density functional theory study of vibrational spectra: 9. structures and vibrational assignments of...

ELSEVIER Spectrochimica Acta Part A 53 (1997) 721-731

SPECTROCHIMICA ACTA

PART A

Density functional theory study of vibrational spectra: 9. Structures and vibrational assignments of dicyanobenzenes

James Higgins, Xuefeng Zhou, Ruifeng Liu * Dtrpurtment c~f Chemistry, East Tennessee St&e University, Johnson City. TN 37614-0695. USA

Received 16 March 1996; accepted 27 July 1996

___-.--

Abstract

Density functional theory BLYP and ab initio HF calculations have been carried out to investigate the structures and vibrational spectra of dicyanobenzenes. The calculated results are in good agreement with reliable experimental data and indicate that the benzene rings of all three isomers are only slightly distorted by the two cyano groups. Vibrational frequencies calculated by BLYP/6-3 lG* force fields agree very well with experimental results, with a mean deviation of about 14 cm-’ for non-CH stretching modes. On the basis of agreement between the calculated and observed results, assignments of the fundamental vibrational modes were examined and some reassignments were proposed. This study demonstrates that the density functional theory BLYP calculation is a powerful approach to understanding the vibrational spectra of organic compounds. 0 1997 Elsevier Science B.V.

Kcyvords: Density functional theory; 1.3-Dicyanobenzene: Phthalonitrile; Terephthalonitrile; Vibrational assignment

1. Introduction

Substituent effects on the structures and spectra of benzene derivatives have been the subject of many experimental and theoretical studies [l-8]. Along these lines, the structures and vibrational spectra of mono- and dicyanobenzenes have been investigated extensively [l-7,9-15]. Although the spectral and structural features of cyanobenzene are well understood by now [S], some uncertainties exist in the assignments of the fundamental vibrational modes of dicyanobenzenes [ 1 1 - 151.

* Corresponding author.

To understand the spectral features of these compounds, both empirical normal coordinate analysis [13,14] and semi-empirical MNDO [14,15] as well as ab initio Hartree-Fock (HF) [15] calculations have been carried out. These calculations have been proven to be very useful for understanding the observed spectral features, and many misassignments of the fundamental vibrational modes have been accordingly corrected [15]. How- ever, owing to various approximations employed in the theoretical methods, the calculated results are associated with certain errors. For example, it is well known that as a result of neglecting electron correlation, the HF frequencies obtained with a basis set of double-zeta quality are gener-

1386-1425/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PIlSl386-1425(96)01794-l

712 J. Higgins et al. / Spectrochimica Acta Part A 53 (1996) 7.?1- 731

ally lo-15% higher than the observed results. As the electron correlation effects are fairly sys- tematic for most organic compounds, an overall seal-ing of the HF frequencies by a constant (i.e. 0.8927 as suggested by Pople et al. [16]) gives a good approximation of the observed result. However, Pulay et al. [17] have shown that owing to extraordinary electron correlation effects in the benzene ring, the raw HF frequency of the Kekule mode (v,~ of benzene) is very close to the experimental result. When the HF frequency of this mode is scaled by the same scale factor that is applied to other modes, the scaled frequency is more than 100 cm ’ lower than the observed result. For this reason, Pulay et al. [17] have proposed an additional scaling procedure for the CC coupling force constants of the benzene ring, although it has not been widely accepted. For example, a recent ab initio study [15] of 1,2-dicyanobenzene obtained a HF/6-31G frequency of the Kekule mode very close to the experimental assignment of Barraclough et al. (1300 cm - ‘) [l 11, but according to scaled HF/6-31G frequency (1221 cm ~ ‘) this mode was reassigned to 1228 cm ~‘.

