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The vibrational spectra and conformations of ethylbenzene
Spectrochimica Acta Part A 60 (2004) 843–853
The vibrational spectra and conformations of ethylbenzene A.I. Fishman a,∗ , A.E. Klimovitskii a , A.I. Skvortsov a , A.B. Remizov b b
a Kazan State University, Kremlevskaya Street 18, Kazan 420008, Russia Kazan State Technological University, K. Marx Street 68, Kazan 420015, Russia
Received 25 November 2002; received in revised form 26 June 2003; accepted 26 June 2003
Abstract Infrared spectra (4000–250 cm−1 ) of the liquid, amorphous, crystalline solids and solutions in liquid krypton and Raman spectra (2500– 20 cm−1 ) of the liquid as well as the amorphous and crystalline solids of ethylbenzene and its deuterated analogue—ethylbenzene-d10 have been recorded. The spectra indicate that in the liquid and amorphous solids a small amount of a second conformer is present, whereas only one conformer remains in the crystalline phases. Assignments of the observed band wave numbers are discussed by comparison with normal mode wave numbers and IR and RS intensities calculated from ab initio 6-31G force fields and optimised geometries for both conformers for two species. All of the normal modes of conformers have been assigned. © 2003 Elsevier B.V. All rights reserved. Keywords: Ethylbenzene; Ethylbenzene-d10 ; Infrared spectra; Raman spectra; Ab initio; Conformational analysis
1. Introduction The different conformations of the ethylbenzene (EB) have been the subject of several experimental and theoretical investigations. The literature contains considerable experimental data on the conformational properties of EB, which have been studied extensively by various experimental techniques. Experimental as well as theoretical methods imply contradicting results for conformational properties of EB. An electron diffraction study [1,2] indicates that the one conformation of EB exists (the bond C(sp3 )–C(sp3 ) is in the plane, which is orthogonal to the benzene ring plane—o-conformation). Indirect evidence from electronic spectra (time-of-flight spectroscopy) [3], low- [4] and high-resolution microwave spectroscopy [5] of EB were interpreted in terms of the o-conformation. This conformation was found to be the most stable by 1 H NMR spectroscopy [6,7]. Using NMR spectroscopy, the authors [6] concluded that the barrier of internal rotation in EB is insensitive to the polarity and internal pressure of the solvent (acetone-d6 , CCl4 , CS2 and perfluoromethylcyclohexane).
∗
Corresponding author. Tel.: +7-8432-315439; fax: +7-8432-382163. E-mail address: [email protected] (A.I. Fishman).
1386-1425/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S1386-1425(03)00309-3
Vibrational spectroscopy, infrared (IR) and Raman were used to solving conformational behaviour of EB, too. The assignment of the observed IR and Raman bands of EB and other alkylbenzenes in spectral interval 650–200 cm−1 was made in [8]. Raman spectra of toluene, EB and n-propylbenzene in a liquid state were also measured at room temperature by Yamakita [9]; assignment of some experimental bands was made. IR spectra of EB have been investigated in liquid, solid phase and CS2 solution [10]. Two solid phases A and B, which was distinguished by IR spectra, were observed. Authors analysed spectral region from 1050 to1030 cm−1 , where two bands at 1034 and 1040 cm−1 were recorded. In the liquid phase the relative intensities of these bands varied with temperature. One band of this doublet is present only in solid phase A, whereas another band is present in the solid phase B. Authors maintain that the IR spectrum of the liquid sample can be obtained by superposition of the spectra of both crystalline forms. This fact was interpreted by the existence of a different conformations in each crystal phase, and in the liquid phase EB exists as a mixture of two conformers—orthogonal and planar (in the latter, C(sp3 )–C(sp3 ) bond is in the plane of the benzene ring). Normal coordinate analysis, using GF-matrix method, was carried out only for o-conformer [11,12]. Ab initio calculations [6,13,14] predicted the only stable o-conformation of EB. Caminati [4] carried out ab initio
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calculations and concluded that planar conformation is a transition state. These results are in contrast with outputs of work [15], where shallow local minimum was founded for p-conformation. It should be noted, that the obtained shape of potential function for internal rotation depends upon the computational method (see, e.g. [15]). Using relative large basis, a minimum at o-conformation and a barrier height at p-conformation of 4.9 (4) kJ mol−1 at the MP2 (B3LYP) level and a flat potential in the vicinity of the p-conformation were obtained [16]. Thus, IR spectroscopy data indicate that EB exists as a mixture of two conformers and these results are not consistent with some experimental and theoretical results. We returned to vibrational spectra analysis and conformational behaviour of EB in this study. Spatial structures of possible conformers of EB are closely related to each other, thus, overlapping of IR and Raman lines, belong to different conformers, should be expected. In order to get a clearer insight into the actual conformational behaviour of EB it was decided to undertake a more elaborate vibrational study involving infrared and Raman spectra deuteroanalogue of EB–EB-d10 . Rather similar nature of internal rotation in these compounds, from one hand, and noticeable frequency shifts under deuteration, from the other, allowed us to make the tenable conclusion about the conformational composition of these molecules. We have analysed the spectra of the liquid and solid phases (glass and crystals) both of EB and EB-d10 . The solid phases of EB were produced both by slow cooling of thin film of liquid between CsI and KBr plates, and by depositing the sample on the KBr window, which was cooled down to 80 K. In order to obtain more complete description of the normal modes of EB and EB-d10 , as well as to support our vibrational assignments, a normal coordinate analysis, used ab initio calculations, both o- and p-conformers was also performed. The results of these spectroscopic and theoretical studies are reported herein.
1.5–3 cm−1 , depending on the bandwidths of the absorption bands. The cells were cooled with liquid nitrogen. The temperature was measured by the thermocouple and was kept constant manually to ensure a standard deviation smaller than 0.5 K. The crystallisation of the films between CsI and KBr plates was observed visually between crossed polarisers. A polariser on the basis of transparent diffraction gratings [17] was used for analysis of the IR bands dichroism. The infrared spectra of EB in liquid krypton were recorded using a Bruker IFS 66v spectrometer equipped with a globar source, a Ge/KBr beamsplitter and LN2 cooled broad band MCT detector. The interferograms were recorded with a resolution of 0.5 cm−1 and Fourier transformed using a Blackman–Harris apodization function. A zero-filling factor of four was used for all experiments. The krypton was obtained from L’Air Liquide and had a stated purity of 99.998%. The cell for liquids used in this study is made of stainless steel, has a pathlength of 70 mm and is equipped with wedged silicon windows. The cell is cooled with bursts of liquid nitrogen. The mole fraction of EB is estimated to be 1:4500. The Raman spectra were recorded by a computercontrolled spectrometer DFS-52 with double monochromator. The system of weak light signals registration included the cooled photomultiplier working in photon counting regime. The Ar+ (488 nm, 50 mW) and He–Ne (632.8 nm, 40 mW) lasers were used for excitation. The Raman scattering was observed under 90◦ angle. Glan prism was placed in front of the slit of the monochromator. The direction of polarisation of the exiting wave was turned using a halfwave plate. The depolarisation ratios are measured accurate to 0.03. The calibration of the monochromator was made using plasma’s lines of the lasers. The solid samples of the EB and EB-d10 for Raman experiments were produced by cooling of the liquid, which was placed into the glass capillary with the internal diameter about 2 mm.
2. Experimental The samples of EB and EB-d10 (Aldrich Chemical Company, purity 99 and 98%, respectively) were used without further purification, Tmp = 178 K. The infrared spectra were recorded using a FTIR Bruker Vector 22 spectrometer (400–4000 cm−1 ) equipped with a globar source, a Ge/KBr beamsplitter and MCT detector. The interferograms were recorded with a resolution of 1 cm−1 and Fourier transformed using a Blackman–Harris apodization function. A zero-filling factor of four was used for all experiments. The number of scans used in a particular experiment was varied from 64 to 128. The IR spectra in the region 200–400 cm−1 were recorded using a computer-controlled SPECORD M-80 spectrometer. The spectra were registered at spectral slit widths of
3. Calculations Ab initio calculations for both conformers of EB were performed in [9] using 6-31++G(d, p) basis functions for normal coordinate analysis and assignment only experimental Raman bands. Density Functional Theory (DFT) calculations for oand p-conformation of EB and EB-d10 were done using the Gaussian-98 suite of programs [18]. We used Becke’s three-parameter exchange functional [19] in combination with the Lee–Yang–Parr correlation functional [20] (B3LYP) and standard double-zeta (6-31G*) basis set. All stationary points were characterized as minima by analysis of the Hessian matrices. The calculated force fields were
A.I. Fishman et al. / Spectrochimica Acta Part A 60 (2004) 843–853
transformed to internal coordinates, and the scaling procedure, using scale factors have taken from [21], was applied with the use of the program described in [22]. Normal coordinate analysis of EB was carried out earlier only for o-conformation [11,12] using the GF method [23,24]. The molecule of EB has 48 normal vibrational modes. There are 38 normal vibrations in the spectral range below 1700 cm−1 . Fragment of monosubstituted benzene ring C6 H5 C(sp3 ) has local symmetry group C2v . A total of 25 vibrations of this fragment, except of stretching vibration of CH, belong to symmetry types: A1 , A2 , B1 , B2 : (8A1 + 3A2 + 8B1 + 6B2 ). Fragment C2 H5 C(sp2 ) has CS symmetry. With exception of stretching vibrations of CH it has 13 vibrations, which divide into 7A and 6A modes.
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Both possible conformers of the EB molecule have a plane of symmetry and belong to the CS point group. o-Conformer has 38 normal modes, 21A and 17A , of these 14 vibrations of benzene ring (8A1 + 6B2 ) should be A , whereas 11 vibrations (3A2 + 8B1 ) should be A . In the spectral range below 1700 cm−1 , p-conformer has 23A and 15A normal modes, of these 16 vibrations of benzene ring (8A1 + 8B1 ) should be A species, whereas 9 vibrations (3A2 + 6B2 ) should be A species. In the Raman spectra of the liquid the vibrations of A and A symmetry should be polarised and depolarised, respectively. A comparison between the observed and calculated infrared frequencies, intensities, depolarisation ratios and potential energy distribution (PED) for EB and EB-d10 are given in Tables 1 and 2.
