Theoretical investigation of monohalogenbenzenes vibronic spectra

Journal of MOLECULAR STRUCTURE

ELSEVIER

Journal of Molecular Structure 349 (1995) 207-210

THEORETICAL INVESTIGATION VIBRONIC SPECTRA

OF MONOHALOGENBENZENES

A.Yu.Slepoukhinea

V.A.Udovenjab

, L.S.Kostyuchenkoa’

aSaratov State Technic University,

410016 Saratov , Russia

bInstitute for Biochemie and Physiology of Phlants and Microorganismus 410015 Saratov , Russia

of AS RF,

The vibrational structure of the first singlet excited electronic states Bl of C!sHsX (X= F, CL, Br) molecules is investigated. The calculated results are compared to experimental data [I] about vapour and crystal spectra analyed in the polarized light at 20 and 4K. The vibronic structure analysis of spectra offered a possibility to assign the vibronic transitions in connection with excitation of definite normal modes. METHOD

AND RESULTS

The calculations of vibrational frequencies of monohalogenbenzenes (MHB) in the B1 state have been carried out by means of Wilson’s GF matrix method. The molecules are assumed here to be planar in their lowest excited electronic states and belong to C2v symmetry group. In this case the vibrations are distributed according to the following symmetry types: F = 1lAi + 3A2 + lOBi+ 6B2. It was supposed also that benzene ring dimensions of excited MHB are equal to that of benzene molecule in excited electronic state B2” : r(CC) = 1.43A and r(CH) = 1.07A. Taking into account the fact, that the spectra are in general conditioned by electronic excitation of benzene ring, the carbon- halogen bond lengths were taken to be the same as in the ground electronic state Al. The benzene ring force constants of MHB in zero-order approximation are taken to be equal to correspondent force constants of ethynyl benzene molecule in excited electronic state Bl. The ethynyl benzene force field was constructed on the basis of the force constants of benzene in the Bzu excited state and experimental frequencies of ethynyl benzene and its three deuterated derivatives. The force constants for C-X bonds and molecular angels C-C-X are taken from correspondent force matrix of MHB in the ground electronic state Al. The assignment of vibrational modes is performed on the basis of calculation of frequencies and forms of vibrations and potential energy distribution through the internal coordinates. The corn plete intepretation of experimental vibronic spectra of the lowest electronic transition Al -Bl in MHB is presented in Tables 1 - 3. 0022-2860/95/$09.50 0 1995 Elsevier Science B.V. 0022-2860(95)08745-l

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208

Table

1

Fundamental frequence electronic state (cm-‘) Symmetry

Wilson

assignments

of fluorbenzene

Vibration

in excited

Y

BI

y experimental

Form of vibration vapour

crystal

1543

1560(4)*

1543 (6)

1515

1512(2)

1509(4)

1488

1492(3)

1496( 1)

Q
l336

1325(4)

1318(3)

Q
1271

1252(2)

1263( 1)

Q
1215

1215(5)

1205(5)

1203

1199(3)

1185(2)

Q
1165

1164(2)

1063(5)

1033

1030(4)

1032(3)

972

968 (10)

968 (10)

Q, y(CCC)

931

920(10)

915(10)

22

Q, B(C CH)

895

883 (5)

1

10

Q
763

777 (7)

766(10)

B2

5

25

P(C H)

724

A2

17a

12

P (C H)

722

710(6)

709 (6)

I32

17b

26

P(C H)

651

A2

10a

13

p(CH)

647

type

number

number

calculated

Al

20a

1

q(C H)

3149

A1

2

2

q(C H)

3130

B1

20b

1.5

q(C H)

3113

A1

7a

3

q(C H)

3110

BI

7b

16

q(C H)

3070

Al

8a

4

Q, v(CCC)

Bl

8b

17

CH),QKO

AI

19a

5

B1

19b

18

B1

14

19

Al

13

6

B1

3

20

Al

9a

7

BI

9b

21

A1

12

8

Q
141

18a

9

B1

18b

AI

B(C

Q,

p(CCH)

B(C C H),

B(C

CW, Q
881

680 (4) 643 (4)

625 (1)

Purely electronic and vibronic absorption bands connected with the totally symmetric vibrations Al are the most intensive in the spectra. The totally symmetric vibrations may be divided into two groups according to their spectral character. To the first group belong vibrations which have multiquantum iterations in spectrum (e.g., the vibrations 12, 18a, 13). The second group may be formed of single-quantum totally symmetric vibration (e.g., 6a,l). The vibronic bands connected with excitation of A2 symmetry vibrations have low intensity in the MHB spectra. And finally, the vibronic transitions in combination with vibrations of B2 symmetry are prohibited in the Czv symmetry group.

