J . Phys. Chem. 1993,97, 11643-1 1648
11643
Spectroscopic Studies on the Monobromotoluenes Walter J. Balfour' and Dusan Ristic-Petrovict Department of Chemistry, University of Victoria, Victoria, British Columbia, Canada Vi3W 3P6 Received: July 21, 1993; In Final Form: August 27, 1993'
Liquid-phase Raman spectra and vapor-phase, near-ultraviolet absorption spectra of a-,0-, m-, and p-bromotoluenes have been studied. A number of new Raman features have been observed and assigned. Medium-resolution UV spectra of the o- and m-isomers have been photographed for the first time, and that for p-bromotoluene, at considerably higher resolution than previously reported. The respective electronic band origins for 0-, m-, and p-bromotoluene lie at 36 841, 36 564, and 36 257 cm-l. The vibrational structure accompanying the T* a transition has been analyzed in detail to reveal a pattern similar to that observed for related substituted benzenes. The corresponding a-bromotoluene spectrum is diffuse.
-
Introduction While the vibrational spectra of disubstituted benzene derivatives have been extensively studied,' the same cannot be said for their electronic spectra. The present research arose from studies on toluene, where, in the course of synthesizing a number of monodeuterated derivatives from their corresponding bromocompounds, we had noticed, in UV studies, sharp impurity absorptions attributable to traces of unreacted starting material. The vibrations of all the bromotoluenes had been investigated by infrared and Hg arc Raman ~pectroscopy.2-~ Mention has been made of the electronic spectra, but no substantive data have been presented for any isomer.8-'O In our study we have obtained laser Raman spectra which show new features, and improved relative intensitiesand depolarizationdata, and UVspectraof theo-, m-, andp-isomersat sufficient resolution to reveal rich vibrational structure.
Experimental Details Commercial samples (Aldrich) of o-,m-, and p-bromotoluene (OBT, MBT, and PBT, respectively) were used without purification. In the case of the a-isomer (ABT), significant photodecompositionwith the release of bromine occurs during 5 14-nm laser irradiation. The bromine gives rise to a fluorescence background in the Raman spectrum. This interference was kept to a minimum by changing the Raman sample frequently. Raman samples were liquid, sealed in glass capillaries, and spectra were recorded, perpendicular to the incident laser beam, in the range from 50 to 3150 cm-l, on a Jobin-Yvon Ramanor HG2S monochromator using 300 mW power and excitation wavelengthsof 496.5,501.7 and 514.5 nm froma Spectra Physics series 2000 argon ion laser. Polarization measurements were made in the conventional way by rotation of the plane of polarization of the incident laser beam with a quartz half-wave plate. The spectra were calibrated using argon plasma lines as internal standards. Measurement errors on well-defined lines are estimated to be within f 2 cm-1. The UV spectra were photographed on a 3.4 m Jarrell-Ash Ebert spectrograph in the first and third orders of an 1180grooves mm-' grating blazed for 750-nm. The background continuum was suppliedby a 150W high pressure xenon lamp and the spectra were calibrated for wavelength by adjacently placed emission from an Fe/Ne hollow cathode lamp. Spectra were photographed at 0 OC and room temperature (-21 "C) using quartz cells up Author to whom correspondence should be addressed. t Current address: Department of Chemistry, University of Alberta, Edmonton, AB T6G 2G2, Canada. Abstract published in Advance ACS Absrrucrs. October 1, 1993.
0022-3654/93/2097- 11643$04.00/0
10
I
I
I
1000
2000
3000
DISPLACEMENT (c"')
Figure 1. Raman spectra of o-,m-, and pbromotoluene liquids.
