I I ELSEVIER

29 November 1996

Chemical Physics Letters 262 (1996) 689-698

CHEMICAL PHYSICS LETTERS

FT-IR spectrum of the 2H-tautomer of benzotriazole in a supersonic jet

Gad Fischer, Xiaolin Cao, Robin L. Purchase Department of Chemistry, The Faculties. Australian National University. Canberra. ACT 0200. Australia

Received 15 March 1996; in final form 17 July 1996

Abstract

The infrared absorption spectrum of benzotriazole and N-deuterobenzotriazole in a cooled molecular beam have been measured and compared to those in the solid state. Small but significant differences in the spectra have been noted and attributed to the occurrence of primarily the 2H-tautomer in the beam and the IH-tautomer in the solid. Optimized geometries and vibrational frequencies have been determined from ab initio molecular orbital and density functional calculations. Neither the molecular orbital calculations nor the spectra support a more quinonoid structure for the 2H-tautomer.

I. Introduction

The relative stabilities of the two tautomers of benzotriazole (BTZ) (Fig. 1) has been an issue of considerable interest and has been studied exten- sively both experimentally and theoretically [1-14]. Early computational and experimental work indi- cated the less symmetrical (C~ point group symme- try) IH-tautomer to be the more stable. Crystal structure measurements [10] showed that the solid state exists almost exclusively as the IH-tautomer. Similarly, the 1H-tautomer was found to be predomi- nant in gas-phase mass spectrometric studies [ l l] . Without exception the electronic spectroscopy of BTZ in a range of solvents was found to be consis- tent with the 1H-tautomer [ 13,14]. Furthermore, from recent measurements of the microwave spectrum of BTZ in a heated cell, Velino et al. [6] found evidence for the existence of only the l H-tautomer. These workers also carried out a high-resolution rotational analysis of the band at 286 nm in the electronic

absorption spectrum in a cell heated to 140°C. They assigned this band as the origin band of a ~r, v * transition, which they attributed to the IH-tautomer [7].

A number of recent studies has suggested that, in contradiction to the earlier works, it is the more symmetrical 2H-tautomer (C2~ point group symme- try) which is the more stable one [1,3]. Ab initio molecular orbital calculations, in which some al- lowance has been made for electron correlation, established that the 2H-tautomer is more stable by

H~ H7 :. HI ' .c9 7 c ajN\ y7 c7

!~a /C~c/ H," ~ c . . / H,

a) b)

Fig. 1. (a) 1H-benzotriazole, (b) 2H-benzotriazole.

0009-2614/96/$12.00 Copyright © 1996 Elsevier Science B.V. All fights reserved PII S0009-2614(96)01043-3

690 G. Fischer et a l . / Chemical Physics Letters 262 (1996) 689-698

some 8 kJ mol- I [3]. An investigation of the temper- ature dependence of the electronic absorption spectra of BTZ and of 1- and 2-methylbenzotriazoles in the gas phase has led to the conclusion that the equilib- rium for the isolated molecule is shifted towards the 2H-tautomer by a few kJ mol-1 [3]. Finally, in an analysis of the rotational fine structure carried out on a high-resolution spectrum of the 286 nm band of BTZ, when present in a cooled molecular beam, only the 2H-tautomer was assigned [1].

Accordingly, the infrared (IR) spectrum of BTZ in solution and in the solid state can be attributed to the 1H-tautomer. No vapour-phase IR spectrum has been reported, but many measurements have been taken of the solid state (KBr disk) and solution spectra [9,12]. Cooled and isolated molecules can be readily produced in a molecular beam. Rotational temperatures of less than 50 K have been estimated for acetylene in our system. Vibrational cooling, however, is far less efficient, and again from com- parison with acetylene vibrational temperatures have only been reduced marginally. With modem FT-IR (Fourier transform infrared) spectrometers it is possi- ble to measure the IR spectra of molecules in a beam. We have undertaken an investigation of the IR spectrum of BTZ in a cooled molecular beam, and have compared the spectrum to that obtained for BTZ in the solid state. To provide confirmation of the identity of the tautomers, and to assist in the assignments, we have carried out some standard ab initio molecular orbital calculations (including den- sity functional theory) of the optimized geometries, and of the force fields and vibrational frequencies at the MP2/6-31G*, B3LYP/6-31G* and HF/6- 31G * levels. Because of size and time restrictions the vibrational frequencies and force fields could not be calculated at the MP2 level for the lower-symme- try tautomer. Spectra have also been measured for N-deutero-benzotriazole (d-BTZ), and the vibra- tional frequencies calculated.

