Transferable valence force fields for substituted benzenes

VIbrational Spectroscopy, 6 (1994) 251-257 Elsevier Science B.V., Amsterdam

Transferable

251

valence force fields for substituted benzenes Part II. Di- and trimethoxybenzenes B. Venkatram Reddy and G. Ramana Rao

Department of Physics, Univer& College, Kakatiya University, Viiyaranyapuri, Warangal506 009 (India) (Received 4th May 1993)

Abstract

A zero-order normal coordinate analysis was made for o-, m- and p-dimethoxybenzenes and 1,2,3-, 1,2,4- and 1,3,5-trimethoxybenzenes by transferring the force constants from our earlier work. The observed and calculated frequencies agree with an average error of 14.7 cm-‘. Unambiguous vibrational assignments of in-plane fundamentals of the six molecules have been made, and several assignments suggested by earlier workers have been revised. Keywords: Infrared spectrometry; analysis; Valence force fields

Raman spectrometry;

In Part I of this series [l] and earlier work [2], the transferability of valence force constants obtained for fluoro- and chloro-substituted anisoles to monohalogenated anisoles has been investigated. In this paper we report the results of similar investigations made using the force constants of nitro-substituted anilines, anisoles and anisidines [3]. For this purpose, we made a zeroorder normal coordinate analysis for (1) p-dimethoxybenzene (PDMB), (2) m-dimethoxybenzene (MDMB), (3) o-dimethoxybenzene (ODMB), (4) 1,2,3-trimethoxybenzene (3TMB), (5) 1,2,4-trimethoxybenzene (4TMB) and (6) 1,3,5-trimethoxybenzene (STMB). The vibrational frequencies of dimethoxybenzenes, 3TMB and STMB were reported by Varsanyi [4], whereas those of trimethoxybenzenes were reported by Sarkar et al. [5].

Correspondence to: G. Ramana Rao, Department of Physics, University College, Kakatiya University, Vidyaranyapuri, Warangal 506 009 (India).

Force constants;

ZERO-ORDER

Methoxybenzenes;

CALCULATIONS

Normal coordinate

AND RESULTS

The molecules being investigated possess different point group symmetries. MDMB exhibits C,, symmetry, whereas the others show C, symmetry. In the C, structure, the 54 fundamentals of ODMB and PDMB fall into 35 a’ species and 19 a” species, whereas the 66 fundamentals of the trimethoxybenzenes fall under 42 a’ species and 24 u” species, while in the C,, structure, the 54 fundamentals of MDMB are distributed as 18a, + 176, + lib, + 8a,. a’ and u” species of the C, symmetry alongwith a,, b, and b, species of the C,, symmetry are active in both infrared and Raman spectra, whereas the a2 species are active in the Raman process only. A zero-order vibrational analysis of in-plane vibrations was made for PDMB, MDMB, ODMB, 3TMB, 4TMB and STMB selecting the molecular parameters, internal coordinates and symmetry coordinates from those employed for monohalogenated anisoles (see part I [l]). The zero-order

0924-2031/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved SSDI 0924-2031(93IE0057-9

B. Venkatram Reddy and G. Ramana Rao / Vii. Spectrosc. 6 (1994) 2.51-257

252 TABLE

1

Zero-order in-plane force constants of di- and trimethoxybenzenes (in units of mdyn A-‘, mdyn rad-’ and mdyn A rad-‘1 No.

Symbol

Coordinates involved

Common atoms

Value

Diagonal force constants Stretch 1 2 3 4 5

KR K, KD,

C-H c-c c-o

KD,

o-c

KD,

C-H

(ring)

(methyl)

-

5.0926 6.5226 4.4512 4.9072 4.6587

Bend 6 7 8 9 10 11

HO H+ H,l H, Ha H,

_

LCCC LCCH LCCO LCOC LOCH LHCH

_ (methyl)

0.8278 0.5170 0.9659 1.5864 0.9325 0.5265

force constants were transferred from the final values obtained for nitro-substituted anilines, anisoles and anisidines [3]. The average error between the observed and calculated frequencies is 17.1, 10.8, 17.2, 17.9, 13.0 and 13.0 cm-’ for PDMB, MDMB, ODMB, 3TMB, 4TMB and STMB, respectively. These should be considered as acceptable as the force constants are not refined in a zero-order calculation. This demonstrates the transferability of force constants presented in Ref. 3. The zero-order force constants used in the present computations are given in Table 1. The observed and calculated frequencies of dimethoxybenzenes and trimethoxybenzenes are presented in Tables 2 and 3, respectively, together with potential energy distributions (PED) and vibrational assignments. PED contributions below 10% are not shown.