Density functional theory (DFT) [18] has be- come very popular recently as a general approach to molecular properties. With a computational cost that scales more favorably than HF theory, DFT recovers electron correlation effectively, and predicts molecular structural and spectral features in good agreement with experimental results. Our recent studies [19] of the vibrational spectra of many organic compounds using DFT with Becke’s exchange [20] and the Lee-Yang-Parr correlation functionals [21] (BLYP) reproduced the observed fundamental vibrational frequencies very well, with a mean deviation of about 15 cm - ’ between the calculated and observed non-CH(D) stretching frequencies. This accuracy enables us to resolve many uncertainties in the assignments of fundamental vibrational modes. In the present study, we carried out DFT BLYP and ab initio HF calculations to better understand the structures and vibrational spectra of dicyanobenzenes.

2. Computational details

All the structural parameters of 1,2-dicyanobenzene (phthalonitrile), 1,3-dicyanobenzene, and 1,4-dicyanobenzene (terephthalonitrile) were fully optimized with the gradient technique at the DFT BLYP and ab initio HF levels of theory using the 6-31G* basis set 1221. Quadratic force fields, dipole moment derivatives, and polarizability derivatives were calculated for the optimized equilibrium structures. The force fields were used in the calculation of normal vibrational modes, and the dipole moment and polarizability derivatives were transformed into normal coordi- nates to obtain infrared intensities and Raman activities. The normal modes calculated by BLYP, 6-3 1 G* force fields were approximately character- ized by total vibrational energy distribution (TED) [23] analysis. To take into account electron correlation which is neglected in the HF approximation, the HF/6-31G* frequencies were scaled by 0.8927 as suggested by Pople et al. [I 61. All quantum mechanical calculations were carried out using the GAUSSIAN 94 program package [24]. The vibrational analysis was carried out using the TX90

program package [25].

3. Results and discussions

3.1. Structurrs

The calculated structural parameters are compared with available experimental data [9,10] in Figs. 1-3. In these figures, the bond lengths are given in Angstroms and the angles in degrees. As all the molecules have pretty high symmetry (CZV or D,,), only symmetry unique structural parameters are given in the figures. A general observation from comparing the calculated and available electron diffraction structural parameters is that most of the HF bond distances are slightly shorter than the experimental results, while most of the BLYP bond distances are slightly longer than the corresponding experimental results. This may be a result of the neglect of electron correlation by HF theory and a slight exaggeration of the effect of electron correlation by the BLYP method. For

J. Higgins et al. /Specirochinlic~rr Ac,tn Part A 53 (1996) 7-71 731 7’1 -.

phthalonitrile, both the calculations and the electron diffraction [9] results indicate the CCC bond angles of the benzene ring to be only slightly distorted from 120”. Except for the C,C2 bond length, the CC bond distances of the six-meme- bered ring of phthalonitrile are predicted very close to 1.40 A, the CC bond length of benzene. These results indicate that the benzene ring in phthalonitrile is only slightly distorted by 1,2-dicyano substitution. For terephthalonitrile, most of the calculated structural parameters are in good agreement with the electron diffraction [lo] results. The deviations in the CC bond lengths and CCC bond angles of the six-membered ring from the corresponding parameters of benzene are also quite small. However, a disagreement between the calculated and electron diffraction results was found for the C,C,C, angle. The calculated results are 119.9” by BLYP/6-31G” and 120.1” by HF/6- 3 1 G*. The corresponding electron diffraction re-

N

III

1.176 I.135 1.161(2)

F 121.0

H

1.384 119.7(8)

I I

I.414

120.4 1.388

1 1.093 ,, 1.074

Fig. 1. Comparison of the calculated and electron diffraction structural parameters of 1.2-dicyanobenzene (bond lengths in Angstrams and angles in degrees). The calculated results were obtained with the 6-31G* basis set. The electron diffraction results are those of Schultz et al. [9]. In the electron diffraction study, only an average value of the CC bond lengths of the benzene ring and an average value ?f the CH bond lengths were determined, which are 1.395(S) A and 1.087(5) A. respectively.