Table 1 Observed IR and Raman frequencies (in cm−1 ) and assignment for ethylbenzene Infrareda
Raman
Calculated frequencies and IR intensities (Km mol−1 ) Orthogonal Planar conformer conformer 1610 A 4.3 1611 A 7.1
PEDb , % or Wilson notations
Liquid
Phase A
Phase B
Liquidc ,
Phase A
Phase B
1605 s
1602 m
1602 w
1582 w
1580 1498 1493 1485 1475
1603 m 1587 m 1583 sh
1602 m 1589 m 1580 sh
1589 A 0.7
1589 A 0.9
o, p, 8b, B1
1497d vw
1495 vw
1494 vw
1506 A 10.4
1508 A 10.9
o, p, 19a, A1
1475 vw
1582 1498 1494 1493 1474
1605 s, 0.63 1588 w 1581 m, 0.72
1462 sh
1462 s
1475 A 6.3 1466 A 5.2
o, 36␣(CH3 ) + 30␣(CH2 ) p, 83␣(CH3 ) o, p, 80␣(CH3 )
1447 A 4.6 1463 A 4.3
o, 51␣(CH2 ) + 23␣(CH3 ) p, 72␣(CH2 ) o, p, 19b, B1
1496 vs
vw m m sh vw
vw w w vw vw
1461 w
1475 A 1.9 1463 vw
1463 A 0.6
1458 vw 1453 vw
1453 vw
1456 A 5.1 1464 A 7.0
1462d w, sh
1455 vs
1455 vs
1454 s
1455d sh
1441 sh
1440 vw
1443 m, br
1446 m 1442 m
1449 m 1439 m
1402 vw 1385 sh 1377 m
1403 vw 1386 vw 1374 m
1448 s 1438 vw 1397 vw, br 1370 m
1373 vw
1372 vw
1369 w
1345 vw 1331 w
1367 m 1345 vw 1334 vw
1322 vw
1323 vw
1326 vw 1318 vw
1323 w, br
1310 w 1296 vw 1244 vw
1311 vw 1297 vw 1243 vw
1296 vw 1243 vw
1204 vw
vw w, br sh w w sh vw w m
1204 1194 1184 1179
ab
1156 1150 1111 1091
w sh w w
1206 1195 1192 1179 1163 1158 1152 1112 1092
1081 1068 1064 1052 1038
vw w w vw s
1068 1064 1052 1040
1190 sh 1178 w
1344 w, br
p, 43␣(CH3 ) + 40(CH3 ) o, 34␣(CH3 ) + 45(CH3 )
1337 A 0.4
1327 A 2.0 1357 A 11.6
1181 A 0.7
o, p, o, o, p, o, o,
1323 w
1318 vw
1296d vw 1241 vw
1244 vw
1297 vw 1242 vw
1202 s, 0.04
1207 m
1204 m
1252 A 0.0 1197 A 0.3
1178 m, 0.55
1177 m
1178 A 0.1
1183 A 1.3
o, p 9a, A1
1156 w
1158 A 0.0
1159 A 0.0
o, p, 9b, B1
1100 A 2.0 1089 A 0.7
o, p 18b, B1 p, 44pa + 31(CH3 )
1338 A 4.6 1313 A 0.0
1153 vw
1155 m, 0.75
1178 m 1165 vw 1152 w
1118 vw, br 1094 w
1093d vw
1093 vw
1094 vw
1100 A 2.1
1065 w, 0.12
1068 sh 1062 w 1054 vw
1061 w 1052 vw
1065 A 8.4
1062 w 1051 vw 1033 a ∼ = b, s
1393 A 2.9
1338 vw 1332 vw
1334d vw
vw vw, br sh vw
1388 A 1.0
o, p, 8a, A1
1292 A 3.7 1277 A 0.3
p 14, B1 27pa + 25(CH2 ) 42(CH2 ) + 32pa p, 3, B1 48(CH2 ) + 26pa + 11(CH3 ) 40(CH2 ) + 16(CH3 ) + 16Qpp p, 2, A1
o, 48(CH3 ) + 22Q 1041 A 1.9
p, 28(CH3 ) + 25Q
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Table 1 (Continued ) Infrareda
Raman
Liquid
Phase A
Phase B
Liquidc ,
Phase A
Phase B
1030 s
1038 a < b, m 1031 a < b 1029 a > b, w
1031 a ∼ = b, m
1031 vs, 0.04
1030 m
1030 m
1022 vw 1003 vw 986 vw
1003 vs, 0.04 992 w
1003 vs 992 w 982vw
1009 sh 1003 vs 991 vw 981 vw
966 m, 0.38
968 vw 960 m
963 m
906 vw, 0.56
907 w
906 vw
846 vw
1004 vw 983 vw
1003 vw 987 vw 983 vw
Calculated frequencies and IR intensities (Km mol−1 ) Orthogonal Planar conformer conformer 1039 A 2.1 1029 A 2.3 1032 A 2.5 996 A 0.1 973 A 0.1
PEDb , % or Wilson notations
o, 32pa + 28Qpp + 20(CH3 ) o, p, 18a, A1
997 A 0.2 974 A 1.1 963 A 0.1
o, o, p, o,
947 A 1.2
948 A 0.0
o, p, 17a, A2
904 A 1.4
902 A 2.0
o, p, 17b, B2
846 A 0.0
845 A 0.0
o, p, 10a, A2
790 A 17.1
p, 32(CH2 ) + 27(CH3 ) + 21 o, 42(CH2 ) + 38(CH3 )
944 A 0.0
p, 12, A1 p, 5, B2 56Q + 13(CH3 ) 72Q
973 vw 964 w
906 m
968 w 960 w 910 sh 908 m 856 vw
845 vw
963 952 910 907 903 898 858 853
vw vw vw sh m w vw w
790 m
846 vw 810 vw, br 790 w,br
786 w
842 vw, 0.75 810 vw 788 vw
772 s 749 vs
776 sh 772 m 754 vs
770 m 746 vs
770 s, 0.05 751 m, 0.07
740 720 700 698
sh w sh vs
556 s
734 ∗ ∗ 699 688 621 558 556
490 m 472 vw 403 vw
493 s ∗ 414 vw
360 vw 296 vw 280 vw
359 vw 306 vw ∗
621 vw
vw
vs sh w sh s
734 ∗ ∗ 701 694 620
788 w
802 vw 788 vw
778 sh 773 m 756 w
770 m 751 w
779 A 0.6 769 A 6.2 751 A 20.1
sh
s m vw
557 s 492 m ∗
702d vw
708 vw
701 A 24.5 627 A 0.0
620 s, 0.75 616d vw 556 w, 0.62
622 w 617 sh 556 w
622 w 617 sh 556 vw
488 m, 0.18 484 sh 403 br, vw
494 m ∗ 407 w
492 w ∗ 422 vw
358 br, vw 302 br, vw 285 sh
359 vw 312 w ∗ 172 w
360 vw 311 vw ∗
158 vs, 0.72
163 m
167 m
559 A 4.7 491 A 3.2 406 A 0.0 349 A 0.0 305 A 0.2 209 A 0.0 135 A 0.5
753 A 1.3
o, 22 + 15Qp + 12c p, 34Qp + 13␥p + 10Qpp o, 24 + 18Qp + 10Qpp (10b, B2 )
719 A 29.7 693 A 7.5
p, 61 + 13(CH2 ) (10b, B2 ) p, 4, B2 o, 4, B2
627 A 0.0
o, p, 6b, B1
540 A 0.3 468 A 4.8 405 A 0.0 397 A 0.8 266 A 0.3 227 A 0.2 173 A 1.0
o, p, o, p, o, p, o, o, p, p, o, p, o,
17␥ + 12␥p + 10c (6a, A1 ) 22␥p + 20␥ + 10Qp (6a, A1 ) 20c + 16Qp + 16␥p (11, B2 ) 50c + 20 (11, B2 ) p, 16a, A2 21␥ + 24␥pa + 11Qp 56␥pa + 26 33␥ + 18 90 27␥ + 48␥pa 84 22c + 37p + 14 31c + 21␥ + 28p
133 s 103 s 92 sh 84 m 69 s 52 m 46 m
84 sh 80 s 65 m 43 A 0.0
12 A 0.0
o, 93p p, 58p + 22
a w: week, m: medium, s: strong, v: very, sh: shoulder, br: broad; a, b represent the intensities of the IR bands in the two orthogonal positions of a polariser; ∗ IR bands and Raman lines, which disappear during the crystallisation. b PED: potential energy distribution; notations used: Q—C(H )–C(H ); Q —C(Ph)–C(H ); Q —C(Ph)–C(Ph); —ring-puck, —C(Ph)–H out of plane; 2 3 p 2 pp c —C(Ph)–C(H2 ) out of plane; ␥—∠C(Ph)–C(H2 )–C(H3 ); ␥p —∠C(Ph)–C(Ph)–C(Ph); ␥pa —∠C(Ph)–C(Ph)–C(H2 ); p —∠H–C(Ph)–C(Ph); (CH2 ) and (CH3 )—∠C–C–H in C–CH2 and C–CH3 groups, respectively; pa —∠C(Ph)–C(H2 )–H; ␣(CH2 ) and ␣(CH3 )—∠H–C–H in the C–CH2 and C–CH3 groups, respectively; p —torsion around C(Ph)–C(H2 ); —torsion around C(H2 )–C(H3 ). c : depolarisation ratio. d Supercooled liquid (77 K).
