J . Phys. Chem. 1986, 90, 2596-2600
2596
the excimer with the configuration at 0 = 0' and R
= 3.5 A.
17203-41-7; 4-methylpyridine, 108-89-4; 4methylpyridine conjugate acid: 16950-21-3;2-ethylpyridine, 100-71-0;2-ethylpyridineconjugate acid, 70199-71-2; 3-ethylpyridine,536-78-7; 3-ethylpyridineconjugate acid, 69966-17-2; 4-ethylpyridine, 536-75-4;4-ethylpyridineconjugate acid, 16950-22-4;2-propylpyridine, 622-39-9; 2-propylpyridine conjugate acid, 70199-72-3;4-propylpyridine,1122-81-2; 4-propylpyridine conjugate acid, 16950-23-5; 2,3-dimethylpyridine, 583-61-9; 2,3-dimethylpyridine conjugate acid, 6688 1-1 5-0; 2,4-dimethylpyridine, 108-47-4; 2.4-dimethylpyridine conjugate acid, 17126-11-3; 2,5-dimethylpyridine, 589-93-5; 2,5-dimethylpyridine conjugate acid, 64343-93-7; 2,6-dimethylpyridine, 108-48-5; 2,6-dimethylpyridineconjugate acid, 1703311-3; 3,5-dimethylpyridine,591-22-0; 3,5-dimethylpyridineconjugate acid, 19495-57-9; 5-ethyl-2-methylpyridine, 104-90-5; 5-ethyl-2methylpyridine conjugate, 101773-1 7-5.
Acknowledgment. We are indebted to Professor Michael Kasha, a Director of the Institute of Molecular Biophysics of Florida State University, for his kind discussions and valuable comments and also to Dr. Masao Kotani, a former President of our University. We also thank Dr. Ken Steckler, National Bureau of Standards, U.S.A., for his kind help in the English version of the manuscript. Registry No. Pyridine, 110-86-1;pyridine conjugate acid, 16969-45-2; 2-methylpyridine, 109-06-8; 2-methylpyridine conjugate acid, 1696946-3; 3-methylpyridine, 108-99-6; 3-methylpyridine conjugate acid,
Intramolecular Hydrogen Bonding. 5. Opposite Asymmetry of the S, and T, States of 6-Hydroxybenzanthrone G. D. Gillispie,* M. H. Van Benthem, and M. Vangsness Department of Chemistry, North Dakota State University, Fargo, North Dakota (Received: November 19, 1985)
581 05
The fluorescence, phosphorescence, and luminescence excitation spectra of 6-hydroxybenzanthrone in an n-hexane Shpol'skii matrix are reported. Deuterium substitution of the intramolecular hydrogen bond proton has little effect on the luminescence spectral distributions but changes the excitation spectrum in a manner similar to that found for 9-hydroxyphenalenone and its methyl derivatives. The vibronic patterns of the fluorescenceand phosphorescence spectra differ in a manner which suggests that the asymmetry in the TI state proton-transferltunneling double minimum potential function is opposite that for the SI and So states.
Introduction
TABLE I: Site Energies for 6-HBA and 6-DBA in n-Hexane Shpol'skii Matrices'
The molecule 6-hydroxybenzanthrone (6-HBA) is structurally related to 9-hydroxyphenalenone (9-HPO). The parent compound
molecule 6-HBA 6-HBA 6-DBA 6-DBA 2
6-HBA
5
state Sl
T1 SI TI
energies, cm-' long short wavelength site wavelength site 22 944 22 976 17 842 17 906 23 077 23 104 17814 17879
Uncorrected to vacuum wavenumbers.
9 -HPO
and the 2- and 5-methyl derivatives of 9-HPO (for which the acronym 9-HPLN has also been used) have been the subject of numerous papers, especially with respect to the double minimum potential function associated with proton transfer/tunneling (PT/T) across the intramolecular hydrogen bond. Rossetti, Haddon, and Brus' (RHB) identified vibronic bands arising from tunneling splitting and used these data to suggest potential functions. In a second paper2 they examined the 2-methyl derivative and discussed the consequences of this weak substitutional perturbation on the shape of the potential function. In addition, they clearly outlined the importance of wave function localization in unsymmetric double minimum potential function problems. More recently, Bondybey et al.394have reexamined 9-HPO and added data for the 5-methyl derivative. With the recognition that vibrational relaxation is incomplete in the S1state of 9-HPO, these workers have been able to validate and extend the original tunneling splitting analysis.
