J. Phys. Chem. 1994,98. 42614270
4261
Vibronic Analysis of 4-, 5-, and 6-Fluoroindole Toni L. 0. Barstis,' Louis I. Grace, T. M. Dum' and David M. Lubman' Department of Chemistry, The University of Michigan. Ann Arbor, Michigan 48109 Received: November 29, 1993; In Final Form: February 14, 1994e
The excitation spectra of 4-, 5-, and 6-fluoroindole have been obtained with jet-cooled one-color resonant two-photon ionization ( I C R2PI) time-of-flight mass spectrometry (TOFMS). The IC R2PI spectra have been vibronically analyzed in the spectral region of 0; to 0; 842 cm-l in 4-fluoroindole and 0; to 0; + 1100 cm-I in 5- and 6-fluoroindole. All transitions observed in the 4-, 5-, and 6-fluoroindole excitation spectra are assigned as ILb 'A' transitions. Assignments of both ground- and excited-state vibrational modes were based upon those previously assigned for i n d ~ l e . ~As . ~ with indole, the ground-state vibrational frequencies were also calculated using ab initio methods as well as being obtained from infrared and Raman studies.
-
+
-
Introduction The excitation spectra of 4-, 5-, and 6-fluoroindole have been studied as one part of the more extensive study of the excitedstatedynamicsofindole. The solution-phaseabsorptionspectrum of indole has been interpreted to have vibronic transitions which result from two closely lying excited electronicstates, labeled 'La and 'Lb.These excited electronic states are selectively stabilized either by substitution on the indole ring or by the solvent polarity of the chemical environment. In vapor-phase studies, the ILb electronic state has been assigned as lower in energy than the lL. electronic state,lJ with an energy gap of >950 cm-1.24-9 However, substitution on the benzene ring of indole has been shown to stabilize the energy of the I L b electronic state, causing a further increase of the energy gap between the two electronic states.'JQJl One would expect substitution of a fluorine atom on the benzene ring of the indole ring to stabilize the ILbelectronic state. The ground-state fundamental vibrational mode assignments and the excited-state vibronic assignments of 4-, 5-, and 6-fluoroindole are both reported in this study. Ground-state fundamental modes were first calculated for all three isomers using an ab initio method, and the modes were then matched to those previously found for indole itself after appropriate changes in mass and the presenceof the fluorine atom were taken into account. These ab iniiio results were then related to the frequenciesobtained from vapor-phase and condensed-phase infrared and Raman studies on all three isomers and to those of indole which had previously been found from a normal coordinate analysis by Takeuchi and Harada" and our own later work.9 A comparison between the ab initio and experimental results has been provided toindicate thedegreeofsimilarityobtained for each set ofresults. The excited-state vibronic assignments have heen made after the relatively small changes from indole in certain vibrations have been noted (such as the benzene ring-hreathing mode and thelowest out-of-planedistortionmodes)coupled with theaddition of the C-F motions which show clearly in the ob initio methods supported by Molecular Editor (see below). Since the fluoroindoles have no conformational isomers, the absence of bands which might be assigned as extravibronic origins, and progressions baseduponthem,has beeninterpretedinfavorofasingleelectronic origin of ILb type, in conformity with the obvious isomorphisms between the excited vibronic structures of indole and the fluoroindoles.
0 "41 ("421
"14 (C-Fmetchl
F i y r e 1. Some fundamental vibrational modes of 4-fluoroindole.
Experimental Section The experimental apparatus used to obtain the jet-cooled IC R2PI spectra was thoroughly discussed in a previous paper? To obtain complete IC R2PI spectra of 5- and 6-fluoroindole, several dyes for the N d Y A G pumped-dye laser were used. The hand intensities over a wavelength region of a particular dye were not normalized to the dye laser intensity curve. In addition, no correction was made in any of the IC R2PI spectra for the backgroundeither by subtracting01 by normalizingtheionization intensity to the dye laser intensity curve. 4-, 5-, and6-FluoroindoleswereobtainedfromSigmaCo.The samples wereused without further purificationsince thespectrum was mass analyzed. The absence of bands in common was a further indication of the isomeric purity of the individual compounds which, after all, have the same mass.
ResuWDiscussion f
Dcprtmcnt of Chemistry and Physics. St. Mary's College, Notre Dame.
IN 465%.