Interaction constants Stretch-stretch VIBRATIONAL 12 13 14 15 16 17 18 19 20 21

FP,,d, F&) FJ,d, FW) F&,R) F&) F&,a) FOXd,

c-c, c-c, c-c, C-H, C-H, C-H, C-H, c-o,

c-c c-c c-c C-C C-H C-H C-H c-c

F(DI,DZ,

c-0,

0-c

Fo,s,osj

C-H,

C-H

c-c, C-H,

LCCC LCCH

C-O, c-o, o-c, C-C,

LCCo LCOC LCOC LCCH

c-c,

LCCO

O-C,

LOCH

C _ C _

C 0 (methyl)

C

0.9227 - 0.4782 0.1783 - 0.0634 - 0.0720 - 0.0708 - 0.0030 0.1527 0.1503 -0.0061

Stretch-bend 22 23 24 25 26 27 28 29

FW, FW) &Dl,+,l) FP1,N F(D2,w) FO.&, [email protected])

FW,B,

c-c C-H c-o c-o c-o c-c c-c o-c

0.3871 - 0.0624 0.1053 0.0757 0.0424 0.1608 0.3106 0.5983

Bend - bend 30 31

Fwe)

32 33 34 35

Fw1.b)

64,4)

Ff41.0)

%.a, Fo%P,

LCCC, LCCH, LCCO,

LCCC LCCH LCCH

c-c c-c c-c

LCCO,

LCOC

c-o

LCOC, LOCH,

LOCH LOCH

o-c o-c

- 0.0995 0.0399 0.1559 0.3005 - 0.0347 - 0.0501

ASSIGNMENTS

The main differences between the present assignments and those suggested by earlier workers [4,5] are: (i) In PDMB, the frequencies now assigned to the modes 2,7b, 14,6,(CH,), y(CH,) and ~(0-0 were thought to be 2Ob, 2, 6,(CH,), 19b, ~(0-0 and y(CH,), respectively, by Varsanyi [4]. (ii) In MDMB, the absorptions now attributed to the vibrations 2,20a, 13, 19a, 6a, 7b, 19b, 6b, 3, 9b, 15, v(O-C) and S(LCOC) were considered to be arising from 20a, 2, 6,(CH,), 19b, 6b, 17b, 19a, 16a, 13, ~(0-0, lOa, y(CH,) and 15, respectively, by Varsanyi [4]. (iii) In ODMB, the bands now identified as 2, 8a, 19a, 19b, 6a, 6b, 3, 18a, 18b, S,(CH,), y(CH,) and ~(0-0 were expected to be 20b, 8b, 19b, SJCH,), 6b, 6a, S,(CH,), y(CH,), 18a, 19a, v(O-C) and 18b respectively, by Varsanyi [4]. (iv) In 3TMB, the fundamentals now ascribed to 2, 7a, 13, 20a, 19b, 14, 12, 3, 6,(CH,), y(CH,) and ~(0-0 were considered to have their origin in 7a, 2, 6,(CH,), 13, 6,,(CH,), 20a, 11, 14, 19b, v(O-C) and y(CH,), respectively, by Varsanyi [41.

-.

v(CC)8a

v(CC)8b v(CCB9a

v(CCN9b

v(CCB4 P(LCCCI6a @(LCCCMb

f?(LCCC)12

N-J+3

B(CH)18a

. ,..>

8

9 10

11

12 13 14

15

16

17

_

I&C)1

7

_

1026

v(CO)13

6

1056

1082 a,

_-

1336

1270 b,

1293

,..

-

812

992 al

719

,,

1291 466 580

1335 b, 466 a, 584 b,

1313 _

.