Although the extra aromatic ring of 6-HBA surely represents a greater perturbation to the electronic structure of 9-HPO than does a methyl group, the electronic spectra are sufficiently similar to warrant a comparative analysis. However, 6-HBA is unique in its own right. In a previous Letter5 we have noted an enormous isotope effect on the phosphorescence lifetime of 6-HBA; deuterium substitution of the hydrogen-bonded proton increases the T, lifetime from 19 to nearly 300 ms. In this paper we consider in greater detail the fluorescence, phosphorescence, and luminescence excitation spectra of 6-hydroxybenzanthrone. The thesis is advanced that the asymmetry in the triplet-state double minimum potential function is opposite that which holds in the So and S, states. Experimental Section
Dilute solutions of 6-HBA in n-hexane were quick frozen to 77 K and then the Shpol'skii matrix was further cooled to ca. 10 K in a closed cycle refrigerator. Luminescence was excited with the output of a homebuilt nitrogen-laser-pumped tunable dye laser at an angle of 45' to the viewing axis. The emission was dispersed through a Spex 0.5-m double monochromator and detected with
(1) Rossetti, R.; Haddon, R. C.; Brus, L. E. J. Am. Chem. SOC.1980, 102,
6913. (2) Rossetti, R.; Rayford, R.; Haddon, R. C.; Brus, L. E. J . Am. Chem. SOC.1981, 103, 1303. (3) Bondybey, V. E.; Haddon, R. C.; English, J. H. J . Chem. Phys. 1984, 80, 5432. (4) Bondybey, V. E.; Haddon, R. C.; Rentzepis, P. M. J . Am. Chem. SOC. 1984, 106, 5969.
0022-3654,I86,12090-2596$01 S O I O I
(5) Van Benthem, M. H.: Gillispie, G . D.; Haddon, R. C. J . Phys. Chem. 1982, 86, 428 1.
0 1986 American Chemical Societv -
SI and TI States of 6-Hydroxybenzanthrone
FLUORESCE NC E
The Journal of Physical Chemistry, Vol. 90, No. 12, 1986 2597
PHOSPHORESCENCE
Figure 2. Site-selected phosphorescence spectra. Bands for 6-HBA appear slightly broader since wider slits had to be used on emission monochromator owing to weakness of 6-HBA phosphorescence.
Figure 1. Site-selected fluorescence spectra of the longer wavelength site for 6-HBA and 6-DBA in n-hexane. See Table 111 for a summary of fundamental frequencies. Abscissa is on a scale linear in wavenumber. The origin bands of the higher energy site can be seen at the extreme left of each trace.
a cooled R C A C31034 red-sensitive photomultiplier tube. The fluorescence signal was processed with a simple sample-and-hold circuit and the resulting signal displayed on a strip-chart recorder. The longer-lived phosphorescence signal was acquired via photon counting. The interpretation of the spectra requires that vibrational intervals in the fluorescence and phosphorescence spectra be carefully compared. Wavelength accuracy of f0.02 nm for individual features was achieved by simultaneously recording the output of a Fe-Ne hollow cathode lamp along with the 6-HBA emission. Air wavelengths of the iron lines were taken from the tabulation by Crosswhite6 and the wavelengths of the 6-HBA bands determined by interpolation.
Results and Discussion The Luminescence Spectra. The spectra reported herein are site-selected for the longer wavelength member of the two major sites of 6-HBA in n - h e ~ a n e .Site ~ energies for the SIand TI states of 6-HBA and its hydroxy-deuterated analogue, 6-DBA, are collected in Table I. Complete spectra have also been acquired for the shorter wavelength site. Although there are minor differences in the spectra of the two different sites, these differences are insufficient to affect the conclusions and analysis to be presented below. The fluorescence spectra of 6-HBA and 6-DBA are shown in figure 1. Vibrational activity is primarily concentrated in the range 200-650 cm-'. There are also weaker fundamentals in the 1200-1250- and 1400-1450-~m-~ regions and these seem to be characteristic of 1,Cquinones such as naphthazarin' and quinizarin.8 Combination bands are of negligible intensity and at least ten fundamental vibrations with frequencies less than 650 cm-' can be assigned with confidence, especially since (6) Crosswhite, H.M. J . Res. Natl. Bur. Stand., Sect. A 1975, 79, 17. (7) Rentzepis, P. M.; Bondybey, V. E. J . Chem. Phys. 1984, 80, 4727. (8) Carter, T. P.; Gillispie, G. D.; Connolly, M. A. J . Phys. Chem. 1982, 86, 192.