to 1 m in length. Measurements were taken directly from photographic enlargements and were reduced to vacuum in the usual way. Near the band origins, where the spectra are sharpest, measurement errors are estimated to be of the order of f l cm-1. 0 1993 American Chemical Society
11644 The Journal of Physical Chemistry, Vol. 97, No. 45, 1993
Balfour and Ristic-Petrovic
TABLE I: Raman Frequencies (cm-1) and Assignments for o.,m, and pBromotoluenP o-bromotoluene raman shift
re1 int
154 dp 217 dp 240 dp 296 p 412 p 435 501 p 543 p 656 p 107 748 800 P 859 940 990 p P 1031 p 1046 p 1124 1136 1158 p 1211 p 1277 p 1381 p 1431 1459 dp 1570 dp 1599 dp 2329 2557 2730 2740 2846 p 2872 p 2910 2927 p 2984 p 3019 p 3061 p
50 6 20 76 19 0.5 2 72 62 0.5 3 61 1 0.5 9 7 43 100 2 2 13 54 9 18 6 5 16 22 3 4 2 9 8 13 sh 68 17 20 97
0
m- bromotoluene
assignmentb C-Br 0-0-p C-Br i-p X-sensitive [7a1 X-sensitive P6bI [16aI [6bI [6al 296 412 [loa1 [121 [10bI [ 17b1 240 748 [2 X 16a] [ 18bI
+ +
PI
+
154 970 [4 + 16b] [gal [I31 ~ 4 1 WH3 bCH3 ~9a1 [8bl Pal [2 X 9a] [2 X 141 2 X 6,CH3
+
[ 19a] 2 X 6&H3 [2 X 19a] vtCH3 vuCH3 [8b + 19a] [2, 13,20a, 20b]
raman shift 172 dp 195 dp 219 dp 307 p 376 519 p 666 p 76 1 833 p 896 999 p 1017 p 1071 p 1094 p 1166 dp 1213 p 1269 1301 1337 1378 p 1448 1472 1572 dp 1602 dp 2333 2732 2740 2892 p 2925 p 2980 3020 3052 p 3061 p
re1 int 23 19 18 60 2 23 36 3 10 1 100 14 18 3 5 14 2 1 1 7 1 0.5 6 8
1 3 4 10 37 6 I 42 44
pbromotoluene
assignment C-Br 0-0-p C-Br i-p X-sensitive C-Br i-p X-sensitive [6a, 16a] [6bI [ 1Ob1 X-sensitive ~7a1 [ 121 ring 172 833 [1 8 4 X-sensitive [9b1 X-sensitive ~ 4 1 131 [2 X 6b] 6sCH3 6&H3, [ 19bl [19a1 [8bI Pal [2 X 9b]
+
2 X 6,CH3 2 X 6&H3 vaCH3 VMCH~ [8b + 19b] 12, 131 [20a, 20b]
raman shift 214 295 p 363 406 478 579 598 632 dp 69 1 794 p 815 p 959 p 1014 p 1072 p 1179p 1209 p 1229 p 1270 1303 1340 1380 p 1451 1489 p 1591 dp 1651 1670 2543 2566 2737 2869 2893 2922 p 2966 3028 p 3062 p
re1 int 1 90 2
assignment
0.5 0.5
0.8 2 13 0.5 100
9 0.8 6 65 11 42 2 1 2 0.5 12 1 1 18 0.8 0.3 2 1 3 10 7 39 6 13 46
Re1 int here means relative peak height. Assignments in the Wilson notationII are indicated in square brackets.
TABLE Ik Raman Frequencies (cm-I) and Assignments for a-Bromotoluene (Benzyl Bromide)
0
1000
20100
3000
DISPLACEMENT (cm-1)
Figure 2. Raman spectrum of a-bromotoluene (benzyl bromide) liquid. The slight curvature in the baseline arises from fluorescence from Brz formed by photodecomposition.
