J . Phys. Chem. 1993,97, 5820-5825
5820
Vibronic Analysis of Indole and 1H-Indole-d6 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: October 12, 1992; In Final Form: February 5, 1993
The excitation spectra of indole and indole-2,3,4,5,6,7-d6 have been obtained with jet-cooled one-color resonant two-photon ionization (1 C R2PI) time-of-flight mass spectrometry (TOFMS). The deuterated indole spectra have been vibronically analyzed in the region 0; to 0; 904 cm-I using the single vibronic level fluorescence (SVLF)results of Bickel et al.5 as a guide for the deuterated species and an a b initio ground-state vibrational calculation of both species. This calculation was obtained by using Gaussian 90, from which the excited-state frequencies of the deuterated species have been estimated from the known excited-state normal protonated values. Comparison of both excited-state vibronic spectra has shown an essentially 1:l porrespondence between all of the bands, suggesting that there is no significant band in the indole spectrum in this interval which can uniquely be assigned as belonging to a system other than the l L b 'A' transition.
+
-
Introduction
The indole molecule has been the center of much discussion in the literaturewith regard to the ground-state normalvibrational mode assignments and the lowest electronic-state vibrational assignments. The ground-state vibrational modes of indole have been assignedusing various methods,'d and the assignments made by different groups show numerous discrepancies. Takeuchi and Harada2 provided a normal coordinate analysis of indole and many of the deuterated indole molecules, while Collier3provided a semiempirical calculation in addition to experimental spectra of indole. In this paper, we providean ab initiocalculation yielding the ground-state normal vibrational modes of both indole (with seven hydrogens) and lH-ind01e-d~(henceforth denoted as indole(-h6) and indole(-d6), respectively). We use calculations performed with indole(-h6) and their comparison with experiment to propose a set of ground-state vibrational frequencies for the indole(-d6) compound. (While this paper was under review, a paper was published' using the same approximations discussed above. For the most part, this paper agrees with our results.) These calculations are an improvement over semiempirical calculations since no initial assumptions or guesses are made. The calculationsprovided here fully support the work of Takeuchi and Haradaa2 Vibronicanalyses of indole have been made by several group^,^,^ mainly in search of the origin of the postulated 'La electronic statee7-I4Both Bickel et al.5and Ito et a1.6 have published SVLF data on the indole molecule which have provided valuable information as to the vibronic nature of the observed bands. In this paper, we provide the jet-cooled one-color (1C) resonant two-photon ionization (R2PI) spectrum of indole(-h6) as well as that of indole(-d6). By carefully comparing the better known ground-state and excited-state analyses of indole(-h6) with those of indole(-d6), the vibronic analysis of the 1C R2PI spectrum of indole(-d6) has been largely accomplished in the region 0; to 0; 904 cm-1.
+
Experimental Section
The experimental arrangement is similarto that used in previous work.I6 The time-of-flight (TOF) mass spectrometer (R.M. Jordan, Co.) is a variation of the basic Wiley-McLaren config~rati0n.I~ It is mounted vertically in a stainless steel six-port cross and pumped by a 6411. diffusion pump. The flight tube is differentially pumped by a 4-in. diffusion pump. The TOFMS serves as a detection system in which a dual-microchannel plate detector monitors the ion signal. 0022-3654/93/2097-5820$04.00/0
An R.M. Jordan PSV- 1 pulsed nozzle provides the beam source in our experiments. The maximum rate of flow through the orifice, i.e., 'choked flow", is reached when the fwhm of the gas pulse is 55 p, so maximum theoretical cooling is achieved. Sample introduction is accomplished by flowing COz carrier gas over a reservoir containing the solid. The reservoir is heated to provide sufficient vapor pressure. A supersonic molecular beam is produced by expanding the seeded mixture from a pressure of 1 atm absolute into the acceleration region of the TOFMS at a pressure of 10-6 Torr. At a fixed distance of 17 cm from the nozzle in the molecular "free flow" region between the TOF acceleration plates, the sample is ionized by laser radiation perpendicular to both the sample jet and the TOFMS device. The laser source consists of the output of a Quanta Ray Nd: YAG (DCR-3) pumped dye laser (PDL-1A). Tunable UV radiation is generated by