2. Experimental

Infrared spectra were recorded with a Bruker IFS66 Fourier transform infrared spectrometer. Spec- tra were obtained of solid state samples pressed into

KBr disks, and of cooled gas-phase molecules in supersonic free jets.

The molecular beam was produced by placing the sample in a reservoir adjacent to the nozzle. Both the reservoir and the nozzle were heated to approxi- mately 100°C using Claybom A-16-1 heat tape. The nozzle with an orifice of about 30 /xm was con- structed from stainless steel. He, acting as a carrier gas, was admitted to the high-pressure side of the nozzle at a pressure of 1 atm ( = 101.325 kPa). The low-pressure side was evacuated to 10 -7 Tor t (--- 1.333 x 10 -5 Pa) using a Varian VHS 6 diffusion pump backed by an Edwards E2M40 rotary pump. The expansion chamber was external to the spec- trometer and equipped with KBr windows. The IR source used was a SiC glowbar. The IR beam was directed from the spectrometer to the expansion chamber via a series of mirrors. The IR beam passed through the molecular beam at a point approximately 5 mm below the nozzle, and then passed to a liquid nitrogen cooled MCT detector. Spectra were taken at a resolution of 2 cm- i.

BTZ was purchased from Aldrich and was used without further purification, d-BTZ was prepared by placing 0.25 g of BTZ into 5-10 ml of D20. The suspension was then stirred for about half an hour whilst being gently heated (-- 50°C). The deuterated sample was evaporated to dryness using a rotary evaporator. A repeated deuteration of the already once deuterated sample showed no change in the FFIR absorption spectrum, indicating that any h-BTZ present was too small to be seen in the spectrum.

3. Calculations

Standard ab initio molecular orbital calculations were carried out using the GAUSSIAN 94 suite of programs [15]. The optimized geometries for the two tautomers have been obtained at the HF/6-31G* and MP2/6-31G* levels, and for the density func- tional technique at the B3LYP/6-31G* level. The IH-tautomer structures are listed in Table 1 together with the averaged bond lengths and angles derived from the measured crystal structure. Good agreement is achieved between the calculated bond lengths and

G. Fischer et al . / Chemical Physics Letters 262 (1996) 689-698 691

Table 1 Optimized structures for lH-benzotriazole (C s) compared to aver- age X-ray diffraction a values (bond lengths in A, angles in °)

H F / M P 2 / B3LYP/ Exp. a 6-31G * 6-31G* 6-31G*

N1-HI 0.993 1.014 1.010 N1-N2 1.332 1.361 1.365 1.346 N2-N3 1.253 1.322 1.292 1.310 C7a-N 1 1.375 1.375 1.382 1.362 C3a-N3 1.355 1.362 1.365 1.377 C7a-C3a 1.384 1.412 1.411 1.389 C3a-C4 1.400 1.407 1.404 1.409 C4--C5 1.369 1.384 1.386 1.368 C5-C6 1.414 1.419 1.416 1.405 C6-C7 1.370 1.386 1.388 1.367 C7-C7a 1.400 1.404 1.402 1.404 C4-H4 1.074 1.086 1.085 C5-H5 1.075 1.087 1.086 C6-H6 1.075 1.087 1.086 C7-H7 1.074 1.087 1.086

N 2 - N I - H I 119.1 118.2 114.8 N I - N 2 - N 3 109.8 108.0 108.7 108.8 N 2 - N 1 - C 7 a 110.5 111.7 111.0 110.3 N 2 - N 3 - C 3 a 108.6 108.0 108.6 108.2 N1-C7a -C3a 103.0 102.8 102.9 104.2 N3-C3a-C7a 108.1 109.4 108.8 108.4 C7a -C3a -C4 121.0 120.5 120.6 120.9 C3a -C4-C5 117.2 117.1 117.4 116.2 C 4 - C 5 - C 6 121.2 121.8 121.4 122.2 C5 - C6- C7 122.2 122.1 122.0 122.6 C 6 - C 7 - C 7 a 116.1 116.0 116.3 115.3 C7-C7a -C3a 122.3 122.6 122.3 122.7 C 3 a - C 4 - H 4 120.7 120.6 120.5 C 4 - C 5 - H 5 120.0 119.6 119.7 C 5 - C 6 - H 6 118.7 118.9 118.9 C 6 - C 7 - H 7 121.7 121.6 121.6

a From Ref. [10].