1469

1441 b,

1442

_

1512

1600 b, 1497 a,

1594 1509

999

1246

672

1320 382 624

1392

1572 1531

1611

1615 1600

1260

1283

3027 3074 3071 3111 * *

1594 al

1234

1258

3000 3066

838

1293 a1

*

- aI

840 b,

*d

3006 a, 3076 a, 3094 b,

721 a,

820

1230

1263

3047 3068 3068 3074 * *

v(CH)2 V(CH)20a v(CH)2Ob ~tCH)7b v(CO)7b v(CH)7a v(C0)7a

5

Calculated frequency

_

-

-

1089

1269

988

1327 460 595

1400

1598

699

1326

866 3102 *

3017 3091 3072 *

-

_

-

1032

1306

787

1311 446 545

1477

1567 1498

1644

782

1171

1269

3016 3102 3072 3092 * *

PDMB MDMB b ODMB PDMB MDMB ODMB

Observed frequency

1 2 3 4

No. Mode

-

-

73(3) + 13(8b)

,

50(14)+31(18b) 31(9b) + 30(6a) + 19(7a) 51(6b)+ 19SUCOC) + 15(3) 34U3) + 246(LCOC) + 17(12)+ 13(19b)

70(8b)+ 17(3) 39(19a)+37U8a)+19(13)

64(8a) + 23(9a)

-,,

33(3)+21(7a)+ 156,,(CH,) + 13y(CH,)+ lo(I) 37(14)+28(18a)+20(13) + lo(12) 44(l) + 17(7a) + 12(6a)

lOl(2) 98(20a) 98(2Ob) 96(7b) * *

Vibrational assignment a

Observed and calculated frequencies (in cm -‘) and vibrational assignments of dimetboxybenzenes

TABLE 2

__

-

I

-

.-

.

-

-

__

.-

(Continued on p. 254)

PED from 14 is replaced by 1 in MDMB and description in ODMB is 21(13)+ 19y(CH,) + 16v(OC)+ 12(3) PED from 7a + 6a is replaced by 12 + 13 and 6b + S(LCOC) in MDMB and ODMB, respectively PED from 9a is replaced by 18a and 18b + 13 + 9b in MDMB and ODMB, respectively 18b replaces 3 in MDMB and ODMB PED from 13 disappears in MDMB and it is replaced by 7a in ODMB PED from 18b is replaced by 6,(CH,)+9b in MDMB and additional PED from 6,,(CH,)+ y(CH,) in ODMB 9b and 9a replace 18b in MDMB and ODMB, respectively PED from 9b + 7a is replaced by 9a + 13 in MDMB PED from 3 disappears in MDMB and it is replaced by 9a + 9b in ODMB PED from SUCOC)+ 19b is replaced by 1 and 18b in MDMB and ODMB, respectively and that from 13 disappears in MDMB PED from 8b is replaced by y(CH,)+ 6,,(CH,) and 13 in MDMB and ODMB, respectively PED from 19b + 12 is replaced by 19a and 1 in MDMB and ODMB, respectively

27(9b) + 26(7b) + 12(6b) 96(7a) PED from 3 + 1 is replaced by 18a in ODMB

Remarks

-

M w

*

y(CH,)

y(CHs)a” y(CHs)a” v(O-CHs) v(O-CH,)

S(XOC)

S(KOC)

35

36 37 38 39

40

41

1180 1180 1032 1032

1182 b, 1182 a2 1034 a, 1053 b,

-b,

-

-

1180

1182 b,

303 a,

* 2958 2958 2836 2836 2935 2935 1469 1469 1448 1448 1469 1469 1180

*

1162

1083

1213 * b, 2OOb, 2959 a, 2959 b, 2836 a, 2836 b, 2940 b, 2940 a2 1456 a, 1456 b, 1441 a, 1441 b, 1456 b, 1456 a2 1182 a,

258a,

*

1131 b,

573

472

1185 1185 1029 1025

1181

234 173 2966 2966 2841 2841 2964 2964 1467 1465 1433 1442 1464 1464 1193

* *

1160

1123

599

338

1185 1185 1026 1062

1167

173 2966 2966 2841 2841 2964 2964 1462 1468 1452 1436 1464 1464 1195

255 1213 *

*

1137

618

372

1185 1185 1022 1023

1170

239 163 2966 2966 2841 2841 2964 2964 1464 1459 1431 1435 1464 1464 1204

* *

1155

1130

376(Lcoc)+ 17(13) + 13(19b)+ llv(OC) 31(9b) + 23f6b) +216(LCOC)+ ll(8b)

