The vibrational spectrum of fluorobenzene

JOURNAL OF MOLECULARSPECTROSCOPY73, 290-304

The Vibrational

(1978)

Spectrum

of Fluorobenzene

E. D. LIPP AND C. J. SELISKAK

of Chemistry,

Department

of Cincinnati,

University

Cincinnati,

Ohio 35221

The high resolution infrared spectra of three fluorohenzene isotopes were obtained and interpreted. Vibrational assignments of most bands were made and values for most of the fundamental vibrations were obtained. Accurate values for the infrared inactive a2 vibrations were also obtained. Several major modifications of literature values for the fundamentals were made and a Fermi resonance

interaction

identified.

INTRODUCTION

The vibrational spectra of fluorobenzene (1) and pentadeuterofluorobenzene (2) have previously been obtained at low resolution. Calculations (3, 4) of the fundamental vibrations of these two isotopes have agreed reasonably well with experimental values. However, the accuracy and reliability with which these fundamentals are known have proved inadequate for an interpretation of the high resolution ultraviolet spectra of these molecules-a study currently underway in our laboratory. Thus, the thrust of the present investigation is an accurate measurement of the fundamental vibrations of the three fluorobenzene isotopes shown below. F

I

I

0

0

:,

0

A Fluorobenzene

:

FC6H4D

FCsHs

F&D5

The vibrational analyses of the spectra of these three isotopes are given with emphasis on revisions of previous analyses necessary in the interpretation of their spectra. EXPERIMENTAL

Samples of commercially available fluorobenzene (Matheson, Coleman, and Bell) and pentadeuterofluorobenzene (Aldrich) were used as purchased. The p-deuterofluorobenzene isotope was prepared from p-bromofluorobenzene by a Grignard technique. Approximately 5 g (0.21 moles) of dry magnesium shavings and 20 ml of p-bromo290 0022-2852/78/0732-0290$02.00/O Copyright Q 1978 by Academic Press. Inc. All rights of reproduction in any form reserved.

FLUOROBENZENE

VIBRATIONAL

‘91

SPECTRUM

TABLE I PR-Branch

Separations (cm-l)

PR - BRANCH SEPARATIONS (cm-') FC6H4D

W’tj

FCb-%

A- type bands talc. obs.

13.5 13.3

13.3 12.9

12.9 12.4

B- type bands talc. obs.

10.8 11.6

10.6 11.2

10.2 9.8

20.4

19.9 (19)

19.3 (18)

C- tyoe bands talc. obs.a

(21)

aThese separations could not be measured as accurately due to their inherent weakness.

fluorobenzene (0.18 moles) were placed in 100 ml of anhydrous diethyl ether. This reaction mixture was gently refluxed and allowed to react for 2 hr at which time formation of the Grignard reagent was complete. Dropwise addition of 10 ml D,O (99.87c D, Stohler Isotope Chemicals) to the reaction mixture produced deuterolysis of the Grignard reagent. The resultant mixture was allowed to react for 1 hr with gentle retluxing and stirring. Precipitated DOMgBr was removed by filtration and a mass spectrum of the product indicated an 84% pure FC&D isotope with the dominant contaminant being normal fluorobenzene. Mass spectra of the purchased isotopes showed the FC’6D5 to be 85% pure while FCsH5 was essentially loOg/, pure. All infrared measurements were performed on a Beckman IR-12 spectrophotometer which was calibrated phase samples

using IUPAC

at room temperature

standards

(5). The spectra

were obtained

using a sample path length

between

on gas

10 cm and

11.25 m. Gas cells with either KBr or CsI windows were used for all measurements 10 cm. Spectra

at longer path

lengths

were obtained

Variable Path gas cell. The spectral region scanned surements

corrected

the long path (mainly

length

to vacuum

and f0.3

at most, a few extremely

interference

severe. Regions particularly

problem were 1400 to 1850,350O to 3950, and is that,

a Wilks

weak bands

20 Meter

was 200 to 4000 cm-’ with all mea-

cm-’ for sharp rotational

used in some measurements,

Hz0 and COZ) was occasionally

employing

at

features.