frequency-doubling the dye output in a phase-matchedKD*Pcrystal. This is performedusing a Quanta Ray WEX-1 wavelength extension device. The UV radiation is collimated with a telescope (positive lens, 30-cm focal length; negative lens, 10-cm focal length) to a -2-mm beam. A Quanta Ray CDM- 1control display module is used to control the stepping motor which tunes the grating in the dye resonance cavity. As an important note, the dye laser calibration reveals an 8-cm-I difference; however, the spectral data are presented as obtained. Experiments were performed at the 10-Hz repetition rate of the laser system. The flight time of the molecular beam requires proper synchronization between the pulsed nozzle and the laser. The oscillator output from the Nd:YAG laser triggers the pulsed nozzle control unit which generates a high-current pulse, causing the valve to open. An adjustable delay between the oscillator pulse and the pulse which triggers the nozzle provides a means of synchronizing the arrival of the molecular beam into the interaction region with the laser pulse. The laser then fires and triggers the LeCroy 9400A digital oscilloscopeand the Stanford Research SystemSRS 250 gated integrator. Thegated integrator unit was used to monitor the molecular ion peak in the TOFMS as a function of wavelength, and the signal was displayed on a strip chart recorder. Nocorrection was made for the background in the 1C R2PI spectra by either subtracting or normalizing the ionization intensity to the dye laser intensity curve. Indole(-h6) was obtained from Sigma Co., and indole(-d6) was obtained from Cambridge Isotopes Laboratories, Inc. The samples were commercially available and used without further purification since the spectrum was mass analyzed. 0 1993 American Chemical Society
Vibronic Analysis of Indole
The Journal of Physical Chemistry, Vol. 97, No. 22, 1993 5821
TABLE I: Fundamental Vibrational Modes of Indole(-M) (cm-1)
G90 VU*r.,ir
v
V*”
Harada2 Collier3 Zwarich4 Lautiel BickeP v
Y
V
V
V
Out-of-Plane (A”) 42 41 40 39 38 37 36 35 34 33
246 282 493 617 665 690 852 883 920 1022
197 226 395 494 532 552 682 707 736 818
225 256 403 426 488 586 607 722 748 780 (767)b
221 252 400 426
235 254 398 426 498 578 612 732 752
224 254 397 423 487 575 608 725 743
208 241 403 422 480 572 605 705 733 763
32
1072
858
853
798 860
898
849
930 968
945
848 873 930 970
31 30
1122 1168
898 934
934 971
29 28 27 26
436 603 687 831
393 543 618 748
398 543 608 763
410 544 608 761
505 546 611 765
25
968
871
870
878
24 23 22 21 20 19 18 17 16 15
997 1107 1151 1209 1243 1281 1320 1362 1390 1415
898 996 1036 1088 1119 1153 1188 1225 1251 1274
867 (873)b 905 1005 1063 1096 1117 1146 1185 1207 1246 1290
897 1012 1066 1089 1121 1149
899 1017 1063 1095 1120 1146
1205 1246 1277 1300
1204 1248 1278
14 13 12 11 10 9 8
1506 1556 1616 1656 1686 1745 1783 3349 3358 3370 3382 3432 3458 3859
1356 1400 1454 1490 1517 1571 1605 3014 3022 3033 3044 3089 3112 3473
575 618 724 739 765
923
In-Plane (A’)
1347 1413 1467 1486 1508 1573 1616
1351 1411 1456 1488 1510 1577 1617 3064 3064 3064 3082 3082 3148 3447
1336 1355 1416 1458 1489 1509 1578 1617
542 607 758 767
395 544 610 760 873
895 1010 1064 1092 1119 1147 1191 1203 1245 1276
903 1015 1068 1085 1123 1142
1334 1352 1412 1455 1487 1509 1576 1616 3050 3050 3050 3050 3106 3123 3419
1334 1350 1410 1459 1479
1208 1248 1288
v * : v 0.80 for out-of-plane; v 0.90 for in-plane. Experimental frequencies from Lautie et al.] reassigned by Takeuchi and Harada.2
Results Ground-State Normal Mode Assignments. The ground-state normal vibrational modes of indole(-h6) and indole(-d6) were calculated using Gaussian 90,18x’9 a widely used computational ab initio program. A geometrical optimization was performed prior to the frequency calculation. Both calculations were performed at the HartreeFock level using a 3-21G basis set*. The Gaussian 90 calculations were performed on a Cray Y/MP at the San Diego Supercomputer Center. The ab initio geometrical optimization of indole confirms that it is a planar molecule belonging to the C, point group. Indole contains 42 normal vibrational modes, of which 29 are in-plane (a’) modes and 13 are out-of-plane (a”) modes, as derived from the C, point group symmetry considerations. The results of the ground-state ab initio frequency calculation of indole(-h6) are given in Table I, along with the experimental results from other research groups.l-4 The frequencies obtained from the Gaussian
Figure 1. Indole out-of-plane (a”) fundamentalvibrationalmcdeassigned and depicted as v37. Vibrational motion as described by Gaussian 9018,19 by Molecular Editor2I software package.