398(LCOC)+ 37(9b) 68(15)+258(LCOC) 99v,KH,) 99v&H 3) loov&H,) 100v,(CH,) 99v&H,) 99v&H,) 69&,.&H,)+ 27y(CH,) 67g,&CH,)+26yKH,l 1038*KH,) 918,(CH,) 738,(CH,)+26yKH,) 738,,(CH,)+26yQ-I,) 39(9a) + 29y(CHs) + ll&,,(CH,) SOy(CH,)+ 188&H,) + 1004) 69y(cH,)+276,,(CH,) 69y(CH,I+276,,(CH,) 76v(OC)+ 12(l) 82v(OC)

32(8a) + 22(9a) + 16y(CH,) + 13(7a) * *

64(14)+ 22(18b)

Vibrational assignment a

PED from 1 disappears in MDMB and ODMB Additional PED from 12 and 14 in MDMB and ODMB, respectively PED from 19b + v(OC) is replaced by 1+ 18a in MDMB. Description in ODMB is S(LCOC) + 7a + 1 PED from 9b + 6b is replaced by 15 + v(OC) in MDMB and that from 6b + 8b is replaced by 19b + v(OC) in ODMB

PED from 9a disappears in MDMB and is replaced by 3 in ODMB PED from 8,,(CH,) is replaced by 9b in MDMB. Description ODMB is y(CH,) + 1+ 18a + 7a

Additional PED from 19b in ODMB Mixing with 19a in MDMB

53(9a) + 43S(LCOC) 67(9b)+ 18(8b)

Additional PED from 7b in MDMB and 19b replaces 14 in ODMB PED from 8a + 7a + y(CI-I,) is replaced by 9a + 14 in ODMB

Remarks

a Results in this column correspond to the para-compound. The number before the parentheses is % PED and that inside the parentheses is mode notation as given by Wilson 161.Major deviations in the case of MDMB and ODMB are shown in remarks column. b In this column the letter that follows the frequency is the symmetry species. ’ Not observed. d Not applicable.

-

-

1179 1179 1049 1049

1179

/3@ZI-I)9b B(C0)9b 1 /3(CO)lS V&H,) v&XI,) @I-Is) #H,) v,(CH,)a” v&ZI-Is)a” S&I-I,) 1466 8,,(CH s) 1466 6&H), 1442 6,KH,) 1442 S,,(CH,)a” 1453 G,,(CHs)a” 1453 Y(CHs) 1179

*

B(C0)9a

20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

1174

B(CID9a

19

1104

PDMB MDMB ODMB

P(CH)18b

Calculated frequency

PDMB MDMBb

ODMB

Observed frequency

18

No. Mode

TABLE 2 (continued)

,”

,,,,

v(CC)8b

v(CCj19a

v(CCj19b

v(CO14

/3(LCCC)6a

,3(LCCC)6b

/%,&CC)12

/3(CH)3

@(CH)9b B(C019b /3(CH)18a

/3(CO)l8b ~KX-I)llb

/3(C0)9a B(CO)lS

9

10

11

12

13

14

15

16

17

19

20 21

18

I#,

v(CC)8a

8

,,

v(CC)l

7

,,,

v(CO)13

6

5

,,

v(CH)2 v(CH)20a v(CODOa v(CHI20b v(C0)7a v(CHMa v(CO)7b

1 2

3 4

Mode

No.

1465 1335

1478

1255

,,,,

1512

1497

993

-c

1482

1482

1605

*

1096

*

,

,.

-

-

1155

* 1158

* 1155

*

1208

1230

194 1140

1323

707

781

1149 *

,,

1304

1494

1497

1644

1606

,,,

154 268

145 *

1086

1152 *

1254

804

341

1611

1595

1605

594

-

1595

1558

448

1341

395

763

619

1340

3093 913

551

1299

1298

398

921

3098 901 852

3075 852 *

3081 1281 *

3040 *

1167 3090 *

3028 *

,,,,

146 248

1139

*

166 1114

*

1205

708

613

540

1341

1478

1508

1657

1587

770

1313

909

3071 1274 *

3040 3099 *

4TMB

157 251

1166

*

157 1172

*

1326

987

390

369

1170

1472

1476

1616

1601

461

1383

891

3090 896 *

3030 3091 *

5TMB

3TMB

3005 3075 *

3020 3007 * b 3081 * 1112

STMB

3TMB

4TMB

Calculated frequency

Observed frequency

18(l)+ 12(20a)