'Abstract published in Adwncc ACS Abstracts, April
1, 1994.
0022-3654/94/2098-426 1$04.50/0
Ground-State Vibratioml Mode Assignments. The groundstate normalvibrational modes of 4-, s-, and 6-fluoroindoles were 0 1994 American Chemical Society
4262 The Journal of Physical Chemistry, Vol. 98, No. 16, 1994 TABLE 1: Theoretical Fundamental Vibrational Modes of 4-Fluoroindole mode no." 42 C-F bend 41 (42) 40 (41) 39 (40+38) 38 (39) 37 (37) 36 (?40) 35 (36) 34 (35) 33 (33) 32 (31) 31 (32) 30 (30) 29 C-F wag 28 (29) 27 (28) 26 (27) 25 (26) 24 (25) 23 (24) 22 (21+22) 21 (22) 20 (23) 19 (21) 18 (19) 17 (18) 16 (16) 15 (17) 14 C-F stretch 13 (14) 12 (13) 11 (12) 10 (10) 9 (11) 8 (9) 7 (8) 6 (7) 5 (5)
vb A. Out of Plane (A") 174 203 264 494 51 1 543 599 685 718 792 852 866 932 B. In Plane (A') 254 471 506 604 658 841 904 1014 1030 1042 1121 1161 1200 1238 1263 1285 1360 1380 1442 1512 1519 1593 1614 3029 3046 3066 3100 3122 3471
intensity
Barstis et al.
TABLE 2 Theoretical Fundamental Vibrational Modes of 5-Fluoroiadde vb A. Out of Plane (A") 140 216 338 406 497 549
mode no." 42 (42) 41 (41) 40 C-F bend 39 (40) 38 (39) 37 (37) 36 (?38) 35 (?36) 34 (34) 33 (33) 32 (32) 31 (?32) 30 (31 or 30)
intensity
580
692 734 788 856 871 924 B. In Plane (A') 304 433 468 609 745 788 897 938 1042 1087 1116 1141 1207 1231 1246 1286 1341 1389 1453 1476 1521 1581 1626 3036 3061 3065 3092 3114 3472
29 C-F wag 28 (29) 27 (28) 26 (27) 25 (26) 24 (25+20) 23 (24) 22 (25) 21 (22) 20 (21) 19 (20) 18 (19) 17 (17) 16 (?16) 15 (15) 14 C-F stretch 13 (14) 12 (13) 11 (12) lO(l1) 9 (10) 8 (9) 7 (8) 6 (7) 5 (5)
4 (4) 3 (3) 2 (2) 1(1) a Mode numbers in parentheses are those of indole12 based on the Mulliken convention.I6 Gaussian 90 ab initio frequencies multiplied by 0.80 for out of plane and by 0.90 for in plane. Theoretical intensities. IR intensities normalized to the strongest peak, 718 (out of plane) or 3471 (in plane). Raman intensities (in parentheses) normalized to the strongest peak, 3066 ( With the TM in a Assignments correspond to indole assignments based on normal the molecular plane, transitions involving the totally symmetric coordinate analysis.12 Nujol IR.= Raman obtained in-house by Ms. K. Liu. a'modes willdominatethespectrum, whereas transitionsinvolving the nontotally symmetric a" modes can only be observed as combinations or overtones where the resulting overall symmetry ments of the in-plane (a') modes than with the out-of-plane (a") is A'. From the 1C RZPI spectra, one can scc that the origin modes. The lack of agreement of the a" modes is expected since these modes are not as well predicted by the ab initio calculation, (0:) transition(s) are the most intense. This is not obvious in the but in general, these two methods are in good agreement, thus 6-fluoroindole 1C RZPI spectrum since the origin transition is making it possible to give reasonable overall ground electronic located in the weak wavelength portion of the dye used. The fact state vibrational mode assignments. that the origins are the most intense peaks leads one to conclude Excited-StateVibronic Assignments. The excited-state specthat, as expected for large molecules, the fluoroindoles do not trum was acquired by using the technique of jet-cooled one-color significantlychange size upon excitation and, accordingly, the SI (1C) resonant two-photon ionization (RZPI) time-of-flight mass excited electronic state is directly above the SOground electronic spectrometry (TOFMS). The 1C RZPI spectra of 4-, 5-, and state and the transition is essentially vertical in the Franck6-fluoroindole are presented in Figures 5-7, respectively. The S1 Condon sense. The vibronic assignments of the fluoroindoles were based on SOexcitation spectra of jet-cooled 5- and 6-fluoroindole correspond relatively well to those reported by Ayachit et ~ l . 1 ~ the vibronic assignments of By assuming that the
-
3387m
4266 The Journal of Physical Chemistry, Vol. 98, No. 16, 1994
Barstis et al.