90 calculation are scaled down by 10-20% to match the experimental frequencies. While the precision can be improved by using a larger, more complete basis set or by performing a configuration interaction calculation to include electron correlations, the sets generated are sufficiently self-consistent to be used after appropriate reduction and are therefore considered as forming a sufficientlygood model from which reliable frequencies for the (-d6) compound can be obtained. The a’vibrational modes of indole as calculated from Gaussian 90 are in good agreement with experiment (-90%), whereas, the a” vibrational modes of indole are not (580%). The general reason that the a’’ modes are not well-predicted by this theoretical technique is probably because the a” modes are more anharmonic and therefore deviate from the harmonic approximation used in the ab initio program. To verify the results of the ab initio frequency calculations of indole(-h6) with the results previously reported in the literature, a comparison of the normal vibrational mode assignments was made. In Table I, one can clearly see the discrepancies of these normal vibrational mode assignments. One obvious discrepancy is the a’ frequencies at 1335 and 1354 cm-I, which were assigned as two fundamental frequencies by Suwaiyan and Zwarich4 and Lautie et al.’ but as only one fundamental frequency at 1349 cm-I by Gaussian 90, Takeuchi and Harada,2 and Colliera3 Harada refers to the frequency pair of 1335 and 1354 cm-I as a Fermi doublet based on both normal coordinate analysis work2 and experimental work.” Another discrepancy is the a” frequency at 491 cm-1 (averaged experimental frequency V38) assigned as a fundamental frequency by all but Collier, who assigns it as an overtone or combination band. Collier assigned the vibration at -491 cm-I as an overtone or combination band because the semiempirical calculation did not predict a fundamental vibrational mode at -491 cm-I. Since the normal coordinate analysis data and the ab initio calculation are more accurate techniques, we assign 491 cm-I as the fundamental vibrational frequency of the v38 mode. The normal vibrational modes predicted by the ab initio calculation were compared with the results of the normal coordinate analysis performed by Takeuchi and Harada2 by focusing on the vibrational motion generated by each of the fundamental modes. This comparison was made possible by visualizing the vibrational motion of the fundamental modes calculated from Gaussian 90 using the software package Molecular Editor.21 Molecular Editor allows thevisualization process by graphically oscillating between the two extremes of the vibrational motion. In brief, the output generated by Gaussian 90 provides the equilibrium geometry and the displacements of the respective fundamental frequency in Cartesian coordinate space. The displacements are both added to and subtracted from the equilibrium geometry to yield the two extremes of vibrational motion. Figure 1 shows the a” fundamental mode assigned to the 575-cm-1 vibration, and Figure 2 shows the a’ fundamental mode assigned to the 758-cm-I vibration. By careful examination of the vibrational motion of the fundamental modes calculated by Gaussian 90 and by Takeuchi and Harada,2 we conclude that there is significant agreement in the identification of the fundamentalvibrational modes. The fundamental modes together with their respective mode numbers are given in Table 11. The numbering of the vibrational modes follows the Mulliken convention.22 There is excellent agreement of the vibrational motion of a’ fundamental frequencies but some disagreement of the vibrational motion of a” fundamental frequencies, namely, v38 and v39,vj4 and v~~~and ~ 3 and 2 v33, in each of which pair the
Barstis et al.