97(20a)

Remarks

,,,

,/

97(7a) 32v(OC) + 300b)

>,

,,,,

*mm

,,,,,,,

,,

,,,

,,

(Continued on p. 256)

Description in 4TMB and 5TMB is 18a + 7a + 19b + S,,(CH,) and 7a + 19b + v(OC), respectively Additional PED from 8a + 12 in 4TMB and that from 19a in STMB 29(13)+29(14)+ 12(12)+ 12(18a) Description in 4TMB and STMB is 13 + 18b + 1 and 13 + 1+ S,,(CH,) + 12, respectively + l&&H,) 20(6b)+ 18(15)+ 16S(LCOQ+ 15(l) PED from 15 disappears in 4TMB. In STMB, 6b + 1303) is replaced by 12 and S(LCOC) disappears 72(8a) + ll(9a) + 11G’b)+ 10(9b) Description in 4TMB and STMB is 8a + 18a + 9b and 8a + 18b, respectively 68(8b) + 15(9a) + 10(20a) + lO(6b) Description in 4TMB and STMB is 8b + 3 and 8b + 18a, respectively 46(19a)+ 31(18a) Description in 4TMB and STMB is 19a + 18b + 7b and S,,(CH,)+ y(CH,)+ 19a, respectively 39(19b)+26(9b)+ 14(13) Description in 4TMB and STMB is S,,(CH,)+ y(CH,)+ 19b Description in 4TMB and 5TMB is 14 + 18b 53(9b)+ 4304) and 14+ y(CH,)+ 3 + SJCH,), respectively Description in 4TMB and STMB is S(LCOC) + 6a 23(6a)+ 20(20a)+ 19S(LCOC) + lO(18b) + 15 and 6a+ 7a+ S(LCOC), respectively Description in 4TMB and STMB is 6b+ 13 + S(LCOC) 24(2Oa)+ 20(6b) + 17(18b) + 12S(LCOC) + 19b and 6b + 7b + S(,XOC) + 9a, respectively Description in 4TMB and STMB is 7b + S(.XOC) + 35(12)+20(2Oa)+ ll(18a) 15 + 12 and 1+ 12, respectively Description in 4TMB and STMB is 3 + y(CH,)+ 1+ 25y(CH,)+ 23(3)+ 20(14) + lSS&H), + lO(20a) S,,(CH,) and 3 + 14, respectively 5604) + 37(9b) * 9b + SGXOC) 28v(OC) + 28(19b) + 14(18a) Description in 4TMB and STMB is 14 + 18a + 7a + y(CH,)+ v(OC) and 18a + y(CH,), respectively 75(18b)+ZlS(LCOC) * Description in 4TMB and STMB is 18b + 14 and 18b + 14 + y(CH s) + 7b, respectively 67(9a)+ 2lS(LCOC) 48S(LCOC)+ 36(15) Additional PED from 13 in 4TMB and that from 1 in STMB

27y(CH,)+20(9b)+ 97(2Ob) *

lOl(2) *

Vibrational assignment a

Observed and calculated frequencies (in cm- ‘) and vibrational assignments of trimethoxybenzenes

TABLE 3

,,,, ,,l.,

_ 356 464

v(O-CHs) &O-CHJ

u(O-CHs)

6(LCOC)

S(LCOC)

S(LCOC)

48

49

50

51

1046

1184 1184 1184 1023 1023

690

542

542

1037

1198 1198 1198 1070 1070

1210

1210

2965 2965 2965 2845 2845 2845 2965 2965 2965 1460 1460 1460 1427 1427 1427 1460 1460 1460 1210

5TMB

716

519

486

1068

1185 1185 1185 1016 1030

1224

1239

2966 2966 2966 2841 2841 2841 2964 2964 2964 1465 1462 1468 1431 1436 1431 1464 1464 1464 1180

3TMB

Calculated

459

358

419

1047

1185 1185 1185 1026 1022

1240

1194

2966 2966 2966 2841 2841 2841 2964 2964 2964 1467 1458 1462 1434 1437 1421 1464 1464 1464 1172

4TMB

697

576

561

1035

1184 1184 1184 1050 1049

1224

1237

2966 2966 2966 2841 2841 2841 2964 2964 2964 1471 1454 1459 1416 1418 1440 1464 1464 1464 1239