raman peak raman peak shift height assignment shift height 111 dp 30 1033p 25 241 p 31 1160dp 7 319 3 1185p 7 454 p 57 1207p 11 28 550 dp 1228dp 83 97 611 dp 1439 1 1 699 1500p 3 7 761 dp 1588 4 805 dp 7 1608dp 31 816 dp} 10 2972p 24 993 5 3009p 6 1009 p 100 3044p sh 3062p 37
assignment
Results and Discussion a. " a n Spectra. The Raman spectra of OBT, MBT, and PBT are presented in Figure 1, and that of ABT is in Figure 2. The corresponding frequency data and assignments are listed in Tables I andII. The30fundamentalmodesofvibrationassociated with a benzene ring divide into l l a l + 3a2 + 6b1 + lob2 in C, symmetry and 21a' + 9a" in C, symmetry. A number of different numbering schemes have been proposed: for the purpose of describing the various bromotoluene spectra, we adopt, following Green," the numbering given in Table 111. Our Raman observations agree in a very satisfactory way with the infrared assignmentsgiven by Green and Raman data quoted by him: OBT,2 MBT,', PBTS4New features have been detected
in all spectra. Many of these new Raman lines can be correlated with features in the correspondinginfrared spectra. For example, Raman displacementsfound for PBT at 406,478,691,959,1270, 1489, and 2966 cm-I match closely infrared assignments at 405, 476, 692, 956, 1270, 1488, and 2974 cm-1. In a few instances Raman features previously claimed have not been detected, namely1074cm-I in OBTs 467,862,943, and 1205cm-' in MBTs and 702 and 840 cm-l in PBT.' A comparison of the present data on ABT with those of Chattopadhyay12 shows broad general agreement. The greater resolution and sensitivity available to us have allowed us to distinguish additional ABT features at displacements of 816, 993, 1588, and 3044 cm-I which lie close to stronger lines. We were unable to substantiateChattopadhyay's
Spectroscopic Studies on the Monobromotoluenes
The Journal of Physical Chemistry, Vol. 97, No.45, 1993 11645
TABLE IIk Fundamental Frequencies and Mode Numbering of Ring Vibrations of Bromotoluenes o-bromom-bromop-bromotoluene toluene toluene no. mode cm-1 no. mode cm-l mode cm-1 3061 a’ 1 3060 3088
a”
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
3060 3060 3060 1592 1565 1467 1456 1274 1252 1208 1158 1120 1045 1031 798 656 543 412 297 217 970 935 855 744 694 50 1 43 3 238 157
3064 3052 3052 1599 1568 1477 1452 1300 1267 1212 1165 1093 1069 996 834 666 521 378 307 198 969 896 868 770 680 513 429 220 171
306 1 3038 3026 1592 1570 1488 1397 1299 1270 1212 1179 1112 1069 1013 793 633 599 364 292 216 956 935 815 801 692 476 405 292 121
TABLE I V Comparison of (T*,T) Band Positions (cm-I) for Monohalo-substitutedToluenes CH3CH3CH3isomer C.&F ref C6H4CI ref C&Br ref 36 873. ortho 37 562 14 16 36 841 this work meta 37386 14 36613 17 36564 thiswork para 36860 14, 15 36294 17 36257 thiswork report of Raman lines at 1315 and 1537 cm-’. There has been a more recent report” of a laser Raman spectrum, purporting to be of ABT, with little in common with the present or previous infrared or Raman observations on this molecule. b. Electronic Spectra. The 260-nm, A1B2u-X1Alg,electronic transition in benzene is forbidden by symmetry selection rules. Disubstituted benzenes have _lower_symmetry, ?nd the corresponding u* u transition (AIBrXlA1 in Cb,AIAtr-%A‘ in C,) is no longer forbidden: indeed the O0o electronic origin band is one of the strongest features in the latter spectra. Such is the case for OBT, MBT, and PBT. For the correspondingtransition in ABT, no structure is evident because of predissociation. The 000 bands of OBT, MBT, and PBT lie at 36 841,36 564, and 36 