Due to

from contaminants susceptible

to this

<400 cm-‘. The upshot of this situation in these isotopes

may not have been

measured. Raman spectra were also obtained at 2 cm-’ resolution on neat samples. The spectra are consistent with previous work (I, Z) done on FCsHs and FCcD5 but this resolution was too low to aid us in positively identifying bands at high resolution. Thus communication is restricted to infrared measurements on gas phase Auorobenzenes.

this

292

LIPP

AND

SELISKAR

TABLE II Infrared Bands of FC& Band P sition , vat (LYll-~ 248.2 (400)

Intensity Band Type br

b, fundamental

m, br

b2 fundamental

S,

442.6

VI,br

462.2

w, br

497.7*

s, C

bT fundamental

517.1

m, A

a, fundamental

648.0

VW, br

654.6 686.?**

ohs.-c lc. (cm-8,

Assignment

894.8 - 248.6 = 646.2 A

1.8

w, br

1066.1 - 413.9 = 652.2 C

2.4

s, C

bl fundamental

754.1

vs, c

bl fundamental

808.J**

s, A

al fundamental

825.1

m, A

2 x 413.9 = 827.8 A

894.8

5, c

bl fundamental

977.8

w. c

bl fundamental

991.2**

m, A

2 x 497.7

1008.6

m, A

al fundamental

3n22.7

m, A

al fundamental

1066.1

m, B

b2 fundamental

= 995.4

A

1100.1

VW, B

686.7 + 413.9 = 1100.6 6

1127.7

VW, B

b2 fundamental

1156.3

s, A

al fundamental

1184.0*

w, sh

497.7 + 686.7 = 1184.4 A

-0.4

1226.5

w, sh

413.9 + 817.6 = 1231.5 A

-5.0

1238.4*

vs, A

al fundamental

1300.7

VW, 6

b2 fundamental

132?.1**

w, A

517.1 + 808.7 = 1325.8 A

-0.5

1.3

RESULTS AND DISCUSSION

Fluorobenzene is a planar molecule with Czy symmetry (6) and there is little doubt that FCsD6 and FCcH4D also possess these characteristics. The 30 fundamental vibrations reduce to symmetry species as follows: llat + lob2 -I- 6bl + 3~2. In designating these symmetry species the z (A) axis has been taken as colinear with the CZ axis and the x (B) axis perpendicular to the plane of the aromatic ring. Consequently, at vibrations produce A-type rotational contours, those of bz symmetry produce B-type contours, and those of b1 symmetry produce C-type contours. The PR-branch separations for bands of differing rotational contour were calculated using the available rotational

FLUOROBENZENE TABLE Band P sition (cm-P, vat)

Intensity Band Type

VIBRATIONAL

SPECTRUM

II-Contimed ohs.-c lc. (cm-8)