5TMB

frequency assignment

a

266fLCOC) + 23(19b) + 13(15)+ 12(13)

41v(OC)+ 16(19b)+ 13(7b) + 12(3) 296UCOC) + 19(l) + 18(13) + 14f18a) 426(LCOC)+ 24f6a) + lo(l)

69y(CHs)u” + 278,S(CHs)u” 69y(CH,)u” +278,,(CH,)u” 69y(CH,)u” +276,,(CH,)u” 7ov(oc)+21(1) 81vfOC)

45y(CH,)+2OS,,(CH,)

27(3)+ 25y(CH,) + 15(7b) + 146,,(CH,)+ 13(8a)

686,,fCH,)+26y(CH,) 62S,,(CH,)+23y(CH,) 678,,(CH,)+28y(CH,) 636&H,) 65&(CH,) 62S,(CH,) 736JCHs)u” + 26y(CH,)a” 73S,,(CH,)u” + 26yfCHs)a” 736&H,)u” + 26yfCHs)u” 46y(CH,) + 1909b) +176&H,)+ 13(3)

99v,,fCH,) 99v,,(CHs) 99v,,(CH,) lOOv&CH,) lOOr#H,) 100&H,) 99v,S(CH&” 99v,&CH,)u” 99v,,(CH,)u”

Vibrational

,, ,,.,

,,

,,

,,,

,,

,,

,,

,,

,,,, ,,

,,

I,

.,

PED PED PED PED PED

from from from from from

19b 19a 19b 19a 19b

in in in in in

4TMB and 5TMB 5TMB 5TMB 5TMB 4TMB

,,,

is mode

as given by Wilson

,,,, 8, ,,,

notation

*

[6].

+ SUCOC) and 6UCOC)+ 6b, respectively Description in 4TMB and 5TMB is 7a + 9a + S(LCOC) + 6a and S(LCOC) + 6a + 9b, respectively Description in 4TMB and 5TMB is 6(LCOC)+7a + 19b and S(LCOC)+ 15 + 14, respectively

PED from 1 is replaced by 19a in 5TMB Additional PED from 14 in 4TMB and that from 19b in 5TMB PED from 7b + 3 disappears in 4TMB and STMB, PED from 19b is replaced by 14 in 4TMB in 4TMB and 5TMB is ‘6b + 15 + 13 + 1 Description

Description in 4TMB and 5TMB is y(CH,) + 14+ 18+ S,,(CH,)+ 13 and y(CH,)+ &(CH,) + 7b, respectively PED from 3 disappears in 4TMB and STMB; PED from 7b + 8a is replaced by 18b + 14 in 4TMB and it is replaced by 18a + 7a in 5TMB Additional PED from 18b in 4TMB and that from 14+ 12+ S(LCOC) in 5TMB

Additional Additional Additional Additional Additional

Remarks

a Results in this column correspond to 3TMB. The number before the parentheses is % PED and inside the parentheses Major deviations in the case of 4TMB and 5TMB are listed in remarks column. b Not applicable. ’ Not observed.

1030

1173 1173 1173 1003 1030

y(CH,W’ y(CH,)u” y(CH,)a”

43 44 45 46 47

1231

1192

y(CH,)

42

1231

1192

2938 2938 2938 2840 2840 2840 2938 2938 2938 1445 1445 1445 1428 1428 1428 1445 1445 1445 1184

4TMB

3TMB

2962 2962 2962 2838 2838 2838 2940 2940 2940 1456 1456 1456 1434 1434 1434 1456 1456 1456 1192

frequency

Observed

y(CH,)

Mode

3 (continued)

41

22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

No.