TABLE 8 A s s i d Fulrdameatal Mode8 of S-FIuoroindole mode no.a
42 (42) 41 (41) 40 C-F bend 39 (40) 38 (39) 37 (38) 36 (37) 35 (36) 34 (34) 33 (33) 32 (32) 30 (31 or 30)
vb A. Out of Plane (A") 140 216 338 406 497 ?580
549 ?692 734 788 856 924 B. h Plane (A') 304 433 468 609 745 788 897 938
vc
mode n 0 . O
255 330 ?372 440 490 579 597 723 ?796 846 939
42 (42) 41 (41) 40 C-F bend 39 (40) 38 (39) 37 (37) 36 (?38) 35 (36) 34 (34) 33 (35) 32 (33) 31 (32) 30 (31)
vb A. Out of Plane (A") 141 218 327 408 504 549 597 686 732 787 ?851 862 925 B. In Plane (A') 305 432 473 602 747 785 897 947 1031 1085 1115 1159 1184 1222
P
260 330 ?360 438 511 601 738
?803 849 930
29 C-F wag ?372 28 (29) 470 27 (28) 61 1 26 (27) 25 (26) 755 24 (25+20) 23 (24) 896 22 (25) 860 ?951 (23) 1042 1067 21 (22) 1087 1089 20 (21) 1116 1120 19 (20) 1141 1138 18 (19) 1190 (18) 1207 1214 17 (17) 1231 1238 16 (16) 1246 1281 15 (15) 1286 1325 14 C-F stretch 1341 1345 13 (14) 1389 1421 12 (13) 1453 1458 1 1 (12) 1476 1483 10 (11) 1521 1511 9 (10) 1581 1583 8 (9) 1626 1626 7 (8) 3036 3010 6 (7) 3061 3045 5 (5) 3883 3065 4 (4) 3092 3113 3 (3) 3114 3139 2 (2) 3472 3445 1(1) Mode numbers in parentheses are those of indolei2 based on the Mulliken convention.i6 b Gaussian 90ab initio frequencies multiplied by 0.80 for out of plane and by 0.90 for in plane. Average experimental freqmciea.
29 C-F wag 28 (29) ?360 27 (28) 540 606 25 (27) 25 (26) 756 24 (25) 871 23 (24) 900 22 (23) ?955 21 (22) 1062 20 (21) 1092 19 (20) 1119 18 (19) 1143 17 (18) 16 (17) 1202 1236 (16) 15 (15) 1295 1269 14 C-E stretch 1319 1287 13 (14) 1341 1345 1380 12 (13) 1412 1 1 (12) 1443 1451 10 (11) 1500 1498 1512 1528 9 (10) 1575 1590 8 (9) 1626 1627 7 (8) 3010 3034 6 (7) 3065 3040 5 (5) 3067 3063 4 (4) 3104 3092 3 (3) 3115 3139 2 (2) 3398 3471 l(1) Mode numbers in parentheses are those of indole12 based on the Mulliken convention.16 Gaussian 90ab initio frequencies multiplied by 0.80 for out of plane and 0.90 for in plane. 'Averaged experimental frequencies.