I.176 I.136

BLYP fIF

Fig. 2. Comparison of the calculated structural parameters of 1.3-dicyanobenzene (bond lengths in kgstr6ms and angles in degrees). The calculations were carried out using the 6.3lG* basis set.

sult is 122.1 + 0.1”. Experience from a large num- ber of ab initio studies [26] indicates that the HF/6-31G* bond angles of most organic compounds are accurate to & 1.5”. Although the deviation between the calculated and electron diffraction results is just out of the calculational uncertainty, it is noteworthy that an X-ray crystal analysis [7] of this compound gave a C,C, C, angle ranging from 120.6 + 0.7” to 121 .l + 0.2”, in reasonable agreement with the calculated results. As this angle is an important parameter for dis- cussing the distortion of the benzene ring due to dicyano substitution, a modern electron diffraction study of this structural feature is desirable.

We did not find experimental structural parameters for 1,3-dicyanobenzene. In view of the agreement between the calculated and reliable experimental results of phthalonitrile and terephthalonitrile, we believe the HF/6-31G* and BLYP/6-31G* bond lengths to be good lower and higher estimates of reliable electron diffraction results of 1,3-dicyanobenzene. The calculated parameters can therefore be used to guide future experimental studies of the structure of this compound.

724 J. Hi,qgins rr al. ; Spectrochimicu .1cto Parr A 5.1 (1996) 721~ 731

3.2. Vihrutional spectra

32.1. Phthalonitrilr Among the three isomers of dicyanobenzenes,

the vibrational spectra of phthalonitrile is perhaps the best understood. A detailed infrared, Raman, and UV study of this molecule was reported by Barraclough et al. [l l] about 20 years ago, and an assignment of all the infrared active modes was given. The infrared and Raman spectra of all three dicyanobenzenes were also studied by Castro-Pedrozo and King [12], and a general vibrational assignment of all the fundamental modes was proposed. Very recently, Navarrete et al. [15] carried out an infrared and Raman study of the vibrational spectra of phthalonitrile, as well as semi-empirical MNDO and scaled ab initio H F/3-21 G and HFj6-3 1 G calculations. Mainly on the basis of their calculated results, a reassignment of the fundamental vibrational modes,

N

Ill

1.176 1.136 1.167(2)

C

I.398 BLYP 1.381 HF 1.397(3) GED

HA,AH I.074

I.438 1.446 I .454(5)

C

/I/ N

Fig. 3. Comparison of the calculated and electron diffraction structural parameters of 1,4-dicyanobenzene (bond lengths in Angstriims and angles in degrees). The calculated results were obtained with the 6-3lG* basis set. The electron diffraction results are those of Schultz ([IO]).

which differs in many aspects from the previous assignments, was proposed. A comparison between our calculated frequencies and infrared and Raman intensities with the assignments of Navar- rete et al. [15] is given in Table 1. In this table, the HF frequencies have been uniformly scaled by 0.8927 to approximately account for the effects of electron correlation. It is shown that the scaled HF calculation is very useful for understanding the observed vibrational spectral features, as most of the reassignments of Navarrete et al. are supported by the BLYP results. The only significant difference between their assignment and the BLYP result is in the assignment of the Kekule mode ~1~. The BLYP frequency of this mode (1309 cm - ’ ) is in good agreement with the early experimental assignment (1300 cm ‘) by Barraclough et al. [ll], but Navarrete et al. reassigned it to a band at 1228 cm ’ based on their scaled HF/6- 31G result of 1221 cm ‘. As pointed out by Pulay et al. [17], this mode is associated with strong electron correlation effects in the HF treat- ment and should be dealt with by extra scaling in the scaled HF approach for a good description. In the case of benzene, the raw HF frequency of this mode is very close to the experimental result. Scaling this mode with the scale factor that ap- plies to most other modes reduces this frequency to an unreasonably Iow value. A similar situation was also found for benzonitrile [5]. As the BLYP method recovers electron correlation effectively, we believe the original assignment of this mode by Barraclough et al. [l l] to be correct. With this revision, the mean deviation between the BLYP frequencies and the assignments of Navarrete et al. for the non-CH stretching modes is 11 cm ‘. This is similar to the mean deviations obtained by us for many other organic molecules.