angles at the MP2/6-31G* and B3LYP/6-31G* levels and the experimentally determined [10] bond lengths and angles. The optimized geometry has also been obtained at the MP2/6-31G* * level [3] but is little different from our MP2/6-31G* geometry. Agreement is less satisfactory for the structure deter- mined at the HF/6-31G * level. For the 2H-tautomer a similar good agreement is obtained between the density functional and the MP2 calculation (see Table 2). Again, there are larger differences with the struc- ture calculated at the HF/6-31G* level. No experi- mentally measured structure of the 2H-tautomer is available for comparison.

It should be noted that at the MP2/6-31G * level the respective bond lengths for each tautomer are similar and the bond length alternation in the ben- zene ring is only slight. Consequently, these results do not lend support to a quinonoid structure for the 2H-tautomer. In contrast, a much larger difference is obtained for the structures calculated at the HF/6- 31G* level, and the bond length alternation in the benzene ring is much more marked for the 2H- tautomer in agreement with the SCF calculations of Palmer et al. [2]. The density functional results are intermediate to the MP2 and HF calculations, but clearly do not give strong support to a more quinonoid structure for the 2H-tautomer than the 1H-tautomer.

Table 2 Optimized structures for the C2v tautomer, 2H-benzotriazole

HF/ MP2/ B3LYP/ 6-31G* 6-31G* 6-31G*

N2- H2 0.995 1.017 1.012 N I -N2 1.297 1.334 1.326 N2-N3 1.297 1.334 1.326 C7a-N 1 1.325 1.369 1.354 C3a-N3 1.325 1.369 1.354 C7a-C3a 1.409 1.423 1.429 C3a-C4 1.425 1.408 1.415 C4-C5 1.351 1.384 1.377 C5 -C6 1.441 1.421 1.429 C6-C7 1.351 1.384 1.377 C7-C7a 1.425 1.408 1.415 C4- H4 1.074 1.086 1.085 C5-H5 1.075 1.087 1.087 C6-H6 1.075 1.087 1.087 C7-H7 1.074 1.086 1.085

N 1 - N 2 - H 2 120.9 120.0 120.4 N I - N 2 - N 3 118.2 120.1 119.2 N2-N l - C 7 a 102.9 101.0 101.9 N2-N3-C3a 102.9 101.0 101.9 N1-C7a-C3a 108.0 108.9 108.5 N3-C3a-C7a 108.0 108.9 108.5 C7a-C3a-C4 121.1 121.3 121.1 C3a -C4-C5 116.8 116.7 116.9 C 4 - C 5 - C 6 122.1 122.0 122.0 C 5 - C 6 - C 7 122.1 122.0 122.0 C 6 - C 7 - C 7 a 116.8 116.7 116.9 C7-C7a-C3a 121.1 121.3 121.1 C 3 a - C 4 - H 4 120.9 121.2 120.9 C 4 - C 5 - H 5 119.8 119.3 119.5 C 5 - C 6 - H 6 118.0 118.7 118.5 C 6 - C 7 - H 7 122.3 122.1 122.2

692 G. Fischer et al. / Chemical Physics Letters 262 (1996) 689-698

Table 3 Energies of the two tautomers of benzotriazole (all energies are in J)

1H-Benzotriazole 2H-Benzotriazole

HF/6-31G* - 1.7152214 x 10- ~5 - 1.7152053 X 10- ~5 MP2/6-31G" - 1.7206681 X 10-t5 - 1.7206870 X I0-J5 B3LYP/6-31G * - 1.7258811 × 10- 15 - 1.7258820 × 10- 15

Table 4 Calculated and observed spectra of the 1H-tantomer of benzotriazole and N-deutero-benzotriazole a

Benzotriazole N -Deuterobenzotriazole

HF/6 -31G ' b B3LYP/6-31G' c Exp.(KBr) HF/6-31G* b B3LYP/6-31G* c Exp.(KBr)