excitation from the ground electronic state to the first excited electronic state is essentially the same for both the fluoroindoles and indole, one can use the vibronic assignments of indole to assign theexcitation spectra of the fluoroindole. Thus, thevibronic assignments of the fluoroindoles were made by using the ratio (mass-weighted) of the excited-state vibronic transitions to ground-state vibrational modes determined for indole and the ground-state vibrational modes of the fluoroindoles presented in Tables 7-9. This is a sufficiently good model, although not exact, for an initial analysis of the excitation spectra. Comparison of the mass-weighted ratios with the actual excited vibronic intervals found in the spectrum allowed assignments to be made, and the correct experimental value for the fundamental of the excited-state mode concerned was then attached to the mode. The excited-state vibronic frequencies so obtained are listed in Tables 10-12 for the 4-, 5-, and 6-fluoroindoles, respectively. The errors reported at the foot of those tables refer to the difference between the frequencies of combination and progression bands and the simple addition of the fundamentals making up the combination or progression. Thus, anharmonicity has not been taken into account, and it is to be expected-as
found-that the experimental frequency sums are slightly lower than the sum of their fundamentals. Assignments which show positive anharmonicities are also noted. The vibronic assignments of 5- and 6-fluoroindole presented in this paper differ from those reported by Ayachit et al.17 The vibronic assignments of the fluoroindoles reported by Ayachit et al. were based on the vibronic assignments of indole reported by Hollas.zl Hollas obtained a vapor-phase absorption spectrum of indole under ambient-temperature conditions and assigned the spectrum by first assigningthe sequence bands and then assigning the remaining bands to ground-state vibrational modes. The vibronic assignments of the fluoroindoles presented in this paper were based on the assignments of indole made from SVLF data reported by Bickel et al.4 Bickel et al. obtained a jet-cooled excitation spectrum of indole in which relatively no sequence bands were observed. The vibronic transitions observed in the spectrum were then directly assigned to ground-state xibrational modes by using SVLF spectroscopy, a technique where known ground-state vibrational modes are mapped to unknown excitedstate equivalents.
The Journal of Physical Chemistry, Vol. 98, No. 16, 1994 4267
Vibronic Analysis of 4-, 5-, and 6-Fluoroindole
I
I
mm
282.w
7
C
1
27350
"o
WAVELENGTH (nm) Figure 5. R2PI spectrum of 4-fluoroindole.
i8
TABLE 1 0 Excited-State Vibronic Assignments of
1
4-F'h0d&&'
I
lezao
217.70
651
49
bandno. 1 2 3 4
9 10 11 12 13 14 15 16 17
nm v Av 280.47 35 644 0 278.42 35 906 262 278.38 35 912 268 278.14 35 942 299 277.94 35 968 324 277.22 36 062 418 277.12 36 075 431 276.92 36 101 457 276.10 36 208 564 275.81 36 246 602 275.50 36 287 643 275.44 36 295 651 275.26 36 318 675 275.17 36 330 686 275.09 36 341 697 274.95 36 360 716 274.86 36 371 727
18 29 20 21 22 23 24 25
274.78 274.70 274.64 274.57 274.40 274.19 274.13 274.00
5
6 7
41
8
I
287.70
281.70
99
30
36 382 738 36 393 749 36 400 757 36 410 766 36 432 788 36 460 816 36 468 824 36 486 842
assgnmt 0:
422 412 40141' 402 392 27' 3g2 26l 39'42) 25' 362/404 35'38l 392412/38140241'/392411421 27l41'42'/391403/271422 3g240141l/38W3 2 7 W 4 1'136'4 1q21/382412/ 27W42' 392402 38240L421 36140'412/2714@/36'40141l42l 24' 23' 361381422/383421/361403 361381412/271381401 35'42)
a Error range of calculated-observed values is 0 to -4 the following: 3g2402,+4 cm-l.
273.50
277M
WAVELENGTH (nm) Figure 6. R2PI spectrum of 5-fluoroindole.
As in the case of indole, the origin transitions of 4-, 5-, and 6-fluoroindole (35 644, 34 342, and 34 979 cm-1, respectively) are 'Lb 'A' transitions, and the vibronic transitions observed
-
cm-l
except for
in the 1C R2PI spectra belong to a single excited electronic state. Thus, the assigned vibronic transitions in the spectral range of 0; to 0; 842 cm-1 of 4-fluoroindole and the assigned vibronic transitions in the spectral range of 0; to 0; 1100 cm-I of 5and 6-fluoroindole are all 'Lb 4- lA' transitions. In the IC R2PI spectra of 5- and 6-fluoroindole, 115 and 33 vibronic bands, respectively (vibronic bands listed in Tables 11 and 12, respectively), remain unassigned since the number of possible assignments suggests a breakdown in their description in terms of local and normal modes.
+
+
-
Barstis& al.
