Fluorescence-Dip Infrared Spectroscopy of Jet-Cooled 5

Oct 17, 1996 - Fluorescence-Dip Infrared Spectroscopy of Jet-Cooled 5-Hydroxytropolone. Rex K. Frost,Fred Hagemeister,Dave Schleppenbach,George Lauren...
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J. Phys. Chem. 1996, 100, 16835-16842

16835

Fluorescence-Dip Infrared Spectroscopy of Jet-Cooled 5-Hydroxytropolone Rex K. Frost, Fred Hagemeister, Dave Schleppenbach, George Laurence, and Timothy S. Zwier* Department of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47907-1393 ReceiVed: June 13, 1996; In Final Form: August 14, 1996X

Ground-state infrared spectra of the syn and anti conformers of jet-cooled 5-hydroxytropolone (5-HOTrOH) monomer have been recorded free from interference from one another using a fluorescence depletion method. The 5-hydroxy group “free” O-H stretch, 2-hydroxy intramolecularly hydrogen-bonded O-H stretch, and C-H stretch transitions are observed for both conformers. The assignment of these bands is clear by comparison with previous infrared studies of bare tropolone and is confirmed by MP2 and DFT level vibrational frequency and intensity calculations. Each of the conformers is predicted to have four allowed C-H stretch transitions. There is a reasonable one-for-one correspondence between calculation and experiment for the anti conformer’s C-H stretch bands, while the syn conformer shows effects of Fermi resonance mixing. The 5-OH stretch bands of the syn and anti conformers are single, sharp transitions located at 3654 and 3664 cm-1, respectively. The OH stretch fundamentals of the 2-OH group involved in the intramolecular hydrogen bond are centered on 3170 and 3195 cm-1. These bands are extensively broadened in both conformers and contain reproducible substructure with irregular spacing of about 10-15 cm-1. The substructure of the intramolecularly H-bonded 2-hydroxy O-H stretch band reflects a selective first-tier vibrational-state mixing, most likely with a set of modes which are in near two-to-one resonance with it. Finally, the 249 cm-1 asymmetry in the double-minimum potential well for H atom tunneling appears to be sufficient to quench syn-anti tunneling in S0 even following excitation of the 2-OH stretch fundamental. No evidence is found for syn T anti “crossover” transitions despite careful searches capable of detecting such transitions at depletion levels of about 1%.

Introduction Tropolone has played an important role as a model system for studying multidimensional effects on intramolecular H atom tunneling.1-17 In the S1 state, mode-specific tunneling splittings have been observed in the low-frequency modes accessible by vibronic spectroscopy which differ by more than an order of magnitude. The high-frequency modes of S1 are not probed by laser-induced fluorescence studies because of the small fluorescence quantum yields and strong state mixing several thousand wavenumbers above the S1 origin. Among the highfrequency modes not readily accessible to electronic spectroscopy is the OH stretch mode, which might be anticipated to be strongly coupled to the H atom tunneling reaction coordinate. The desire to study the mode-specific effects of the OH stretch motivated a recent study17 of tropolone by our group using the double-resonance method of fluorescence-dip infrared spectroscopy (FDIRS)18 to probe the O-H and C-H stretch fundamentals in the ground state. A tentative assignment of the OH(V)1) tunneling levels in S0 suggests a 12 cm-1 splitting for S0 OH(V)1), a 12-fold increase over the zero-point level splitting (0.99 cm-1). This is a relatively modest increase given the high frequency of the OH stretch and its expected close coupling to the reaction coordinate. If this splitting stands up to future experimental tests, it raises the possibility that the OH stretch mode may not be a strong promoter mode for the H atom tunneling in tropolone, possibly because the OH stretch oscillation is directed more nearly perpendicular to than along the O-O separation coordinate.17 The present article reports the analogous FDIR study of 5-hydroxytropolone (5-HOTrOH), building on earlier work19,20 * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, October 1, 1996.

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of the S1 r S0 spectroscopy of the molecule. Hydroxy substitution in the 5-position produces an asymmetry of 249 cm-1 in the double-minimum potential well for H atom tunneling in S0, as shown in Figure 1. This asymmetry is sufficient to quench the tunneling at the zero-point level, thereby producing distinct syn and anti conformers (Figure 1). These conformers each possess an S1 r S0 origin,19 labeled red (anti) and blue (syn) in the laser-induced fluorescence excitation spectrum shown in Figure 2. While the S0 zero-point levels are localized in the syn or anti wells, the single vibronic levels of S1 show pronounced modespecific syn-anti mixing. Many states are nearly pure syn or anti in character, but others are highly mixed either by accidental Fermi resonance or by possessing strong syn-anti coupling matrix elements. Many of the mixed-character excited-state levels can be reached by transitions out of both syn and anti ground-state levels, thereby producing vibronic “crossover” transitions in the spectrum (Figure 1a). These transitions effectively drive syn f anti or anti f syn photoisomerization in a mode-specific fashion by exciting out of a localized syn (anti) state to a mixed state which is primarily anti (syn) in character. As in tropolone, the present near-infrared vibrational study complements the electronic spectroscopy because it allows a search of high-frequency regions of the ground-state potential energy surface for similar mode-specific effects to those observed in the S1 state. Several prospects provide specific motivations for our work. First, using FDIR spectroscopy, the effects of the H atom tunneling asymmetry on the OH stretch spectroscopy can be probed from the vantage point of either conformer by selectively monitoring either conformer’s localized S0 zero-point level in the fluorescence spectrum. Second, the presence or absence of syn f anti or anti f syn infrared “crossover” transitions (Figure 1b) can be probed with good © 1996 American Chemical Society

