J. Phys. Chem. 1994,98, 9221-9232
9227
Low-Frequency Vibrations in SOand SI States of 1,2,3,4-Tetrahydronaphthalene(Tetralin) from Fluorescence in a Seeded Jet Nikhil Guchhait, Tapas Chakraborty, Devashis Majumdar, and Mihir Chowdhury’ Department of Physical Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Calcutta- 700 032, India Received: March 3, 1994’
The laser-induced fluorescence excitation and the single vibronic level luminescence (SVL) spectra of 1,2,3,4tetrahydronaphthalene ( T H N ) or tetralin in a supersonic jet have been investigated. The origin for the S1-So transition has been found at 36 791 cm-l. Both excitation and S V L spectra show a prominent progression of a low-frequency vibration which is assigned to the puckering “twist” motion involving CZand C3 atoms. The vibronic features are consistent with a symmetrically twisted minimum energy configuration of the pseudosix-membered cyclohexene ring. A rough estimate of the barrier to interconversion between different forms in the S1 state has been obtained from the excitation spectrum which revealed an interaction between “twist” and “bend” vibrations.
Introduction A cyclic ring system consisting of N atoms has N - 3 out-ofplane vibrations. The two out-of-plane vibrations of fivemembered rings, such as cyclopentane and its analogues, have been investigated extensively, one of which may be interpreted as normal “radial” vibration and the other as “pseudorotation” with or without barrier.132 A psuedo-six-membered ring with one rigid (double) bond, such as cyclohexene, has one relatively high-frequency torsional mode around the double bond and two low-frequency ring-puckering modes. It has been shown that the torsional motion around the double bond does not couple with the other two out-of-plane ring-puckering modes, but the two lowfrequency modes are strongly coupled. Both the low-frequency puckering modes, twist and bend, have double-minima potentials, but the twisted symmetrical form is lower in energy than the bent configuration. Extensive studies have been carried out on cyclohexene and analogous six-membered rings over the last four decades.3-5 An analysis of the far-IR and Raman spectra shows that the barriers to planarity are of the order of 4500 cm-I. There is another route from one twisted form to the mirror-equivalent one via the bent form where the barriers are only of the order of 3000 cm-Is3 Although far-IR and Raman spectroscopies are capable of providing detailed information about the conformation and barrier of the SO state, these techniques cannot be applied to excited states. Absorption spectroscopy or fluorescence excitation spectroscopy of vapours can in principle yield information on vibrations of the excited states. However, congestion caused by the hot bands and collision-induced bandwidths make the task nearly impossible, particularly so for low-frequency bands. Cooling by seeded jet expansion has been found useful for eliminating the hot bands and reducing the bandwidths. We have, therefore, undertaken a study of S1 state conformations of pseudo-five- and pseudo-six-membered rings by laser-induced fluorescence excitation spectroscopy as part of our general program of determining S1 potential energy surfaces involving various large-amplitude motion^.^^^ The spectrum of indoline, an example of a five-membered ring system, has recently been reported by 118.6 The present report deals with a pseudo-sixmembered ring system. The absorption frequency of the smallest molecule having a six-membered ring with one double bond, namely cyclohexene, is too far in the UV region to be amenable to electronic excitation by our laser. We have, therefore, chosen e Abstract
published in Aduance ACS Abstracts, August 15, 1994.
0022-365419412098-9227$04.50/0
a compound, tetrahydronaphthalene (THN), where a benzene ring is fused to the six-membered saturated ring system. This was expected to allow us to investigatethe change in the saturated ring vibronic pattern on exciting the molecule from the SOto the S1 state of benzene. Our analysis, however, shows that out-ofplane phenyl ring and substituent motions get strongly coupled with cyclohexene-type ring puckering motion.