257 cm-1, respectively. There is extensive sequence structure associated with these bands in all three molecules. The trend toward longer wavelengths along the series 0, m,p is similar to that seen in the spectra of monofluoro- and monochlorotoluenes (Table IV). Our bromotoluene000 assignments differ somewhat from previous proposals: Viswanatha gives for OBT 34 846 cm-1, which may be a misprint; Sen9 gives for MBT 36 526 cm-1, and JoshiIo gives for PBT 36 263 cm-1. The band origin in unsubstituted toluene is found at 37 479 cm-1. ~Bromotdwne.Banded absorption has been observed throughout the 279-250-nm region. The strongest band at the lowenergy end, and one of the most prominent bands in the spectrum (36 841 cm-I), is unquestionably the electronic system 0-0.It is the origin of several excited-state progressions, and a number of major bands to longer wavelengths lie at intervals which correspondclosely to a’ fundamental vibrational frequenciesseen
-
in the infrared and/or Raman spectra. Bands in the region u0 up to approximately vo + 1000 cm-1 display well-defined, reddegraded heads; to higher energies, diffuseness becomes increasingly evident. The most important features of the spectrum to the red of the system origin occur at displacements of 1036,799,656,542, and 217 cm-l and may be assigned as 15O1,1601,1701,1801,and 2101, respectively. The principal excited-state intervals found (see also ref 8) are 276,479,761,959,1017, and 1183 cm-1. It is assumed that these vibrational frequencies represent the upper state analoguesof the totally symmetricmodes numbered in the ground state 20, 18, 17, 16, 15, 14, and 11, respectively. It is precisely these modes which are strongly active in the spectra of other substituted benzenes that have been analyzed in detail.18 This observation is a reflection of the fact that similar geometry/ bonding modifications accompany the benzenoid T * u promotion in all instances. The major part of the intensity in the spectrum can be accounted for in terms of progressions and combinations involving the above seven excited-state vibrational modes. The origin band and many of the stronger bands in the spectrum have considerable sequence structure to their low-energy side. The most prominent sequence has an interval of -65 cm-1 and can be followed in the most favorableinstanceto the fifth member. It is therefore evident that the vibration involved must be of low frequency. An attribution to V I S , even though the numbers agree (478-543 cm-I), seems highly improbable. Furthermore, were the 45-cm-1 interval to arise from the lowest ground-state vibrational level ( v j o = 157 cm-I), an upper state vibrational frequency of 92 cm-I is implied, which seems very low. There is a more attractive alternative, namely v” = 217 cm-1, v’ = 152 cm-I, especially since there is a band at YO + 152 cm-l in the spectrum. For these various reasons the 2111 assignment is preferred for the -65-cm-I sequence. A second sequenceinterval of -21 cm-I has appreciable intensity. A plausible candidate for the vibration involved is vibration 20, where v” = 297 cm-1, and this assignment is reinforced by the observation of a band, albeit weak, at vo 275 cm-’. Sequences at +8 and -99 cm-1 are more difficult to assign with the same degree of confidence. They are thus labeled A and B, respectively, in Table V, where observations and assignments are listed. The +8-cm-I sequence deserves comment, since most vibrations would be expected to decrease in frequency upon u* u excitation. The fairly strong band at YO + 8 cm-’ and somewhat weaker bands at YO + 20 