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Table 2 Observed IR and Raman frequencies (in cm−1 ) and assignment for ethylbenzene-d10 Infrareda
Raman
Liquid
Phase A
Phase B
1604 vw
1612 w
1604 w 1594 sh
1578 1575 1571 1563 1556 1547
1582 w, br
1582 1575 1570 1563 1556 1547 1460 1453 1446 1422 1415 1410 1401 1397 1388 1384 1376 1371 1349 1341 1337 1330 1324 1313 1305 1293
sh sh m sh vw vw vw vw vw w vw vw vw vw sh m, br br vw vw vw vw s w vw vw sh
1285 1280 1270 1265 1258
w vw vw vw vw
sh sh s sh w vw
1446 w 1421 vw 1416 vw
1397 vw 1385 1376 1370 1350 1342 1338 1330 1324 1314 1304 1294
vs vw vw vw vw vw m vw vw vw sh
1285 1280 1274 1264
m sh sh vw
1231 vw 1208 sh 1201 vw
1183 vw
1154 vw 1143 vw 1121 w, br
1569 1562 1556 1546 1460 1454 1448 1423 1414
m sh vw vw vw vw vw vw vw
1401 1397 1392 1385 1376 1372 1350 1342 1340 1330 1324 1315 1306 1295 1290 1286 1278 1273 1265 1258 1230 1213 1211 1202
vw vw vw s vw vw vw vw vw s vw sh vw sh vw w vw vw br vw vw vw vw vw
1199 1193 1184 1172 1158 1155 1143 1134 1120
w vw vw vw vw vw vw vw vw
Liquid,
Calculated frequencies and IR intensities Phase A
Phase B
1610 vw 1597 vw
1611 1597 1586 1580
1570 m
Orthogonal conformer
Planar conformer
1570 m
1573 A 5.0
1573 A 7.0
o, p, 8a, A1
1558 vw 1546 w
1560 vw 1547 w
1547 A 0.1
1547 A 0.2
o, p, 8b, B1
1386 vw, 0.11 1366 vw
1383 vw 1368 vw
1382 vw
1388 A 4.4
1387 A 5.3
o, p, 19a, A1
1332 wv
1330 vw
1336 A 0.7
1338 A 1.3
o, p, 14, B1
1291 A 0.3
1292 A 1.2
o, p, 19b, B1
1200 A 4.9
p, 28Qp + 25Q (2, A1 ) o, 38Qp + 12Q (2, A1 )
1120 A 1.0
p, 23Q + 13␣(CD2 ) + 12␣(CD3 ) o, 37Q + 13␣(CD2 ) + 8(CD3 )
1578 sh 1573 m, 0.62 1561 sh 1547 w, 0.73 1467 vw 1457 vw
vw vw w, br vw
1313 vw 1293 vw 1285 vw
1286 vw
1287 vw
1202 s, 0.06
1200 s
1201 m
1184 w, 0.06
1183 w 1166 vw
1189 vw
1262 vw 1233 vw
1218 sh 1215 vw 1200 w
1193 vw, br 1183 vw 1166 vw
1188 A 0.9
1159 vw 1155 vw, br 1141 vw 1132 vw 1123 vw
1148 vw
1121 w
1120 w 1117 A 0.0
1108 sh 1093 vw, br 1079 vw 1067 s
1116 vw 1102 vw 1092 1081 1076 1066
vw vw vw s
PEDb , % or Wilson notations
1115 1105 1100 1090
vw vw sh vw
1077 vw 1066 s
1116 w 1102 w, sh
1069 m, 0.08
1103 vw
1065 w
1068 m
1068 A 2.0
1068 A 3.2
o, 45␣(CD2 ) + 25␣(CD3 ) p, 37␣(CD3 ) + 36␣(CD2 )
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Table 2 (Continued ) Infrareda Liquid
Raman Phase A
Phase B
1054 vs
1058 vw
1053 s
1050 sh
1050 vs
1044 sh
1043 sh
1026 w
1034 vw 1028 w
1046 1040 1032 1028
1022 sh 1009 w 1002 vw
1022 vw 1014 vw 1006 vw 1001 vw 993 vw
970 vw
972 m
961 vw 950 vw 931 vw
960 951 930 926 920
m vw vw vw vw
910 w
912 w
895 vw
897 vw 886 vw
872 sh 866 vw
840 s
820 s 815 sh 790 vw
755 vw
866 861 852 846 843 839
w, br vw vw sh m m
821 819 796 790
sh s vw vw
sh vw sh m
962 vs, 0.03 952 w 931 vw
933 927 920 918 910
vw vw vw vw m
903 897 890 869 865 861
sh vw vw vw vw vw
843 841 835 826 820 815 801 792
sh m m sh s s vw w
668 vw
667 vw
660 vw, br
661 643 633 ∗ 605 598
vw vw
1057 w
1051 w
1051 w
1029 w
1029 s
Orthogonal conformer 1054 A 3.8 1053 A 1.9 1045 A 0.0 1030 A 3.1
Planar conformer 1057 A 3.3
1047 A 0.9
p, o, p, o, o,
1019 A 6.4
o, 24pa + 22(CD2 ) + 11(CD3 ) p, 21p + 18(CD3 ) + 11pa
972 A 0.1 953 A 0.0
o, 52(CD2 ) + 42(CD3 ) p, 53(CD3 ) + 41(CD2 ) o, p, 12, A1
905 A 0.4
o, 33(CD3 ) + 16Q + 11␥ + 10c p, 50pa + 17c
888 A 1.4
p, 28Q + 20(CD3 ) + 7pa
1054 A 3.4
82␣(CD3 ) 92␣(CD3 ) 73␣(CD3 ) + 18␣(CD2 ) 79␣(CD3 ) + 12␣(CD2 ) p 3, B1
1010 vw
961 m
691 m
632 m 626 sh 605 vw
1028 m, 0.05
975 w
710 702 699 692
vw vw s
1058 sh
1045 sh
976 sh 970 m
710 701 696 691
vw vw vw m
Phase B
1015 vw 1006 vw 1001 vw
757 vw 749 vw 734 w
708 vw, br
1055 m, 0.55
Phase A
PEDb , % or Wilson notations
1022 sh
752 vw, br 741 vw 738 sh
736 w
Liquid,
Calculated frequencies and IR intensities
vw vw vw m
975 vw 970 m
974 w
961 vs 952 vw 930 vw
961 vs 951 vw
953 A 0.0
911 m
905 A 4.7
920 vw 910 s, 0.02
870 s, 0.24
841 s, 0.21
910 m
868 sh 865 m
886 vw 866 s
841 A 0.9 865 A 0.0
841 A 1.5 868 A 0.1
o, p, 9b, B1 o, p, 9a, A1
845 w 841 w 835 w
848 m 843 s
839 A 1.2
838 A 1.2
o, p, 18a , A1
A 1.0 A 1.8 A 0.2 A 0.0
825 A 2.2 803 A 0.0
770 A 0.0
770 A 0.8
o, p, 18b, B1 o, p, 5, B2 o, 46pa o, 39 + 20Q + 6(CD3 ) p,23(CD3 ) + 14Q + 19␥pa + 12(CD2 ) o, p, 17a, A2
734 A 1.0
725 A 0.9
o, p, 17b, B2
684 A 1.8
o, 19Qp + 12p + 12(CD3 ) + 12␥p p, 20Q + 15Qp + 12␥p + 11(CD3 ) + 12p
658 A 0.0
o, p, 10a, A2
623 A 3.6
o,10b, B2 p, 24 + 22 + 14c (10b, B2 )
822 vw
792 br, vw
825 vw 819 vw 794 vw
796 vw
736 vw
736 vw
707 vw, br
710 vw
739 w 716 vw 709 vw
692 vs, 0.07
692 s
697 w 694 m
667 vw 665 vw
825 817 814 783
686 A 2.8
767 A 0.0
667 w 658 w, 0.7
643 vw 628 m ∗ 608 vw
973 A 0.2
630 w, 0.74
633 w
661 m 637 w 634 w
658 A 0.0 631 A 1.6
A.I. Fishman et al. / Spectrochimica Acta Part A 60 (2004) 843–853
849
Table 2 (Continued ) Infrareda
Raman
Calculated frequencies and IR intensities
Liquid
Phase A
Phase B
Liquid,
Phase A
Phase B
Orthogonal conformer
Planar conformer
596 584 574 568 553 550 537
595 586 ∗ 569 553 551 ∗ 506 504 500 496 494
594 582 ∗ 571 554 551 ∗
vw m
597m, 0.72 583 sh
593 w 581 w
598 w 589 vw
602 A 0.0 579 A 0.4
600 A 0.0
vw sh vs, br
551 w, 0.62
556 sh 553 w
553 vw
549 A 15.1
497 w
494 A 6.1
vw vw, br vw, br vw sh vs sh
504 vw
494 vs
vw w,br w sh vs, br vw vw sh s vs
503 w 497 sh 495 vs
569 A 4.6
o, p, 6b, B1 o, 46(CD3 ) + 40(CD2 ) p, 30 + 29(CD3 ) + 27(CD2 )
535 A 16.2
o, 4, B2 p, 4, B2
503 vw
495 m, 0.22
494 w
490 A 0.6 489 sh 438 s
442 vs
489 sh 447 sh 442 vs
408 vw
∗
∗
PEDb , % or Wilson notations
437 w, 0.17
439 m
444 m
413 vw
∗ 370 vw
∗
357 br, vw
273 br
405 A 6.5
356 vw 306 273 221 164
w w m w
139 vs, 0.72
148 m
123 sh
123 m 93 s 78 sh 71 m 62 vw 35 w 28 w
298 280 223 164 152 147
440 A 8.5
vw w vw w w w
353 A 0.0 292 A 0.0 273 A 0.1
154 A 0.0 120 A 0.4
358 A 0.7 352 A 0.0 205 A 0.6 199 A 0.1
o, 19␥p + 12Qp + 10 + 10␥ (6a, A1 ) p, 20␥p + 16Qp + 13␥ + 10c (6a, A1 ) o, 21c + 11Qp + 18 + 11 (11, B2 ) p, 40c + 25 (11, B2 ) p, o, o, o, p, p,
25␥pa + 21␥ + 9Q + 7Qp p, 16a, A2 56␥pa + 21 32 + 31␥ 63 37pa + 25␥ + 22
144 A 0.5
o, 88 p, 38 + 25 + 16c o, 30c + 28 + 23␥
9 A 0.0
o, 93p p, 80p
85 sh 76 m
39 sh 30 m
34 A 0.0
a
See notations in Table 1. Notations used: Q—C(D2 )–C(D3 ); Qp —C(Ph)–C(D2 ); Qpp —C(Ph)–C(Ph); —ring-puck, —C(Ph)–C(D) out of plane; c —C(Ph)–C(D2 ) out of plane; ␥—∠C(Ph)–C(D2 )–C(D3 ); ␥p —∠C(Ph)–C(Ph)–C(Ph); ␥pa —∠C(Ph)–C(Ph)–C(D2 ); p —∠D–C(Ph)–C(Ph); (CD2 ) and (CD3 )—∠C–C–D in C–CD2 and C–CD3 groups, respectively; pa —∠C(Ph)–C(D2 )–D; ␣(CD2 ) and ␣(CD3 )—∠D–C–D in C–CD2 and C–CD3 groups, respectively; p —torsion around C(Ph)–C(D2 ); —torsion around C(D2 )–C(D3 ). b
Wilson notations [24] are used for assignment benzene ring vibrations in spectral interval 1700–300 cm−1 .
4. Results and discussion Conformational behaviour of EB and EB-d10 was analysed by comparison of IR and Raman spectra of the liquid (or glassy state) and solid phases and results of normal coordinate analysis. It has been pointed out that an identification of frozen bands in crystal phase spectra of molecules is difficult, because of Davydov’s splitting and some changes of band intensities and peak positions during crystallisation take place. To gain more valuable information, we used both
the investigations of the solid films formed by condensing of vapours on a cold window (glassy state) and the analysis of the IR band’s dichroism in the spectra of the crystals. The observed IR and Raman bands (below 1700 cm−1 ) of the EB and EB-d10 and the results of the normal coordinate analysis are listed in the Tables 1 and 2. Some experimental spectra of liquid and crystalline EB and EB-d10 are given in Figs. 1–6. As in [10], two different solid phases of EB were obtained. The metastable solid phase A is observed in the temperature range 100–150 K. This phase observed both after annealing of the solid film, produced by depositing the vapours of EB on the cooled window and by cooling of liquid film (5 K min−1 ) down 150 K with subsequent annealing. The
A.I. Fishman et al. / Spectrochimica Acta Part A 60 (2004) 843–853
Absorbance
Intensity
850
a
a
b
b 0.1 c
c
1060
1040
1020 -1
Wavenumber / cm
400
360
320
280 Wavenumber / cm
Fig. 1. IR spectra of liquid EB at 170 K (a); crystal phase A at 120 K (b); and crystal phase B at 160 K (c).
Absorbance
irreversible phase transition to solid phase B occurs at 150 K. Phase B is stable in the temperature range from 80 K up to melting point (178 K). Let us consider the region 1050–1020 cm−1 in the spectra of EB (Fig. 1), which is discussed in details in [10]. Two absorption bands with maximum at 1030 and 1038 cm−1 in the spectra of liquid were assigned earlier to different conformers [10]. However, according to our calculations, both conformations have two fundamentals in this spectral range (Table 1). One of these vibrations has A symmetry and belongs to characteristic vibration of benzene ring (18a type in Wilson notations). The strong IR band at 1030 cm−1 and polarised Raman line at 1031 cm−1 have to be assigned to this vibration of both conformations. The second fundamental belongs to vibration of the ethyl fragment. This vibration
-1
Fig. 3. Raman spectra of liquid EB at 300 K with parallel (a) and perpendicular (b) polariser settings; and crystal phase B at 80 K (c).
is not observed in Raman spectra [9,25]. We assign the IR band at 1038 cm−1 to this vibration. In the IR spectra of solid phase A in this region there are two doublets (1029, 1031 and 1038, 1040 cm−1 ) with opposite dichroic ratio of components (Table 1). So, in phase A there are bands of fundamental 18a and vibration of ethyl fragment of o-conformer, having Davydov’s splitting. The doublet 1031, 1033 cm−1 is observed in the spectra of solid phase B (Fig. 1c, Table 1). These components cannot be explained by Davydov’s splitting because they have close dichroic ratio in the IR spectra. Therefore, in solid phase B, bands at 1031 and 1033 cm−1 belong to analogous fundamentals. Thus, the obtained data
a
0.1
b
400
360
320
280 -1
Wavenumber / cm
Fig. 2. IR spectra of liquid EB at 300 K (a); and crystal phase A at 120 K (b).