K 398 (4) 397 (6) 389 (7) 388 (10) 521 ( < 1) 525 ( < 1) 519(1) 523 (1) 6 1 4 ( < 1) 614(< 1) 612(< 1) 6 1 2 ( < 1) 759 (10) 768 (9) 753 s 756 (13) 763 (7) 753 s 877 ( < 1) 876 ( < 1 ) 902 (18) 880 (9) 972 (2) 966 (7) 976 (4) 958 (6) 995 (18) 986 (44) 1023 m 1000 (9) 997 (I 2) 1017 m

1064 (34) 999 (7) 1079 m 849 (14) 838 (33) 866 w 1101 (10) 1108 (3) 1123 m 1092 (38) 1103 (30) 1123 w I 117 (7) 1137 (7) 1147 m 1115 (16) 1116 (66) 1146 m 1211 (14) 1213 (31) 1210 vs 1175 (61) 1143 (5) 1180 s 1248 (3) 1242 (28) 1210 ? 1221 (4) 1216 (10) 1210 m 1278 (1) 1254 (3) 1262 w 1258 (1) 1250 (3) 1268 w 1282 (19) 1297 (42) 1280 m 1282 (32) 1296 (70) 1274 m 1391 (39) 1373 (2) 1384 s 1329 (15) 1325 (7) 1333 m 1428 (4) 1378 (2) 1419 m 1426 (14) 1376 ( < 1) 1419 w 1482 (13) 1449 (9) 1458 m 1480 (36) 1441 (17) 1449 m 1510 (4) 1489 (2) 1511 w 1489 (12) 1481 (5) 1491 m 1607 (1) 1581 (3) 1594 m 1602 (2) 1578 (4) 1593 m 1637 (10) 1615 (10) 1623 m 1631 (12) 1612 (I 1) 1619 m 3012 (2) 3074 (2) 3012 (4) 3074 (3) 3026 (10) 3086 (14) 3026 (18) 3086 (21 ) 3036 (16) 3095 (22) 3036 (28) 3095 (34) 3046 (8) 3105 (11) 3046 (14) 3105 (16) 3515 (100) 3526 (100) 2585 (100) 2591 (88)

A" 217(1) 214(< I) 215(3) 212(2) 259 (2) 254 (3) 256 (6) 251 (8) 427 (18) 421 (13) 427 m 429 (3) 422 (3) 427 m 447 (75) 443 (95) 407 s 344 (90) 345 (99) 570 (11) 566 (15) 563 (3) 558 (5) 689 (20) 671 (34) 706 m 671 (4) 650 (7) 670 m 755 (47) 735 (55) 740 vs 754 (94) 734 (100) 741 vs 770 (9) 753 (8) 779 s 770 (14) 752 (1 I) 776 m 862 ( < I) 833 (1) 862 (< 1) 833 ( < 1) 968 (1) 912 (1) 968 (2) 912 (2) 999(< I) 9 5 0 ( < 1) 1005 m 999(< I) 950 (< 1) 1004m

a Intensities are given after the frequencies. Calculated intensities (in parentheses) are arbitrarily scaled such that the greatest intensity is equal to 100. Observed intensities are labelled qualitatively (w = weak, m = medium, s = strong, vs = very strong). b Scaled by a factor of 0.895. c Scaled by a factor of 0.963.

G. Fischer et al. / Chemical Physics Letters 262 (1996) 689-698 693

In agreement with previously reported ab initio computations the 2H-tautomer is found to be the more stable at the higher levels of theory, that is MP2/6-31G * and the density functional B3LYP/6- 31G*, but not at the HF/6-31G* (see Table 3). However, the energy differences are small.

Force constants and harmonic vibrational frequen- cies were obtained by analytic second differentiation of the energy with respect to nuclear displacements. The scaled vibrational frequencies of BTZ and d- BTZ at the HF/6-31G* and B3LYP/6-31G* levels are listed in Table 4 for the 1H-tautomer. In Table 5

Table 5 Calculated and observed spectra of the 2H-tautomer of benzotriazole and N-deutero-benzotriazole a

Benzotriazole N- Deuterobenzotriazole

HF/6-31G* b B3LYP/6-31G* c MP2/6-31G' a Exp. HF/6-31G* b B3LYP/6-31G* c MP2/6-31G* a Exp. (beam) (beam)