The only experimental assignment of the fundamental vibrational modes of rpz-dicyanobenzene is that given by Castro-Pedrozo and King [12]. Comparison of the calculated results and the experimental assignment is given in Table 2. For the non-CH stretching a, modes rj-r,2, the BLYP frequencies agree reasonably with the assignments of Castro-Pedrozo and Wang. the mean deviation

J. Higgins et ~1. ~Spectrocltimica Acta Part A 53 (1996) RI- 731

Table I Comparison of the calculated and observed vibrational frequencies” of 1,2-dicyanobenzene

775

Sym.

a ,

a2

b,

1’ BLYP HF -

Freq. I IR Freq.h I,,

I 3144 5.64 3046 6.67 2 3129 7.62 3032 6.17 3 2243 5.44 2334 26.18 4 1554 0.70 1580 0.75 5 1473 9.92 1484 21.92 6 1309 2.61 1194 6.18 I II98 0.73 1182 0.28 8 1173 0.00 1091 10.09 9 1035 0.73 1016 0.15

10 699 3.17 685 3.73 II 555 0.01 556 0.27 I? 458 0.70 462 2.66 13 113 1.48 117 2.17 14 965 0.00 1018 0.0 I5 871 0.00 902 0.0 16 730 0.00 737 0.0 I7 566 0.00 568 0.0 I8 365 0.00 371 0.0 I9 109 0.00 112 0.0 20 930 I .38 980 2.66 21 756 41.51 176 62.13 22 534 13.94 542 24.77 23 383 1.01 3s7 1.31 24 I65 4.02 166 5.03 35 3141 6.53 3041 4.77 26 3117 0.87 3021 0.64 27 2247 3.32 2334 14.04 28 1583 2.17 1605 3.00 29 1440 3.31 1439 IO.10 30 1267 0.02 1264 0.02 31 1182 0.04 1157 0.67 31 1089 2.05 1071 1.90 33 799 1.45 780 2.91 34 594 0.05 605 0.48 35 408 0.08 400 0.50 36 I85 4.50 I92 7.42

Expt.’ Mode description”

I Ra Freq.

241.1 3080 97.38 3041

309.2 2232 17.43 1573

9.50 1485 16.39 1228’ 38.18 1206 26.64 1155 27.4 1038 13.06 706 2.31 563

13.51 473 5.24 148 0.38 944 3.79 885 I.28 739 I .50 531 3.90 364 1.65 120 0.79 928 0.02 769

31.15 525 3.00 381 0.18 175

24.14 3105 48.92 3029

160.6 2232 77.76 1590

0.17 1447 0.52 1296 2.80 1182 0.60 1095 1.06 807 0.08 610 3.94 414 0.91 201

I IR

m m S

m s m ms

VW m w m

w vs vs

m w S

m ms m w VW

m VW VW

s

VW

vs

W

W

wm m W ms m vm m sh VW

VW

VW

w

w

W

vm W

m W

w vs m VW VW

w

VW

W

m

CH str. CH str. CN str. C=C str. + CH bend CH bend. C=C str. C-C str.+C--C str. CH bend C-C str. Ring def. CC bend. + ring def. CC+CCN bend CCN + CC bend CH o-o-p bend CH o-o-p bend Ring tor.+C-C o-o-p bend Ring tor. +C--C o-o-p bend CCN o-o-p bcnd+ring tor. Ring tor.+C -C o-o-p bend CH o-o-p bend CH o-o-p bend CCN + C ~C o-o-p bend Ring tor. + CCN bend CC+CCN o-o-p bend CH str. CH str. CN str. C=C str. CH bend CH bend Ring def. Ring def. Ring def.+C--C str. CC bend+CCN bend Ring def. + C- C str. CCN bend. + C --C bend

“Frequencies are in cm -‘, the cdlcutated IR intensities are in km mol- ‘, and the calculated Raman activities are in A’ u -‘. ‘HF frequencies have been scaled by 0.8927. “Experimental assignments of Navarrete et al. 1151. “Based on vibrational energy distribution analysis. ‘Assignment of this mode is in error; the original assignment of Barraclough et al. [I l] to a band at 1300 cm ’ is supported by the calculations (see discussions in text).