the spectrum of 6-DBA so resembles that of the normal isotopic species. The phosphorescence spectra of 6-HBA and 6-DBA are compared in Figure 2. Although the 6-DBA emission is more than an order of magnitude more intense than that of the normal isotopic species, the vibronic distributions for the first 600 cm-I are virtually identical. As is the case for the fluorescence spectrum, the bulk of the vibronic activity is concentrated in the low-frequency region; most of the strong vibrational bands in the fluorescence apparently have counterparts in the phosphorescence although with different relative intensities. Many weaker bands show up only in the phosphorescence. The likely reason is that the spin-orbit coupling pathways make both in-plane and outof-plane vibrations allowed in the phosphorescence, whereas only the former are allowed for a planar 6-HBA in fluorescence. With regard to the deuterium isotope effects on the phosphorescence spectra, we suggest that the 711-cm-' band of 6-HBA, the strongest vibronic band in the entire spectrum, be correlated with the (662,670) doublet in 6-DBA. The 931-cm-' mode in 6-HBA is then matched with the 720-cm-' band in 6-DBA; the frequency shift and the absence of similar bands in the fluorescence spectra make the assignment of this vibration as an 0-H out-of-plane bend plausible. Relationship of 6-HBA to the Hydroxyphenalenones. A symmetric double minimum potential function exists for motion of the hydrogen-bonded proton in 9-HPO as the molecule tunnels back and forth between two C, symmetry forms. As first reported by Rossetti, Haddon, and Brus,' a temperature-sensitive band in the fluorescence excitation spectrum of 9-DPO (deuterated 9HPO) in neon can be assigned as a transition from the upper member of a tunneling split pair of vibrationless levels in So. From the temperature dependence of the intensity, a ground-state tunneling splitting of ca. 10 cm-' was inferred. The corresponding S, splitting was found to be increased to 170 cm-l, indicative of a lower barrier to proton tunneling in SI compared to So. By assuming a two-parameter model for thepotential function and by equating the reduced mass for the tunneling motion with the proton (deuteron) mass, RHB inferred a barrier height of 24 12 kcal/mol for proton tunneling in So. No temperature-dependent feature could be found for 9-HPO and this was ascribed to a greater tunneling splitting, which precluded sufficient thermal population of the upper member of the split pair.
*
2598
The Journal of Physical Chemistry, Vol. 90, No. 12, 1986
Kunze and de la Vegag have calculated the potential function for proton transfer/tunneling (PT/T) in 9-HPO at the ab initio STO-3G S C F level with full geometry optimization. A barrier height of 5.2 kcal/mol was found. Unfortunately, the predictional accuracy of such a calculation is impossible to assess at this time. The proton transfer is nominally an isodesmic reaction, for which the S C F level is generally adequate, but there are clearly substantial electronic rearrangements taking place along the transfer coordinate. For example, a similar calculation at the 3-2iG* S C F level gives a barrier height of 7.4 kcal/mol for malonaldehyde but the barrier drops to 4.8 kcal/mol when the basis set is expanded to 6-31G* and correlation is introduced at the MP2 level.1° Even greater changes are brought about by basis set expansion and correlation energy corrections in the C,H,OS analogue to malonaldehyde.I0 A methyl substituent at the 2-position introduces a slight asymmetry into the double minimum potential. From the matrix isolation fluorescence and excitation spectra of 2-Me-9-HP0, Rossetti et aL2inferred that one well was preferentially stabilized by 180 cm-' over