A t 5 2 3 ( < 1 ) 530 ( <1 ) 519 ( <1 ) 521 (<1 ) 528(<1) 5 1 7 ( < 1 ) 747 (6) 767 (4) 755 (3) 752 m 744 (9) 764 (6) 752 (5) 752 m 944 (2) 953 (9) 925 (5) 944 (4) 949 (15) 921 (9) 989(7) 988(< 1) 980(1) 1022m 985(10) 987(< 1) 980(2) 1020m

1118(9) 1133(3) 1122(< 1) 1145m 1111(20) 1119(11) 1103(< 1) 1144m 1166(9) 1147(4) 1143(1) 1209? 1154(8) 1139(< 1) 1138(< 1) l180m 1311 ( < 1) 1293(1) 1287(2) 1303m 1307(< 1) 1293(1) 1286(< I) 1302w 1396(2) 1381 (4) 1375(6) 1414m,b 1396(4) 1379(9) 1374(11) 1414m,b 1443(< 1) 1436(< 1) 1433(2) 1459m 1441 ( < 1) 1435(< 1) 1432(4) 1450m 1557 (4) 1549 (4) 1540 (2) 1593 m 1558 (6) 1548 (6) 1540 (4) 1596 m 3024 (8) 3084 (10) 3052 (4) 3024 (13) 3084 (16) 3052 (7) 3046(5) 3104(7) 3068(5) 3045(8) 3104(11) 3068(9) 3501 (100) 3509(100) 3424(100) 2577 (100) 2580(100) 2519(100)

A 2 263 262 240 263 262 240 563 566 422 563 566 422 782 769 580 782 769 580 862 845 768 861 845 768

1007 957 840 1007 957 840

Bj 214(1) 219(2) 204(3) 208(3) 213(5) 198(5) 439(5) 430(5) 387(4) 436(13) 428(13) 386(8) 632(11) 593(21) 586(9) 517(31) 481 (43) 490(46) 719 (24) 691 (31) 682 (6) 659 (1) 637 (2) 612 (4) 676 w 759 (54) 742 (59) 695 (99) 742 vs 757 (67) 742 (80) 692 (96) 742 vs 983(< 1) 924(1) 830(3) 1007m 983(< 1) 924(1) 830(< 1) 1005m

B 2 407 (4) 409 (5) 396 (3) 401 (7) 403 (10) 390 (6) 605 (3) 612 (2) 601 (1) 605 (4) 612 (3) 601 (2) 888 (1) 880 (4) 857 (6) 868 (10) 858 (18) 836 (18)

1124(6) 1115(6) 1097(3) 1125w 1128(1) 1120(1) 1104(1) 1124w 1205 (40) 1183 (47) 1178 (4) 1209 vs 983 (30) 972 (26) 961 (8) 1245(18) 1233(9) 1249(2) 1264m 1239(3) 1233(12) 1221 ( < 1) 1263m 1348 (9) 1317 (6) 1318 (8) 1278 m 1348 (26) 1300 (41) 1288(2) 1278 m 1484(< 1) 1450(2) 1427(5) 1376(32) 1334(1) 1362(1) 1334m 1538(< 1) 1497(< 1) 1462(4) 1504w 1523(1) 1495(1) 1462(5) 1463 m? 1655(1) 1619(< 1) 1596(< 1) 1621 m 1654(1) 1619(< 1) 1596(< 1) 1623 m 3011 (3) 3072 (3) 3041 (2) 3010 (5) 3072 (5) 3041 (3) 3044(8) 3101 (12) 3065 (6) 3043 (13) 3101 (20) 3065 (10)

a Intensities are given after the frequencies. Calculated intensities (in parentheses) are arbitrarily scaled such that the greatest intensity is equal to 100. Observed intensities are labelled qualitatively (w = weak, m = medium, s = strong, vs = very strong, b = broad). b Scaled by a factor of 0.895, c Scaled by a factor of 0.963. d Scaled by a factor of 0.9427.

694 G. Fischer et a l . / Chemical Physics Letters 262 (1996) 689-698

the scaled vibrational frequencies for the 2H-tautomer are listed at the HF/6-31G*, B3LYP/6-31G* and MP2/6-31G* levels of theory. The scaling factors used have been found to give good fits with experi- mental data for a wide range of compounds, 0.895 for the HF/6-31G*, 0.9427 for the MP2/6-31G ~ calculations [16], and 0.963 for the density functional B3LYP/6-31G* [17].