5822 The Journal of Physical Chemistry, Vol. 97, No. 22, I993
TABLE III: Assigned Fundamental Vibrational Modes of
0
Indole(-d6) (cm-l) G90 Y
Figure 2. Indole in-plane (a’) fundamental vibrational mode assigned as V26, the “ring breathing” mode. Vibrational motion as described by Gaussian 901*.19 and depicted by Molecular Editor2’ software package.
TABLE E. Assi ed Fundamental Vibrational Modes of Indole(-h6) (cm-l$ G90 V
V
u*O
42 41 40 39 38 37 36 35 34 33 32 31 30
246 282 494 617 665 690 852 883 920 1022 1072 1122 1168
197 226 395 494 532 552 682 703 736 818 858 898 934
29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
436 603 687 831 968 997 1107 1151 1209 1243 1281 1320 1362 1390 1415 1506 1556 1616 1656 1686 1745 1783 3349 3358 3370 3382 3432 3458 3859
393 543 618 748 871 898 996 1036 1088 1119 1153 1188 1225 1251 1274 1356 1400 1454 1490 1517 1571 1605 3014 3022 3033 3044 3089 3112 3473
Haradaz v
Lautiel v
Out-of-Plane (A”) 225 256 403 426 488 586 607 722 748 780 (767)b 853 934 97 1 In-Plane (A’) 398 543 608 763 867 (873)* 905 1005 1063 1096 1117 1146 1185 1207 1246 1290 1347 1413 1467 1486 1508 1573 1616
224 254 397 423 487 575 608 725 743 848 930 970
542 607 758 895 1010 1064 1092 1119 1147 1191 1203 1245 1276 1352 1412 1455 1487 1509 1576 1616 -3050 -3050 3050 -3050 3106 3123 3419
BickelS Y 208 24 1 403 422 480 572 605 705 733 763 849 923
395 544 610 760 876 903 1015 1068 1085 1123 1142 1208 1248 1288 1350 1410 1459 1479
-
0.80 for out-of-plane; v 0.90 for in-plane. Experimental frequencies from Lautie et ale1reassigned by Takeuchi and Harada.2 v*:
Y
vibrational assignment has been switched from the previous assignments. The ab initio results of indole(-d6) are given in Table 111, along with the results from previous work.1.2 Again, the in-plane (a’) vibrational modes are in excellent agreement with experiment, whereas the out-of-plane (a”) vibrational modes are not. The normal vibrational modes calculated using Gaussian 90 and normal coordinate analysis methods are in excellent agreement, and we have adopted it in our analysis. Vibronic Assignments. The excited-state spectrum was acquired by using the technique of jet-cooled onecolor (1C) resonant
Y
v*4
Lautiel
42 41 40 39 38 37 36 35 34 33 32 31 30
227 260 425 568 592 620 667 703 754 888 913 93 1 1013
Out-of-Plane (A”) 182 207 208 236 340 366 455 474 469 496 503 534 539 562 576 603 589 710 730 710 744 76 1 810 860
29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
402 582 661 770 863 879 916 948 963 1003 1049 1161 1214 1299 1335 1348 1483 1489 1581 1610 1706 1753 247 1 2480 2495 2508 2535 258 1 3859
In-Plane (A’) 362 369 524 524 595 584 693 697 777 764 791 787 824 818 854 844 867 860 902 90 1 944 947 1044 1093 1130 1169 1195 1202 1213 1232 1335 1323 1340 1387 1423 1434 1449 1452 1535 1549 1578 1594 2224 2232 2246 2257 2282 2323 3473
v*:
Y
Y
Harada2 v 21 1 236 367 372 469 499 530 566 585 635 710 756 857 368 524 580 707 761 784 815 838 858 897 946 1005 1119 1192 1209 1243 1321 1382 1422 1454 1557 1599
0.80 for out-of-plane; Y 0.90 for in-plane
two-photon ionization (R2PI) time-of-flight mass spectrometry (TOFMS) with the experimental setup already described. Figure 3 showstheRZPIspectrumofindole(-h6).ThisS, +Soexcitation spectrum of jet-cooled indole(-h6) corresponds well to spectra previously reported in the l i t e r a t ~ r e ~ I. ~except J ~ J for some differences in the relative intensities of sets of bands. This does not affect the frequency differences,which are essentiallyidentical with those found in previous work. The SI-SO transition of indole(46) is generally assumed to be a ?r* ?r transition with the transition moment (TM) in the plane of the m o l e c ~ l e . ~Taking ~ - ~ ~ the TM to be in the plane of the molecule, transitions involving the totally symmetric a’ modes will dominate the spectrum, whereas transitions involving the nontotally symmetric a” modes can be observed only as combinations or