Assignment

1392.6

m, A

497.7 + 894.8 = 1392.5 A

0.1

1440.1

w, A

686.7 + 754.1 = 1440.8 A

-0.7

1475.1

m, A

497.7 + 977.8 = 1475.5 A

-0.4

1500.3

vs. A

al fundamental 517.1 + 1008.6 = 1525.7 A

1526.8

m, A

1604.8

vs. A

1712.4

m, 8

894.8 + 817.6 f 1712.4 B

0.0

1775.0

m, A

817.6 + 957.4 = 1775.0 A

0.0

1853.0

m. B

894.8 + 957.4 = 1852.2 B

0.8

1935.6

m, 8

1127.7 + 808.7 = 1936.4 8

-0.8

1957.1

w. A

686.7 + 754.1 + 517.1 = 1957.9 A

-0.8

2044.2

w, A

1.1

al fundamental

2 x 1022.7 = 2045.4 A

2175.4

VW, B

400.4 + 817.6 + 957.4 = 2175.4 A

2222.8

VW, B

1066.1 + 1156.3 = 2222.4 6

0.4

2279.4

VW. sh

1460.5 + 817.6 = 2278.1 C

1.3

2312.8

VW, B

1127.7 + 497.7 + 686.7 = 2312.1 B

0.7

A

2393.8

VW,

2409.0

VW, A

A

0.0

977.8 + 817.6 + 517.1 + 2312.5 B

0.3

686.7 + 817.6 + 808.7 = 2313.0 B

-0.2

413.9 + 957.4 + 1022.7 = 2394.0 A

-0.2

808.7 + 1604.8 = 2413.5 A

-4.5

497.7 + 754.1 + 1156.3 = 2408.1 A

constants

203

0.9

2 x 1232.4 = 2464.8 A

2462.2

VW,

2474.6

VW, c

977.8 + 1500.3 = 2478.1 C

-3.5

2506.8

VW, A

1008.6 + 1500.3 = 2508.9 A

-2.1

2519.2

VW, A

1022.7 + 1500.3

-3.8

2572.6

VW, sh

754.1 + 808.7 + 1008.6 = 2571.4 C

q

2523.0 A

1.2

and an algorithm developed by Seth-Paul (7, 8). Rotational constants for F&H5 and FCcHdD were obtained from microwave data (6) while those for FCCDC were calculated using a model geometry consistent with microwave results for the previous two isotopes. The results of these calculations and comparison with averages of measured PR-branch separations are given in Table I. The accuracy of calculated separations is seen to be excellent. A complete tabulation of our infrared measurements is given by isotope in Tables II, III, and IV along with band assignments. The wave numbers in the tables denote Q-branch maxima for A- and C-type bands, while values for B- type bands represent the minimum of intensity between P- and R-branch intensity lobes. For bands of mixed

LIPP AND

294

Band Position (cm-l, vat)

Intensity Band Type

TABLE

SELISKAR

II-Codnued ohs.-c lc. (cm-?)

Assignment

2615.4

My,

B

1460.5 + 1156.3 = 2616.8 B

-1.4

2652.2

VW,

A

1156.3 + 1500.3 = 2656.6 A

-4.4

2744.0

VW, A

2787.8

VW, A

497.7 + 686.7 + 1604.8 = 2789.2 A

-1.4

2886.4

VW, br

2917.4

VW,

A

2956.6

VW, 8

497.7 + 1156.3 + 1232.4 = 2886.4 C

-0.0

754.1 t

-1.8

977.8 + 1156.3 = 2888.2 A

2 x 1460.5 = 2921.0 A 1460.5 + 1500.3 = 2960.8 B

-4.2

3061.0

s, A

aT fundamental

3080.2

s, sh

al fundamental

3094.4

s, A

a, fundamental

3116.0

w, sh

1066.1 + 817.6 + 1232.4 = 3116.1 8

-0.1 -1.0 0.0

3327.8

VW. c

248.6 + 3080.2 = 3328.8 C

3483.4

VW, c

3069.5 + 413.9 = 3483.4 C

vs- very strong, w= weak,

s= strong, VW- very weak,

m= moderate, br= broad,

sh- sharp

* = higher intensity branch in a split Q-branch **= average of a split Q-branch

or indefinite rotational contour, the maximum of the band is recorded. Relative intensities of the bands are also given. A tabulation of the fundamental vibrations deduced from these measurements is given in Table V with calculated values (3, 4) shown for comparison. Several computer programs were written to aid us in our assignment of overtone and combination bands. These programs took a specified set of fundamental vibrations and assigned measured bands according to symmetry and numerical fit. AEEpossible combination and difference bands of double and triple quanta were considered in this analysis. Our final assignments were based on symmetry, numerical fit, homology among isotopes, and repeated rotational profile peculiarities associated with a specific vibration. Most of the measured bands were assigned unambiguously and, by in large, are consistent with the literature. These are listed without further comment. Those assignments warranting comment are discussed by symmetry type in following sections of this paper.

All of the al fundamentals shown in Table I have been identified with reasonable certainty except for vibration 13 in FCsH,D. Recent calculations (9) performed on