being 15.7 cm-‘. The experimental assignment of r13 is, however, 264 cm - ’ higher than the BLYP frequency 113 cm .- ‘. The calculations indicate that v,~ gives a medium to strong infrared band and a very weak Raman band. As it appears at

around 100 cm - ‘, a region in which KBr and polyethylene (used to make sample discs for IR measurements) absorb significantly, the band due to this mode might be obscured in the experimental study. For the four infrared inactive a2 modes,

116

Table 2

J. Higgins et al. /Spec~ochimica Acta Part A 53 (1996) 721 - 731

Comparison of the calculated and observed vibrational frequencies” of 1,3-dicyanobenzene

Sym.

a /

a,

b,

,’ BLYP HF Expt.’ Newd Mode description”

Freq. I IR Freq.b I,, I Ra Freq. I,, I K.l

1 3150 0.00 3045 5.08 208. 3090 2 3144 5.48 3044 0.27 19.1 3089 3 3122 3.14 3025 1 .YO 50.7 3060 4 3248 3.42 2332 14.6 379. 2241 5 1561 2.38 I589 0.74 26.4 1576 6 1404 3.79 1410 12.3 4.59 1421 1 1226 0.23 1206 0.20 54.6 I248 8 1093 2.51 1067 3.18 2.24 1100 9 9X4 0.26 974 0.35 36.2 1005

IO 696 0.23 682 0.58 7.98 709 II 505 0.04 521 0.67 5.88 524 12 449 0.65 438 1.26 5.20 470 13 113 7.88 I16 11.2 6.31 377 I4 906 0.00 959 0.00 1.58 906 15 619 0.00 621 0.00 9.36 620 16 361 0.00 376 0.00 8.13 215 17 157 0.00 161 0.00 0.32 152 I8 958 0.25 1011 0.10 0.28 988 19 x90 9.29 940 11.4 2.09 865 20 791 24.4 815 32.5 0.26 793 21 676 14.3 676 29.7 0.77 672 22 504 10.3 514 17.0 3.87 460 23 382 1.37 388 1.56 0.86 158

24 122 5.75 123 7.31 0.77 62 25 3138 I.88 3036 2.22 62.4 3109 76 2241 16.2 2332 54.2 152. 2225 27 1586 0.02 1616 0.20 63.8 I595 28 1477 8.41 1476 23.6 0.00 1475

29 1327 0.77 1301 1.52 0.34 1329 30 1291 0.01 1184 0.00 0.17 1271

31 1180 0.38 1129 2.51 0.30 1195 32 1139 0.71 1096 5.44 9.44 1060 33 888 5.41 865 9.18 0.86 900 34 577 0.10 586 0.80 2.45 590 35 451 0.02 441 0.03 2.64 502

36 I86 1.37 193 2.33 2.20 343

5

m

m m m m VW

m w VS

m

w m VS

vs VS

s m w VS

m s VW m m VW S

m vs m

m sh w \s 5 m 5 W

vs m W

m ‘Y m

VW m m 377 VS

VW

W

m 502 398

W’

s

w w VW

?

W 460 w 180

-

CH str. CH str. CH str. CN str. C--C str. + CH bend CC str.+CH bend C C str. + ring def. CC str.+CH bend Ring def. Ring def. CCN bend.+CCC bend Ring def. CCC bend+CCN bend CH wag C-CN wag Ring tor. Ring tor.+CCN wag CH wag CH wag CH wag Ring tor. CCN wag+ring tor. Ring tor. C-CN wag+CCN wag CH str. CN str. c--c str. CH bend+C-C str. CH bend+C=C str. C-C str.+CH bend CH bend CH bend+C=C str. C C str. + ring def. C c‘ str.+ring def. Ring def. CCN bend