Experiment T H N was obtainedfromAldrichChemica1 Co. and was distilled before use. Our experimental setup for measurement of fluorescence excitation and SVL spectra has been described in detail elsewhere.* In brief, the sample along with H e as carrier gas was expanded into the vacuum chamber through a 0.5 mm diameter orifice of a nozzle valve (General Valve Corporation) with 10-Hz repetition rate. The sample was heated to about 100 OC to increase the sample concentration in vapor phase. The background pressure of the chamber during the experiment was maintained at less than 10-4 Torr by a 6-in. oil-diffusion pump backed by a mechanical rotary pump. The output of a PDL-3 dye laser, pumped by a Nd:YAG laser (Quanta Ray DCR 11) was frequency-doubled by a KDP crystal. The second harmonic output crossed the free jet at 1 cm downstream of the nozzle orifice. The laser-induced fluorescence signal was collected by a two-lens optics, detected by a EM1 9781R photomultiplier and averaged by a boxcar integrator (PAR model 64). The normalised output was then fed to an X-Y chart recorder which was synchronised with the wavelength drive. The 0; SVL spectrum was obtained by exciting at the 0-0 band, dispersing the fluorescence in a monochromator, and counting photons at each wavelength.
Results and Discussion The molecule T H N has two fused rings-a benzene and a cyclohexene system. The vibrational frequenciesof pure benzene moiety are high-both in SOand S1 ~tates.~JOLow frequencies originate from the out-of-plane vibrations of the pseudo-sixmembered cyclohexene ring system3 and from the inter-ring butterfly folding as in indane.ll The liquid and solid IR and Raman,12 the vapor I R and Raman,3-5 and microwave13 spectra of cyclohexene itself in the SO state have been reported and exhaustively a n a l y ~ e d .The ~ cyclohexene ring has three out-ofplanevibrations. The torsion around thedouble bond (DB torsion) is of frequency 380 cm-1 and does not interact with other two 0 1994 American Chemical Society
9228
The Journal of Physical Chemistry, Vol. 98, No. 37, 1994
Guchhait et al.
(31
I
(51
--\-_ ,
I-
-
It
ioaoo (b)
Figure 1. Tetralin (THN) conformers: (a) half-boat (bent); (b) symmetric
half-chair (twisted). puckering frequencies involving C2 and C3 atoms. The puckering motion of the Cz and Cs atoms may be coupled in-phase and out-of-phase to generate bend and twist vibration. In the So state, cyclohexene has been found to have a double-minima potential curve with respect to twist coordinate. The minimum energy configuration corresponds to about 30' twist and Oo bend. The 0-1 bend frequency and degenerate (0,l)-(2,3) twist frequency are found to be 163 and 275 cm-1, respectively. Low frequencies may also originate from out-of-plane substituent motions in substituted benzenes. For the SOstate of o-xylene, the observed 255 cm-l band has been interpreted as in-phase out-of-plane C-CH3 vibration and the 178 cm-I band as the corresponding out-of-plane vibration.gb The other two molecules that are relevant to the present discussion of THN spectrum are indane and 1,Cbenzodioxan. The gas-phase IR,l4a the vapor-phase electronic absorption,l5 and the supersonic jet SVL" and the microwave14b spectra of this molecule, indane have been reported and analyzed. The five-membered ring has the C2 puckering bend frequency at 143 cm-l in the SOstate and 115 cm-I in the SIstate. It has another low-frequency mode at 248 cm-1 in the SOstate, which has been assigned to butterfly mode involving bending around the bond common to the two rings. There is still one more band at 370 cm-1 in the SOstate, which shows B contour and has been assigned to the DB twist frequency. Both in SOand S1 states the molecule has been found to have nonplanar bend configuration (C,symmetry) with doubleminima potential with respect to C2 puckering motion. There is wide difference in estimated barrier height. In contrast to 1979 cm-l obtained from far IR14aand SVL spectra," OW- doubling in microwave spectrum gave a barrier height of only 433.5 cm-1. The molecule 1,4-benzodioxan has recently been studied by Gordon and Hollaslsb by fluorescence excitation, SVL, and vapor absorption. The molecule has been found to be rigidly nonplanar with twisting C2 conformation with large barrier in both SOand SIstates. Thesix-membered-ring puckering bend and twist modes are found to be 105 and 166 cm-I, respectively, in the SOstate; the corresponding numbers are 80.5 and 140 cm-l for the S1 state. There are several possibilities for the minimum energy configuration of the molecule THN. Although the planar form of symmetry is energetically higher with respect to either the twisted form of symmetry Cz or the bent form of symmetry C, (see Figure l), we feel that the planar C, group is the most conuenient startingpoint for discussing the symmetries of vibronic states. The fundamentals of puckering bend and butterfly bend vibrations belong to b2 representation (denoting reflection in
10.00
1
i
9wo RAMAN SHIFT(cm-I)
25000
500.00
750.00
I oow 0
RAMAN SHIFT(Cm-l 1
Figure 2. Raman frequencies of tetralin (THN) in the liquid state at room temperature. Inset shows the spectrum close to exciting light with narrower slit width. The frequencies of thedesignatedbandsareindicated in Table 1.