FLUOROBENZENE

VIBRATIONAL

295

SPECTRUM

bromobenzene and p-deuterobromobenze:ne suggest that vibration 13 should shift significantly between these two isotopes, while vibrations .?O and 2 should shift onlv slightly. Our spectra show A-type bands at 2304.4 and 2274.4 cm-’ in FC’aHJ), both of which possess sufficient intensity to be considered as fundamentals. Since only one fundamental is expected in this region, the appearance of two bands may suggest a Fermi resonance interaction although the resonance partners are as yet unspecified. Both of the values 2304.4 and 2274.4 cm-l can be used to explain combination bands so it is still uncertain which value corresponds to vibration 13 and which bands may lx in resonance. Vibration IYa in FC6Hs is interesting because in each of the five times it appears in combination the observed value of the band is 2.1 to 4.2 cm-l lower than expected for harmonic combination. The vibrational combination differences with fundamentals 5 (bl), 28a (al), 1 (al), 9a (al), and 19b (b2) have a common al state which can only be IYo and is 1406. i cm-‘. yet the 1Ya fuadamental has a distinct 11-type rotational profile with a sharp (j-branch at 1500.3 cm-l. No other bands of comparable intensity appear in the 1500 cm-’ region of the spectrum and this argues against a harmonic resonance eq)lanation for these observations. One possible exljlanation is that the off-diagonal anharmonicity constants, X lyu,k, are surprisingly large.

J

\ FC,H,D

II

820

800 WAVENUMBER

1340

1320

1300

( cm-‘)

FIG. 1. Vibration 12 and combination band 12 + 6a in F&H& and FCsHaD. The Q-branch positions of vibration 12 are 810.1 and 807.3 cm-1 (FCsHs) and 807.8 and 805.4 cm+ (F&HID). For combination 12 + 6e, the positions are 1328.7 and 1325.6 cm-’ (FGHs) and 1321.4 and 1319.0 cm-’ (FCaHdD).

296

LIPP AND

SELISKAR

The combination band appearing at 1327.1 cm-’ in FCsHs has previously been misidentified (I, 2) as a bt fundamental, and calculated values (3) have mistakenly confirmed this assignment. When observed at long path length (see Fig. 1) the band appears to exhibit A-type contour with a distinctive split Q branch remarkably similar to the split Q branch found in vibration 12 at 808.7 cm-l. In FC6H4D a similar A-type band is found at 1320.2 cm-‘. This latter band also possesses a split Q branch similar in appearance to that found in vibration 12 at 806.6 cm-‘. Both bands in question can be assigned as the combination 12 + 6a in the appropriate isotope. The homology between bands and rotational band contours argues conclusively for the assignments as al combination rather than bz fundamental. There appears to be a possible Fermi resonance interaction in both FCGHS and FCeHkD between vibration 7a and the combination band 10~ + 16~. The fundamental is shifted to higher wavenumber in each instance with the final positions of the analogous TABLE

III

Infrared Bands of FCcHbD Band Position (cm-l, vat)

Intensity Band Type

Assignment

247.9

s, br

bl fundamental

(400)

m, br

b2 fundamental

416.6

w, br

442.7

w, br

492.0*

s, 6

bl fundamental

513.2

m, A

a, fundamental

558.3

w, c

603.0

s. c

654.4

VW, sh

715.6

s, c

bl fundamental

806.6**

s, A

al fundamental

825.0

m, sh

2 x 413.9

851.1

s, 6

bT fundamental

922.2

w, 6

bl fundamental

938.6

w, br

979.9

w, A

990.9

w, sh

al fundamental

1021.6**

m, A

al fundamental

1093.6

m, B

b2 fundamental

1156.1

s, A

aT fundamental

1226.8

m, sh

413.9 + 817.7 = 1231.6 A

1239.2

vs, A

1276.3

VW, sh

ohs.-talc. (cm-l)

bT fundamental

q

827.8 A

2 x 492.0 = 984.0 A

al fundamental

-4.8

FLUOROBENZENE

VIBRATIONAL

TABLE Band PO ition (cm-'I, vat) 1288.9

Intensity Band Type YW,

SPECTRUM

207

III-Continr~d Assignment

ohs.-talc. (cm-l)