the other. For the relatively high barrier which obtains for the ground electronic state, this stabilization is sufficient to localize the u = 0 wave function in the lower energy well. Extensive delocalization remains in the SI excited state owing to a much lower barrier to PT/T. The calculations of Kunze and de la Vega, limited to the ground electronic state, also agree qualitatively with this analysis. Significant new data on 9-HPO (referred to as 9-HPLN by the authors) were made possible by the recognition of incomplete vibrational relaxation in the frozen rare gas matrix environment. Bondybey et aL3 verified the 9-DPO tunneling splittings assigned by R H B and were further able to also clearly identify the corresponding splittings in the normal isotopic species. The splittings in 9-HPO are about half those predicted by RHB from their potential modeling analysis. Bondybey, Haddon, and English suggest that this discrepancy is likely a consequence of the simplifying assumption that the reduced mass for tunneling is close to that of the proton (deuteron) mass. The 5-methyl derivative of 9-HPO was also examined. If the methyl group is assumed to be freely rotating, this is also a case of a symmetric double minimum potential function. The methyl substituent increases the tunneling splittings somewhat, presumably indicative of a slightly reduced barrier to tunneling. The parent compound or a symmetrically substituted derivative such as 5-Me-9-HPO nominally represents the situation of a molecule undergoing proton tunneling in the ground electronic state between two equivalent minimum energy conformations. The tunneling leads to splitting of the zero-point-level and other zeroth-order levels below the barrier into symmetric and antisymmetric components; these are usually given (+) and (-) designations. A similar situation applies to the SI excited state. In the case of strictly symmetric double minimum potential functions in both So and SI,the selection rule O+-O+ and O--Oapplies. The transitions variously labeled To1and 0,- (Le., from the lower member of the tunneling split pair in So to the upper member of the corresponding pair in SI)are nominally forbidden in 9-HPO and 5-Me-9-HPO (assuming free methyl group rotation) but are nonetheless observed in the excitation spectra. This selection rule breakdown has been ascribed to matrix perturbations by Rossetti et al., whose assignments of bands with frequency 31 1 cm-' in 9-HPO and 172 cm-I in 9-DPO to O+- were later confirmed by Bondybey et al. An assignment of 197 cm-' was made for 2-Me-9-DPO by RBH and values of 189 cm-I in 5-Me-9-DPO and 431 cm-I for 5-Me-0-HPO were deduced by Bondybey et al. These latter authors also noted that the bands associated with the tunneling transition were broadened and related this to matrix perturbation observed for Jahn-Teller-type distortions in other molecules. The Fluorescence Excitation Spectra of 6-HBA and 6-DBA. The luminescence excitation spectra of 6-HBA and 6-DBA are (9) Kunze, K. L.; de la Vega, J. R. J . Am. Chem. SOC.1984, 106, 6528. (10) Johnson, G.: Gordon, M. S.; Gillispie, G. D., unpublished work.
Gillispie et al.
N
It
6-DBA
Figure 3. Luminescence excitation spectra. The excitation spectra are independent of whether fluorescence or phosphorescence is used for monitoring. These spectra are uncorrected for variations in laser power across the profile of the coumarin 440 dye and therefore the 0-0 bands
appear artificially weak. TABLE 11: Tunneling SDlittinns (in cm-') ~
molecule
9-HPO 2-Me-9-HPO 5-Me-9-HPO 6-HBA
H" 69 (199)c 92
~~~~
S, state
So state
Db 12
(180)'
H 311 577 431 522
D 172 197 189 178
a Normal isotopic species. Deuteriohydroxy species. Inferred from model calculations; see ref 2.