4268 The Journal of Physical Chemistry, Vol. 98, No. 16, 1994
TABLE 11: Excited-State Vibronic Assignmeats of 5-Fluoroindole. bandno. 1 2 3 4
14 15 16 17 18 19 20 21 22 23 24 2s
nm 291.10 290.82 289.12 288.94 288.37 288.14 287.90 287.63 287.54 287.47 287.41 287.19 287.07 286.74 286.70 286.54 286.49 286.31 286.21 286.12 285.88 285.74 285.33 285.23 285.14
26
285.00 35 077
27 28 29 30
284.97 284.84 284.75 284.67
35081 35097 35 108 35 118
31 32 33 34 3s 36 37
284.63 284.49 284.44 284.34 284.24 284.12 284.03
35 123 35 140 35 146 35 159 35 171 35 186 35 197
38
283.96 35 206
5
6 7 8
9 10 11 12 13
v
Av
34342 3437s 34578 34599 34668 34695 34724 34757 34768 34776 34783 34810 34 824 34865 34869 34889 34895 34917 34929 34940 34969 34987 35 037 35049 35 060
0 33 235 257 325 353 382 414 425 434 441 468 482 522 527 546 553 574 587 598 627 644 694 707 718
assgnmt 0;
412 28l 38l42l 27l 381411 402 37'411 392/424 361421/381401 36'411 3g2 372 26l 391423 413421 371423 27l42' 426/271411421 2S1 362/371412421/381401422/
26141142'/382412/271371411/
28'38'39l 863 361391411421/382401421/ 281371391/371392421/381402411/
39 40 41 42
283.88 283.78 283.67 283.57
35 216 35 228 35 242 35 254
873 886 899 912
43 44 45 46
283.54 283.48 283.35 283.27
35 258 35 265 35 282 35 292
916 923 939 949
47
283.21 35 299
48
283.10 35 313
49
283.06 3s 318
so
51
282.99 35326 282.91 35 336
52
282.84 35 345
53
282.73 35 359
54 55
282.62 35 373 282.54 35 383
56
282.44 35 395
57
282.42 35398
58
nm v Av assgnmt 282.37 35 404 1062 37)391/28135'401/36'39'/
59
282.28 35 415 1073
60
282.22 35 423 1080
61
282.16 35 430 1088
62
282.04 35 446 1103
63
282.00 35 450 1108
64
281.90 35 463 1121
65 66 67 68 69 70 71 72 73 74 7s 76 77 78 79 80 81 82 83 84
281.83 28 1.79 28 1.72 28 1.67 28 1.64 28 1.60 28 1.47 28 1.42 281.32 28 1.28 281.18 281.11 28 1.04 280.94 280.88 280.74 280.71 280.66 280.58 280.55 280.48 280.44 280.42 280.29 280.24 280.18 280.08 280.02 279.96 279.90. 279.78 279.67 279.66 279.60 279.51 279.40 279.34 279.31 279.26 279.20 279.17 279.14 279.10 279.05 279.00 278.86 278.82 278.71 278.67 278.60 278.56 278.28 278.22 278.1 1 278.01 277.94 277.79 277.74
42=
402411421 735 28138142'/381391422/28"41'/ 391401411421 739 271412 755 3S1381 766 271391421/382422/371413 776 371381422/39140'412/ 371401411421 781 23l 798 27l38142l 804 281391401/392401421/361412421 816 381402421/403411/361401422 829 361391422/371#421/272 844 27'38l41' 855
bandno.