molecular plane as a") while both the puckering twist and the DB torsion belong to a2 representation. In Raman transitions all are allowed, but in IR absorption the a2 is forbidden. If the molecule has a double-minima potential curve with large barrier height, one can still adopt the Cb notations, but it is necessary to remember that then = 0 , l states are degenerate and belong to two different representations of the C, group. Alternatively, if the molecule is symmetrically twisted in its equilibrium configuration (Figure lB), one may deal with the appropriate lower group C2; the b2(C,) and a2(Cb) representations then reduce to b(C2) and a(Cz) representations, respectively; in such a case, all the vibrations become allowed, b being polarized perpendicular to the C, axis and a parallel to it. If the molecule is bent (Figure 1A) at equilibrium, the appropriate group becomes C,,and the b2(Cb) and az(C,) representations reduce to a'(C,) and a"(C,) representations, and again all the transitions become allowed. It may be worth pointing out however, that in electronic transitions only the totally symmetric vibrations of the group are likely to appear as a progressionof bands withsmoothlychanging intensitypattern. Figure 2 shows the low-frequency Raman spectrum of T H N in the liquid state. It may be observed that the spectrum has a close similarity with 1,Cbenzodioxan (BD),15bindane,ll,14,15* and cyclohexene3-5 spectrum. Figure 3 shows the SVL spectrum for the 0-0 excitation. The frequencies observed in Raman spectrum of liquid T H N and in 0;SVL spectrum ofjet, along with relevant information for benzodioxan, cyclohexene, and indane, are collected in Table I. In cyclohexene the lowest observed frequency is 163 cm-1 in vapor state (and 175 cm-1 in the liquid state) and has been interpreted as puckering bend frequency involving in-phase motion of C2 and C, atoms. In indane, the lowest frequency of 143 cm-1 in 0-0SVL of jet has been similarly interpreted as the puckering bend frequency of the C2 atom. It is therefore likely that in T H N also the lowest frequency will be puckering bend. In the present case the lowest frequency is observed at 105 cm-l in the Raman spectrum. Although this band could be interpreted as difference frequency, as in cyclohexene, our calculations discussed later show that this is a fundamental frequency and corresponds to puckering bend. This assignment agrees with that of BD.*5b It may be noted that the bend frequency of 105 cm-l does not appear in the SVL (jet) spectrum. Indeed, a 143 cm-I appears as progression in the 0; SVL spectrum of THN. As discussed in previous paragraph, only totally symmetric vibrations can appear as progression. Since the bend vibration is not appearing as a progression in jet SVL spectrum, unlike the case of indane,
The Journal of Physical Chemistry, Vol. 98, No. 37, 1994 9229
SOand SIStates of Tetralin
only the twisting vibration can appear as a progression. We therefore assign the 143 cm-1 band in SVL uet) spectrum to twisting frequency of symmetry a. This assignment is supported by our calculation of vibrational frequencies, but is in contrast to indane assignment where 143 cm-1 (progression) is assigned to bend frequency. It may be pointed out that although the bend frequency cannot appear as a progression on a Cz (twisted) equilibrium geometry, group-theoretic arguments can not explain why one quanta of such vibration does not appear over the A A electronic transition. The overall symmetry of a vibronic state with one quanta of b vibration is B1; thus, Cz polarized A B vibronic transition can not borrow intensity from the close-lying 0-0 band of the same electronic (A- A) transition. The stealing of intensity from the A B electronic transition is less effective because the energy gap between the A electronic state and the B electronic state is large. This extra-group-theoretic argument explains the extreme weakness of the "bend" vibronic band. The conformational analysis of the tetralin molecule has been carried out by us using the AM1 technique.18 The total energy of the molecule has been optimized using Davidson-FletcherPowell a l g ~ r i t h m . ~Both ~ * ~the ~ half-chair and half-boat conformers were considered during the geometry optimization. No geometrical constraint was imposed during energy minimisation and the results are presented in Table 3. The half-chair conformer has been found to be the lower energy form and the energy difference between the two forms is 2.1 kcal mol-'. The relevant topographical features of the half chair conformer have been compared with the MM2 optimized parameters of cyclohexene, 1-methyltetralin, 2-methyltetralin,I6and they are shown in Table 4. Similar parameters from electron diffraction studies on cyclohexene21 are also included in the table for comparison. The results are fairly comparable. It may be noticed from Table 3 that the twisted form (half-chair) is symmetric. The fully
-
+
+
L , 0
I
I
-400
I
-1200 WAVENUMBER SHIFT A9 (CM-')
-800
I
1
-1600
1
-2000
Figure 3. SVL spectrum of tetralin (THN) in seeded jet for 0; excitation. Frequency shifts are counted from the 0; excitation frequency. The vibrational frequencies of numbered bands are indicated in Table 1.