16836 J. Phys. Chem., Vol. 100, No. 42, 1996

Frost et al. Experimental Section

Figure 1. Schematic S0 and S1 potential energy curves for the H atom tunneling reaction coordinate in 5-HOTrOH. The syn conformer’s zeropoint level is 249 cm-1 below that of the anti conformer (ref 17). (a) Vibronic crossover transitions from a syn (anti) ground-state level to levels in S1 which are primarily anti (syn) have been observed (ref 17). These transitions gain intensity by the small amount of syn (anti) character in the partially delocalized excited vibronic states. (b) In the S0 state, infrared spectra in the O-H stretch region can in principle be used to probe the degree of delocalization in the S0 OH(V)1) levels by selective excitation out of either the syn or anti ground-state zeropoint levels.

Details of the fluorescence-dip infrared technique used in this study have been discussed previously.17 A brief summary of our implementation is as follows. A solid sample of 5-HOTrOH is heated to 120 °C directly before a solenoid pulsed valve (General Valve, 0.5 mm diameter orifice). Helium gas at a pressure of roughly 5 bar is passed over the sample and supersonically expanded through the nozzle into a vacuum chamber. The ground-state populations of both the syn and anti conformers of 5-HOTrOH can be monitored separately via their respective near-ultraviolet origins (Figure 2). The near-UV is set at a wavelength were it can monitor the ground-state population of one of the isomers via its fluorescence. The UV beam (0.05 mJ/pulse, 0.2 cm-1 bandwidth) is loosely focused with a 50 cm lens to maximize overlap with the infrared laser. Ground-state infrared spectra of the syn and anti conformers are recorded by depleting these fluorescence signals with a spatially overlapped but temporally preceding infrared pulse generated using a Nd:YAG-pumped OPO system. The LiNbO3 OPO is typically operated under conditions where 1-3 mJ/pulse of idler is delivered to the supersonic jet in a 12 ns pulse, loosely focused to a spot size of 2 mm diameter. The resolution of the OPO is about 1.5 cm-1. The infrared light is tuned, and when its wavelength is resonant with a vibrational transition of the monitored species, ground-state population is removed to an excited vibrational level. This is manifested as a dip in the monitored fluorescence signal and therefore allows the recording of ground-state infrared spectra. Active base line subtraction is used to remove long-term drift from the spectrum and to normalize the spectrum for ultraviolet power fluctuations. Given the shot noise on the fluorescence, depletions of order 1% can be observed by the method as implemented. A particular advantage of the present method is that it enables the acquisition of the infrared spectra of each of the conformers free from interference from one another. 5-HOTrOH was synthesized by Steven Hardinger at Cal State Fullerton using the procedure of Russell et al.28 and used without further purification. Ab Initio Calculations

Figure 2. Fluorescence excitation spectrum of the low-energy region of the S1 r S0 transition in 5-HOTrOH. The syn and anti origin transitions used as monitor transitions in the fluorescence-dip infrared spectroscopy are marked in the figure.

sensitivity to test whether the OH(V)1) levels are delocalized appreciably over both syn and anti wells. Third, the simultaneous presence of the free OH and intramolecularly H-bonded OH in the same molecule enables a direct comparison of their spectral characteristics. Finally, the spectroscopic consequences of the coupling between the high-frequency stretch modes and background states can be compared in 5-HOTrOH to the much more numerous studies of state mixing in small molecules.21-27

In order to provide a basis for comparison with experiment, ab initio calculations of the minimum-energy structures, harmonic vibrational frequencies, and infrared intensities of syn and anti conformers of 5-HOTrOH have been carried out. Previous computations on 5-HOTrOH20 have used MøllerPlesset second-order many-body perturbation theory (MP2) to calculate the isomeric structures, vibrations, and H atom tunneling transition-state properties.20 Here, we apply density functional theory (DFT) methods to 5-HOTrOH, both for comparison with the MP2 calculations on 5-HOTrOH and to provide a seamless comparison with the analogous DFT results on tropolone (TrOH) and its 1:1 complex with water.29 The DFT methods employ the Becke3LYP nonlocal exchange-correlation functional30,31 which has been found to provide results of comparable accuracy to MP2 calculations for TrOH,17 TrOH‚H2O,29 (H2O)n, and benzene-(H2O)n clusters.32 In the Becke3LYP procedure, the energy terms contain contributions from local and nonlocal exchange and correlation functionals31 as well as from exact exchange. Two different basis sets,33 6-31+G* and 6-31G**, were used for this study. Calculations employing the first basis set allow direct comparison with the TrOH and TrOH‚H2O work. The second basis set was used for survey calculations and for best comparison with the results of the MP2 calculations on 5-HOTrOH.