Fig. 4. IR spectra of liquid EB at 170 K (a); and crystal phase A at 120 K (b).
Intensity
A.I. Fishman et al. / Spectrochimica Acta Part A 60 (2004) 843–853
a
b c 440
420
400 -1
Wavenumber / cm
Fig. 5. Raman spectra of liquid EB-d10 at 300 K with parallel (a) and perpendicular (b) polariser settings; and crystal phase B at 80 K (c).
Absorbance
show that during crystallisation of the EB, there is no freezing of the bands in this spectral region. Calculations show that the fundamental 18a in EB-d10 has frequency 839 cm−1 for o-conformer and 838 cm−1 for p-conformer (Table 2). So, it is reasonable to assign strong IR band at 840 cm−1 and strong polarised Raman line at 841 cm−1 to the characteristic vibration 18a [26]. The frequencies of the vibrations of ethyl fragment in EB-d10 , which are similar to vibration with frequency 1038 cm−1 of EB, are quite different for conformers and equal 814 and 888 cm−1 , for o- and p-conformers, respectively (Table 2). However, comparison of this spectral range
X
0.5
a 0.5
b
640
600
560
440
400 -1
Wavenumber / cm
Fig. 6. IR spectra of liquid EB-d10 at 170 K (a) and crystal phase B at 120 K (b).
851
of the liquid and solid states of EB-d10 shows, that no IR or Raman bands disappear upon going from one state to another. The careful analysis of IR and Raman spectral data allowed the freezing of some vibrations in other spectral regions to find out, during crystallisation of EB and EB-d10 . In the spectral region 420–250 cm−1 there are four very weak IR bands and Raman lines in spectra of the liquid phase of EB: 280, 296, 360 and 403 cm−1 (IR, Fig. 2a, Table 1) and 285, 302, 358 and 403 cm−1 (Raman, Fig. 3a and b, Table 1). Low temperature studies showed that IR band at 280 cm−1 and Raman line at 285 cm−1 disappear upon crystallisation. It is seen that the calculated frequencies in this range significantly differ for o- and p-conformations. As ab initio calculations predict, that the o-conformer is more stable [4,14,15], we assign the vanished IR band at 280 cm−1 and Raman line at 285 cm−1 to the torsion vibration around C(H2 )–C(H3 ) bond of p-conformer (Table 1). In the spectral interval from 510 to 460 cm−1 , the doublets 472, 490 cm−1 in IR (Fig. 4a) and 484, 488 cm−1 in Raman spectra (Table 1) are observed in the liquid EB. Low-frequency components of these doublets vanish in the IR and Raman spectra of the solid phases A and B (Fig. 4b, Table 1). As it was pointed out in [26], the frequency of the vibration 11, B2 (in Wilson notation) falls into this spectral interval in spectra of monosubstituted benzenes. This vibration is not characteristic, and its frequency depends on the substituting group. Our calculations showed, that PED of this vibration includes considerable contribution of —ring-puck and c —C(Ph)–C(H2 ) out of plane coordinates. Besides, frequency of this vibration depends on of ethyl fragment orientation in respect of the benzene ring plain. The 11-type vibration gives rise to IR bands in the liquid phase of EB-d10 at 438 and 408 cm−1 and Raman lines at 437 and 413 cm−1 for o- and p-conformers, respectively (Fig. 5). As in EB low frequency components, which disappear during crystallisation, are assigned to pconformer. Comparison of IR spectra of liquid and solid phases of EB shows that there are freezing of the shoulder at 700 cm−1 and weak band at 720 cm−1 in the range 760–680 cm−1 (Table 1, Fig. 4a and b). These lines are not observed in the Raman spectra. Two characteristic vibrations of benzene ring (4 and 10b) [26] fall in this region and it seems likely to ascribe strong-intense IR bands at 698 and 749 cm−1 to these vibrations, respectively. The calculation shows that the frequency of 4-type vibration differs in 8 cm−1 in oand p-conformers. Therefore, shoulder at 700 cm−1 , which vanished upon crystallisation, is assigned to 4-type vibration of p-conformer. The similar situation was observed in IR spectra of isotopomer (Table 2). According to the normal analysis, a 4-type vibration has frequencies 535 and 549 cm−1 for pand o-conformations, respectively. Very intensive IR band at 550 cm−1 is detected in this spectral range; at the same time
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its shoulder at 537 cm−1 disappeared in both solid phases. Consequently, we assign the band at 537 cm−1 to vibration 4 of the p-conformer. The frequency of benzene ring vibration of EB, 10b is sensitive to orientation of ethyl fragment. Calculated frequency of the o-conformer (751 cm−1 ) differs from that one of the p-conformer (719 cm−1 ). Thus, we assign the band at 720 cm−1 in IR spectra of EB, which vanished during crystallisation, to mode 10b of p-conformer. The frequencies of 10b vibrations shift to 631 and 623 cm−1 upon deuteration, for o- and p-conformers, respectively (Table 2). There are two bands at 632 and 626 cm−1 observed in this region in the IR spectra of liquid EB-d10 . The shoulder at 626 cm−1 , which disappears under crystallisation, we assigned to 10b vibration of p-conformer, whereas band at 632 cm−1 belongs to the same type vibration of the o-conformer. The very weak IR band at 574 cm−1 disappears in spectral interval 600–560 cm−1 of EB-d10 during crystallisation (Table 2, Fig. 6). According to normal coordinate analysis, vibrations of ethyl fragment of each conformer fall into this range. So, band at 574 cm−1 can be assigned to vibration of ethyl fragment of p-conformer. Thus, the comparison of liquid and crystal phases of EB and its isotopomer—EB-d10 , shows that some IR and Raman bands disappear during crystallisation. Normal coordinate analysis predicts that these bands belong to conformation-sensitive vibrations. This indicates that ethylbenzene in liquid phase exists as a mixture of two conformers. The determination of the standard enthalpy difference was not pursued, because the bands of p-conformer have low intensities and noticeable overlapping with neighbour bands. According to calculations, absorption coefficients of the bands, of p-conformer (at 719, 693, 468 cm−1 for EB and at 623, 569, 535, 405 cm−1 for EB-d10 ), have great values. However, corresponding IR bands have weak intensities. Based on it, we can conclude that proportion of p-conformer in liquid is small enough. The electron diffraction [1,2], time-of-flight spectroscopy [3], low- [4] and high-resolution microwave spectroscopy [5] of EB have shown that in the gas phase only o-conformation exists. The appearance of small portion of second conformer in liquid EB probably is due to influence of aggregative state. It is interesting to analyse the variation of conformational composition upon going from liquid to gaseous phase. However, remarkable complication of spectral contours in gas phase, is caused by rotational structure, does not allow to observe the absorption bands of p-conformer. In this case, analysis of IR spectra of samples dissolved in liquid rare gases is a powerful tool. These solvents have great chemical inertness and transparency in a wide spectral region, that allows to use a very small concentrations of samples (typical value near 1:5000 in mole fraction [27]). Sufficiently large liquid phase temperature intervals can be achieved at slightly increased pressure [28,29].
Fig. 7. IR spectra of liquid EB at 135 K (a) and EB dissolved in LKr at 135 K (b).
We have investigated the spectra of EB, dissolved in liquid krypton (LKr). It showed that intensities of bands, which belong to less stable p-conformer, are significantly less than corresponding ones in the spectra of liquid (or glassy state) at the same temperature. As an example the spectral region from 780 to 450 cm−1 is shown in Fig. 7. The intensities of p-conformer bands at 472 and 720 cm−1 are strongly reduced in LKr. Therefore, the conformational equilibrium in LKr as well as in the gas phase considerably shifts towards o-conformer in rare gas solution. This fact is in a good correlation with experimental data, which were obtained for gas phase by other experimental methods. Acknowledgements Financial support from Russian Foundation for Basic Research (grant no. 02-03-32404) and INTAS (grant no. 172). We are grateful to Dr. S. Katsyuba for ab initio calculations and Prof. V. Furer for fruitful discussion. We also acknowledge Prof. B.J. van der Veken, who helped us recording the spectra of solutions in LKr. References [1] P. Scharfenberg, B. Rozsondai, I. Hargittai, Z. Naturforsch. A 35 (1980) 431. [2] P. Scharfenberg, J. Chem. Phys. 77 (1982) 4791. [3] P.J. Breen, E.R. Bernstein, J.I. Seeman, J. Chem. Phys. 87 (1987) 3269. [4] W. Caminati, D. Damiani, G. Corbelli, B. Velino, C.W. Bock, Mol. Phys. 74 (N4) (1991) 885. [5] B. Mate, R.D. Suenram, C. Lugez, J. Chem. Phys. 113 (2000) 192. [6] T. Schaefer, G.H. Penner, R. Sebastian, Can. J. Chem. 65 (1987) 873. [7] C. Algieri, F. Castiglione, G. Celebre, G. De Luca, M. Longeri, J.W. Emsley, PCCP 2 (2000) 3405.
A.I. Fishman et al. / Spectrochimica Acta Part A 60 (2004) 843–853 [8] L. Verdonck, G.P. van der Kelen, Spectrochim. Acta A 28A (1972) 51. [9] K. Ohno, J. Kimura, Y. Yamakita, Chem. Phys. Lett. 342 (2001) 207. [10] J. Stokr, H. Pivcova, B. Schneider, S. Dirlikov, J. Mol. Struct. 12 (1972) 45. [11] M.A. Kovner, A.M. Bogomolov, Optika i Spectrosc. (in Russian) 1 (N3) (1956) 364. [12] L.A. Gribov, V.A. Dement’ev, A.T. Todorovskii, Interpretirovaniye kolebatelnye spectry alkanov, alkenov i proizvodnikh benzola (in Russian), Nauka, Moscow, 1986. [13] J.B. Lagowski, I.G. Csizmadia, G.J. Vancso, J. Mol. Struct. (Theochem.) 258 (1992) 341. [14] R. Benassi, F. Taddei, J. Mol. Struct. (Theochem.) 418 (1997) 59. [15] O. Farkas, S.J. Salpietro, P. Csaszar, I.G. Csizmadia, J. Mol. Struct. (Theochem.) 367 (1996) 25. [16] J. Klocker, A. Karpfen, P. Wolschann, Chem. Phys. Lett. 367 (2003) 566. [17] V.A. Yakovlev, F.M. Gerasimov, Optiko-mech. Prom. (in Russian) 10 (1964) 28. [18] M.J. Frisch, G.W. Trucks, M. Head-Gordon, P.M.W. Gill, M.W. Wong, J.B. Foresman, B.G. Johnson, H.B. Schlegel, M.A. Robb, E.S. Replogle, R. Gomperts, J.L. Andres, K. Raghavachari, J.S. Binkley,
[19] [20] [21] [22] [23] [24] [25]
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C. Gonzales, R.L. Martin, D.J. Fox, D.J. DeFrees, J. Baker, J.J.P. Steward, J.A. Pople, GAUSSIAN-98, Gaussian Inc., Pittsburgh, PA, 1998. A.D. Becke, J. Chem. Phys. 98 (1993) 5648. C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. G. Rauhut, P. Pulay, J. Phys. Chem. 99 (1995) 3093. V.A. Sipachev, J. Mol. Struct. (Theochem.) 121 (1985) 143. M.V. Volkenstein, M.A. El’yashevich, B.I. Stepanov, Kolebaniya molekul (Vibrations of Molecules), Gostehizdat, Moscow, 1949. E.B. Wilson, J.C. Decius, P.C. Cross, Molecular Vibrations, McGraw-Hill, New-York, 1955. H.E. Sterin, V.T. Alexanyan, G.N. Zhizhin, Catalog spectrov kombinatsionnogo rasseyania uglevodorodov (Catalogue of Raman Spectra of Hydrocarbons), Nauka, Moskow, 1976, p. 360. L.M. Sverdlov, M.A. Kovner, E.P. Krainov, Vibrational Spectra of Polyatomic Molecules, Halstead Press, New York, 1974. A.I. Fishman, W.A. Herrebout, B.J. van der Weken, J. Phys. Chem. A 106 (2002) 4536. M.O. Bulanin, J. Mol. Struct. 19 (1973) 59. B.J. van der Veken, Rare gases, in: R. Fausto (Ed.), Low Temperature Molecular Spectroscopy, Kluwer Academic Publishers, Dordrecht, 1996.