b2 fundamental

B

w, A

513.2 + 806.6 = 1319.8 A

0.4

1343.2

w, A

492.0 + 851.1 = 1343.1 A

0.1

1436.3

w, A

400 + 1036 = 1436 A

1320.2**

1493.0*x 1504.3

a, fundamental

vs, A w, sh

513.2 + 990.9 = 1504.1 A

0.2

al fundamental

1600.4

vs, A

1668.8

w, B

851.1 + 817.7 = 1668.8 B

1774.9

w, A

817.7 + 957.2 = 1774.9 A

0.n -0.1

0.0

1797.4

VW, A

806.6 + 990.9 = 1797.5 A

1829.9

VW, A

806.6 + 1021.6 = 1828.2 A

1.7

19n1.5

w, B

1093.2 + 806.6 = 1899.8 8

1.7

806.6 + 1156.1 = 1962.7 A

2.6

1965.3

VW, sh

2041.8

w, A

2249.0

VW, B

2274.4

m, A

2304.4

m, A 1

2404.6

2 x 1021.6 = 2043.2 A 1093.2 + 1156.1 = 2249.3 B 1

VW, A

-0.3

a, fundamental (see text) 806.6 + 1600.4 = 2407.0 A

-2.4

492.0 + 922.2 + 990.9 = 2405.1A

-0.5

2462.8

VW, A

2 x 1232 = 2464 A

2519.4

VW, B

1288.9 + 413.9 t 817.7 = 2520.5 B

-1.1

2573.2

w, br

2614.4

VW, B

R51.1 + 957.2 + 806.6 = 2614.9 B

-0.5

2644.6

VW, A

1156.1 + 1493.0 = 2649.1 A

-4.5

2686.0

VW, B

922.2 + 957.2 + 806.6 = 2686.0 B

2729.2

VW, B

31162.4

m, B

715.6 f 413.9 + 1600.4 = 2729.9 B

0.0 -0.7

bp fundamental

3190.0

VW, sh

1093.2 + 1288.9 + 806.6 = 3188.7 A

1.3

3216.4

VW, A

1288.9 + 1415.3 + 513.2 = 3217.4 A

-1.0

3266.2

VW, A

990.9 + 2274.4 = 3265.3 A

3295.6

VW, A

1021.6 + 2274.4 = 3296.0 A 990.9 + 2304.4 = 3295.3 A

3381.6

0.9 -0.4 0.3

VW, br

resonance components being nearly identical in the two isotopes. The bands in question are shown in Fig. 2. Vibrations 10a and 16aare of a2 symmetry, and the resultant al combination should be invariant between FC6Hs and FCGHID (cf. section on a~ vibra-

LIPP AND SELISKAR

298

TABLE IV Infrared Bands of FCeDs Band Position (cm-l, vat)

Intensity Band Type

Assignment

233.7

s, c

bl fundamental

384.9

m, B

b2 fundamental

4X.6**

m, C

bl fundamental

457.6

w, sh

472.5

w, sh

502.8

m, A

al fundamental

552.9

s, c

bl fundamental

624.7

s, c

bl fundamental

659.3

VW, sh

718.6

w, A

233.7 + 426.6 = 660.3 A

752.5

vs. A

al fundamental

s, c

bl fundamental

806.7

s, B

b2 fundamental

819.8

s, A

al fundamental

843.2

m, B

b2 fundamental

851.0

w, sh

2 x 426.6 = 853.2 A

877.2

m, A

al fundamental

883.0a

w, sh m, sh

9il.9a

w, sh

922.3a

w, sh

951.7a

w, A

963.9

m, A

981.4

w, A

-1.0

2 x 362.6 = 725.2 A

759.0

890.4a

ohs.-c lc. (cmwp)

al fundamental 426.6 + 552.9 = 979.5 A

1.9

-

tions). Vibration 7a also apparently shifts little since the amount of resonance interaction is approximately the same in each isotope. The harmonic value of 1Oa + 16a is 1231.5 cm+ in FCeH6 and 1231.6 cm-r in FCcHdD, corresponding to a perturbation of about 5 cm-r in both isotopes. Further evidence for this assignment is the appearance of the overtone of 7a in each isotope, occurring at 2462.2 cm-r in F&H6 and 2462.8 cm-’ in FCeHbD. Vibration 7a is also found in combination in F&H5 (3116.0 and possibly 2886.4 cm-l) from which a harmonic value of 1232.4 cm-l is calculated for 7~. It was not observed in combination in FCP,H~D. Although the rotational contour of the lower energy component in the resonance interaction is partially obscured, the appearance of the overtone 2 X 7a and homology between the isotopes argues strongly for the Fermi resonance explanation.