IR Spectroscopy of Jet-Cooled 5-Hydroxytropolone

J. Phys. Chem., Vol. 100, No. 42, 1996 16837

TABLE 1: Calculateda Vibrational Frequencies and Intensities for Normal Modes above 1480 cm-1 in Both the Syn and Anti 5-HOTrOH Conformations syn conformer freq (cm

-1)

1483.2 (1501.7) 1519.8 (1565.1) 1561.4 (1623.0) 1636.0 (1648.1) 1671.8 (1679.9) 1698.9 (1722.8) c 3147.8 (3229.5) c 3183.2 (3260.0) c 3190.8 (3265.2) c3199.8 (3271.1) d3358.3 (3484.2) e 3826.1 (3878.8)

scaled

freqb

(cm-1)

1417.7 (1403.5) 1452.6 (1462.8) 1492.4 (1516.9) 1563.7 (1540.4) 1597.9 (1570.1) 1623.8 (1610.2) 3008.7 (3018.4) 3042.5 (3046.9) 3049.8 (3051.8) 3058.4 (3057.3) 3209.9 (3256.5) 3657.0 (3625.3)

anti conformer intensity 323.4 (153.5) 182.1 (257.7) 78.9 (27.2) 197.7 (187.7) 37.7 (42.2) 31.5 (13.7) 21.2 (12.8) 2.9 (1.2) 5.3 (0.6) 3.2 (3.1) 106.8 (100.9) 63.1 (99.6)

freq

(cm-1)

1480.2 (1490.1) 1518.3 (1562.8) 1554.0 (1602.9) 1631.0 (1653.5) 1670.2 (1670.2) 1696.0 (1719.9) 3134.5 (3205.5) 3185.5 (3253.0) 3195.4 (3258.3) 3199.9 (3266.9) 3335.7 (3439.5) 3831.0 (3885.8)

scaled freqb (cm-1)

intensity

1414.8 (1392.7) 1451.2 (1460.6) 1485.3 (1498.1) 1558.9 (1545.4) 1596.4 (1561.0) 1621.0 (1607.5) 2996.0 (2996.0) 3044.7 (3040.4) 3054.2 (3045.3) 3058.5 (3053.4) 3188.3 (3214.7) 3661.7 (3631.8)

242.6 (200.1) 117.8 (190.6) 118.9 (92.0) 206.5 (193.9) 19.2 (22.2) 33.2 (21.8) 20.8 (15.5) 6.8 (4.4) 3.7 (2.4) 1.8 (0.7) 103.7 (87.3) 70.7 (85.9)

a Frequencies and intensities are given for calculations at the Becke3LYP/6-31G* level of theory with those from the MP2/6-31G** level of theory in parentheses. b Vibrational frequencies scaled at each level of theory by a factor which brings the lowest frequency anti conformer C-H stretch mode into agreement with experimentally observed frequency C-H stretch fundamental. c C-H stretch normal modes. d 2-Hydroxy group intramolecularly H-bonded O-H stretch normal mode. e 5-Hydroxy group “free” O-H stretch normal mode.

Not surprisingly, the structures of the syn and anti conformers of 5-HOTrOH are very similar to the tropolone parent. A slight increase in the O-O separation in the intramolecular H-bond region (0.014 and 0.0103 Å for syn and anti at the MP2/631G** level, respectively) suggests a slight weakening of the intramolecular H bond induced by the 5-OH group. The energy differences between syn and anti isomers calculated with the Becke3LYP 6-31G* and 6-31G** levels are 240 and 234 cm-1, respectively, fortuitously close to the MP2 6-31G** level result (245 cm-1) and to experiment (249 cm-1). Table 1 lists experimentally observed vibrational frequencies and calculated harmonic vibrational frequencies and intensities in the CH and OH stretch region of relevance here. Results are included for both the syn and anti conformers of 5-HOTrOH using both DFT and MP2 levels of theory. Vibrational frequencies scaled for anharmonicity are also given to facilitate a more direct comparison with experiment. The scale factors used are those which bring the experimental and calculated frequencies of the lowest C-H stretch mode into correspondence with one another. Figure 3a presents a stick diagram of the calculated frequencies and intensities for the entire hydride stretch region, with a close-up view of the C-H stretch region shown in Figure 3b. Results and Discussion Overview FDIR Spectra. Figures 4 and 5 present FDIR spectra of the syn and anti conformers of jet-cooled 5-HOTrOH covering the region from 2600 to 4200 cm-1. As mentioned earlier, these spectra were recorded monitoring the blue (syn) and red (anti) origin transitions shown in Figure 2. The signal to noise of the scans is sufficient to observe depletions of a few percent, consistent with the estimated shot-noise limit of our fluorescence signal (∼104 photons/pulse). The spectra of both conformers consist of three sets of transitions: (i) a single sharp transition near 3650 cm-1, (ii) a set of bands near 3200 cm-1, and (iii) another set of bands near 3000 cm-1. Based on a comparison with the FDIR spectrum of tropolone monomer,17 these three regions are nominally assigned to the “free” 5-hydroxy O-H stretch, the intramolecular H-bonded O-H stretch, and the C-H stretch, respectively. In tropolone, the assignment of the C-H and intramolecularly H-bonded OH absorptions was confirmed by the study of deuterated tropolone (C7H5OD). In 5-HOTrOH, both MP2 and DFT level ab initio calculations (Table 1) lead to a similar assignment of the transitions near 3200 cm-1 to the 2-hydroxy O-H stretch transitions and the 3000 cm-1 bands to the C-H stretch