3613g142* 371402411/361371422 391403/271392 261412/271381401 271381391/361#422/383421/
261401421 381403/361371411421/271381391 371382421/393401/371381401411 394/371381391411/271361411 381392401/372391411/361381412/ 373421/281361391/351401422/
3613g2421 957 261381421/3614@411/362422/ 383411/271371381/261401411
970 361371401421/261391411/ 261371421/37138241 '/3813g3/ 271372 97s 36'381391421/3713a'402/ 361391401412 984 3S2 994 351381422/371381391401/373411/ 27I36l4O1 1003 372391401/361381401411/ 26138'411 1017 361371381421/231422/ 36*371401411 1030 36137242'/361391#/38339' 1040 371382391/271361381/ 3S1381411421 1052 362401421261361421/351391412/ 372381391/36138241 1/271361371 1055 26'381401
85
86 87 88
89 90 91 92 93 94 9s 96 97 98 99 100 101 102 103 104 105 106 107
1oa
109 110 111 112 113 114 115 116 117 118 119 120 121 122
35 472 3s 477 35 486 35 492 35 496 3s 501 35 517 35 524 35 536 35 541 3s 554 35 563 3s 512 35 584 35 592 35 610 35 613 35 620 35 630 35 634 35 643 35 648 35 650 35 667 35 673 35 681 35 694 35 701 3s 709 35 717 35 732 35 746 3s 747 3s 755 35 766 35 780 35 788 35 792 35 798 35 806 35 810 35 814 35 819 35 825 35 832 35 850 35 855 35 869 35 874 35 883 35 888 35 924 35 932 35 946 35 959 35 968 35 988 3s 994
1130 1135 1143 1150 1153 1159 1175 1181 1194 1199 1212 1220 1229 1242 1250 1267 1271 1277 1288 1291 1300 1305 1308 1324 1331 1338 1351 1359 1366 1374 1389 1403 1405 1412 1424 1438 1446 1450 1456 1464 1468 1471 1476 1483 1489 1SO7 1512 1527 1532 1541 1546 1582 1590 1604 1617 1626 1645 1652
The Journal of Physical Chemistry, Vol. 98, No. 16, 1994 4269
Vibronic Analysis of 4-, 5-, and 6-Fluoroindole
TABLE 11 (Continued) band no. nm
AY
nm Y 275.84 36 242 123 277.68 36 002 1660 275.80 36 247 36 018 1675 124 277.56 275.66 36 266 125 277.48 36 028 1686 275.58 36 276 36 038 1696 126 277.40 275.54 36 282 127 277.34 36 046 1704 275.50 36 287 36 062 1719 128 277.20 275.48 36 290 36 077 1735 129 277.10 275.42 36 297 36 080 1738 130 277.08 275.38 36 303 36 088 1745 131 277.02 275.28 36 316 1757 132 276.93 36 100 275.20 36 326 36 107 1765 133 276.87 275.12 36 337 36 110 1768 134 276.85 275.02 36 350 36 119 1777 135 276.78 36 356 274.98 36 126 1783 136 276.73 274.94 36 361 36 132 1790 137 276.68 274.86 36 371 36 139 1796 138 276.63 274.82 36 377 36 154 1812 139 276.51 274.78 36 382 36 156 1813 140 276.50 274.75 36 386 36 161 1819 141 276.46 274.72 36 390 1828 142 276.39 36 170 274.70 36 393 1829 143 276.38 36 171 274.62 36 403 144 276.36 36 174 1832 274.48 36 422 36 184 1842 145 276.28 274.40 36 432 36 196 1854 146 276.19 274.34 36 440 36 203 1860 147 276.14 274.30 36 446 36 209 1867 148 276.09 274.22 36 456 36 216 1874 149 276.04 274.14 36 467 150 275.97 36 225 1883 274.04 36 480 1895 151 275.88 36 237 * Error range of calculated - observed values is 0 to 4cm-l except for the following: 381411, +1 cm-l; 36]42l, -5 cm-l; +6 cm-I; 371423, +3 cm-l; 27l42*, +5 cm-'. Y
assgnmt
1
I aim
I aa6.00
26
o1.m
17620
ES
band no. 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180
AY
assgnmt
1900 1905 1923 1934 1939 1945 1947 1955 1960 1974 1984 1995 2008 2013 2018 2029 2034 2040 2044 2048 2050 2061 2079 2090 2098 2103 2114 2125 2138 39'423, +1 cm-'; 413421,
coordinate analysis.I2 The excited-state vibronic modes of the fluoroindoles have been assigned on the basis of direct comparison to indole as determined from SVLF workq and are more accurate than those reported by Ayachit et a1.I' since the indole vibronic assignments used were more accurate. Thevibronic assignments of the fluoroindoles strongly suggest that the vibronic transitions are all 'Lb 'A' transitions. No 'La 4- 'A'origin transition was assigned, thus increasing the energy gap of the 'La and 'Lb excited electronic states from >950 cm-1 in indole to >1100 cm-1 in the fluoroindolessubstituted at the five and six positionsof the indole ring. As a final note, the complexity of the excitation spectrum appears to increase in the order of 4-fluoroindole < 6-fluoroindole < 5-fluoroindole, as shown in Figures 5-7. Substitution at the five position of the indole ring appears to strongly affect the excitation s p e c t r m , whereas substitution at either the four or six position of the indole ring only slightly affects the excitation spectrum. Whether the complexity of the excitation spectrum is somehow greater in other indole molecules substituted in the five position and whether the spectral complexity exhibited in these molecules corresponds to unique excited-state dynamics must be answered with additional research.