4. Spectra

The FTIR spectra of BTZ when present as a finely divided solid in KBr, and as isolated cooled molecules in a supersonic jet are presented in Figs. 2 and 3, respectively. Although closely similar, it will be noted that the solid state and beam spectra show some important differences. The moderately strong bands at 1384 and 706 cm-l in the KBr spectrum are almost absent in the beam spectrum. They are replaced by very weak bands at 1383 and 703 cm- t.

A very qualitative estimate based on relative band heights would suggest that in the beam the IH- tautomer (see below) is present to the extent of only about 10 percent. These bands, according to the ab initio calculations, have large N - H in-plane-bend, and N - H out-of-plane-bend characters, respectively. These are precisely the vibrations that could be expected to be most effected in the two tautomers. The ab initio calculations indicate that in the beam the N -H in-plane-bend character is largely found in the band at 1209 cm-~, down from 1384 cm-~ in KBr.

For d-BTZ similar observations are made, Figs. 4 and 5. The bands at 866 and 670 cm- t are absent or very largely reduced. According to our ab initio calculations these bands are in the region of the N -D in-plane and out-of-plane bends, respectively. The differences between the spectra are too large to be attributed to environmental effects. Most bands are only shifted by a few cm-~ in going from the solid KBr to the gas phase beam. Futhermore, no evidence

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Wavenumber (cm-0 Fig. 2. The solid state FT-IR spectrum of benzotriazole in KBr.

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650

G. Fischer et al . / Chemical Physics Letters 262 (1996) 689-698 695

is found in the spectra for the presence of dimers or larger clusters, either mixed with the carrier gas, or p u r e .

Two conclusions follow from these observations. First, we are dealing with two different forms of BTZ, most probably corresponding to the two tau- tomers. Second, the forms in the cooled beam and in the KBr disk are largely pure to the extent that in the FFIR spectra one form predominates in each case. It has been shown that the 1H-tautomer is present in the solid state [10], it thus follows that the 2H- tautomer must be the BTZ species in the cooled molecular beam. This is in agreement with the obser- vations of Berden et al. [1], who assigned the elec- tronic transition in jet-cooled BTZ to the 2H- tautomer. It is also in agreement with the theoretical calculations which show the 2H-tautomer to be the more stable, Table 3, and with the temperature de- pendent studies of Catalan et al. [3] on the UV absorption of gaseous BTZ and 1- and 2-methyl-ben- zotriazoles.

At the HF/6-31G* and B3LYP/6-31G* levels only small differences are obtained between the cal- culated frequencies despite significant differences between the calculated optimized geometries, Tables 1 and 2. For the in-plane vibrations all the calcula- tions are in good agreement. However, for the out- of-plane modes the MP2 calculations give substan- tially lower frequencies. Overall the agreement be- teen the ab initio computed vibrational frequencies and absorption intensities and the measured FTIR spectra is excellent. However, there are a few bands, particularly in the d-BTZ spectrum for which it has not been possible to achieve a fully acceptable as- signment. Thus, no computed band could be found to match the observed strong band at 1212 cm- t in the d-BTZ spectrum.

Upon mono-deuteration the computations and measured spectra (Tables 4 and 5, and Figs. 4 and 5) show that, as expected, the majority of vibrations only experience very small frequency changes. In KBr prominent bands that are present for d-BTZ

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696 G. Fischer et al. / Chemical Physics Letters 262 (1996) 689-698

only occur at 1333, 1234, 1180, 866 and 670, while bands at 1100, 1078 and 706 are very much stronger for BTZ and d-BTZ. Prominent bands present only in the d-BTZ beam spectrum occur at 1334, 1180 and 833 cm - t . The fact that almost all the BTZ bands also appear in the d-BTZ spectrum suggests that N-deuteration may be less than complete, al- though repeated deuteration did not give evidence of this.

Clearly, the bands experiencing the most promi- nent changes upon deuteration are those which are characterized by a large component of - N - H / - N - D motion. Thus the computations indicat that the fre- quency of the N - H stretching vibration is reduced from about 3500 to about 2580 cm- l, and the out-of plane H-wag is reduced from about 690 and 450 to about 670 and 340 cm-~ respectively for the 1H- tautomer upon deuteration, and largely from about 630 to about 520 for the 2H-tautomer. For both the KBr and beam spectra of d-BTZ broad bands are

seen in the range 2000 to 2500 cm-l which may be attributed to the N - D stretch. No clear band can be identified as the N - H stretch in the BTZ spectra. Since the structured absorption in the range 2700 to 2300 cm-1 in the beam spectra is the same for the two isotopic species it is proposed that a broad band corresponding to the N - H stretch could possibly be overlapped by this structured absorption and thus hidden. In the KBr spectra some out-of-plane bend character is assigned to the bands at 706 and 670 cm- 1 for BTZ and d-BTZ respectively. Correspond- ing bands have not been clearly identified in the beam spectra. Spectral cut-off in the beam was 600 cm- ~.