Figure 3. Calculated scaled vibrational frequencies and infrared intensities of the C-H and O-H stretch fundamentals of the anti conformer of 5-HOTrOH. (a) An overview of both regions. (b) A close-up view of the C-H stretch region. The scale factor used is 0.956.

transitions. The following sections focus attention on each region in turn. 5-Hydroxy Group “Free” O-H Stretch Region. Expanded views of the “free” O-H stretch regions of the anti and syn conformer infrared spectra are presented in Figure 6a, and b, respectively. Not surprisingly, the free OH stretch transitions are sharp, single transitions of width about 4 cm-1. Given the large depletions observed for these transitions, the widths probably have contributions from saturation broadening. Comparison of the experimental and theoretical frequency shifts summarized in Table 1 leads to an assignment of the sharp transitions at 3654 and 3664 cm-1 in the syn and anti infrared spectra, respectively, to the “free” O-H stretch of the 5-hydroxy group in each conformer. By comparison, the frequency of the OH stretch fundamental in phenol34 is 3657 cm-1, in almost exact correspondence with the values for syn and anti 5-HOTrOH. The ab initio calculations correctly predict a slightly lower 5-OH stretch frequency for syn (3826 cm-1) than for anti (3831 cm-1) conformers, providing some further confirmation that the blue

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Frost et al.

Figure 4. Overview fluorescence-dip infrared spectrum of the syn conformer of 5-HOTrOH. Figure 6. Close-up view of the 5-OH “free” OH stretch transitions in (a) anti and (b) syn 5-HOTrOH.

Figure 5. Overview fluorescence-dip infrared spectrum of the anti conformer of 5-HOTrOH.

and red S1 r S0 origins are to be assigned to syn and anti conformers, respectively. The scaled 5-OH stretch frequencies (3649 and 3662 cm-1 for syn and anti, respectively) are in good agreement with experiment. The OH stretch vibrational frequency is known to be a sensitive probe of hydrogen-bonding interactions. In water clusters, for instance, the different H-bonding environments of the OH groups produce infrared absorption frequencies which can be spread over more than 500 cm-1. In the present case, the 5-OH stretch is not H bonded, and the two preferred orientations of the 5-OH group (syn and anti) produce only 0.71 kcal/mol (250 cm-1) difference in energy between the conformations.19 Nevertheless, the 5-OH group responds to these subtle differences with a 10 cm-1 frequency shift which, under the conditions of the present experiment, is easily measured.

Thus, even nominally “free” OH stretch fundamentals can serve as a diagnostic of different conformations in isolated molecules, much as OH groups serve as a diagnostic for H bonding in intermolecular complexes. The weak feature at 3651 cm-1 in the anti conformer infrared spectrum (Figure 6a) is not assigned. Since this frequency is close to that of the 5-hydroxy O-H stretch in the syn conformer infrared spectrum, one might suspect that this transition arises from a contamination of the anti conformer near-UV monitor transition by the syn conformer; i.e., this small feature might correspond to the syn and not the anti conformer. This is not likely since the fluorescence signature used to monitor the anti conformer ground state is the red origin and not the blue origin (Figure 1). The more likely source of this band is a weak Fermi resonance unique to the anti conformer. The source of the large depletions observed (g90%) for the 5-OH stretch bands deserves brief comment. In earlier work29 on the tropolone-H2O complex, such large depletions were ascribed to predissociation of the complex in the vibrationally excited state, but in 5-HOTrOH, this is not an open channel. State mixing with dark background states (IVR) does not change the integrated absorption strength of the band and, as such, will not effect the magnitude of the depletions observed as long as the state mixing does not extend to background levels outside of the bandwidth of the OPO. Even if it did, such broadening would only decrease the magnitude of the depletions from their maximum level in the unmixed case. By process of elimination, then, absorption of further photons out of the vibrationally excited state must be responsible for producing depletions greater than 50%, as observed. C-H Stretch Region. The ab initio calculations predict that all four C-H stretch transitions of each conformer will carry some intensity and will occur in a 50-60 cm-1 region located about 100-150 cm-1 below the 2-OH stretch band. The theoretical anti C-H stretch vibrational frequencies and intensities are presented in Figure 3, with numerical results for both conformers summarized in Table 1. Using either MP2 or DFT levels of theory, the syn and anti conformers are both predicted

IR Spectroscopy of Jet-Cooled 5-Hydroxytropolone

Figure 7. Close-up view of the C-H stretch and intramolecularly H-bonded OH stretch transitions in (a) anti and (b) syn 5-HOTrOH. The C-H stretch transitions are below 3100 cm-1 while the structure due to 2-OH stretch is above 3100 cm-1.