The vibrational spectra and conformations of ethylbenzene A.I. Fishman a,∗ , A.E. Klimovitskii a , A.I. Skvortsov a , A.B. Remizov b b
a Kazan State University, Kremlevskaya Street 18, Kazan 420008, Russia Kazan State Technological University, K. Marx Street 68, Kazan 420015, Russia
Received 25 November 2002; received in revised form 26 June 2003; accepted 26 June 2003
Abstract Infrared spectra (4000–250 cm−1 ) of the liquid, amorphous, crystalline solids and solutions in liquid krypton and Raman spectra (2500– 20 cm−1 ) of the liquid as well as the amorphous and crystalline solids of ethylbenzene and its deuterated analogue—ethylbenzene-d10 have been recorded. The spectra indicate that in the liquid and amorphous solids a small amount of a second conformer is present, whereas only one conformer remains in the crystalline phases. Assignments of the observed band wave numbers are discussed by comparison with normal mode wave numbers and IR and RS intensities calculated from ab initio 6-31G force fields and optimised geometries for both conformers for two species. All of the normal modes of conformers have been assigned. © 2003 Elsevier B.V. All rights reserved. Keywords: Ethylbenzene; Ethylbenzene-d10 ; Infrared spectra; Raman spectra; Ab initio; Conformational analysis
1. Introduction The different conformations of the ethylbenzene (EB) have been the subject of several experimental and theoretical investigations. The literature contains considerable experimental data on the conformational properties of EB, which have been studied extensively by various experimental techniques. Experimental as well as theoretical methods imply contradicting results for conformational properties of EB. An electron diffraction study [1,2] indicates that the one conformation of EB exists (the bond C(sp3 )–C(sp3 ) is in the plane, which is orthogonal to the benzene ring plane—o-conformation). Indirect evidence from electronic spectra (time-of-flight spectroscopy) [3], low- [4] and high-resolution microwave spectroscopy [5] of EB were interpreted in terms of the o-conformation. This conformation was found to be the most stable by 1 H NMR spectroscopy [6,7]. Using NMR spectroscopy, the authors [6] concluded that the barrier of internal rotation in EB is insensitive to the polarity and internal pressure of the solvent (acetone-d6 , CCl4 , CS2 and perfluoromethylcyclohexane).
∗
Corresponding author. Tel.: +7-8432-315439; fax: +7-8432-382163. E-mail address: [email protected] (A.I. Fishman).
1386-1425/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S1386-1425(03)00309-3
Vibrational spectroscopy, infrared (IR) and Raman were used to solving conformational behaviour of EB, too. The assignment of the observed IR and Raman bands of EB and other alkylbenzenes in spectral interval 650–200 cm−1 was made in [8]. Raman spectra of toluene, EB and n-propylbenzene in a liquid state were also measured at room temperature by Yamakita [9]; assignment of some experimental bands was made. IR spectra of EB have been investigated in liquid, solid phase and CS2 solution [10]. Two solid phases A and B, which was distinguished by IR spectra, were observed. Authors analysed spectral region from 1050 to1030 cm−1 , where two bands at 1034 and 1040 cm−1 were recorded. In the liquid phase the relative intensities of these bands varied with temperature. One band of this doublet is present only in solid phase A, whereas another band is present in the solid phase B. Authors maintain that the IR spectrum of the liquid sample can be obtained by superposition of the spectra of both crystalline forms. This fact was interpreted by the existence of a different conformations in each crystal phase, and in the liquid phase EB exists as a mixture of two conformers—orthogonal and planar (in the latter, C(sp3 )–C(sp3 ) bond is in the plane of the benzene ring). Normal coordinate analysis, using GF-matrix method, was carried out only for o-conformer [11,12]. Ab initio calculations [6,13,14] predicted the only stable o-conformation of EB. Caminati [4] carried out ab initio
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calculations and concluded that planar conformation is a transition state. These results are in contrast with outputs of work [15], where shallow local minimum was founded for p-conformation. It should be noted, that the obtained shape of potential function for internal rotation depends upon the computational method (see, e.g. [15]). Using relative large basis, a minimum at o-conformation and a barrier height at p-conformation of 4.9 (4) kJ mol−1 at the MP2 (B3LYP) level and a flat potential in the vicinity of the p-conformation were obtained [16]. Thus, IR spectroscopy data indicate that EB exists as a mixture of two conformers and these results are not consistent with some experimental and theoretical results. We returned to vibrational spectra analysis and conformational behaviour of EB in this study. Spatial structures of possible conformers of EB are closely related to each other, thus, overlapping of IR and Raman lines, belong to different conformers, should be expected. In order to get a clearer insight into the actual conformational behaviour of EB it was decided to undertake a more elaborate vibrational study involving infrared and Raman spectra deuteroanalogue of EB–EB-d10 . Rather similar nature of internal rotation in these compounds, from one hand, and noticeable frequency shifts under deuteration, from the other, allowed us to make the tenable conclusion about the conformational composition of these molecules. We have analysed the spectra of the liquid and solid phases (glass and crystals) both of EB and EB-d10 . The solid phases of EB were produced both by slow cooling of thin film of liquid between CsI and KBr plates, and by depositing the sample on the KBr window, which was cooled down to 80 K. In order to obtain more complete description of the normal modes of EB and EB-d10 , as well as to support our vibrational assignments, a normal coordinate analysis, used ab initio calculations, both o- and p-conformers was also performed. The results of these spectroscopic and theoretical studies are reported herein.
1.5–3 cm−1 , depending on the bandwidths of the absorption bands. The cells were cooled with liquid nitrogen. The temperature was measured by the thermocouple and was kept constant manually to ensure a standard deviation smaller than 0.5 K. The crystallisation of the films between CsI and KBr plates was observed visually between crossed polarisers. A polariser on the basis of transparent diffraction gratings [17] was used for analysis of the IR bands dichroism. The infrared spectra of EB in liquid krypton were recorded using a Bruker IFS 66v spectrometer equipped with a globar source, a Ge/KBr beamsplitter and LN2 cooled broad band MCT detector. The interferograms were recorded with a resolution of 0.5 cm−1 and Fourier transformed using a Blackman–Harris apodization function. A zero-filling factor of four was used for all experiments. The krypton was obtained from L’Air Liquide and had a stated purity of 99.998%. The cell for liquids used in this study is made of stainless steel, has a pathlength of 70 mm and is equipped with wedged silicon windows. The cell is cooled with bursts of liquid nitrogen. The mole fraction of EB is estimated to be 1:4500. The Raman spectra were recorded by a computercontrolled spectrometer DFS-52 with double monochromator. The system of weak light signals registration included the cooled photomultiplier working in photon counting regime. The Ar+ (488 nm, 50 mW) and He–Ne (632.8 nm, 40 mW) lasers were used for excitation. The Raman scattering was observed under 90◦ angle. Glan prism was placed in front of the slit of the monochromator. The direction of polarisation of the exiting wave was turned using a halfwave plate. The depolarisation ratios are measured accurate to 0.03. The calibration of the monochromator was made using plasma’s lines of the lasers. The solid samples of the EB and EB-d10 for Raman experiments were produced by cooling of the liquid, which was placed into the glass capillary with the internal diameter about 2 mm.
2. Experimental The samples of EB and EB-d10 (Aldrich Chemical Company, purity 99 and 98%, respectively) were used without further purification, Tmp = 178 K. The infrared spectra were recorded using a FTIR Bruker Vector 22 spectrometer (400–4000 cm−1 ) equipped with a globar source, a Ge/KBr beamsplitter and MCT detector. The interferograms were recorded with a resolution of 1 cm−1 and Fourier transformed using a Blackman–Harris apodization function. A zero-filling factor of four was used for all experiments. The number of scans used in a particular experiment was varied from 64 to 128. The IR spectra in the region 200–400 cm−1 were recorded using a computer-controlled SPECORD M-80 spectrometer. The spectra were registered at spectral slit widths of
3. Calculations Ab initio calculations for both conformers of EB were performed in [9] using 6-31++G(d, p) basis functions for normal coordinate analysis and assignment only experimental Raman bands. Density Functional Theory (DFT) calculations for oand p-conformation of EB and EB-d10 were done using the Gaussian-98 suite of programs [18]. We used Becke’s three-parameter exchange functional [19] in combination with the Lee–Yang–Parr correlation functional [20] (B3LYP) and standard double-zeta (6-31G*) basis set. All stationary points were characterized as minima by analysis of the Hessian matrices. The calculated force fields were
A.I. Fishman et al. / Spectrochimica Acta Part A 60 (2004) 843–853
transformed to internal coordinates, and the scaling procedure, using scale factors have taken from [21], was applied with the use of the program described in [22]. Normal coordinate analysis of EB was carried out earlier only for o-conformation [11,12] using the GF method [23,24]. The molecule of EB has 48 normal vibrational modes. There are 38 normal vibrations in the spectral range below 1700 cm−1 . Fragment of monosubstituted benzene ring C6 H5 C(sp3 ) has local symmetry group C2v . A total of 25 vibrations of this fragment, except of stretching vibration of CH, belong to symmetry types: A1 , A2 , B1 , B2 : (8A1 + 3A2 + 8B1 + 6B2 ). Fragment C2 H5 C(sp2 ) has CS symmetry. With exception of stretching vibrations of CH it has 13 vibrations, which divide into 7A and 6A modes.