For the in-plane H-wag the correlation between BTZ and d-BTZ is more difficult to make because more than one band has a large component of in-plane H(D) wag. For the lH-tautomer at the HF/6-31G* level, the bands at 1391 and 1064 cm- l have a large component of in-plane N - H wag and correlate with

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G. Fischer et al . / Chemical Physics Letters 262 (1996) 689-698 697

the bands at 902 and 849 cm-I of d-BTZ which have a similarly large component of N-D wag. In the KBr spectra, N - H and N-D in-plane bends are assigned to bands at 1384 and 866 cm -1 respec- tively. Similar observations can be made for the 2 H-tautomer, in the beam spectra. The N - H in-plane bend is assignmed to the strong band at 1209 cm- (HF/6-31G* puts it largely at 1205 cm- ~ and strong, Table 5). For d-BTZ the bands at 983 and 868 cm- are calculated to have a large component of N-D in-plane bend, but no strong bands are seen at these frequencies.

The IR spectra of BTZ and the mono-deuterated species in the solid state (KBr) have been previously reported [9,12] and empirical assignments of the bands, based on inspection and similarities to related molecules, were given. In the solid state BTZ is present as the 1H-tautomer, and its only element of symmetry corresponds to the molecular plane. There are 11 out-of-plane and 25 in-plane vibrations. A tentative assignment of most of the prominent bands

based largely on the calculations is given in Table 4. Comparison with the published IR spectrum of benz- imidazole [18] and indole [19] support the conclusion that the 1H-tautomer of BTZ is the species in the solid state. As noted by Jalviste and Treshchalov [5], benzimidazole, indole and BTZ (1H-tautomer) have very similar geometric configurations and atomic masses, and are therefore expected to have similar vibrational frequencies.

The vapour phase IR spectrum of BTZ has not been reported, and because of its very low volatility the spectrum would normally have to be taken at elevated temperatures. At the higher termperatures the equilibrium favours the 1H-tautomer, thus ensur- ing that such an approach would not permit a suc- cessful measurement of the IR spectrum of the 2H- tautomer. The spectra of the molecules in a super- sonic jet overcomes the problem of high tempera- tures. The beam spectra are presented in Figs. 3 and 5. As noted above, they show significant difference to the KBr spectra, and we attribute them to the

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0

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0

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1525 1350 1175 I000

Wavenumber (cm-1) Fig. 5. The FT-IR spectrum of N-deuterobenzotriazole in a molecular beam.

¢,1 tin,

I

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650

698 G. Fischer et al. / Chemical Physics Letters 262 (1996) 689-698

2H-tautomer. The 2H-tautomer belongs to the Czv symmetry point group and its 36 fundamental vibra- tions can be reduced to 13a~ + 5 a z + 6 b l + 12b 2. Assignments based on the correlation between most of the prominent bands in the spectrum and the calculations are given in Table 5.

The 2H-tautomer could be expected to be less aromatic, more quinonoid, than the 1H-tautomer in the way that isobenzofuran is less aromatic than benzofuran [20], and this could be expected to have ramifications on the spectra. However, both the higher level ab initio calculations, particularly the MP2/6-31G*, and the observed spectra indicate that this is not the case. The carbon ring modes at about 1600 c m - I are little changed in the KBr and beam spectra indicating that no clear argument can be made for greater bond-length alternation, or aro- maticity of one or the other tautomer. This is in agreement with the calculations where significant differences are also predicted in the spectra, but the differences are not such as to allow for conclusive differentation in terms of a greater quinonoid struc- ture between the two tautomers.

Acknowledgement

We are grateful to D.M. Smith and Dr. J. Mc- Grady for help with some of the calculations, and GF wishes to thank the Australian Research Council for its support. The calculations for this work were carried out on the Fujitsu VP of the ANU Supercom- puter Facility.

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