to possess a single strong transition flanked by three weaker transitions at slightly higher frequency. The strong C-H stretch transition in the anti conformer is calculated to lie 13 cm-1 lower in frequency than in the syn conformer. Figure 7a,b shows an expanded view of the FDIR spectrum encompassing the C-H stretch and 2-OH stretch regions. By comparison with previous experimental results on tropolone and tropolone-H2O and using the ab initio calculations as a further guide, it is clear that the clump of transitions beginning at 2996 cm-1 in the anti conformer and 3002 cm-1 in the syn conformer is due to C-H stretch transitions. However, unlike tropolone17 and tropolone-H2O,29 the C-H stretch regions of the syn and anti conformers show markedly different substructure. The total integrated intensities of the C-H stretch bands of the syn and anti conformers are quite similar, however, suggesting that the different appearance of the C-H stretch regions lies in Fermi resonant mixing with dark background states. The matchup of the experimental C-H stretch spectrum (Figure 7a) with the ab initio calculations (Figure 3b) is easily made for the anti conformer. This is consistent with vibrational state mixing which is either quite weak or occurs in a nonstate-specific way with a high density of background states so that only broadening results. In contrast, the different substructure and poor correspondence with ab initio results for the syn conformer point to Fermi resonant mixing of the upper state responsible for the strong C-H stretch transition with a single dark state. In such a two-state Fermi resonance, the two transitions in the syn spectrum at 3002 and 3020 cm-1 share the intensity of the dominant CH stretch band and reflect the percent character of the bright C-H stretch mode in the Fermi resonant mixing. By analogy with other aromatics, the most likely Fermi resonant dark state is the overtone of one of the C-H bending modes.25,26,35 Assuming that the relative intensities of the features at 3002 and 3020 cm-1 in the syn conformer spectrum reflect the states’ percent C-H stretch (i.e., bright state) character, simple two-

J. Phys. Chem., Vol. 100, No. 42, 1996 16839 state first-order nondegenerate perturbation theory predicts a coupling matrix element of roughly 8 cm-1 and an energy separation of about 12 cm-1 between the unmixed C-H stretch state and the dark state. The lack of such splitting in the anti conformer would mean that this same background state is shifted enough that the percent C-H stretch character of the background state is no more than about 5%. Assuming the same coupling matrix element for the anti conformer, an energy separation of about 30 cm-1 is required to attain two bands of roughly 95% and 5% purity C-H stretch character. As seen in Table 1, the ab initio calculations predict that the anti conformer fundamentals most likely involved in this 2:1 Fermi resonance are redshifted by 5-15 cm-1 compared to the corresponding syn levels. This is roughly consistent with the increased energy separation required to explain the different band structure of the syn and anti conformer C-H stretch regions. 2-Hydroxy Group Intramolecularly H-Bonded OH Stretch Region. The intramolecularly H-bonded O-H stretch fundamental produces a broad, structured set of bands centered near 3170 cm-1 for the anti conformer and 3200 cm-1 for the syn conformer. This represents a frequency shift of 450-500 cm-1 relative to the free OH stretch fundamentals. The integrated intensities and overall widths (∼100 cm-1) of the syn and anti 2-hydroxy O-H stretch bands are similar, but the substructure of the bands is unique to each conformer. The syn conformer 2-hydroxy O-H stretch band is dominated by a single transition at 3195 cm-1. Satellite peaks are observed to the blue and the red, most notably at 3178, 3202, and 3218 cm-1. The anti conformer 2-hydroxy O-H stretch band has no single dominant transition, but rather a series of similar intensity bands at 3161, 3170, 3183, and 3195 cm-1 flanked by less pronounced transitions at 3129 and 3146 cm-1. The Breadth and Substructure of the OH Stretch Bands. The breadths and substructure of the 5-OH and 2-OH stretch bands are dramatically different and clearly run counter to density-of-states arguments. The higher frequency 5-OH stretch band is a single, narrow transition while the intramolecular H-bonded 2-OH stretch band is broad and shows definite substructure unique to each isomer. In both cases, this substructure has an average spacing of about 10-15 cm-1, comparable to that observed in the intramolecularly H-bonded O-H stretch band in the tropolone-H2O complex.29 This, together with the similar overall widths of the bands, implies that the syn and anti conformer 2-OH stretch V ) 1 states are mixed with a specific set of dark states common to all these systems. As a basis for considering the vibrational state mixing in more detail, a direct count of the total density of vibrational states of a given symmetry was carried out as a function of energy.36 The 42 harmonic vibrational frequencies for each conformer are taken from the DFT ab initio calculations. Negligible tunneling between the conformer wells is assumed, so that syn and anti calculations are separate from one another. All a′ symmetry vibrational states at a given energy were calculated, including even overtones and combination bands of a′′ symmetry. The results for the syn conformer are shown in Figure 8. At the energy of the 2-OH stretch (V ) 1) (3200 cm-1), the total density of a′ states for a given conformer is about 900 states/cm-1, while the 5-OH stretch (V ) 1) level (3650 cm-1) has a bath of background levels of density several thousand states/cm-1. Similar values are found for the anti conformer. As anticipated, the total state density of a′ vibrational states in the intramolecularly H-bonded O-H stretch region is 3-4 orders of magnitude larger than the observed spacing of the substructure in the 2-OH stretch band. As with the C-H stretch, the most likely source of the sparse density of states responsible for primary coupling to the 2-OH (V ) 1) level are with

16840 J. Phys. Chem., Vol. 100, No. 42, 1996

Figure 8. Total density of a′ vibrational states of the syn conformer of 5-HOTrOH as a function of energy above the zero-point level. The harmonic frequencies from the DFT 6-31G* ab initio calculations are used as input for the calculation.