“Frequencies are in cm ’ , the calculated IR intensities are in km mall’, and the calculated Raman activities are in A“ tt ‘. ‘HF frequencies have been scaled by 0.8927. “Experimental assignments of Castro-Pedrozo and King [ 121. %iuggested reassignments of the fundamental vibrational modes based on the calculated results. ‘Based on vibrational energy distribution analysis, only the major components of each mode are given.

the experimental assignments of v,~, \I,~, and r,, was incorrectly assigned to 11,~ g a rees well with the agree reasonably well with the calculated results. calculated frequency of \‘i6 and therefore should For I’,~, the experimental assignment to a medium be accordingly reassigned. Raman band at 2 15 cm ~ ’ is 152 cm - ’ lower than For the b, modes, significant differences be- the BLYP frequency 367 cm ~ ‘. However, a tween the calculated results and the experimental medium IR band observed at 377 cm - ’ which assignments were found for Vet, \lz3, and 1~~~. For

J. Higgins et al. /Sp~ctrochitnica Acfa Part A 53 (1996) 721 - 731 ,127

‘Jam, the BLYP frequency is 44 cm - ’ higher than the experimental assignment of 460 cm -- ‘. It is, however. in good agreement with an infared band observed at 502 cm-’ which was assigned to the b, mode 11~~. The BLYP frequency of vX5 (451 cm -- ‘) is, in contrast, very close to 460 cm- ‘. Thus, it is reasonable to exchange assignments for “2Z and 11~~. The experimental assignment of v13 to an infrared band at 158 cm ’ is 224 cm ~ ’ lower than the BLYP frequency 382 cm - ‘. The latter is very close to a weak infrared band observed at 398 cm --’ which was assigned as an overtone of 215 cm ~ ‘. However, the calculations indicate that the band at 215 cm ’ is unlikely to be a fundamental. According to agreement between the calculated and observed results, it is likely that the band at 398 cm- ’ is due to rZ3. Castro-Pedrozo and King assigned an infrared band at 62 cm -- ’ to I’~+ but the calculations indicate that vL1 should be around 122 cm ~ ‘. As this frequency region is not easily probed by the infrared technique, more experimental studies are desirable to resolve the discrepancies.

For 1,3-dicyanobenzene, the b2 mode ‘130 corresponds to the Kekule mode of benzene. The BLYP frequency of this mode (1291 cm - ‘) agrees reasonably well with the assignment of Castro-Pe- drozo and King (1271 cm- ‘). The scaled HF frequency is about 90 cm ~’ lower. Significant differences between the BLYP frequencies and the experimental assignments are found for ‘I~~, rj5, and ‘xJh. The discrepancy for I’~~ has been dis- cussed above and reassignment was proposed. For 11~~. the BLYP frequency is 79 cm - ’ higher than the assignment of Castro-Pedrozo and King. In view of the agreement between the calculated and observed frequencies of the other modes, the difference for this mode is unusually large. For Vet, the assignment of Castro-Pedrozo and King is 157 cm -- ’ higher than the BLYP frequency 186 cm -~ ‘. In the Raman spectrum of Castro-Pedrozo and King. there is a weak band at 180 cm ’ which was considered to be a candidate of rZ3. On the basis of agreement between the calculated and observed results, this band is probably due to the b3 mode 1~~~.

With the reassignments proposed above, the mean deviation between the BLYP and observed

non-CH stretching frequencies is 12 cm - ‘. As only one experimental study of the vibrational spectrum of this compound has been carried out. we could not find reasonable candidates for I’,+ r14, and ‘I~~. More experimental studies are re- quired to give reliable assignments of these modes.