we conclude that the molecule belongs to Cz (twisted form) and not to C, (bent form). This conclusion is consistent with the symmetrical twisted geometry proposed for cyclohexene and BD at equilibrium geometry. Several NMRl6 studies and semiemperical calculationsl7 point out that the half-boat form of THN is higher in energy than the half-chair form or the twisted conformer. Our own calculations (discussed later) supports this view. If the equilibrium geometry corresponds to the C2 group,
TABLE 1: Comparison of So Vibrational Frequencies of Cyclohexene, Indane, Tetralin (THN), and 1,4-Benzodioxan along with Their Assignments ~~
cyclohexene' (IR and Raman in vapor)
indaneb SVLUet)
freq
assgn
freq
110, 163d
twist bend bend
275
twist
320,
[bend]:
380 430,
DB torsion bend twist
540
assgn
143
277 (bend)
285 (248)' (bend)
27: 26:
+
DB torsion bend
+
*
37 1 412 512 607
51: 25: 24; 221
736 748
210 20i
859
19:
1025
18:
1,4-benzodioxane* freq
THNCRamang S V V (liq), Uet)
assgn
freq
assgn
105 166
puckering bend puckering twist
(a) 105 (b) 160 (1) 143
puckering bend ( ~ 1 ) puckering twist ( 4
190(?) 248.4
DB torsion butterfly bend
(c) 265 (2) 284 (d) 310
Butterfly bend (v3) [twist]: ( 2 ~ 2 ) DB torsion (v4)
(3) 431 (e) 433 (f) 454 (4) 451 (9) 475 (h) 505 (5) 507 (i) 581 (6) 585 (7) 646 (8) 686 U) 702 (9) 705 (k) 725 (10) 728 (I) 806 (1 1) 804 (m) 818 (12) 843 (n) 867 (13) 870 ( 0 ) 987 (14) 987 (15) 1010 (16) 1046 (17) 1159 (18) 1186 (19) 1217
[twist]! ( 3 ~ ) (v2
+ v3)
(4 v4
+ v2
bz ring mode bz ring mode
bz ring mode bz ring mode
bz ring mode bz ring mode
0 References 3-5. Reference 11. C Present work. d The frequency is shifted to 175 cm-I in the liquid side.12 C This is observed in SVL from the first vibronic band. fSeen in the d-substituted molecule. g The band numbers a, b, ..., or 1, 2, ..., correspond to those in Figures 2 and 3, respectively. * Reference 15b.
Guchhait et al.