Figure 9. Energy level diagram (above) showing all possible a′ overall symmetry 2:1 or (1 + 1):1 Fermi resonant levels calculated for the syn conformer of 5-HOTrOH in the region from 2600 to 4000 cm-1. This manifold of dark states is compared directly with the FDIR spectrum of the syn conformation of 5-HOTrOH (below). See the text for further discussion.

overtones (2:1) or combination bands ((1 + 1):1) in Fermi resonance with it. Such Fermi resonances are known21-27 to play an important role in the state mixing of vibrationally excited hydride stretch levels in a number of molecules, arising from cubic anharmonicity in the molecular potential. Figure 9 juxtaposes the FDIR spectrum of the syn conformer with an energy level diagram showing the calculated energies of all first overtone and 1 + 1 combination a′ symmetry levels in the energy range from 2600 to 4000 cm-1. One notes, first, that the average spacing of the 2:1 and (1 + 1):1 Fermi resonant states in the region of the 2-OH stretch band (3200 cm-1) is about 10-15 cm-1, roughly comparable with the spacing of the substructure in the 2-OH stretch of the syn and anti conformers. The makeup of these levels is also interesting. Almost all the levels in the region near 3200 cm-1 are overtones or 1 + 1 combination bands involving six modes which in the syn conformer range in frequency from 1483 to 1699 cm-1. These vibrations include C-H and O-H bends, CC stretches,

Frost et al. and the CdO stretch, forming a total of 21 overtones and (1 + 1) combination bands with a typical spacing between states of about 15 cm-1. The large shift down in the frequency of the 2-OH stretch due to the intramolecular H bond thus has the important consequence of moving the 2-OH(V)1) level into Fermi resonance with a number of levels which would not be in close proximity with a free OH stretch such as the 5-OH vibration. The overall breadths of the 2-OH stretch bands indicate strong anharmonic coupling with these Fermi resonant levels, as might be anticipated due to the intramolecular H bond with its short O-O separation (2.50 Å). In particular, the 2-O-H bend, C1C2 stretch, and CdO stretch are physically plausible candidates for anharmonic coupling to the intramolecularly H-bonded 2-OH stretch. Further studies employing isotopic labeling will be required to test this intuition more thoroughly. Despite the higher density of background states at 3650 cm-1, the narrowness of the 5-OH stretch transitions compared to the intramolecularly H-bonded O-H stretch bands indicates significantly less mixing of the 5-OH stretch (V ) 1) levels with background states. This reduced mixing can arise either from a reduced coupling matrix element or from an increased energy separation from the background states which have large matrix elements. The 5-OH stretch, as a “free” OH stretch, involves oscillation away from the rest of the molecule and thus undoubtedly has smaller coupling matrix elements with most other vibrations in the molecule. The energy level diagram of Figure 9 indicates that there are indeed background levels which could be coupled to the 5-OH level via cubic anharmonicity. What is not obvious from a first glance at the figure, however, is that the levels which are in close proximity with the 5-OH stretch (V ) 1) level are almost entirely (1 + 1) combination levels in which a very low-frequency mode is built off either the C-H or 2-OH stretch (V ) 1) levels. Due to a gap in the frequencies of the normal mode fundamentals between 1700 and 3100 cm-1 (Table 1), there is a corresponding absence of possible 2:1 Fermi resonances involving two higher frequency modes such as are thought to be important for the broadening of the 2-OH stretch. Crossover Transitions. A primary motivation for this work has been the prospect of observing infrared crossover transitions analogous to those observed in the S1 r S0 vibronic spectroscopy of 5-HOTrOH.19,20 As shown schematically in Figure 1, the term “crossover” transition is used to indicate a transition from a state whose vibrational wave function is localized in one of the conformer wells to a level whose wave function is primarily in the other conformer well. If this excited state has enough amplitude in both wells, it can be used to drive a transition which in some sense “crosses over” the H atom tunneling barrier, thereby effecting syn T anti photoisomerization in a mode-selective fashion. In the S0 state (Figure 1b), if the delocalized vibrationally excited levels are accessible out of either conformer’s zero-point level, they will differ in infrared frequency by the energy difference between the conformer’s zero-point levels, known19 to be 249 cm-1 in the present case. One would then expect a crossover transition in the syn conformer infrared spectrum 249 cm-1 to the blue of the observed anti conformer 2-hydroxy O-H stretch frequency (Figure 1b). Similarly, a crossover transition in the anti conformer infrared spectrum would appear 249 cm-1 to the red of the observed syn conformer 2-hydroxy O-H stretch frequency. An analogous pair of crossover transitions are also conceivably associated with the 5-hydroxy “free” O-H stretch transitions. The anticipation of such crossover transitions results from the intuition that the 2- and 5-hydroxy group O-H stretch