A comparison between the calculated frequencies and experimental assignments of the fundamental vibrational modes of terephthalonitrile is given in Table 3. Most of the calculated results are in reasonable agreement with the recent experimental assignments of Arenas et al. [14], but there are some discrepancies. For the af. funda- mentals r4 and v5, both the calculated frequencies and Raman activities are in reasonable agreement with the early assignments of Barraclough et al. [ll] and Castro-Pedrozo and King [12], while agreement with the assignments of Arenas et al. [14] is not as satisfactory. On the basis of agreement between the calculated and observed results. we believe the early assignments of these two modes to be correct. Barraclough et al. [ll] assigned the bzg mode v19 and the b,, mode r?, to two weak Raman bands at 561 and 520 cm ‘, respectively, but they noted that the assignments could be exchanged. Their assignments were fol- lowed by Arenas et al. [14], but the calculated frequencies of the two modes indicate that the assignments should indeed be exchanged. For the b,, mode vX7. the BLYP frequency 183 cm -’ is in much better agreement with the assignment of Barraclough et al. (206 cm ~ ‘) than with the as-

signments of Castro-Pedrozo and King (380 cm 1) [12] and Arenas et al. (378 cm- ‘) [14]. The calculated frequency of v36 (77 cm - ‘) is too low compared to the experimental assignment of Rarr- aclough et al. [l l] ( 155 cm ‘), but is in mar- ginally acceptable agreement with the assignments of Castro-Pedrozo and King [12] and Arenas et al. [14]. vZh has been assigned to a medium IR band at 360 cm ’ [11,12], but the calculated frequency of vZo is more than 200 cm ’ lower. On the basis of the calculated frequency and IR inten- sity, it seems that a possible candidate of rZh is the medium IR band at 155 cm ’ reported by Barra-

Tabl

e 3

Com

paris

on

of

the

calcu

late

d an

d ob

serve

d \,ib

ratio

nal

frequ

encie

s”

of

1,4-

dicya

nobe

nzen

e

I’ BL

YP

HF

Frey

.

I 31

47

2 22

41

3 15

97

4 11

97

5 11

60

6 79

8 I

369

8 93

6 9

394

10

825

11

3132

12

22

45

13

1498

14

11

95

15

1009

16

63

1 17

94

7 18

71

4 19

51

2 20

19

0 21

31

45

22

1406

23

13

00

24

1114

25

51

6 26

11

9 27

31

33

28

1530

29

13

07

30

648

31

552

32

183

_---

~

I IR

Freq

.h

0.00

30

41

0.00

23

31

0.00

16

23

0.00

11

69

0.00

11

51

0.00

78

4 0.

00

360

0.00

99

5 0.

00

399

0.00

85

7 2.

01

3027

21

.0

2331

1.

65

1503

0.

30

1170

2.

71

1000

1.

18

609

0.00

98

8 0.

00

723

0.00

52

2 0.

00

196

5.35

30

40

8.15

13

93

2.72

11

49

2.46

10

68

0.10

52

9 12

.0

123

0.00

30

28

0.00

15

66

0.00

13

01

0.00

64

4 0.

00

560

0.00

19

0

I IK 0.

00

0.00

0.

00

0.00

0.

00

0.00

0.

00

0.00

0.

00

0.00

2.

77

66.9

24

.1

0.02

3.

20

3.48

0.

00

0.00

0.

00

0.00

3.

08

12.7

23

.1

5.80

2.

00

16.7

0.

00

0.00

0.

00

0.00

0.

00

0.00

Expt

.’ ~~

I,,

Freq

. 4,

I,,

.~~

-~~

226.

30

73

m

742.

22

38

s 26

5.

1610

s

0.13

11

86

w 11

5.

1176

5

20.4

81

3 m

4.

63

372

w 0.

00

0.00

2.

67

790

w 0.

00

3058

m

0.

00

2236

s

0.00

15

09

s 0.

00

1207

s

0.00

10

32

m

0.00

64

8 m

4.

66

965

m

6.64

72

0 w

18.3

56

1h

w 0.

21

198

w 0.

00

3102

m

0.

00

1405

s

0.00

12

82

s 0.

00

1127

w

0.00

65

6 s

0.00

36

4 m

91

.4

3064

m

0.

22

1610

sh

3.

94

1320

VI

1.