845
Both possible conformers of the EB molecule have a plane of symmetry and belong to the CS point group. o-Conformer has 38 normal modes, 21A and 17A , of these 14 vibrations of benzene ring (8A1 + 6B2 ) should be A , whereas 11 vibrations (3A2 + 8B1 ) should be A . In the spectral range below 1700 cm−1 , p-conformer has 23A and 15A normal modes, of these 16 vibrations of benzene ring (8A1 + 8B1 ) should be A species, whereas 9 vibrations (3A2 + 6B2 ) should be A species. In the Raman spectra of the liquid the vibrations of A and A symmetry should be polarised and depolarised, respectively. A comparison between the observed and calculated infrared frequencies, intensities, depolarisation ratios and potential energy distribution (PED) for EB and EB-d10 are given in Tables 1 and 2.
Table 1 Observed IR and Raman frequencies (in cm−1 ) and assignment for ethylbenzene Infrareda
Raman
Calculated frequencies and IR intensities (Km mol−1 ) Orthogonal Planar conformer conformer 1610 A 4.3 1611 A 7.1
PEDb , % or Wilson notations
Liquid
Phase A
Phase B
Liquidc ,
Phase A
Phase B
1605 s
1602 m
1602 w
1582 w
1580 1498 1493 1485 1475
1603 m 1587 m 1583 sh
1602 m 1589 m 1580 sh
1589 A 0.7
1589 A 0.9
o, p, 8b, B1
1497d vw
1495 vw
1494 vw
1506 A 10.4
1508 A 10.9
o, p, 19a, A1
1475 vw
1582 1498 1494 1493 1474
1605 s, 0.63 1588 w 1581 m, 0.72
1462 sh
1462 s
1475 A 6.3 1466 A 5.2
o, 36␣(CH3 ) + 30␣(CH2 ) p, 83␣(CH3 ) o, p, 80␣(CH3 )
1447 A 4.6 1463 A 4.3
o, 51␣(CH2 ) + 23␣(CH3 ) p, 72␣(CH2 ) o, p, 19b, B1
1496 vs
vw m m sh vw
vw w w vw vw
1461 w
1475 A 1.9 1463 vw
1463 A 0.6
1458 vw 1453 vw
1453 vw
1456 A 5.1 1464 A 7.0
1462d w, sh
1455 vs
1455 vs
1454 s
1455d sh
1441 sh
1440 vw
1443 m, br
1446 m 1442 m
1449 m 1439 m
1402 vw 1385 sh 1377 m
1403 vw 1386 vw 1374 m
1448 s 1438 vw 1397 vw, br 1370 m
1373 vw
1372 vw
1369 w
1345 vw 1331 w
1367 m 1345 vw 1334 vw
1322 vw
1323 vw
1326 vw 1318 vw
1323 w, br
1310 w 1296 vw 1244 vw
1311 vw 1297 vw 1243 vw
1296 vw 1243 vw
1204 vw
vw w, br sh w w sh vw w m
1204 1194 1184 1179
ab
1156 1150 1111 1091
w sh w w
1206 1195 1192 1179 1163 1158 1152 1112 1092
1081 1068 1064 1052 1038
vw w w vw s
1068 1064 1052 1040
1190 sh 1178 w
1344 w, br
p, 43␣(CH3 ) + 40(CH3 ) o, 34␣(CH3 ) + 45(CH3 )
1337 A 0.4
1327 A 2.0 1357 A 11.6
1181 A 0.7
o, p, o, o, p, o, o,
1323 w
1318 vw
1296d vw 1241 vw
1244 vw
1297 vw 1242 vw
1202 s, 0.04
1207 m
1204 m
1252 A 0.0 1197 A 0.3
1178 m, 0.55
1177 m
1178 A 0.1
1183 A 1.3
o, p 9a, A1
1156 w
1158 A 0.0
1159 A 0.0
o, p, 9b, B1
1100 A 2.0 1089 A 0.7
o, p 18b, B1 p, 44pa + 31(CH3 )
1338 A 4.6 1313 A 0.0
1153 vw
1155 m, 0.75
1178 m 1165 vw 1152 w
1118 vw, br 1094 w
1093d vw
1093 vw
1094 vw
1100 A 2.1
1065 w, 0.12
1068 sh 1062 w 1054 vw
1061 w 1052 vw
1065 A 8.4
1062 w 1051 vw 1033 a ∼ = b, s
1393 A 2.9
1338 vw 1332 vw
1334d vw
vw vw, br sh vw
1388 A 1.0
o, p, 8a, A1
1292 A 3.7 1277 A 0.3
p 14, B1 27pa + 25(CH2 ) 42(CH2 ) + 32pa p, 3, B1 48(CH2 ) + 26pa + 11(CH3 ) 40(CH2 ) + 16(CH3 ) + 16Qpp p, 2, A1
o, 48(CH3 ) + 22Q 1041 A 1.9
p, 28(CH3 ) + 25Q
846
A.I. Fishman et al. / Spectrochimica Acta Part A 60 (2004) 843–853
Table 1 (Continued ) Infrareda
Raman
Liquid
Phase A
Phase B
Liquidc ,
Phase A
Phase B
1030 s
1038 a < b, m 1031 a < b 1029 a > b, w
1031 a ∼ = b, m
1031 vs, 0.04
1030 m
1030 m
1022 vw 1003 vw 986 vw
1003 vs, 0.04 992 w
1003 vs 992 w 982vw
1009 sh 1003 vs 991 vw 981 vw
966 m, 0.38
968 vw 960 m
963 m
906 vw, 0.56
907 w
906 vw
846 vw
1004 vw 983 vw
1003 vw 987 vw 983 vw
Calculated frequencies and IR intensities (Km mol−1 ) Orthogonal Planar conformer conformer 1039 A 2.1 1029 A 2.3 1032 A 2.5 996 A 0.1 973 A 0.1
PEDb , % or Wilson notations
o, 32pa + 28Qpp + 20(CH3 ) o, p, 18a, A1
997 A 0.2 974 A 1.1 963 A 0.1
o, o, p, o,
947 A 1.2
948 A 0.0
o, p, 17a, A2
904 A 1.4
902 A 2.0
o, p, 17b, B2
846 A 0.0
845 A 0.0
o, p, 10a, A2
790 A 17.1
p, 32(CH2 ) + 27(CH3 ) + 21 o, 42(CH2 ) + 38(CH3 )
944 A 0.0
p, 12, A1 p, 5, B2 56Q + 13(CH3 ) 72Q
973 vw 964 w
906 m
968 w 960 w 910 sh 908 m 856 vw
845 vw
963 952 910 907 903 898 858 853
vw vw vw sh m w vw w
790 m
846 vw 810 vw, br 790 w,br
786 w
842 vw, 0.75 810 vw 788 vw
772 s 749 vs
776 sh 772 m 754 vs
770 m 746 vs
770 s, 0.05 751 m, 0.07
740 720 700 698
sh w sh vs
556 s
734 ∗ ∗ 699 688 621 558 556
490 m 472 vw 403 vw
493 s ∗ 414 vw
360 vw 296 vw 280 vw
359 vw 306 vw ∗
621 vw
vw
vs sh w sh s
734 ∗ ∗ 701 694 620
788 w
802 vw 788 vw
778 sh 773 m 756 w
770 m 751 w
779 A 0.6 769 A 6.2 751 A 20.1
sh
s m vw
557 s 492 m ∗
702d vw
708 vw
701 A 24.5 627 A 0.0
620 s, 0.75 616d vw 556 w, 0.62
622 w 617 sh 556 w
622 w 617 sh 556 vw
488 m, 0.18 484 sh 403 br, vw
494 m ∗ 407 w
492 w ∗ 422 vw
358 br, vw 302 br, vw 285 sh
359 vw 312 w ∗ 172 w
360 vw 311 vw ∗
158 vs, 0.72
163 m
167 m
559 A 4.7 491 A 3.2 406 A 0.0 349 A 0.0 305 A 0.2 209 A 0.0 135 A 0.5
753 A 1.3
o, 22 + 15Qp + 12c p, 34Qp + 13␥p + 10Qpp o, 24 + 18Qp + 10Qpp (10b, B2 )
719 A 29.7 693 A 7.5
p, 61 + 13(CH2 ) (10b, B2 ) p, 4, B2 o, 4, B2
627 A 0.0
o, p, 6b, B1
540 A 0.3 468 A 4.8 405 A 0.0 397 A 0.8 266 A 0.3 227 A 0.2 173 A 1.0
o, p, o, p, o, p, o, o, p, p, o, p, o,
17␥ + 12␥p + 10c (6a, A1 ) 22␥p + 20␥ + 10Qp (6a, A1 ) 20c + 16Qp + 16␥p (11, B2 ) 50c + 20 (11, B2 ) p, 16a, A2 21␥ + 24␥pa + 11Qp 56␥pa + 26 33␥ + 18 90 27␥ + 48␥pa 84 22c + 37p + 14 31c + 21␥ + 28p
133 s 103 s 92 sh 84 m 69 s 52 m 46 m
84 sh 80 s 65 m 43 A 0.0
12 A 0.0
o, 93p p, 58p + 22
a w: week, m: medium, s: strong, v: very, sh: shoulder, br: broad; a, b represent the intensities of the IR bands in the two orthogonal positions of a polariser; ∗ IR bands and Raman lines, which disappear during the crystallisation. b PED: potential energy distribution; notations used: Q—C(H )–C(H ); Q —C(Ph)–C(H ); Q —C(Ph)–C(Ph); —ring-puck, —C(Ph)–H out of plane; 2 3 p 2 pp c —C(Ph)–C(H2 ) out of plane; ␥—∠C(Ph)–C(H2 )–C(H3 ); ␥p —∠C(Ph)–C(Ph)–C(Ph); ␥pa —∠C(Ph)–C(Ph)–C(H2 ); p —∠H–C(Ph)–C(Ph); (CH2 ) and (CH3 )—∠C–C–H in C–CH2 and C–CH3 groups, respectively; pa —∠C(Ph)–C(H2 )–H; ␣(CH2 ) and ␣(CH3 )—∠H–C–H in the C–CH2 and C–CH3 groups, respectively; p —torsion around C(Ph)–C(H2 ); —torsion around C(H2 )–C(H3 ). c : depolarisation ratio. d Supercooled liquid (77 K).