TABLE

v

z

e 8 P $ 01 2 g e ti

S

-F

P

s

3

F

6

9

g fl ;: 3 b B *

F

B. Venkatram Reddy and G. Ramana Rao / vib. Spectrosc. 6 (1994) 251-257

(v) In 4TMB, the frequencies now attributed to 7b, 14, y(CH,), ~(0-0 and &LCOC) were thought to be arising from r(CH,), S,(CH,), &O-C), r(CH,) and one of the u” modes, respectively, by Sarkar et al. [5]. (vi) In STMB, the absorptions now identified as 2; 20a and 20b; 7a, 7b, 13, 1, 3, S,(CH,), y(CH,), v(O-C),6 (LCOC) and 6 (LCOC) were expected to be 20a and 20b; 2; 17a, 17b, 6,(CH,); 16a and 16b; S,(CH,), S,,(CH,), ~(0-0, r(CH,); (6a, 6b); and 4, respectively, by Varsanyi 141. The vibrational assignments of C-C stretching modes, ring vibrations and frequencies having their origin in the methoxy groups can be understood by referring to the PED tables and corresponding discussion in part I [l] and Ref. 2. Further, a detailed discussion of C-H stretching vibrations is unwarranted as they are pure vibrations. Hence the discussion is confined to C-H bending vibrations only. Modes 3,9a, 18a and 18b in PDMB and ODMB vibrations, 3, 9b, 18a and 18b in MDMB, modes 3,9b and 18a in 3TMB, vibrations 3, 18a and 18b in 4TMB and STMB are known as C-H bending vibrations in the present set of molecules. The phase relations for these modes in the dimethoxybenzenes are + + + + for mode 3, + - + - for mode 9a or 9b, + + - - for vibration 18a, and + - - + for mode 18b, whereas in 3TMB these are +l, +l, +1 for mode 3, 0, +2, -2 for vibration 9b, and 2, - 1, - 1 for mode 18a, while in 4TMB and STMB the phase relations are + 1, + 1, + 1 for mode 3, +2, - 1, - 1 for mode 18a, and. 0, + 2, - 2 for mode 18b. These are approximately true in the case of molecules investigated due to asymmetric substitution. For example in PDMB actual eigenvectors obtained for the modes 9a, 18a and 18b are (0.23, -0.17, 0.23, -0.17); (0.19, 0.18, -0.19, -0.18); and (0.23, -0.20, - 0.23, 0.20), respectively. These phase relations along with potential energy distributions (PEDs) were used to identify and assign the C-H in-plane bending vibrations. The highest C-H in-plane bending vibration is mode 3. Its assign-

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ment is usually difficult, as mode 14, in which alternate C-C bonds either increase or decrease, appears in its vicinity. Varsanyi [4] assigned the higher frequency near 1335 cm-’ to mode 14 and the lower frequency at 1310 cm-’ to the C-H bending mode 3 in MDMB. In ODMB and 3TMB, mode 14 was assigned around 1291 and 1230 cm-‘, respectively, by Varsanyi [41, whereas he could not propose any assignment for mode 3 in these molecules. According to the calculations made here, the bands around 1293, 1270, 1336, 1230, 1208 and 1323 cm-’ are due to mode 3 in PDMB, MDMB, ODMB, 3TMB, 4TMB and STMB, respectively. It is interesting to note that this vibration mixes with the KekulC mode in 3TMB and STMB. It also mixes with y(CH,) in 3TMB and 4TMB. Other vibrations contributing to this mode can be seen in the PED tables. On the basis of calculations, the pair of bands at 1026 and 1104 cm-‘, 1082 and 1131 cm-‘, 1056 and 1083 cm-‘, 1140 and 1158 cm-‘, and 1155 and 1155 cm-’ are attributed to modes 18a and 18b, respectively, in PDMB, MDMB, ODMB, 4TMB and STMB. In 3TMB, the pair of bands at 1149 and 1096 cm-’ are ascribed to modes 9b and 18a, respectively. It should be noted that the bands at 1056 and 1083 cm-’ were assigned to y(CH,I and 18a, respectively, in ODMB by Varsanyi [41.

REFERENCES 1 B. Venkatram Reddy and G. Ramana Rao, Vib. Spectrosc., 6 (1994) 231. 2 B. Lakshmaiah and G. Ramana Rao, J. Raman Spectrosc., 20 (1989) 439. 3 D. Vijaya Kumar, V. Ashok Babu, G. Ramana Rao and G.C. Pandey, Vib. Spectrosc., 4 (1992) 39. 4 G. Varsanyi, Assignments for Vibrational Spectra of Seven Hundred Benzene Derivatives, Vol. I, Adam Hilger, London, 1974, pp. 233, 189, 110, 300 and 284. 5 A.K. Sarkar, S.C. Charkravorthi and S.B. Banerjee, Indian J. Phys. Part B, 51 (1977) 71. 6 E.B. Wilson, Jr., Phys. Rev., 45 (1934) 706.