and YO 29 cm-1 may originate from transitions between internal rotational levels of the methyl group in the ground and excited states. Okuyama, Mikami, and Ito have been able to make definitive assignments in the case of o-fluorot01uene.l~ mBromotoluene. The UV absorption spectrum of MBT has been briefly mentioned by Sen9 By comparison with the corresponding spectra of OBT and PBT, it is relatively diffuse and, as such, much lessvibrational structure is measurable. Only two strong bands, separated by 962 cm-l, are evident in the spectrum. The lower energy one, found at 36 564 cm-’, is taken to be the system origin; the 962-cm-l interval then represents the first excited-state quantum of the “symmetric” ring-breathing vibration, which has a value in the ground state of 996 cm-I. This decrease in frequency upon excitation is entirely analogous to the situation in m-fluoro- and m-chlorotoluene: in the former molecule the ring-breathing mode is 1003cm-l in the ground state and 965 cm-1 in the first electronic l(u*,u)state, and in the latter the corresponding values are 998 and 962 cm-I. The origin band shows three intensity maxima: 36 564,36 563, and 36 560 cm-1. Its sequence structure consists of two weak features on the high-energy side (+17 and +29 cm-I), which possibly arise from CH3 torsional motion, and bands at -1 5 , -34, a n d 4 6 cm-1, of which the -66-cm-l feature is moderatelystrong. Part of the observed intensity of the -66-cm-1 band may be due
-
+
-
+
11646 The Journal of Physical Chemistry, Vol. 97,No. 45, 1993
TABLE V band 35 805 35 978 35 991 36 042 ww 36 051 36 112 36 120 36 180 36 185 vw 36 235 36 299 vw 36 376 36 384 36 416 vw 36 504 36 526 36 554 36 573 36 5741 36 589 w w 36 604 36 624 36 652 w 36 676 vw 36 680 w w
Balfour and Ristic-Petrovic
-
Band Positions (cm-l, Vacuum) and Assignments in the AIA" 31A1, r* + r System of o-Bromotoluene int assignment band int assignment band int assignment 36 841 ws origin 37 779 m -1036 +959 - 21 -799
- 64
-799 -799
+9
-656
- 65
36 849 w 36 861 vw 36 870 w w 36 902 w w 36 925 vw 36 933 w w 36 996 vw 37 054 ww 37 096 ww 37 117 w 37 128 w w 37 167 w w
-656 -542 - 64 -542
5(-63) 5(-63)
6901
- 22
-64- 63 - 23 -64 - 63 -99
- 21
-99 -64 - 22
I
36755 36 754 vw 36 770 vw 36 776 36 777 vs 36 779 m 36 783 m 36 801 m w 36 822 ms
1
-64 3(-21) -64+8 -19 - 21 -19
A'I
+152 +276 - 63 +276 - 21 +276 +479 - 3(64) +479 - 66 - 62 - 25
37 192 1 9 7 mw +479 - 66 - 62 37 201 ww +479 -66 -62 8 37 208 w w +479 - 62 - 2(23) 37 222 vw +479 - 98 37 231 w w +479 - 66 - 23
+
1
4(-63) -217 - 20 -217 3663) -99 - 66
36691 vw 36 36 706 vw 36 713 36 7141 m 36 721 w w 36 737 vw 36 742 w
+8
20'12122
37 253 m 37254 37 261 vw 37 265 vw 37 276 vw 37 298 m w 37 320 ms 37 328 vw 37 379 w 37 412 vw 37 423 vw 37 445 m 37 452 w w 37 471 vw 37 513 ww 37 537 m w 37 582 w 37 602 m 37611~ 37 673 m 37711 m w 37 736 ms 37 774 m
+479
- 66
+479 - 66
+479 - 22 - 22 +479 - 22 +479 +479 8 +604 - 68
+
+604 - 22 +604 +604 7 +761- 65 - 66 +761 - 65 - 24 +761- 65 +761- 20 +761 +761 + 9 +959 - 64 - 63 +959 - 64 - 25 +959 - 64 +933
+