FLUOROBENZENE

VIBRATIONAL

TABLE Band P sition (cm-9, vat)

Intensity Band Type

998.7

VW,

1035.0

m,

IV-Continrwd ohs.-ca c. (cm-i)

Assignment

sh

362.6 + 636.1 = 998.7 A

a

bp fundamental

1105.3

VW, sh

1138.8

w, sh

1172.4

vs. A

1189.0

m, A

299

SPECTRUM

0.0

2 x 552.9 = 1105.8 A 362.6 + 776.2 = 1138.8 A

0.0

al fundamental 384.9 + 806.7 = 1191.6 A 426.6 + 759.0

-2.6

= 1185.6 A

3.4 0.0

1205.8**

w, sh

843.2 + 362.6 = 1205.8 B

1231.5

w, C

233.7 + 362.6 + 636.1 = 1232.4 C

-0.9

1244.6

w, C

426.6 + 819.8 = 1246.4 C

-1.8

1277.7

w, A

1311.9

m,

a

m, B

(1385)

b2 fundamental 384.9 + 362.6 + 636.1 = 1383.6 B

1.4

1395.9

vs,A

1466.8

m, A

502.8 + 963.9 = 1466.7 A

0.1

1567.3

m, C

426.6 + 362.6 + 776.2 = 1565.4 C

1.9

1579.2

vs.,A

1673.5

m, A

502.8 + 1172.4 = 1675.2 A

1790.4

W, a

1035.0 + 752.5 = 1787.5 I3

2.9

1926.4

w, A

752.5 + 1172.4 = 1924.9 A

1.5

1993.5

VW, A

819.8 + 1172.4 = 1992.2 A

1.3

2047.2

w, A

877.2 + 1172.4 = 2049.6 A

-2.4

2146.4**

w. A

752.5 + 1395.9 = 2148.4 A

-2.0

2197.2

VW,

2291.4

YS, A

al fundamental

al fundamental -1.7

a al fundamental

-

The bz fundamentals were the most difficult to identify experimentally, mainly because of their weak intensity compared to al and bl fundamentals. The only b2 fundamentals of moderate intensity in FCEHS and FCsH4D were 15 and 9b, respectively. However, several appeared in FCeDs. The remainder were observed only at long path length or were identified from their appearance in combination bands. The assignment of vibration 9b in FC6H6, 1127.7 cm-l, differs significantly from its calculated value of 1154 cm-‘. Indeed, no R-type band can be observed at 1154 cm-’ due to the intense al fundamental (vibration 9~) at 1156.3 cm-r. A weak B-type band is observed at 1127.7 cm-‘, and this value appears in combination with vibration 12 (808.7 cm-l) to produce a B-type band of moderate intensity at 1935.6 cm-r. The

300

LIPP AND TABLE Band Ppsition (cm- , vat)

Intensity Band Type

2426.4

w, A

2454.2

VW, A

2564.8

m, A

SELISKAR

IV-Continued ohs.-talc. (cm-l)

Assignment 502.8 + 752.5 f 1172.4 = 2427.7 A

-1.3

877.2 + 1579.2 = 2456.4 A

-2.2

1172.4 + 1395.9 = 2568.3 A

-3.5

2578.0

VW, A

2678.6

w, B

362.6 + 636.1 + 1579.3 = 2577.9 A

0.1

384.9 + 2291.4

2.3

2747.6

VW, A

752.5 + 819.8 t 1172.4 = 2744.7 A

2.9

2862.4

w, A

502.8 + 963.9 + 1395.9 = 2862.6 A

-0.2

3076.8

w, A

233.7 + 552.9 + 2291.4 = 3078.0 A

-1.2

3158.0

w, B

877.2 + 2280.8 = 3158.0 B

0.0

3256.0

w, A

963.9 + 2291.4 = 3255.3 A

0.7

3323.0

vw. br

q

2676.3 8

1035.0 + 2291.4 = 3326.4 B

-3.4

aProbable impurity bands.