A.I. Fishman et al. / Spectrochimica Acta Part A 60 (2004) 843–853
847
Table 2 Observed IR and Raman frequencies (in cm−1 ) and assignment for ethylbenzene-d10 Infrareda
Raman
Liquid
Phase A
Phase B
1604 vw
1612 w
1604 w 1594 sh
1578 1575 1571 1563 1556 1547
1582 w, br
1582 1575 1570 1563 1556 1547 1460 1453 1446 1422 1415 1410 1401 1397 1388 1384 1376 1371 1349 1341 1337 1330 1324 1313 1305 1293
sh sh m sh vw vw vw vw vw w vw vw vw vw sh m, br br vw vw vw vw s w vw vw sh
1285 1280 1270 1265 1258
w vw vw vw vw
sh sh s sh w vw
1446 w 1421 vw 1416 vw
1397 vw 1385 1376 1370 1350 1342 1338 1330 1324 1314 1304 1294
vs vw vw vw vw vw m vw vw vw sh
1285 1280 1274 1264
m sh sh vw
1231 vw 1208 sh 1201 vw
1183 vw
1154 vw 1143 vw 1121 w, br
1569 1562 1556 1546 1460 1454 1448 1423 1414
m sh vw vw vw vw vw vw vw
1401 1397 1392 1385 1376 1372 1350 1342 1340 1330 1324 1315 1306 1295 1290 1286 1278 1273 1265 1258 1230 1213 1211 1202
vw vw vw s vw vw vw vw vw s vw sh vw sh vw w vw vw br vw vw vw vw vw
1199 1193 1184 1172 1158 1155 1143 1134 1120
w vw vw vw vw vw vw vw vw
Liquid,
Calculated frequencies and IR intensities Phase A
Phase B
1610 vw 1597 vw
1611 1597 1586 1580
1570 m
Orthogonal conformer
Planar conformer
1570 m
1573 A 5.0
1573 A 7.0
o, p, 8a, A1
1558 vw 1546 w
1560 vw 1547 w
1547 A 0.1
1547 A 0.2
o, p, 8b, B1
1386 vw, 0.11 1366 vw
1383 vw 1368 vw
1382 vw
1388 A 4.4
1387 A 5.3
o, p, 19a, A1
1332 wv
1330 vw
1336 A 0.7
1338 A 1.3
o, p, 14, B1
1291 A 0.3
1292 A 1.2
o, p, 19b, B1
1200 A 4.9
p, 28Qp + 25Q (2, A1 ) o, 38Qp + 12Q (2, A1 )
1120 A 1.0
p, 23Q + 13␣(CD2 ) + 12␣(CD3 ) o, 37Q + 13␣(CD2 ) + 8(CD3 )
1578 sh 1573 m, 0.62 1561 sh 1547 w, 0.73 1467 vw 1457 vw
vw vw w, br vw
1313 vw 1293 vw 1285 vw
1286 vw
1287 vw
1202 s, 0.06
1200 s
1201 m
1184 w, 0.06
1183 w 1166 vw
1189 vw
1262 vw 1233 vw
1218 sh 1215 vw 1200 w
1193 vw, br 1183 vw 1166 vw
1188 A 0.9
1159 vw 1155 vw, br 1141 vw 1132 vw 1123 vw
1148 vw
1121 w
1120 w 1117 A 0.0
1108 sh 1093 vw, br 1079 vw 1067 s
1116 vw 1102 vw 1092 1081 1076 1066
vw vw vw s
PEDb , % or Wilson notations
1115 1105 1100 1090
vw vw sh vw
1077 vw 1066 s
1116 w 1102 w, sh
1069 m, 0.08
1103 vw
1065 w
1068 m
1068 A 2.0
1068 A 3.2
o, 45␣(CD2 ) + 25␣(CD3 ) p, 37␣(CD3 ) + 36␣(CD2 )
848
A.I. Fishman et al. / Spectrochimica Acta Part A 60 (2004) 843–853
Table 2 (Continued ) Infrareda Liquid
Raman Phase A
Phase B
1054 vs
1058 vw
1053 s
1050 sh
1050 vs
1044 sh
1043 sh
1026 w
1034 vw 1028 w
1046 1040 1032 1028
1022 sh 1009 w 1002 vw
1022 vw 1014 vw 1006 vw 1001 vw 993 vw
970 vw
972 m
961 vw 950 vw 931 vw
960 951 930 926 920
m vw vw vw vw
910 w
912 w
895 vw
897 vw 886 vw
872 sh 866 vw
840 s
820 s 815 sh 790 vw
755 vw
866 861 852 846 843 839
w, br vw vw sh m m
821 819 796 790
sh s vw vw
sh vw sh m
962 vs, 0.03 952 w 931 vw
933 927 920 918 910
vw vw vw vw m
903 897 890 869 865 861
sh vw vw vw vw vw
843 841 835 826 820 815 801 792
sh m m sh s s vw w
668 vw
667 vw
660 vw, br
661 643 633 ∗ 605 598
vw vw
1057 w
1051 w
1051 w
1029 w
1029 s
Orthogonal conformer 1054 A 3.8 1053 A 1.9 1045 A 0.0 1030 A 3.1
Planar conformer 1057 A 3.3
1047 A 0.9
p, o, p, o, o,
1019 A 6.4
o, 24pa + 22(CD2 ) + 11(CD3 ) p, 21p + 18(CD3 ) + 11pa
972 A 0.1 953 A 0.0
o, 52(CD2 ) + 42(CD3 ) p, 53(CD3 ) + 41(CD2 ) o, p, 12, A1
905 A 0.4
o, 33(CD3 ) + 16Q + 11␥ + 10c p, 50pa + 17c
888 A 1.4
p, 28Q + 20(CD3 ) + 7pa
1054 A 3.4
82␣(CD3 ) 92␣(CD3 ) 73␣(CD3 ) + 18␣(CD2 ) 79␣(CD3 ) + 12␣(CD2 ) p 3, B1
1010 vw
961 m
691 m
632 m 626 sh 605 vw
1028 m, 0.05
975 w
710 702 699 692
vw vw s
1058 sh
1045 sh
976 sh 970 m
710 701 696 691
vw vw vw m
Phase B
1015 vw 1006 vw 1001 vw
757 vw 749 vw 734 w
708 vw, br
1055 m, 0.55
Phase A
PEDb , % or Wilson notations
1022 sh
752 vw, br 741 vw 738 sh
736 w
Liquid,
Calculated frequencies and IR intensities
vw vw vw m
975 vw 970 m
974 w
961 vs 952 vw 930 vw
961 vs 951 vw
953 A 0.0
911 m
905 A 4.7
920 vw 910 s, 0.02
870 s, 0.24
841 s, 0.21
910 m
868 sh 865 m
886 vw 866 s
841 A 0.9 865 A 0.0
841 A 1.5 868 A 0.1
o, p, 9b, B1 o, p, 9a, A1
845 w 841 w 835 w
848 m 843 s
839 A 1.2
838 A 1.2
o, p, 18a , A1
A 1.0 A 1.8 A 0.2 A 0.0
825 A 2.2 803 A 0.0
770 A 0.0
770 A 0.8
o, p, 18b, B1 o, p, 5, B2 o, 46pa o, 39 + 20Q + 6(CD3 ) p,23(CD3 ) + 14Q + 19␥pa + 12(CD2 ) o, p, 17a, A2
734 A 1.0
725 A 0.9
o, p, 17b, B2
684 A 1.8
o, 19Qp + 12p + 12(CD3 ) + 12␥p p, 20Q + 15Qp + 12␥p + 11(CD3 ) + 12p
658 A 0.0
o, p, 10a, A2
623 A 3.6
o,10b, B2 p, 24 + 22 + 14c (10b, B2 )
822 vw
792 br, vw
825 vw 819 vw 794 vw
796 vw
736 vw
736 vw
707 vw, br
710 vw
739 w 716 vw 709 vw
692 vs, 0.07
692 s
697 w 694 m
667 vw 665 vw
825 817 814 783
686 A 2.8
767 A 0.0
667 w 658 w, 0.7
643 vw 628 m ∗ 608 vw
973 A 0.2
630 w, 0.74
633 w
661 m 637 w 634 w
658 A 0.0 631 A 1.6
A.I. Fishman et al. / Spectrochimica Acta Part A 60 (2004) 843–853
849
Table 2 (Continued ) Infrareda
Raman
Calculated frequencies and IR intensities
Liquid
Phase A
Phase B
Liquid,
Phase A
Phase B
Orthogonal conformer
Planar conformer
596 584 574 568 553 550 537
595 586 ∗ 569 553 551 ∗ 506 504 500 496 494
594 582 ∗ 571 554 551 ∗
vw m
597m, 0.72 583 sh
593 w 581 w
598 w 589 vw
602 A 0.0 579 A 0.4
600 A 0.0
vw sh vs, br
551 w, 0.62
556 sh 553 w
553 vw
549 A 15.1
497 w
494 A 6.1
vw vw, br vw, br vw sh vs sh
504 vw
494 vs
vw w,br w sh vs, br vw vw sh s vs
503 w 497 sh 495 vs
569 A 4.6
o, p, 6b, B1 o, 46(CD3 ) + 40(CD2 ) p, 30 + 29(CD3 ) + 27(CD2 )
535 A 16.2
o, 4, B2 p, 4, B2
503 vw
495 m, 0.22
494 w
490 A 0.6 489 sh 438 s
442 vs
489 sh 447 sh 442 vs
408 vw
∗
∗
PEDb , % or Wilson notations
437 w, 0.17
439 m
444 m
413 vw
∗ 370 vw
∗
357 br, vw
273 br
405 A 6.5
356 vw 306 273 221 164
w w m w
139 vs, 0.72
148 m
123 sh
123 m 93 s 78 sh 71 m 62 vw 35 w 28 w
298 280 223 164 152 147
440 A 8.5
vw w vw w w w
353 A 0.0 292 A 0.0 273 A 0.1
154 A 0.0 120 A 0.4
358 A 0.7 352 A 0.0 205 A 0.6 199 A 0.1
o, 19␥p + 12Qp + 10 + 10␥ (6a, A1 ) p, 20␥p + 16Qp + 13␥ + 10c (6a, A1 ) o, 21c + 11Qp + 18 + 11 (11, B2 ) p, 40c + 25 (11, B2 ) p, o, o, o, p, p,
25␥pa + 21␥ + 9Q + 7Qp p, 16a, A2 56␥pa + 21 32 + 31␥ 63 37pa + 25␥ + 22
144 A 0.5
o, 88 p, 38 + 25 + 16c o, 30c + 28 + 23␥
9 A 0.0
o, 93p p, 80p
85 sh 76 m
39 sh 30 m
34 A 0.0
a
See notations in Table 1. Notations used: Q—C(D2 )–C(D3 ); Qp —C(Ph)–C(D2 ); Qpp —C(Ph)–C(Ph); —ring-puck, —C(Ph)–C(D) out of plane; c —C(Ph)–C(D2 ) out of plane; ␥—∠C(Ph)–C(D2 )–C(D3 ); ␥p —∠C(Ph)–C(Ph)–C(Ph); ␥pa —∠C(Ph)–C(Ph)–C(D2 ); p —∠D–C(Ph)–C(Ph); (CD2 ) and (CD3 )—∠C–C–D in C–CD2 and C–CD3 groups, respectively; pa —∠C(Ph)–C(D2 )–D; ␣(CD2 ) and ␣(CD3 )—∠D–C–D in C–CD2 and C–CD3 groups, respectively; p —torsion around C(Ph)–C(D2 ); —torsion around C(D2 )–C(D3 ). b
Wilson notations [24] are used for assignment benzene ring vibrations in spectral interval 1700–300 cm−1 .
4. Results and discussion Conformational behaviour of EB and EB-d10 was analysed by comparison of IR and Raman spectra of the liquid (or glassy state) and solid phases and results of normal coordinate analysis. It has been pointed out that an identification of frozen bands in crystal phase spectra of molecules is difficult, because of Davydov’s splitting and some changes of band intensities and peak positions during crystallisation take place. To gain more valuable information, we used both
the investigations of the solid films formed by condensing of vapours on a cold window (glassy state) and the analysis of the IR band’s dichroism in the spectra of the crystals. The observed IR and Raman bands (below 1700 cm−1 ) of the EB and EB-d10 and the results of the normal coordinate analysis are listed in the Tables 1 and 2. Some experimental spectra of liquid and crystalline EB and EB-d10 are given in Figs. 1–6. As in [10], two different solid phases of EB were obtained. The metastable solid phase A is observed in the temperature range 100–150 K. This phase observed both after annealing of the solid film, produced by depositing the vapours of EB on the cooled window and by cooling of liquid film (5 K min−1 ) down 150 K with subsequent annealing. The
A.I. Fishman et al. / Spectrochimica Acta Part A 60 (2004) 843–853
Absorbance
Intensity
850
a
a
b
b 0.1 c
c
1060
1040
1020 -1
Wavenumber / cm
400
360
320
280 Wavenumber / cm
Fig. 1. IR spectra of liquid EB at 170 K (a); crystal phase A at 120 K (b); and crystal phase B at 160 K (c).