9230 The Journal of Physical Chemistry, Vol. 98, No. 37, 1994 TABLE 2 SIVibrational Frequencies from the Fluorescence Excitation Spectrum of Tetralin (THN) band senaration band frbm0-0 no.’ band (cm-I) assignmentb 0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
95 179 189 270 282 295 329 353 363 375 390 417 422 446 48 1 511
18 19
520 544
20 21 22 23
566 574 679 765
twist bend harmonic twist harmonic bend harmonic + twist twist harmonic DB torsion (coupled mode) inter-ring folding (coupled mode) bend even harmonic bend harmonic + twist harmonic twist harmonic DB torsion + twist benzene mode inter-ring folding + twist bend even harmonic + twist DB torsion + twist harmonic benzene mode + twist inter-ring folding + bend harmonic inter-ring folding + twist harmonic bend even harmonic + twist harmonic
TABLE 3: Energy-Minimized Position Coordinates of the Main Atoms, Heats of Formation (AH,), and Relative Energies (AE) for the Half-Chair (Twisted) and Half-Boat (Bent) Conformers of Tetralin (THN) positional coordinates (A) AHf AE conformer atom x Y z (kcal mol-’) (kcal mol-’) half-chair C 6 0.0000 0.0000 0.0000 C7 1.3955 0.0000 0.0000 C8 2.0891 1.2064 0.0000 C9 1.3999 2.4266 0.0016 C10 -0,0033 2.4268 -0.0055 C5 -0.6928 1.2068 -0.0032 C1 2.1833 3.6921 0.0311 C2 1.3680 4.9093 -0.3528* C3 0.0294 4.9066 0.3521* C4 -0.7861 3.6926 -0.0414
0.0000 0.0000 C6 1.3945 0.0000 C5 2.1002 1.2040 C10 1.4045 2.4122 C9 -0.0023 2.4149 C8 -0.6999 1.2062 C4 2.0875 3.7322 C3 1.4501 4.6752 C2 -0.0608 4.7684 C1 -0.6990 3.7280
1-2-3-4 2-3410 3410-9
63.00 47.00 16.00
62.00 47.00 16.00
64.00 48.00 16.00
60.20 44.90 15.20
-61.54 46.42 -17.7
a Refer to Figure 1 for the definition of atoms. MM2 calculation results. For methyl cyclohexene and cyclohexene 3 and 4 atoms are the sp2 carbons and this numbering is kept as it is for comparison. Results of present AM1 calculation.
DB torsion + twist harmonic benzene ring mode benzene ring mode
0 The band numbers correspond to those indicated in Figure 5. * The ilabelsof vis correspond to those for the SOstate. Note that the frequency orderingof v3 and v4 has been changed from that of the ground state (see text).
half-boat C7
TABLE 4 Relevant Topographic Features of Methylcyclohexene, I-Methyltetralin, 2-Methyltetralin, Cyclohexene, and Tetralin molecules dihedral methyl- 1-methyl- 2-methyl- cycloangle‘ (deg) cyclohexene tetralinb tetralid hexenebsc tetralid
0.0000 0.0000 0.0000 -0.0236 -0.0086 -0.0026 -0.0387 0.9664 0.8588 -0.0456
0.21
0.00
2.36
2.15
optimized twisted (half-chair) form of tetralin has been used for normal-mode vibrational analysis. The results of calculation at the low-frequency range are presented in Figure 4. To get an idea of the nature of vibration, the atomic displacement vectors at each frequencies are marked along the three orthogonal axes. The frequencies obtained from AM 1 calculations do not generally reproduce the experimental data. For a complex system such as tetralin, the calculated values are on average 30 cm-I different from the experimental results. Thus we haveused theeigenvectors of individual vibrational frequencies for assignments. The
-3,=z7~cm-1(zeccm-V
-+
~oocm-~(431cm-ll
Figure 4. Calculated and observed frequencies of tetralin for five lowfrequency modes. The calculatedvalues are uniformly scaled by a factor of 1.063 (see text) and the atomic displacement vectors at the three mutual orthogonal directions are marked by arrow headed lines. The experimentally observed frequencies are presented within parentheses. calculated values are then uniformly scaled by a factor of 1.063 for comparison with experimental values. The scale factor has been obtained by comparing the experimentally observed double bond torsional mode of cyclohexene (380 cm-I) with that of it’s theoretically calculatedvalue (357.5 cm-I). This theoretical result has been obtained from the fully optimized half-chair geometry of cyclohexene at the AM1 level, and since tetralin is the next higher homologue of cyclohexene, we consider this factor to be fully transferable. The results presented in Figure 4, show that the calculated scaled frequencies are comparable to the experimental results. An accurate ab initio calculation needs to be done to confirm the vibrational frequencies of the proposed symmetric structure. We have calculated the change in twist coordinate between SO and SI states. In our calculation the So state is represented by a harmonic oscillator and the SIstate by a shifted harmonic oscillator, described by the Hamiltonian H , where H = p 2 / 2 m+ ~ ( x ) , with V ( x ) = ‘ / J r ( x- xq)’
and xq = 0