analogous combination band appears in FCeHaD at 1901.5 cm-’ with the contributing fundamentals also being vibrations 12 and 9b. This homology between isotopes supports the assignment of 9b in F&H6 as 1127.7 cm-‘. Vibration 18b was difficult to identify in all three isotopes due to band overlap with low wavenumber water vapor intensity. The exact position of this fundamental could only be approximated in F&H, and FCCHP. The more exact value 400.4 cm-’ listed for FCcH6 was deduced from its appearance in the combination band at 2175.4 cm-l. bl Vibrations All bl fundamentals in the three isotopes were intense bands excepting vibration 5, which was weak in F&Ha and FCeHhD, and could not be seen due to possible band overlap in FC6D5. The assignment of this fundamental in F&H6 differs markedly from its calculated (4) value of 992 cm-r and its literature experimental value of 997 cm-’ (10). Indeed, there is a band at 992.8 cm-l, but as shown in Fig. 3, it has A-type rotational contour and is better assigned as the anharmonic overtone of vibration 16b (2 X 497.7 = 995.4 A). The analogous overtone is seen in both FCsH4D and FGD6. The correct position of vibration 5 in FCGHb appears to be 977.8 cm-’ where there is found a Q branch on the P-branch lobe of the previous overtone. The band is sharp with obvious C-type contour. The fact that it is observed in combination, 977.8 + 497.7 = 1475.5 A, confirms this assignment. a2 Vibrations The a2 vibrations are infrared inactive in single quanta. They may be expected to appear in combination and overtone bands and, in fact, did so quite often. Using

FLUOROBENZENE VIBRATIONAL SPECTRUM

301

TABLE V Fundamental Vibrations

a1

Fundamental 20a

obs. 3094.4

2

3080.2

13

3061.0

4) )

ob,'"6"5 talc. (3,4) _______ 2298

3071

__-_--

2291.4

2286

3055

(2274.4)

_______

2262

8a

1604.8

1607

1600.4

1579.2

1591

1500.3

1498

1493.0

1395.9

1385

1172.4

1166

7a

1232.4

1246

(1232)

9a

1156.3

1158

1156.1

963.9

952

18a

1022.7

1020

1021.6

877.2

854

1

1008.6

1002

990.9

819.8

816

12

808.7

800

806.6

752.5

747

517.1

513

513.2

502.8

501

___--_

3082

______

______

2296

7b

3069.5

3061

3062.4

2280.8

2282

8b

______

1604

__-_--

______

1577

20b

19b

1460.5

1466

1415.3

1311.7

1327

14

______

1322

_____-

_-___-

1266

3

1300.7

1290

1283.9

1035.0

1031

9b

1127.7

1154

1093.2

843.2

840

1066.1

1065

806.7

812

_____.

594

384.9

392

15

h

'W -___--

3085

19a

6a

bz

FC6H5 ca1c (3

(1036)

6b

____-_

_-_-_-

618

18b

400.4

411

5

977.8

992

922.2

__-__-

817

17b

894.8

897

851.1

759.0

762

lob

754.1

747

715.6

624.7

620

4

686.7

682

603.0

552.9

550

(400)

16b

497.7

503

492.0

426.6

430

11

248.6

242

247.9

233.7

229

17a

957.4

960

957.2

776.2

777

10a

817.6

816

817.7

636.1

634

16a

413.9

408

413.9

362.6

357

( )=approximate

the calculated (4) values of these fundamentals as a guide, the a2 fundamentals in FCCDG were derived from appropriate overtone and combination bands. These results are given in Table V where comparison with calculation shows excellent agreement.

302

LIPP AND SELISKAR

--

1260

1240

WAVENUMBER

1220

1200

( cm”)

FIG. 2. Vibration 7a and the combination IOa + 16a in FCeH5 and F&HID. The Q-branch positions of 7a are 1238.4 cm-’ (FCsHs) and 1239.2 cm+ (F&J&D) and for the other resonance component, 10a + 16a, 1226.5 cm-1 (FCsHb) and 1226.8 cm-l (FCaHdD).

The situation in FCJ&, and FC&D was even better due to the invariance of the uz fundamentals from one isotope to the other. This invariance stems from the fact that an uz vibration in a CzV molecule cannot involve vibrational amplitude on any atom along the CZ axis. Since the two isotopes differ only in an axial atom, the corresponding a2 fundamentals should be identical. Using the calculated (4) values as a guide, an A-type band at 1775.0 cm-’ in FCsHh was tentatively identified as the 17a + 10a combination. This assignment appears certain since the corresponding combination appears at 1774.9 cm-’ in FCBH~D. Neither a2 vibration is individually identified at this point. B-type bands appearing at 1712.4 cm-l in F&H6 and 1668.8 cm-l in FC6HhD were thought to be homologous combination bands involving a2 vibrations. Two combinations were possible : 17a + 10b or IOU + 17b, since both sums yield bands of correct numerical magnitude and rotational contour. To deduce the correct combination, the a2 fundamental isotope invariance was employed. If the two observed bands are homologous combinations, then the isotopic shift from 1712.4 cm+ to J668.8 cm-l must be due solely to the bl component of each combination. The observed isotope shift is 43.6 cm-l. The shift in vibration lob between isotopes is 38.5 cm-l while that of 17b is 43.7 cm-‘. The correct assignment for the combination bands must then be 10a + 176. This allows calculation of lOa, which in