overtones where the resulting overall vibronic symmetry is A’. From the 1C RZPI spectrum, one can see that the origin transition (0;)is the most intense peak. We conclude that, as expected for large molecules, indole does not significantly change size upon excitation. Accordingly,the SIexcited electronic state is directly above the SOground electronic state, and the transition is essentially vertical in the Franck-Condon sense. The most conclusive method one can use to spectroscopically analyze an excitation spectrum is the technique of single vibronic level fluorescence (SVLF). In this method, a single vibronic
-
Vibronic Analysis of Indole
The Journal of Physical Chemistry, Vol. 97, No. 22, 1993 5823 TABLE I V Vibronic Assignments of Indole(-b6)'
3Mo
band
X,nm
v
Av
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 11 18 19 20 21 22 23 24 25 26 21 28
283.68 283.41 283.26 281.16 280.17 280.65 280.21 280.06 279.86 219.18 219.52 219.39 219.33 219.25 218.00 211.87 211.73 211.11 211.60 277.52 217.40 271.01 211.00 216.96 216.88 216.66 216.56 216.41
35241 35214 35293 35556 35 606 35621 35611 35696 35122 35131 35165 35782 35189 35800 35961 35 911 35996 35998 36012 36023 36038 36 081 36090 36096 36 106 36 135 36 148 36 160
34 52 316 365 380 436 456 48 1 490 524 54 1 549 559 120 131 155 158 112 182 198 84 1 850 855 865 894 901 919
29
216.41
36 161
921
30
276.39
36 110
929
31 32 33
216.21 216.09 216.06
36 194 36209 36213
953 969 913
34 35
215.96 215.93
36226 36230
986 990
36
215.88
36231
9966
31
215.85
36241
1000
1 , ,
274
127
276
276
zn
278
279
I
280
Figure 3. Jet-cooled R2PI spectrum of indole(-h6) in the wavelength range (a, top) 285-211 nm and (b, bottom) 280-274 nm.
transition is probed by collecting the dispersed fluorescence spectrum following excitation. The strongest feature in most SVLF spectra is the Av = 0 transition, and built on the main Av = 0 transition are ground-state vibrational modes. Thus, SVLF allows one to map known ground-state vibrational modes to the unknown excited-state equivalents. SVLF data on the indole(-h6) molecule have been provided in the literature by Bickel et aL5and Ito et a1.6 The assignments used in this paper are based on Bickel et al.'s work supplemented by a scaled comparison of the calculated ground-state frequencies for both the (-h6) and (-d6) species. Those assignments are shown in Table IV. Bickel et al. found that in the region above -0; + 950 cm-l the spectrum is complicated by combinations of many vibrational modes causing direct assignments of the vibronic transitions to be complex, and our assignments show this clearly. The 1C R2PI spectrum of indole(-d6) is shown in Figure 4. Again, the origin transition is the most intense peak in the spectrum, so one can assume, as in the case of indole(-h6), that the geometry of the molecule does not significantly change upon excitation and the transition is vertical in the Franck-Condon sense. By also assuming that essentially the same type of behavior occurs during the excitation of both indole(-h6) and indole(-d6) molecules, one can use the vibronic assignments of indole(-h6) to assign the indole(-d6) excitation spectrum. Thus, the vibronic assignments of indole(-d6) were made by comparing the ratio of the excited-state modes to the ground-state modes calculated for indole(-h6) and the ground-statemodes of indole(-d6) as computed from Gaussian 90 ab initio calculations and normal coordinate analysis.2 This is not, of course, exact, but it is a sufficiently good model for the initial analysis. These vibronic assignments are given in Table V. Discussion
In this work, we are confining the discussion to indole(-h6) and indole(-d6) in the gas phase so that the assignments of bands
0
assignment 0 :