to overlapping by the second member of the -34-cm-1 sequence, and this may explain why the -66-cm-l sequence does not apparently show a second member. It is interesting to observe that the corresponding m-chlorotoluene spectrum exhibits structure a t vo 26, +12, -16, -39, and -65 cm-1 and again the -65-cm-I sequence is strong. The MBT spectrum is made up entirely of a repetition of the 000 band's sequence pattern, with the same relative intensity distribution, superimposed upon progressions and combination bands with excited-state fundamental frequencies of 293, 453, 807, 962, and 1200 cm-l. We assume that the sequence activity involves vibrations similar to those found in the spectrum of the o-isomer. The experimental data are summarized in Table VI. pBromotoluene. The system of PBT bands extends from about 282 to 252 nm. At longer wavelengths the bands clearly degrade to the red, but below 263 nm they become diffuse, making it difficult to fix their positions precisely. The strong band at 36 257 cm-1 is taken as the 0-0 transition. With this assignment three transitions may be identified as originating on levels in which single quanta of totally symmetric ground-state vibrations are excited. The a l vibrations concerned are those of lowest energy, in cm-I: v11 = 292 (IR), 295 (R), 292 (UV); v10 = 599 (IR), 598 (R), 601 (UV); and v9 = 793 (IR), 794 (R), 793 (UV). There is also a band a t vo - 240 cm-I, and one possible assignment is 20°2, vz{ at 121 cm-1being the lowest energy out-of-plane bending freq~ency.~ A second explanation is that -240 cm-1 represents a 1-1 sequence in the CC twisting vibration generally referred to as 16a in the Wilson n0tation.I' An interval in the range -200 to -250 cm-1 has been attributed to such a sequence band in the
+
+7
37 790 m 37 800 s 37 822 vw 37 833 w 37 843 w 37 858 ms 37 895 vw 37 926 mw 37 950 w 37 959 w 37 977 w w 38 002 w 38 024 m 38 078 mw 38 088 w w 38 150vw 38 216 vw 38 255 ww 38 279 w 38 301 vw 38 332 m w 38 356 vw 38 363 vw 38 402 vw 38 425 vw 38 434 vw 38 457 vw 38 497 vw 38 501 w 38 560 w 38 693 w 38 734 w 38 757 m w 38 780 m w 38 811 mw 38 844 vw 38 914 vw 38 980 w 39 021 vw 39 035 w 39 201 w
+lo17 - 68 +959 +lo17
- 25
+lo17 +1183 - 65 - 64 +1183 - 65 - 23 +959 + 150 +1183 - 65 +1183 - 22 - 25 +1183-22 +1183 +761 476 +959 + 479 - 3(64) +959 479 - 2(64) +959 479 - 63 +959 + 479 - 24 +959 + 479 +761 761 - 62 +lo17 + 474
+ + +
11102011 11'0
+
+
+761 761 +1183+477-99 +1183
+ 477 - 67
+1183
+ 477
+959 + 957 - 64 +959 957 - 23 +959 + 957 +1183 756 +lo17 + 953 +2(761) + 481 +1183 + 956 - 66 +1183 956
+ + +
+1183+ 1011 +1183+ 1177
spectra of a number of monosubstituted and p-disubstituted benzenes, including p-fluorot~luene.~~ Here the second interpretation is considered less probable. The frequency of the 16a vibration is known to be rather stable: the 16a ground-state values for toluene, toluene-wd3, toluene-4-d, and PBT are respectively 407,406,403, and405 cm-1. The 1-1 sequencein toluene, toluenea-Q, and toluene-4-d is found at -177, -176, and 176 cm-*, respectively,20 and it seems doubtful that the frequency change upon excitation in PBT will be markedly different. We note that no such sequence is evident in the OBT or MBT spectra. Seven excited-state fundamental frequencies have been identified. These are 330,535,757,800,1016,1193, and 1479 cm-1. Their progressions, alone or in combination, account for all of the stronger bands lying toward higher frequencies. The modes responsible for this activity are all in-plane motions of the benzene ring and loosely correlate with vibrations numbered 29,28,9,8, 7, 5, and 4 in the ground state. Many of the principal spectral features have sequence bands, the strongest of which occur at displacements of approximately -30 and -87 cm-1. The ground-state C H j rotational barrier in PBT should be very small, and there is no discernible sequence structure on the high-energy side of the system origin. Okuyama et al.14 report that the bands due to internal rotation in p-fluorotoluene are extremely weak in their fluorescence spectrum. The -30-cm-1 interval can be suitably explained as a 1-1 sequence in vibration 29. However, this attribution is considered unlikely on Boltzmann grounds for a vibration whose groundstate value is 364 cm-1, since three members of the sequence are found attached to the 0 0 0 band and the sequence is also observed