For the So state the k l m ratio is chosen in such a way that the fundamental frequency equals the observed one. The vibrational energy states and the corresponding wave functions ($; and $ix) are obtained by solving the Schroedinger equation for the system by Fourier grid Hamiltonian method (FGH).22,*3 The absorption intensity (I:, S: - S): is assumed to be proportional to the overlap integral square ( ($il$ix)2). The observed intensity ratio has been obtained for the shift value xq = 0.02AO.Since
The Journal of Physical Chemistry, Vol. 98, No. 37, 1994 9231
SOand SI States of Tetralin
I presumably reflects the release of ring strain when the fivemembered ring is enlarged to a six-membered one. There is a regular progression of 95 cm-I consisting of band numbers 1, 2, 4, 6, and 11. This is assigned to the “puckering twist” vibration. The sensitivity of this frequency to the nature of the *-electronic state is interesting. On electronic excitation the *-electronic charge densities a t C9 and Clo atoms increase. These charges influence the puckering potential presumably by interacting with the C2-H and C3-H bonds. In principle, the barrier with respect to twist can be evaluated from the expression
B = ’/,,Av(v/Av
I , 0
I
I
1
100 200 300 400 WAVENUMBER SHIFT A* (CM-’ )
1
500
6W
700
Figure5. Fluorescence excitationspectrumof tetralin (THN) in a seeded jet. Frequency shifts are counted from the 0-0band. The 0-0transition
is saturated; the inset shows relative intensities of first three and hot bands. The vibrational frequencies and assignments of numbered bands are indicated in Table 2. the difference in the minimum-energy twist coordinate between Soand SIstates is small, we consider it unlikely that the potential energy curve with respect to the twist coordinate is a single minimum in the SOstate and a double minimum with a higher barrier (see later) in the SIstate. We therefore propose a doubleminimum potential with a barrier for both SOand SI states. The third and fourth lowest fundamental frequencies of T H N in Raman spectrum are at 265 and 310 cm-I, respectively. However, these frequencies do not appear in the SVL spectrum. In the case of cyclohexene, a band appears at 275 cm-’ in Raman and IR spectra and has been interpreted as puckering twisting frequency. In the case of indane, a 248 cm-I fundamental is noted in SVL from the first vibronic band (but not in 0; SVL) which has been interpreted as butterfly bending around the bond common to the two rings. Our calculations on tetralin shows that the two broad bands in Raman spectrum correspond to a bending (Le., v3) and a twisting (Le., v4) mode, respectively. None of these two appear in the SVL spectrum; this fact needs to be rationalized. The nonappearance of v4 is easily understandable, for the displacement vectors are all localized in the saturated ring system (see Figure 4), and hence any r-excitation is unlikely to affect it. The v 3 mode is also not expected to appear as progression because of its “bending” B symmetry; but one quanta may in principle appear over the 0-0 band. As explained earlier, such a vibronic transition will have to borrow intensity from the energetically distant A B (C, polarized) transition, and hence, the intensity of such a vibronic band will be very small. Other high-frequency vibrations are assigned by corresponding with indane spectrum. It is worth repeating that there is a very close correspondence in relative intensities and frequencies between the indane 0; SVL and the T H N 0; SVL bands. This is shown in Table 1. Figure 5 shows the excitation spectrum of T H N in jet. The band positions and assignments are shown in Table 2. The 0-0 band is very intense as expected for a strongly allowed transition. The lowest excited state of benzene IBz,,(D6h group) reduces to AI (C2J in C2”group and the transition is polarized along the CZ axis. The frequency of the 0-0 band 36 791 cm-I, is about 100 cm-l to the red of indane. The red shift with respect to indane
-
+ 1)2
where v is the frequency of transition and Av is the difference between the first interval (Le. energy difference between n = 1 and n = 0) and the second interval (Le. energy difference between n = 2 and n = 1) of the sequence. This handy expression was deduced by Durig et al.4 and modified by Smithson and W e i ~ e r , ~ utilizing the results of perturbation calculations developed originally to deal with pseudorotation in five-membered rings by Ikeda et al.24 In the present case the barrier with respect to the twist-coordinate change (keeping the bend coordinate unchanged) is calculated to be 1700 f 700 cm-l. It is difficult to identify the puckering bend frequency which is expected to be below the twist frequency of 95 cm-I. Although, for reasons discussed earlier, the vibronic band with one quanta of bend mode is