sequence sequence *422 *412 *291 *39'42l *39'41L *28' 402 *311421 *211 *311411 *392 '261 *312 25 292/391423 *361391/40'412421 29'401421/39'41 '422 28'42l 311423/391401422/38141z421 28'412 402412/291381421 38'41 3/39140141'42l/29'281 311412421/39'40141~/40~421 211412/291311421/291391401 281391421/21129'/ 311413/403411 311401422/392412/38 '40'4 121 39'402421 31140142z/392412/381401412/ 39IW42l 31'40141 1421/39140241 281402/27140141I 361423/382411421/29'381391/ 28'402 38'40241 1/39240141 371391411421/38'40241~/ 3g240141I 382412/361411422/393421/ 38'39'40'42' 362/311381422/361411422/ 393421/382412/381391401421
* refers to the vibronic assignmentsof Bickel et aLs v in cm-I. Error range of calculated-observedvalues (where calculatedrepresentsharmonic values of assigned fundamental modes) is 0 to 4cm-1 except for the following: 39'421, +1 cm-I; 39l411, +5 cm-I; 31'42l, +2 cm-l; 311411, +2 cm-1; and 36139', +6 cm-1. b The error range is +2 cm-l. to the 'La- 'A' transition in, e.g., the 2,3-dialkylindoles,l0,*6and the results of the room temperature polarized single crystal by Yamamoto and Tanaka2' will not be included. The literature contains only a single claim for the assignment of bands belonging to the 'La 'A' transition of indole(-h6). In that paper,l4 the indole(-h6) bands at 0; 455 cm-l and 0; 480 cm-l are assigned as the split origins of the 'La 'A'transition since there is no evidence of any bands to lower energies with the requisite polarization characteristics. The authors do not, however, suggest any mechanism whereby the origin band of a molecule with the molecular symmetry of indole could be split by 25 cm-I. On the other hand, there is a remarkable correspondence between the number of bands in the (-d6) isomer and their locations (vis-a-vis the (-h6) compound), which would require that the 480-cm-1 band be a regular member of the 'Lb system unless the zero-point shift of the 'La origin is exactly the same as that for 'Lb (-140 cm-1 to higher frequencies). In addition, the assignments of bands 21 and 23 in Table IV fit well as combinations of 422 and 412 with v'28 as do bands 21 and 21 of Table V, and this defines the band at 0; 480 cm-1 in (-h6) as a normal part of the 'Lb system. The assignment of the band at 0; + 466 cm-1 in (-d6) is also exactly what is to be
-
+
-
+
+
Barstis et al.
5824 The Journal of Physical Chemistry, Vol. 97, No. 22, 1993 TABLE V
"1
I'
I
ran"
Figure 4. Jet-cooled R2PI spectrum of indole(-d6) in the wavelength range (a, top) 284-277 nm and (b, bottom) 280-274 nm.
expected for v'28 based upon the frequency of v"28 coupled with the difference in upper and lower state frequencies for the (-h6) compound. Our assignment of the band at 0; 456 cm-I as 39'41' rests upon two issues. First, the calculated frequencyof the combination from the 392and 412overtone assignments is 5 cm-' higher than 456 cm-I, Le., 461 cm-I. By itself, this might suggest that the assignment is in error, but we find exactly the same behavior for the indole(-d6) assignment (Table V), where the difference is 4.5 cm-I (421.5 cm-1 calculated from the overtone assignments and 417 cm-I found experimentally). We ascribe this difference to the often encountered problem of negative anharmonicities for overtones and higher harmonics of vibrations-particularly outof-plane modes-where multiple quanta do not result in dissociative behavior. The qI mode is of this type, and the similar nature of the indole(-d6) assignment suggests this conclusion. There is some difference between the absolute intensities of the 39'41 I bandsin the indole(-h6) and indole(-d6) spectra, but until more is known about the precise nature of the modes themselves and their mixture with other totally symmetric modes it is impossible to further clarify the issue. The net result of these assignments is to suggest that all of the significant intensity in the first 500 cm-1 or so of indole itself is at least explicable in terms of a single 'Lborigin and that the search for the elusive 'La origin should probably be sought elsewhere. Further work is in progress which we hope will clarify the question of the 'L, electronic state.