IR Spectroscopy of Jet-Cooled 5-Hydroxytropolone

J. Phys. Chem., Vol. 100, No. 42, 1996 16841

Figure 10. FDIR spectra of (a) anti and (b) syn 5-HOTrOH plotted on a frequency scale showing the energy above the syn zero-point level. In this plot, the positions of crossover transitions which access the same excited-state level from either conformer’s zero-point level are located vertically above or below one another, as indicated by the dotted lines. Such crossover transitions are not observed. High-sensitivity scans in the crossover regions place an upper bound of 1% depletion on the magnitude of the crossover transitions.

motions are closely associated with the two paths19 for synanti isomerization in 5-HOTrOH. The obvious path is H atom transfer from the 2-hydroxy group to the adjacent intramolecularly H-bonded keto oxygen, resulting in the mirror image conformer with respect to this moiety, but leaving the in-plane 5-hydroxy group in its original position. The second tunneling path is the simple internal rotation of the 5-hydroxy group in the absence of H atom transfer between the 2-hydroxy and keto groups. In S0, the barriers for these two pathways are calculated to be about 2000 and 1300 cm-1, respectively, at the MP2/631G** level of theory.20 In the 2-OH stretch, vibrational excitation brings the molecule nominally toward the first transition state. The 5-OH stretch is not along the internal rotation reaction coordinate, but its excitation nevertheless deposits 3650 cm-1 of energy into a mode localized on the 5-OH group. Anharmonic coupling of the 5-OH stretch with the internal rotor coordinate could potentially facilitate isomerization via internal rotation of 5-OH. We have searched carefully for these infrared crossover transitions under conditions where they could be observed if they produced fluorescence depletions as small as 1%. Figure 10 replots the FDIR spectra with the anti spectrum shifted by 249 cm-1 so that the frequency scale corresponds to the energy above the syn zero-point level. Then the expected positions of the crossover transitions, which access the same excited-state level from the syn and anti zero-point levels, are directly above or below the corresponding transitions in the other conformer. As can be seen from the figure, no evidence for either 2-hydroxy or 5-hydroxy O-H stretch crossover transitions is observed. Infrared syn f anti crossover transitions out of the localized syn zero-point level, ψsyn(0), will have an intensity determined by

〈ψsyn(0)|µ|{Rψanti(V) + βψsyn(V)}〉 * 0

(1)

where R and β are the syn-anti mixing coefficients of the vibrationally excited state. In the general case, ψanti(V) and ψsyn(V) will themselves be admixtures of harmonic anti and syn states. The integral in eq 1 is nonzero if (i) the excited state is partially delocalized (β * 0) so that the ground- and excitedstate wave functions have a region of nonzero overlap and (ii)

the contributors to ψsyn(V) are vibrationally excited levels which carry oscillator strength from the syn zero-point level. One obvious scheme for inducing intensity in crossover transitions would be by direct coupling between, for example, the 2-OH (V ) 1) states of the syn and anti conformers. In this case, criterion i is physically plausible while criterion ii is guaranteed by the appearance of the 2-OH stretch band in the FDIR spectrum of both conformers. The results from tropolone17 can serve as a basis for estimating the value of β within this simple two state approximation. In tropolone, the 2-OH (V ) 1) tunneling splitting has been tentatively assigned to be 12 cm-1. In this symmetric case, the observed tunneling splitting is just twice the syn-anti coupling matrix element Vsa ) 6 cm-1. Assuming a similar value of Vsa in 5-HOTrOH, the 249 cm-1 asymmetry in the potential produces a first-order perturbation theory estimate of β ) 0.024. The prediction of this two-state estimate is that the syn f anti 2-OH stretch crossover transition intensity will be just 0.06% of the syn f syn 2-OH stretch band, a level too small to be observable in depletion. IR-UV Gain Spectroscopy. The breadth and substructure of the 2-OH stretch bands indicate that these levels are strongly mixed with a subset of background states. The sharpness of the 5-OH transitions reflects much weaker coupling. Nevertheless, it is likely that significant state mixing is present even in this latter case but is simply unresolved at the modest resolution of our experiment. In order to test this, we have carried out IR-UV gain scans in which the OPO is set to one of the 5-OH or 2-OH IR transitions and the UV is tuned to search for new features in the spectrum due to the vibrationally excited species. These would appear as gains in the fluorescence signal35 interspersed between the IR-UV depletion transitions of the cold bands. However, following excitation of either conformer’s 5-OH or 2-OH stretch bands, no UV gain signals were observed in a 1300 cm-1 region centered on the blue (syn) origin. The reason for this null result is not entirely clear. One possibility is that the mixed states accessed in infrared excitation still have largely ∆V ) 0 Franck-Condon factors in the S1 r S0 transition and thus produce S1 vibronic levels well above the origin where fluorescence quantum yields may be low. In this case, the S1 r S0 absorptions from the background levels could occur but do not appear in fluorescence. Conclusion The major result of this work is that the OH stretch fundamentals of 5-hydroxytropolone appear, under all measures available to the present experiment, as localized transitions associated with one or the other conformer. The absorption frequencies of the 2-OH and 5-OH stretch transitions of the two conformers show little difference from one another. These transitions occur out of zero-point levels which differ in energy by 249 cm-1, suggesting little change in the height or shape of the barrier to H atom tunneling when accessed from either side of the barrier. The slightly lower frequency of the anti conformer 2-OH stretch (3170 versus 3200 cm-1 for the syn isomer) is consistent with the higher energy of the anti conformer reaching higher up the barrier. Nevertheless, one is led to believe on the basis of the FDIR spectroscopy that the effective barrier for H atom tunneling is still large following OH stretch (V ) 1) excitation. The state mixing of the 2-OH and 5-OH bands is strikingly different for both conformers and clearly runs counter to densityof-states arguments. The breadth and substructure of the 2-OH stretch bands indicate a strong mixing with a sparse set of background levels. The source of these background states probably does not come from the other side of the well. Instead,