-
3
~
WAVELENGTH (nm) Figure 7. R2PI spectrum of 6-fluoroindole.
Conclusion The ground-state vibrational modes of the fluoroindoles have been assigned on the basis of direct comparison to indole as determined from ab initio frequency calculations9 and normal
Acknowledgment. This work has been supported by the National Science Foundation under Grant No. CHE-90226 10. L.I.G. acknowledges support by the Department of Chemistry at The University of Michigan under a Dow-Britton Fellowship and support from the Research Partnership Award sponsored by the Dean of the Rackham School of Graduate Studies of the Office of the Vice President for Research of the University of Michigan. We gratefully acknowledge Kei-Lee Liu at The University of Michigan for obtaining the Raman spectra of the fluoroindoles. We also acknowledgethe time contribution made available on the Cray Y/MP by the San Diego Supercomputer Center.
Barstis et al.
4270 The Journal of Physical Chemistry, Vol. 98, No. 16, 1994
TABLE 12: Excited-State Vibronic Assienments of 6-FluoroindolP ~~
bandno. 1
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 31 32
nm 285.80 284.14 283.60 283.23 283.07
v
Av
34979 35 184 35250 35297 35316
282.69 282.61 282.50 282.41 282.35 282.15
35364 35374 35388 35399 35407 35432
281.75 281.61 281.40 281.30 281.22 281.08 280.95 280.70 280.52 280.44 280.23 280.16 280.07 280.02 279.86 279.82 279.71 279.58 279.47
35482 35500 35 526 35539 35549 35567 35583 35615 35638 35648 35674 35683 35695 35701 35722 35727 35741 35757 35771
0 204 271 317 337 353 385 395 408 420 427 452 465 503 520 547 560 570 587 604 635 658 668 695 704 716 722 742 747 762 778 792
assgnmt
bandno. nm v 44 278.34 35 917 45 278.26 35927
~
~
assgnmt 937 37'391402l281351421 948
Av
412 28' 38'42' 39'41l 27' 381411/424
46
278.19 35936
957
47 48
278.11 35 946 277.94 35968
967 989
49
277.86 35979
999
402 392 381401 38l 412422/401423 26l 372/381423 4l342I 361391/371423 27'422 4@422/40141 2421/414 392422/27141 '42' 361423/281391421 28l38142l 391413/39140141 l42l 25' 362/28139141'/28'371421 381391411421 28138'4l1/4024l2/4O342' 23l
50 51 52
277.68 36002 1023 277.60 36012 1033 277.56 36018 1038
53 54
277.50 36025 1046 277.45 36032 1053
55 277.06 36083 1103 56 277.00 36090 1 1 1 1 57 276.92 36 101 1122 58 276.75 36 123 1144 59 276.63 36 139 1160 60 276.52 36 153 1174 61 276.42 36 166 1187 276.21 36 194 1215 62 63 276.16 36200 1221 64 276.05 36215 1236 65 275.82 36245 1266 66 275.76 36253 1274 271371421/281391401/28'37141' 67 275.63 36270 1291 68 371381411421/361391422/ 275.55 36280 1301 281392/272 69 275.53 36283 1304 70 279.33 35789 810 371401412/371402421/26141 '4211 275.47 36291 1312 33 71 275.44 36295 1316 3813g2421 72 279.21 35805 826 3g2401411/27'4@/371391412/ 275.41 36299 1320 34 73 37l39l4O142I 275.36 36305 1326 74 279.08 35821 842 28'382/38l4O24l1 35 275.34 36308 1329 1422/404/ 75 278.95 35838 859 383421/271361421/35141 275.33 36309 1330 36 76 28136'4 1 275.20 36326 1347 77 278.86 35850 870 281371381/391403/37140241 275.14 36334 1355 37 78 278.78 35860 881 392402/371391401411 275.11 36338 1359 38 79 39 278.73 35866 887 371382421/261281 275.08 36342 1363 80 40 278.62 35881 901 36'37141 1421/271371391/271382/ 275.00 36353 1374 28l37l 81 274.92 36363 1384 278.50 35896 917 37138139141 1/361392421/383411/ 82 274.88 36369 1390 41 83 372381421/271361411 274.77 36383 1404 278.41 35908 928 271371381/362422/351391422/ 84 274.68 36395 1416 42 2g136l39I 85 274.60 36406 1427 278.38 35912 932 382402/372401411/261391411/ 86 274.51 36418 1439 43 87 381393/36138'412/36138140'421/ 274.30 36446 1467 26'37'42l 88 274.21 36458 1479 a Error range of calculated - observed values is 0 to -4 cm-1 except for the following: 38'421, +1 cm-l; 38'411,+2 cm-I; 381401,+1 cm-I; 412422, +1 cm-I; 41342l,+6 cm-1; 27l42l, -5 cm-l.