67

658

w 10

.3

520h

w

7.19

20

6 s

.- Expt.

d ~~

~.

~~~

Fw.

I,,

I,,

~...

...~~~

~

3090

w

2218

vs

16

15

s 11

93

sh

1177

s

817

s 47

0 w

975

395

w w

794

W

3060

m

s 22

20

vs

1502

vs

I2

78

s 10

25

m

643

s 97

5 w

w 71

9 w

213

5 16

5 5

3102

m

s 14

03

vs

1294

w

1120

w

535

mw

360

m

3037

W

1610

sh

13

28

v w

662

M 52

8 m

____

~~

Ex

pt.’

New’

Freq

. I,,

3088

22

42

1619

11

83

1134

82

0

977

392

794

3053

m

22

34

vs

1504

m

12

00

m

1028

w

636

m

971

725

566

213

3099

m

14

01

m

1277

m

11

23

VW

530

VW

3078

I6

09

I328

66

2 52

4 38

0 37

8

I Il.1

m vs

S m

VW

s VW

VW

W m m

W Sh

W

m

m \h’

3088

22

42

1619

11

93

1177

82

0 37

2 97

7 39

2 79

4 30

53

2234

15

04

1200

10

28

636

971

725

524

213

3099

14

01

1294

11

23

530

155(

?)

3078

1328

66

2 56

6 20

6

___~

~-

-. Ct

i str

. CN

str

. C-

C str

. CH

be

nd+C

-C

str.

C C

str.

C-C

str.

Ring

de

f. CH

wa

g Ri

ng

tor.

CH

wag

CH

str.

CN

str.

CH

bend

C-

C str

. +r

ing

def.

Ring

de

f. C-

C str

. +

ring

def.

CH

wag

Ring

to

r. Ri

ng

tor.+C

-C

wag

C-C

wag+

CCN

wag

CH

str.

GC

str.+

CH

bend

C-

C str

. CH

be

nd+C

=C

str.

CCN

bend

C

~C

bend

+CN

bend

CH

str

. C-

C str

. CH

be

nd

Ring

de

f. Ri

ng

def.

+ CC

N be

nd

CNN

bend

+C

C be

nd

1. Higgins et al. ,I Sprctrochirnica AOU Part A 53 (I 996) i’.?l~ 7.3 I

clough et al. [l 11. However, the calculated frequency of this mode (119 cm ~ ‘) seems too low. A new experimental study of this spectra1 feature is desirable to resolve this discrepancy.

The Kekule mode of benzene corresponds to rZ3 of terephthalonitrile. As in the cases of phthalonitrile and 1,3-dicyanobenzene, the raw HF frequency of this mode agrees very well with the observed frequency, but after being scaled by 0.8927. the frequency is about 130 cm I lower. However, the BLYP result agrees well with the observed frequency, indicating that BLYP recovers electron correlation efficiently. The similarity between the HF!6-31G* results of the Kekule modes of all three dicyanobenzenes and that of benzene indicate that the benzene ring is only slightly disturbed by the two cyano substituents. This is in agreement with the calculated structural parameters.

4. Conclusions

Density functional theory BLYP and ab initio HF calculations were carried out to investigate the structures and vibrational spectra of dicyanobenzenes. The calculated results are in good agreement with reliable experimental data and indicate that the benzene rings of all three isomers are only slightly distorted by the two cyano groups. Vibrational frequencies calculated by BLYP/&3 1G* force fields agree very well with experimental results, with a mean deviation of about 14 cm ~ ’ for non-CH stretching modes. On the basis of agreement between the calculated and observed results, assignments of the fundamental vibrational modes of dicyanobenzenes were examined and some reassignments were proposed. This study demonstrates that density functional theory BLYP calculation is a powerful approach for understanding the vibrational spectra of organic compounds.

Acknowledgements

This study was partially supported by the Re- search and Development Committee of East Ten-

nessee State Corporation.

University and Research

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PI

[31

[41

[51

PI

[71

181

[91

I101 IllI

1121

[I31

[I41

1151

[I 61

[I71

1181

(191

WI Pll

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