16842 J. Phys. Chem., Vol. 100, No. 42, 1996 it is more likely that the large red shift in the 2-OH stretch frequency due to the intramolecular H bond brings this level into near 2:1 Fermi resonance with a number of CC stretch, CH bend, OH bend, and CdO stretch levels in the same conformer well, thereby producing the observed substructure of the 2-OH band and its overall width. On the other hand, the 5-OH stretch is at about 3650 cm-1, well removed from such resonances, and is instrumentally narrow. Experiments aimed at selectively detecting the amount of syn (anti) character in the nominally OH stretch (V ) 1) anti (syn) state have met without success. The search for crossover transitions attempted to observe the syn (anti) character by driving transitions to the OH(V)1) anti (syn) level out of the syn (anti) conformer’s zero-point level. Alternatively, the IRUV gain experiments attempted to excite out of these syn character (anti character) background states levels to the syn (anti) conformer’s S1 state. Thus, while electronic excitation of 5-hydroxytropolone can produce substantial delocalization within a few hundred wavenumbers above the S1 origin,19 the infrared absorption spectrum shows no direct evidence for such delocalization in spite of excitation 3000-3700 cm-1 above the S0 origin. Three possible reasons for these differences can be put forward. First, the 2-OH stretch may not be as strongly coupled to the syn-anti reaction coordinate as one might have expected. This is consistent with the small tunneling splitting tentatively assigned to the 2-OH (V ) 1) level in tropolone.17 In fact, the highly nonlinear, sterically hindered intramolecular H bond results in a 2-OH stretch vibration which is more nearly perpendicular to the keto oxygen than directed toward it.17 Second, vibrational state mixing within the syn (anti) well at the energy of the syn (anti) hydride stretch may so dilute the OH(V)1) character with states which are not strongly coupled to the anti (syn) states that the OH(V)1) coupling is effectively lost. This “single-well” IVR is a common enemy of modeselective behavior in all molecules. Third, the syn-anti coupling in the syn (anti) OH(V)1) levels could be substantial, but the nature of the anti (syn) background levels with which it is coupled may make them undetectable by either the crossover or IR-UV gain experiments; that is, the transitions may simply lack oscillator strength from the anti (syn) zero-point level. In IR-UV gain experiments, the ultraviolet excitation out of the anti (syn) states may access levels which do not fluoresce and therefore cannot be detected in the present experiment. Acknowledgment. The authors gratefully acknowledge the support of the Petroleum Research Fund, administered by the American Chemical Society, for this work. Partial support was also provided by the National Science Foundation (NSF CHE9404716). The authors thank Prof. Steven Hardinger of the California State University at Fullerton for synthesizing the 5-hydroxytropolone used in this work. References and Notes (1) Alves, A. C. P.; Hollas, J. M. Mol. Phys. 1972, 23, 927-45; 1973, 25, 1305-14. (2) Redington, R. L.; Redington, T. E. J. Mol. Spectrosc. 1979, 78, 229-47. (3) Rossetti, R.; Brus, L. E. J. Chem. Phys. 1980, 73, 1546-50. (4) Sekiya, H.; Nagashima, Y.; Nishimura, Y. Chem. Phys. Lett. 1981, 160, 581-85. (5) Tomioka, Y.; Ito, M.; Mikami, N. J. Phys. Chem. 1983, 87, 440105. (6) Alves, A. C. P.; Hollas, J. M.; Musa, H.; Ridley, T. J. Mol. Spectrosc. 1985, 109, 99-122. (7) Redington, R. L.; Chen, Y.; Scherer, G. J.; Field, R. W. J. Chem. Phys. 1988, 88, 627-33. (8) Sekiya, H.; Nagashima, Y.; Nishimura, Y. Bull. Chem. Soc. Jpn. 1989, 62, 3229-31. (9) Sekiya, H.; Nagashima, Y.; Nishimura, Y. J. Chem. Phys. 1990, 92, 5761-69.

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