FLUOROBENZENE

VIBRATIONAL

303

SPECTRUM

-

1000

1020

WAVENUMBER FIG. 3. Overtone

2 X 166 and

for 2 X 16b and 977.8 cm-l

vibration

980

960

(cm”)

5 in F&H&. Q-branch

positions

are 992.8 and 959.6 cnl-’

for 5.

turn allows calculation of 17~ from the combination lOa + 17~. The results of these calculations, shown in Table V, show indeed that the a2 fundamentals are invariant between the isotopes. Vibration 16~ was observed as an overtone at 825.1 cm-’ in FC6H6 and 825.0 cm-i in FCGHID. The combination f6u + 4 was then found in FCsHs, from which a value of 413.9 cm-’ was determined for 16~. The corresponding combination in FC6H4D would overlap a strong al fundamental and was not observed. Based on the invariance of the overtone in both isotopes, the harmonic value of the 16~ fundamental in FC6H4D was also taken as 413.9 cm-‘, identical with that in FCeHs. The invariance of the u2 fundamentals was a beautiful feature of these spectra which allowed us to measure the (I? fundamentals with certainty. CONCLUSIOSS

The vibrational spectroscopy of fluorobenzene has received much attention in recent literature (II, 12). Our results modify the published experimental data in a significant manner. Specifically, several fundamentals have been reassigned in response to measured rovibrational band contours in the gas phase spectra of three isotopes. The agreement of these results with vibrational fundamentals deduced from high-resolution electronic spectra of the 2644 A fluorobenzene band system will be the subject of a forthcoming communication. In addition, the assignments of the u2 fundamentals from combination and overtone bands made possible by judicious isotope substitution is a technique which can be used by others investigating the vibrational spectra of large polyatomic CPUmolecules. RECEIVED: April 12, 1978

304

LIPP AND SELISKAR REFERENCES

1. D. C. SMITH, E. E. FERGUSON,R. L. HUDSON, AND J. RUD NIELSEN, J. Chem. Phys. 21, 1475-1479

2. 3. 4. 5. 6. 7. 8. 9. 10.

(1953). D. STEELE, E. R. LIPPINCOTT,AND J. XAVIER, J. Chem. Phys. 33, 1242-1247 (1960). V. J. EATON AND D. STEELE, J. Mol. Spectrosc. 48, 446-458 (1973). R. A. R. PEARCE, D. STEELE, AND K. RADCLIFFE,J. Mol. Stuz&. 15, 409420 (1973). IUPAC, “Tables of Wavenumbers for the Calibration of Infrared Spectrometers,” Butterworths, Washington, D.C., 1961. B. BAK, D. CHRISTENSEN,L. HANSEN-NYGAARD, AND E. TANNENBAUM,J. Chem. Phys. 26, 134-137 (1957). W. A. SETH-PAUL, J. Mol. Structure 3, 403-417 (1968). W. A. SETH-PAUL AND G. DIKSTRA, Spectrochim. Ada 23A, 2861-2870 (1967). T. UNO, A. KUWAE, AND I;. MACHIDA, Spectrochim. Acta 23A, 607-614 (1977). D. W. SCOTT, J. P. MCCULLOUGH, W. D. GOOD, J. F. MESSERLY, R. C. PENNINGTON,T. C. KINCHELOE,I. A. HOSSENLOPP,D. R. DOUSLIN, AND G. WADDINGTON,J. Am.

5463 (1956). 11. M. PIERRE, J. Phys. (Paris) 38, 39-45 (1977). 12. M. PIERRE, J. Chem. Phys. 66,3814-3821 (1977).

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