency at 63 cm-~ for the torsional fundamental. The barrier height comes out to be 4.91 kCal/mole, in good agreement with the value reported in literature [5]. CONCLUSIONS All the 36 normal modes of vibration of benzoyi fluoride could be assigned. The trigonal C - C - C angle bending mode interacts with other modes which is reflected in its reduced magnitude from 1012 to 769 c m - t . The ring breathing mode does not show

909

such interaction and its magnitude is retained as that for benzene (992 cm-~). However, its magnitude increases by 18 cm-~ in going to the gas phase. This evidences that the intermolecular interactions affect the breathing mode to some extent. The CFO wagging mode, exhibits a reduced frequency in the vapour phase which may be due to the absence of intermolecular and some other vibrational coupling forces resulting in a more easier motion (lower frequency) during the to(CFO) mode. Contrary to the case of CFO wagging mode, the v(C=O) and v(C-F) modes show a shift to higher frequency side in going from the solid/liquid to vapour phase suggesting weakening of the C - F and C=O bonds in solid/liquid phase as compared to those in vapour phase. Acknowledoement--R. SHANKER is thankful to U.G.C., Government of India, for financial assistance. REFERENCES

[1] H. SEEWANN-ALBERTand L. KAHOVEC,Acta Phys. Aust. 1, 352 (1948). [2] J. H. S. GREENand D. J. HARRISON,Spectrochim. Acta 33A, 583 (1977). [3] R.N. KNISELEY,V. A. FASSE~E. L. FARQUHARandL. S. GRAY,Spectrochim. Acta 18, 1217 (1962). [4] R. K. KAKAR,J. chem. Phys. 56, 1189 (1972). [5] J. R. DURra, H. D. BraT, K. FURIC~J. QIu and T. S. LI/'rLE, J. melee. Struct. 129, 45 (1985). [6] D. H. WHII~EN,Trans. R. Soc. 248A, 131 (1955). [7] H. HOLLENSTEINand H. H. GUNTHARD,Spectrochim. Acta 27A, 2027 (1971). [8] J. H. SCHACHTSCHNEID~Project No. 31450, Tech. Rept. No. 231-64, Shell Development Co., Emeryvile California U.S.A. [9] R. ZWARICH,J. SMOLAREKand L. GOODMAN,J. molec. Spectrosc. 38, 336 (1971). [10] G. HERZBERG, Infrared and Raman Spectra of Polyatomic Molecules. Von Nostrand Inc. (1945). [11] N. B. COLTHUP, L. H. DALe and S. E. WmERLEY, Introduction to Infra-red and Raman Spectroscopy. Academic Press, New York (1964). [12] J. O. HIRSHFELDER,J. chem. Phys. 8, 431 (1940). [13] K. S. PITZERand W. D. GWlNN,J. chem. Phys. 10, 428 (1942~. [14] G. VARSANYI,Assignments for Vibrational Spectra of Seven Hundred Benzene Derivatives. Halsted Press, New York (1974). [15] H. D. BIST, J. C. D. BRANDand D. R. WILLIAMS,J. melee. Spectrosc. 24, 402 (1967). [16] J. H. S. GREENand D. J. HARRISON,Spectrochim. Acta 32A, 1265 (1976). [17] C.B. HARRIS,R. M. SHELBYandP. A. CORNELIUS,Phys. Rev. Lett. 38, 1415 (1977). [18] S. H. W. HAKIN, O. S. KHALILand L. GOODMAN,J. melee. Spectrosc. 72, 383 (1978). [19] R. A. YADAVand I. S. SINGH,Ind. J. Phys. 58B, 556 (1984). [20] E. B. WmSON, Jr, J. C. DEOUS and P. C. CROSS, Molecular Vibrations. McGraw-Hill, 252 (1955). [21] D. C. MCKEAN,Spectrochim. Acta 29A, 1559 (1973). [22] S. RAM,Can. J. Chem. 62, 1845 (1984). [23] S. RAM,K. RAMand J. S. YADAV,J. de Chem. Phys. 81, 577 (1984). [24] D. R. LIDEJr and D. E. MANN, J. chem. Phys. 29, 914 (1958).