Spectroscopic Studies on the Monobromotoluenes
The Journal of Physical Chemistry, Vol. 97, No. 45, 1993 11647
-
TABLE VI: Band Positions (cm-l, Vacuum) and Assignments in the AIAff band int assignment band 36 387 vw 37 387 w -66 - 64 37 459 ms 36 434 vw -66 - 39 36 459 vw 37 490 mw -66 -34 - 17 -34 -15
36 498 s 36 513 w 36 530 mw
1
36 545 m 36549 36 560 36 563) 36564 vs
5781
37 513 m 37 526 vs 37 543 m 37 553 mw 37 693 w 37 731 vw 37 745 w 37 764 mw 37 915 w 37 941 w 37 979 w 37 991 w 38 008 vw 38 264 w w 38 333 vw 38 420 w 38 486 mw 38 502 mw 38 514 mw 38 657 w 38 688 w 38 723 mw 38 738 w 38 750 w w 38 966 w
$17
36 36581 m 36 593 mw 36 655 w w 36 789 ww 36 857 ww 36 873 ww 36 954 ww 36 984 vw 37001 vw 37 017 w 37 032 vw 37 048 w 37 113 ww 37 180ww 37 304 ww 37 354 vw 37 371 mw
+29 +9 1 +293 - 68 +293 +293 16 +453 - 63 +453 - 33 +453 - 16 +453 +453 15 +453 31 +616 - 67 +616 +807 - 67 +807 - 17 +807
+
+ +
-
%'Af, r *
-
1 36 36056 053 1ww 36 109 vw 35 35965 964 vw 36 016 vw
36 138 w 36 162 mw 36 167) 36 170 s 36 180mw 36 196 ms
1
36227 36 225 vs 36 235 ms 36 245 ms 36 251 ms 36 257 w s 36 525 w w 36 554 ww 36 587 vw 36 601 vw 36 609 vw
-292
1101
-24 1
2002
-87 - 32 - 29 -87 - 32 -30 - 31 - 34 -87
19'120~2 19'120'1 20% 19'1
-77 -30 - 31
2022
-30
20'1
-22 -6
origin
+330 - 33 - 29 29102022 29'020'1 +330 - 33 +330 29'0
36 674 ww 36 702 vw 36 710 vw 36 724 vw 36 737 vw 36 764 mw 36 792 m 36 918 w w 36 927 w 36 955 vw 36 983 s 37 014 vs 37 057 m 37 123 vw
int
assignment
+807 + 16 +962 - 67 +962 - 36 +962 - 13 +962 +962 + 17 +962 + 27 +1200 - 71 +1200-33 +1200- 19 1200 +962 + 453 - 64 +962 + 453 - 38 +962 453 +962 + 453 + 12 +962 453 + 29 +962 + 807 - 69 +962 + 807 +962 + 960 - 66 +962 + 960 +962 960 + 16 +962 + 960 28 +1200 + 959 - 66 +1200 + 959 - 35 +1200 + 959 +1200 + 959 15 +1200 959 + 27 +1200 + 1202
+
+ + +
+
-
TABLE VU: Band Positions (cm-l, Vacuum) and Assignments in the A1B2 %'AI, T * band int assignment band int assignment band 35 433 35 446 35 464 35 612 35 656 35 727 35 781
r System of mBromotoluene
T
+ +
System of pBromotoluene int assignment
+535 - 90 - 28 +535 - 90 +535 - 82
37 450 vs +1193 37 522 mw +757 + 535 - 27 37 549 m +757 535 3 7 7 3 6 m +1479 +535 - 28 - 27 37 777 w +lo16 + 532 - 28 +535 - 28 37 805 w +lo16 532 37 940 w +lo16 + 757 - 90 +535 37 988 m +1193 538 +330 + 331 +757 - 87 37 992 m +lo16 + 757 - 38 +757 - 31 - 28 38 022 m +lo16 757 - 8 +757 - 31 38 030 ms +lo16 757 +757 38 076 w +lo16 + 803 +800 38 176 mw +1193 + 756 - 30 +535 + 331 28I029Io 38 206 m +1193 756 {+1016- 88 - 33 - 29 71~202221L138 251 mw +lo16 1014- 36 37 152 w +1016- 88 - 33 7'020'121'1 38 287 m +lo16 + 1014 37 185 ms +1016- 88 7'021'1 38 343 vw +1193 + 1015 - 92 - 30 37 212 m +lo16 - 32 - 29 7'020~2 3 8 3 7 3 ~ +1193+ ~ 1015-92 +lo16 - 32 7'020'1 37 241 s 38 402 vw +1193 + 1015 - 32 - 31 37 249 m +1016- 24 3 8 4 3 3 ~ +1193+ 1015-32 37 266 m +lo16 - 7 38 465 m +1193 1015 38 493 w +1479 + 757 37 273 w s +lo16 7'0 37 301 ww +1193 - 90 - 30 - 29 5'020~221'1 38 635 mw +1193 + 1185 38 952 vw +lo16 + 1014 + 750 - 85 +535 + 532 2iQ0 37 330 {+1193 - 90 - 30 5'020'121'1 39 005 vw +lo16 + 1014 + 750- 32 5'021'1 37 360 m +1193 - 90 39 037 mw +lo16 + 1014 + 750 37 389 mw +1193 - 32 - 29 5'020~2 39 216 m +1193 1015 751 5'020'1 37 418 ms +1193 - 32 39 472 m +1193 1015 + 1007 37428mw +1193-22 39 651 mw +1193 + 1185 + 1016 37 443 m +1193 - 7