References and Notes (1) Phillip, L. A.; Levy, D. H. J . Chem. Phys. 1986,85, 1327. (2) Bersohn, R.; Even, U.; Jortner, J. J. Chem. Phys. 1983,80(3), 1050. (3) Hager, J. W.; Demmer, D. R.; Wallace, S.C. J. Phys. Chem. 1987, 91, 1375. (4) Bickel, G. A.; Demmer, D. R.; Outhouse, E. A.; Wallace, S. C. J. Chem. Phys. 1989, 91(10),6013. (5) Nibu, Y.;Abe, H.; Mikami, N.; Ito, M.J . Phys. Chem. 1983, 87,
3898.
(6) Strickland, E. H.; Horwitz, J.; Billip, C. Biochemistry 1970,9(25),
4914.
(7) Cable, J. R. J . Chem. Phys. 1990, 92(3), 1627. (8) Catalan, J.; Perez, P.; Acuna, A. U. J. Mol. Struct. 1986,142, 179. (9) Bantis, T. L. 0.;Grace, L. I.; Dunn, T. M.; Lubman, D. M. J. Phys. Chem. 1993,97, 5820. (10) Eftink, M.R.; Selvidge, L. A.; Rehms, A. A.; Callis, P. R. J . Phys. Chem. 1990, 94, 3469. (11) Lami, H.; Glasser, N. J. Chem. Phys. 1985,84(2), 597. (12) Takeuchi, H.;Harada, I. Spectrochim. Acta 1986,42A, 1069. (13) Frisch, M. J.; Head-Gordon, M.; Trucks, G. W.; Foraman, J. B.; Schlegel, H. B.; Raghavachari, K.;Robb, M.; Binkley, J. S.; Gonzalez, C.; Defrees, D. J.; Fox, D. J.; Whitcside, R. A.; Seeger, R.; Melius, C. F.;Baker,
J.; Martin, R. L.; Kahn, L. R.; Stewart, J. J. P.; Topiol, S.;Pople, J. A. Gaussian 90, revision F; Gaussian, Inc.: Pittsburgh, PA, 1990. (14) Frisch, M. Gaussian 90 User's Guide and Programmer's Reference, Revision I Version; Gaussian, Inc.: Pittsburgh, PA, 1991. (15) Smith, A. Molecular Editor, softwarepackage for Apple Macintosh; Intellmation: Santa Barbara, CA. (16) Mulliken, R. S.J . Chem. Phys. 1955, 23(11), 1997. (1 7) Ayachit,N. H.; Huralikoppi,A. M.; Shashidhar,M. A. Spectrochim. Acta 1986, 42A(7), 781. (18) Lambert, P.; Lecomte, J. Compt. Rend. 1939, 208, 1148. (19) Lautie, A.; Lautie, M. F.; Gruger, A.; Fakhri, S . A. Spectrochim. Acta 1980, 36A, 85. (20) Collier, W.J. Chem. Phys. 1988, 88(12), 7295. (21) Hollas, J. M. Spectrochim. Acta 1963, 19, 753.
(22) Nujol infrared spectrum of Cfluoroindoleobtained through private communication with Sigma Chemical Co., Inc., St. Louis, MO. (23) Kellcr, R. J. The Sigma Library of FT-IR Spectra, 1st ed.; Sigma Chemical Co., Inc.: St. Louis, MO, 1986;Vol. 2,p 573. (24) Pouchert, C. J. The Aldrich Library of FT-IR Spectra Vapor Phase, 1st ed.; Aldrich Chemical Co., Inc.: Milwaukee, WI. 1989; Vol. 3, p 1500.