Absorbance
irreversible phase transition to solid phase B occurs at 150 K. Phase B is stable in the temperature range from 80 K up to melting point (178 K). Let us consider the region 1050–1020 cm−1 in the spectra of EB (Fig. 1), which is discussed in details in [10]. Two absorption bands with maximum at 1030 and 1038 cm−1 in the spectra of liquid were assigned earlier to different conformers [10]. However, according to our calculations, both conformations have two fundamentals in this spectral range (Table 1). One of these vibrations has A symmetry and belongs to characteristic vibration of benzene ring (18a type in Wilson notations). The strong IR band at 1030 cm−1 and polarised Raman line at 1031 cm−1 have to be assigned to this vibration of both conformations. The second fundamental belongs to vibration of the ethyl fragment. This vibration
-1
Fig. 3. Raman spectra of liquid EB at 300 K with parallel (a) and perpendicular (b) polariser settings; and crystal phase B at 80 K (c).
is not observed in Raman spectra [9,25]. We assign the IR band at 1038 cm−1 to this vibration. In the IR spectra of solid phase A in this region there are two doublets (1029, 1031 and 1038, 1040 cm−1 ) with opposite dichroic ratio of components (Table 1). So, in phase A there are bands of fundamental 18a and vibration of ethyl fragment of o-conformer, having Davydov’s splitting. The doublet 1031, 1033 cm−1 is observed in the spectra of solid phase B (Fig. 1c, Table 1). These components cannot be explained by Davydov’s splitting because they have close dichroic ratio in the IR spectra. Therefore, in solid phase B, bands at 1031 and 1033 cm−1 belong to analogous fundamentals. Thus, the obtained data
a
0.1
b
400
360
320
280 -1
Wavenumber / cm
Fig. 2. IR spectra of liquid EB at 300 K (a); and crystal phase A at 120 K (b).
Fig. 4. IR spectra of liquid EB at 170 K (a); and crystal phase A at 120 K (b).
Intensity
A.I. Fishman et al. / Spectrochimica Acta Part A 60 (2004) 843–853
a
b c 440
420
400 -1
Wavenumber / cm
Fig. 5. Raman spectra of liquid EB-d10 at 300 K with parallel (a) and perpendicular (b) polariser settings; and crystal phase B at 80 K (c).
Absorbance
show that during crystallisation of the EB, there is no freezing of the bands in this spectral region. Calculations show that the fundamental 18a in EB-d10 has frequency 839 cm−1 for o-conformer and 838 cm−1 for p-conformer (Table 2). So, it is reasonable to assign strong IR band at 840 cm−1 and strong polarised Raman line at 841 cm−1 to the characteristic vibration 18a [26]. The frequencies of the vibrations of ethyl fragment in EB-d10 , which are similar to vibration with frequency 1038 cm−1 of EB, are quite different for conformers and equal 814 and 888 cm−1 , for o- and p-conformers, respectively (Table 2). However, comparison of this spectral range
X
0.5
a 0.5
b
640
600
560
440
400 -1
Wavenumber / cm
Fig. 6. IR spectra of liquid EB-d10 at 170 K (a) and crystal phase B at 120 K (b).
851
of the liquid and solid states of EB-d10 shows, that no IR or Raman bands disappear upon going from one state to another. The careful analysis of IR and Raman spectral data allowed the freezing of some vibrations in other spectral regions to find out, during crystallisation of EB and EB-d10 . In the spectral region 420–250 cm−1 there are four very weak IR bands and Raman lines in spectra of the liquid phase of EB: 280, 296, 360 and 403 cm−1 (IR, Fig. 2a, Table 1) and 285, 302, 358 and 403 cm−1 (Raman, Fig. 3a and b, Table 1). Low temperature studies showed that IR band at 280 cm−1 and Raman line at 285 cm−1 disappear upon crystallisation. It is seen that the calculated frequencies in this range significantly differ for o- and p-conformations. As ab initio calculations predict, that the o-conformer is more stable [4,14,15], we assign the vanished IR band at 280 cm−1 and Raman line at 285 cm−1 to the torsion vibration around C(H2 )–C(H3 ) bond of p-conformer (Table 1). In the spectral interval from 510 to 460 cm−1 , the doublets 472, 490 cm−1 in IR (Fig. 4a) and 484, 488 cm−1 in Raman spectra (Table 1) are observed in the liquid EB. Low-frequency components of these doublets vanish in the IR and Raman spectra of the solid phases A and B (Fig. 4b, Table 1). As it was pointed out in [26], the frequency of the vibration 11, B2 (in Wilson notation) falls into this spectral interval in spectra of monosubstituted benzenes. This vibration is not characteristic, and its frequency depends on the substituting group. Our calculations showed, that PED of this vibration includes considerable contribution of —ring-puck and c —C(Ph)–C(H2 ) out of plane coordinates. Besides, frequency of this vibration depends on of ethyl fragment orientation in respect of the benzene ring plain. The 11-type vibration gives rise to IR bands in the liquid phase of EB-d10 at 438 and 408 cm−1 and Raman lines at 437 and 413 cm−1 for o- and p-conformers, respectively (Fig. 5). As in EB low frequency components, which disappear during crystallisation, are assigned to pconformer. Comparison of IR spectra of liquid and solid phases of EB shows that there are freezing of the shoulder at 700 cm−1 and weak band at 720 cm−1 in the range 760–680 cm−1 (Table 1, Fig. 4a and b). These lines are not observed in the Raman spectra. Two characteristic vibrations of benzene ring (4 and 10b) [26] fall in this region and it seems likely to ascribe strong-intense IR bands at 698 and 749 cm−1 to these vibrations, respectively. The calculation shows that the frequency of 4-type vibration differs in 8 cm−1 in oand p-conformers. Therefore, shoulder at 700 cm−1 , which vanished upon crystallisation, is assigned to 4-type vibration of p-conformer. The similar situation was observed in IR spectra of isotopomer (Table 2). According to the normal analysis, a 4-type vibration has frequencies 535 and 549 cm−1 for pand o-conformations, respectively. Very intensive IR band at 550 cm−1 is detected in this spectral range; at the same time
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its shoulder at 537 cm−1 disappeared in both solid phases. Consequently, we assign the band at 537 cm−1 to vibration 4 of the p-conformer. The frequency of benzene ring vibration of EB, 10b is sensitive to orientation of ethyl fragment. Calculated frequency of the o-conformer (751 cm−1 ) differs from that one of the p-conformer (719 cm−1 ). Thus, we assign the band at 720 cm−1 in IR spectra of EB, which vanished during crystallisation, to mode 10b of p-conformer. The frequencies of 10b vibrations shift to 631 and 623 cm−1 upon deuteration, for o- and p-conformers, respectively (Table 2). There are two bands at 632 and 626 cm−1 observed in this region in the IR spectra of liquid EB-d10 . The shoulder at 626 cm−1 , which disappears under crystallisation, we assigned to 10b vibration of p-conformer, whereas band at 632 cm−1 belongs to the same type vibration of the o-conformer. The very weak IR band at 574 cm−1 disappears in spectral interval 600–560 cm−1 of EB-d10 during crystallisation (Table 2, Fig. 6). According to normal coordinate analysis, vibrations of ethyl fragment of each conformer fall into this range. So, band at 574 cm−1 can be assigned to vibration of ethyl fragment of p-conformer. Thus, the comparison of liquid and crystal phases of EB and its isotopomer—EB-d10 , shows that some IR and Raman bands disappear during crystallisation. Normal coordinate analysis predicts that these bands belong to conformation-sensitive vibrations. This indicates that ethylbenzene in liquid phase exists as a mixture of two conformers. The determination of the standard enthalpy difference was not pursued, because the bands of p-conformer have low intensities and noticeable overlapping with neighbour bands. According to calculations, absorption coefficients of the bands, of p-conformer (at 719, 693, 468 cm−1 for EB and at 623, 569, 535, 405 cm−1 for EB-d10 ), have great values. However, corresponding IR bands have weak intensities. Based on it, we can conclude that proportion of p-conformer in liquid is small enough. The electron diffraction [1,2], time-of-flight spectroscopy [3], low- [4] and high-resolution microwave spectroscopy [5] of EB have shown that in the gas phase only o-conformation exists. The appearance of small portion of second conformer in liquid EB probably is due to influence of aggregative state. It is interesting to analyse the variation of conformational composition upon going from liquid to gaseous phase. However, remarkable complication of spectral contours in gas phase, is caused by rotational structure, does not allow to observe the absorption bands of p-conformer. In this case, analysis of IR spectra of samples dissolved in liquid rare gases is a powerful tool. These solvents have great chemical inertness and transparency in a wide spectral region, that allows to use a very small concentrations of samples (typical value near 1:5000 in mole fraction [27]). Sufficiently large liquid phase temperature intervals can be achieved at slightly increased pressure [28,29].
Fig. 7. IR spectra of liquid EB at 135 K (a) and EB dissolved in LKr at 135 K (b).
We have investigated the spectra of EB, dissolved in liquid krypton (LKr). It showed that intensities of bands, which belong to less stable p-conformer, are significantly less than corresponding ones in the spectra of liquid (or glassy state) at the same temperature. As an example the spectral region from 780 to 450 cm−1 is shown in Fig. 7. The intensities of p-conformer bands at 472 and 720 cm−1 are strongly reduced in LKr. Therefore, the conformational equilibrium in LKr as well as in the gas phase considerably shifts towards o-conformer in rare gas solution. This fact is in a good correlation with experimental data, which were obtained for gas phase by other experimental methods. Acknowledgements Financial support from Russian Foundation for Basic Research (grant no. 02-03-32404) and INTAS (grant no. 172). We are grateful to Dr. S. Katsyuba for ab initio calculations and Prof. V. Furer for fruitful discussion. We also acknowledge Prof. B.J. van der Veken, who helped us recording the spectra of solutions in LKr. References [1] P. Scharfenberg, B. Rozsondai, I. Hargittai, Z. Naturforsch. A 35 (1980) 431. [2] P. Scharfenberg, J. Chem. Phys. 77 (1982) 4791. [3] P.J. Breen, E.R. Bernstein, J.I. Seeman, J. Chem. Phys. 87 (1987) 3269. [4] W. Caminati, D. Damiani, G. Corbelli, B. Velino, C.W. Bock, Mol. Phys. 74 (N4) (1991) 885. [5] B. Mate, R.D. Suenram, C. Lugez, J. Chem. Phys. 113 (2000) 192. [6] T. Schaefer, G.H. Penner, R. Sebastian, Can. J. Chem. 65 (1987) 873. [7] C. Algieri, F. Castiglione, G. Celebre, G. De Luca, M. Longeri, J.W. Emsley, PCCP 2 (2000) 3405.
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