+
Vibronic Assignments of Ind0le(-d6)~ v Av assignment
band
X,nm
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
282.56 282.28 282.10 280.26 279.88 279.79 279.43 279.27 279.08 278.89 278.66 278.59 278.47 277.60 277.53 277.40 277.10 277.04 276.78 276.72 276.66 276.59 276.56 276.48 276.43 276.38 276.31 276.21 276.18 276.09
35380 35415 35438 35 671 35719 35730 35777 35797 35821 35846 35875 35884 35900 36012 36022 36038 36077 36085 36 119 36 127 36 135 36 144 36 148 36158 36 165 36 171 36 180 36 194 36198 36209
0 35 58 290 339 350 396 417 441 466 495 504 520 632 641 658 697 705 739 747 754 764 768 778 784 791 800 813 817 829
31 32 33 34 35 36
275.95 275.86 275.82 275.70 275.60 275.52
36228 36240 36245 36261 36274 36284
847 859 864 880 893 904
0: sequence sequence 422 412 29' 39'42' 39'41' 402 28'/37'42l 37'41' 392 27 I 37l38' 372 26l 361391/292 40'41 2421 29'40'411 29l39'42' 281422/371423/40241 '42' 38141'422 381411422/29'39141'
37'411422/28141'42'/402412 39140141'42' 38141242'/291402/392422 37141242'/281412/29'38'42' 28129'/29137'421/38'413 39241'42'/38'401422 29' 381411/371401422/281401421/ 40341'/27]41 l42' 381391422 281391421/37'391422/39141 '402 38'401412 29'38140'/36'423/29138140'/404 39240'41I/38'4O242'
36'41'422/37'40242'/28'402/382422
in cm-1. Error range of calculated - observed values (where calculated represents harmonic values of assigned fundamental modes) is 0 to -4 cm-I except for the following: 39'41', +4.5 cm-I; 37'42', -1 cm-1; 37'411, -5 cm-I; 37'381, -2 cm-I; and 40141242',-1 cm-I. av
frequency. This theoretical approach thus proved to be a useful and powerful technique in assigning the ground-state vibrational manifold in such a large molecule, and, for cases where there is only a very small change in the force field in the excited state, scaled ground-state values can be useful in the excited-state analysis. The vibronic analysis of the indole(-h6) excitation spectrum was made possible through the use of SVLF dataesv6Detailed comparison between the vibronic analyses of indole(-h6) and indole(-d6) leads us to conclude that the vibronic transitions in the range 0; to 0; + 1000 cm-' in indole(-h6) are all associated with the ILb excited state. Acknowledgment. This work has been supported by the National Science Foundation under Grant CHE-9022610. L.G. gratefully acknowledgessupport by the Department of Chemistry at The University of Michigan under a Dow-Britton Fellowship and support under 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 also acknowledge the time contribution made available on the Cray Y/MP by the San Diego Superconductor Center.
Conclusion
The agreement between ab initio calculations, normal coordinate analysis,2 and deuterated studies192 on the ground-state vibrationalmodes of indole(-h6) is excellent. The scaled Gaussian 90 ab initio calculation of indole(-h6) accurately predicted the fundamental vibrational modes in both vibration form and
References and Notes (1) Lautie, A.; Lautie, M. F.; Gruger, A,; Fakhri. S. A. Spectrochim. 85. (2) Takeuchi, H.; Harada, I. Spectrochim. Acta 1986, 4 2 4 1069. (3) Collier, W. J . Chem. Phys. 1988.88 (12), 7295. (4) Suwaiyan, A,; Zwarich, R.Spectrochim. Acru 1986, 42A, 1017.
Acta 1980, 36A,
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