near-forbidden, thevibronic band with two quanta of bend mode is of A symmetry and can derive small amount of intensity from the strongly allowed 0-0 band. In the excitation spectrum there are four bands which could not be assigned as twist harmonic, namely, band numbers 3, 5, 7, and 8; but the question is, which among these is the puckering bend frequency? As stated, the bend second harmonic frequency is expected to be less than the twist second harmonic frequency. Second, the vibronic band should be a weak one, for no corresponding band is observed in the SVL spectrum. A third criterion we applied is as follows: Of the different modes-DB torsion, butterfly bending and puckering bend-the last one is expected to interact most strongly with the puckering twist mode; for, both involve puckering motion of same C2 and C j atoms. The signature for such an interaction is that the bend second harmonic frequency riding over the twist fundamental should differ from the bend second harmonic frequency riding over the 0-0 band. From the above three considerations we assign the band number 3 as the puckering bend second harmonic transition. The observed frequency (89 cm-I) is close to the bend frequency of 80.5 cm-l observed for BD.ISb The barrier to bend coordinate (keeping twist coordinate unchanged) can be calculated by use of eq 1. This gives a barrier height of 1200 f 250 cm-I. The twist frequency that rides over the bend second harmonic is measurably lower in magnitude than the corresponding frequencyriding over the zeroth level. Durig et al. have suggested: without providing theoretical justification, that the same empirical formula (eq 1) could be used for calculating the barrier to interconversion between two identical forms. One only has to take Av as the difference between the first bend harmonic frequency on the zeroth level and the first bend harmonic frequency on the first twist vibration. If we follow Durig et ale’s empirical prescription, the barrier to interconversion between the mirror-image forms comes out to be between 450 and 1500 cm-I. The two extreme numbers have been arrived a t taking v as twist and bend harmonic frequency respectively. However, justification for eq 1 is hard to obtain. Other three fundamentals that can be identified are v4 (295 cm-I), v3 (329 cm-1) and v5 (417 cm-1). The v3 and v4 have been assigned to inter-ring butterfly bend and DB torsion respectively; these are all coupled modes as shown by our AMI calculation. From band-shape considerations, we ascribe the 295 cm-I band
9232
Guchhait et al.
The Journal of Physical Chemistry, Vol. 98, No. 37, 1994
to DB torsion ( ~ 4 ) . The contour of the 329 cm-1 band is distinctly different, which indicates that it belongs to a state of different symmetry. We assign it to the inter-ring butterfly bend ( 4 . A rotational contour analysis is needed to confirm this assignment. v 5 is close to one of the S1 frequency of indane and may be assigned to a ring mode. The weak bands near the origin increase in intensity when the pressure of the vacuum chamber is increased, indicating that they are hot. These can be interpreted as transitions originating from the hot V I level a t 105 cm-1 above ground to v1 and v2 levels of the excited SI state.
Concluding Remarks We have shown from analysis of the jet fluorescence excitation and emission spectra that the molecule T H N has symmetrically twisted configuration in both states. This point, although corroborated by AMI calculation for the So state, needs the attention of ab initio theoreticians. Second, there occurs an interaction between “bend” and “twist” puckering motions. Although our spectrum is not sufficiently detailed to carry out an unambiguous analysis with two-dimensional multi-parameter potential of the form3 Aa4 Ba2 + Cp4 + OB2+ EaZO2,it may be interesting to generate the full potential energy surface for SO and SIstates by accurate theoretical calculations and find out whether the theoretical potential energy surface is compatible with the experimental spectrum.
+
Acknowledgment. The work has been carried out with financial support from the Department of Science and Technology, Government of India. N.G. would like to thank the Council of Scientific and Industrial Research for providing a fellowship. We thank Prof. T. N. Misra’s research group for recording the Raman spectrum for us, and Dr. D. N . Nath for instrumental assistance. The FGH calculations were done by using the program of our colleagues Prof. S. P. Bhattacharyya and Dr. P. Dutta, to
whom we are indebted. We thank a referee for drawing our attention to two recent papers and for pointing out an error in our assignment.
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