209

Table 2 Fundamental frequence electronic state (cm-‘)

assignments

of m onochlorbenzene

Y

Symmetry Wilson Vibration number

Form of vibration

in excited B I

y experimental

type

number

calculated

Al

20a

1

q(C H)

3149

Al

2

2

q(C H)

3130

Bl

20b

15

q(C H)

3113

Al

7a

3

q(C H)

3110

Bl

7b

16

q(C H)

3070

Al

8a

4

Q
Bl

8b

Al

vapour

crystal

PC C H)

1535

17

s(C C II), Q
1500

1480

1480( 1)

19a

5

s(C CH),Q(CC)

1431

1448

1442( 1)

Bl

19b

18

1337

1337

1437(2)

Bl

14

19

Q, s6JCI-U

1270

1251

Bl

3

20

1203

1193

Al

9a

6

/J(C C H), Q
1151

1145

1145(4)

7

s(C CH),Q(CC)

1046

1042

1044(8)

21

B(C CH),Q(CC)

1006

999

996 (8)

Al Bl

13 9b

1524(l)*

Al

12

8

Q, B(CCH)

959

952

969UO)

Al

18a

9

Q
901

914

924(10)

Bl

18b

22

Q
879

899

894(l)

5

25

P(C H)

690

A2

17a

12

P(C H)

715

B2

17b

26

P(C II)

669

Al

1

lo

Q(CCl), v(CCC)

651

A2

1Oa

13

P(C H)

647

I32

*In parenthesis the relative bands are presented

intensities

B(CCH)

of vibrational

714(l) 670

lines in Al-B1

680 (4) 625(l) absorption

Examining the structure of MHB vibronic spectra there can be found the chawhich do not alter when substituent is changing from racteristic frequencies, F to Br (8a, 8b, 19a, 19b, 14, 3, 9a, 9b, 12, 18a, 18b, 17a, 6b, lOa, 16a). Also vibrations dependent on substitution are observed - 1, 6a, 15. Frequency 15, corresponding to C-C-X bending vibrations and frequency 1 - mixed vibration of ring and C-X bond - both decrease in the time of transition from F to Br because of halogen mass increase. Frequency 6a, correspondent in gene-

210

Table

3

Fundamental frequence electronic state (cm-‘) Symmetry

Wilson

assignm ents of m onobrom

benzene

Y

Vibration

in excited

B1

Y experimental

Form of vibration type

number

A1

20a

1

qtC H)

3149

A1

2

2

q(C L-U

3130

B1

20b

15

q(C H)

3113

AI

7a

3

q(C f-U

3110

B1

7b

16

q(C H)

3070

A1

8a

4

Q
1504

B1

8b

17

Q
1498

1476

A1

19a

5

B(C C H), s(C C H),

Q
1465

1447

B1

19b

18

Q, p(CCH)

1339

1328

BI

14

19

Q
1272

1257

B1

3

20

s(CCH),Q(CC)

1202

1199

A1

9a

6

B(C C H),

1139

1141

7

Q, B(CCW

1007

1017

A1 BI

13 9b

number

calculated

21

B(C C H), B (C C HI,

Q
vapour

1511

Q
1006

Q (C C) Q, B(CCH)

944

959

925

929

879

871

A1

12

8

A1

18a

9

Bl

18b

22

Q
B2

5

25

P(C H)

689

A2

17a

12

P(C I-I)

715

B2

17b

26

P(C H)

669

A2

1Oa

13

P(C H)

647

Al

1

10

Q
638

650

Bl

6b

23

Y(C C C)

512

519

B(CCH)

ral to mixed ring vibrations, decreases bond participation in the vibration.

from

crystal

709

F to Br owing to increase

955

686

of C-X

REFERENCES 1. G.V. Klimusheva, L.S. Kostyuchenko, L.M. i Spektroskopia (USSR), No.2 (1976) 245.

Sverdlov

and G.T. Soroka,

Optica