to stack on the -87-cm-1 one. The preferred assignment is 20l1. Vibration 20, which is a substituent-sensitive mode, is the lowest lying ground-state fundamental, and the corresponding vibration in some related systems also gives rise to ubiquitous sequence structure: -35 cm-1 is observed for p-fluorotoluene,lg -66 cm-l for fluorobenzene,21and -60 cm-l for chlorobenzene.22 It is probable that the other intense sequence (-87 cm-1) involves either V ~ O " (=216 cm-I) or VI;' (=292 cm-1). Here again a comparison with behavior upon excitation in similar molecular systems is instructive and suggests that the frequency change for
+ + + + + + +
+
+ +
+
5'07'020'121'1 5'07'021'1 5'07'020~2 5'07'020'~ 5'07'0 4'09'0 520
72~9'~211~ 72~9'~201~ 72~910 51~7L~9L~ 5'07~0 52~7'o
~ 3 should 0 be small whereas that for ~ 1 may 9 be appreciable. On this basis the -87-cm-1 sequence is tentatively assigned to the latter mode. A number of weaker bands belong to other sequences or cross-sequences, but speculation on their origin is unwarranted. Table VI1 lists the observations and their assignments. a-Bromotoluene. Attempts to photograph a spectrum of ABT met with no success. Its T * T spectrum is apparently completely diffuse. Benzyl halides are known to photodecompose, predominantly by homolytic cleavage of the carbon-halogen bond, and the resulting benzyl radical, which is resonance stabilized, can
-
The Journal of Physical Chemistry, Vol. 97, No. 45, 1993
11648
TABLE VIII: Correspondence between Selected Vibrational Frequencies in the Ground and First l(a*,a) Excited States of Haloeenotoluenes. CH&HIX (cm-l) ~
~
~~
~
ortho-Isomers
x = c1
X=F grd st
exst
426 525 576 754 840 1037 1233 ref2
392 317 498 705 924 1230 14,23
442 527 728 1003 1266 ref 3
426 464 685 962 1261 14,23
424 638 842 1017 1157 1214 ref4
398 585 802 845 1012 1194 14,23
grdst
exst
366 345 445 420 553 487 678 642 805 772 1042 964 1207 2 16 metu-Isomers 387 522 455 684 632 1002 962 1272 1207 3 17 pura-Isomers 374 634 797 1017 1090 1209 4
X = Br grdst ex st 297 276 412 543 479 656 604 798 761 1031 959 1208 1183 2 this work
Balfour and Ristic-Petrovic transition is forbidden, the principal progressions appear in combination with one quantum of the doubly degenerate (eZ8) ring vibration, 6 in the Wilson numbering convention. Its frequency decreases from 608 cm-1 in the ground state to 522 cm-* in the excited state. We note similar behavior in the substituted toluenes. In the lower symmetry the corresponding electronictransition is allowed, the vibrationalpair 6a,6b is active, and the vibrational frequency decrease is roughly the same.
Acknowledgment. The authors wish to thank Dr. Ram S.Ram and Ms.Yvonne Fried for experimental assistance. They are grateful to the University of Victoria and the Natural Sciencts and Engineering Research Council of Canada for financial support. References and Notes
307 521 666 996 1267 3
293 453 616 962 1200 this work
364 633 793 1013 1069 1212 4
330 535 757 800 1016 1193 this work
be detected. In the same context we recall that we saw evidence of Br2 formation upon prolonged irradiation during the Raman experiments. Table VI11 shows a summary of the more important observations from the analyses of the T*-T electronic spectra of the halogen-substituted toluenes. The transition is localized primarily on the ring, and for the most part, the frequency changes that occur upon excitation correlate very well within the family of compounds. In the benzene spectrum, where the purely electronic
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