shown in Figure 3. As opposed to the fluorescence and phosphorescence spectra, deuterium substitution effects are substantial in the excitation spectrum. The band at 178 cm-' for 6-DBA stands out quite notably. By analogy to the spectra for 9-HPO and its derivatives, an assignment of this band to O+-is indicated. The situation for 6-HBA is perhaps not quite so clear but we believe that 522 cm-' is the likeliest candidate for assignment as O+-primarily on the basis of intensity. Since the 522-cm-' bands is also broader than the rest, it is tempting to cite this as additional evidence in favor of the analysis. However, we must point out that the corresponding band is of comparable width to the others in the excitation spectrum for the shorter wavelength site. A search for evidence of vibrationally unrelaxed fluorescence in 6-HBA, which could confirm the tunneling splitting assignments, was unsuccessful. Since we have not found vibrationally unrelaxed emission for 9-HPO in hexane as well, the phenomenon is to this time specific to frozen rare gas matrices. Nevertheless, the tunneling splitting trends summarized in Table I1 lend credence to the provisional 6-HBA analysis. The So and SI Proton-Transfer Potential Functions. The fluorescence 0-0 band wavenumber for 6-HBA in hexane is 22 960 cm-' (averaged over the values for the two main sites), which is to be compared with the value of 22 650 cm-I for 9-HPO in the same solvent. In frozen rare gases the fluorescence origins occur at 23 199 cm-I for 9-HPO, 23 165 cm-' for 2-Me-9-HP0, and 22 694 cm-I for 5-Me-9-HPO. There is probably not very much
S1 and TI States of 6-Hydroxybenzanthrone
The Journal of Physical Chemistry, Vol. 90, No. 12, 1986 2599
TABLE III: Comparison of Vibrational Intervals (in cm-’) in the Fluorescence Swctra of CHBA and CDBA 6-HBA 6-DBA A 232.5 -0.6 231.9 320.9 -0.6 320.3 2.3 311.2 319.5 1.1 392.1 393.2 -1.7 412.7 411.0 4.9 457.1 452.2 4.8 510.1 505.3 1 .o 538.3 539.3 -5.7 549.5 543.8
additional conjugation brought about by the extra benzene ring of 6-HBA in comparison to 9-HPO. Still, it is reasonable to expect that the ring introduces a greater asymmetry into the PT/T potential function than would the methyl substituent of 2-Me9-HPO. In the latter case the inferred preferential lowering of one well of the double minimum potential function by 180 cm-l would be sufficient to localize roughly 80% of the O+ and 0-wave functions in one well or the other. Accordingly, we propose that in 6-HBA the O+ and 0-wave functions are essentially completely localized in individual wells. [The +/- notation is retained although strictly it is only applicable to more symmetric forms such as 9-HPO.] There is no evidence on the size of the tunneling splitting at this time but it is expected to be somewhat smaller than in 2-Me-9-HPO for which the inferred value is ca. 200 cm-l, dropping to about 180 cm-’ for the deuterated species. The SIPT/T potential function must also be unsymmetric in 6-HBA, but the localization effect on the wave functions will be less pronounced owing to a lower barrier and hence greater tunneling splitting. Rossetti et al. have pointed out that wave function localization becomes significant when the asymmetry introduced by a structural modification is comparable to the tunneling splitting in the absence of the asymmetry. The fact that the O++and the O+-transitions are of comparable intensity is strong evidence for the substantial delocalization of the O+ and 0-wave functions in SI. The fluorescence vibronic structure of 9-HPO and that of its methyl derivatives is not much affected by deuterium replacement of the hydrogen bonded proton. In Table I11 we compared the fluorescence vibrational intervals for 6-HBA and for 6-DBA and it can be seen that the differences are for the most part minimal. This tends to rule out any large coupling between the protontransfer/tunneling mode and the other vibrational modes of the molecule. The T I State Proton-Transfer Tunneling Potential Function. If anything, the phosphorescence intervals are even more invariant to the isotopic substitution, leading to a similar conclusion about the separability of the proton-transfer/tunneling mode from the other vibrational modes in the TI state. Most molecules with intramolecular hydrogen bonds do not exhibit phosphorescence so there is virtually no precedent for the discussion here of the TI state potential function. In Figure 4 we show a superposition of the fluorescence and phosphorescence spectra of 6-DBA (for the longer wavelength of the two main sites.). At first glance the spectra seem unremarkable; for example, there are five modes active in fluorescence with frequencies between 230 and 420 cm-l and each of these seems to have a counterpart in the phosphorescence spectrum. However, the accurate wavenumbers determined via the hollow cathode lamp calibration reveal without question that two of these five bands have a different interval from the origin in fluorescence than in phosphorescence. In both cases (321 vs. 329 cm-’ and 413 vs. 420 cm-l) the interval is greater in the phosphorescence spectrum than in the fluorescence. Although the hollow cathode calibration was not applied to the 6-HBA phosphorescence owing to its low intensity, there is a similar wavenumber interval mismatch for the normal isotopic species. If this were the only evidence, further discussion would probably be unwarranted. Although it would be surprising if 329 cm-’, the strongest phosphorescence vibronic band, should be silent in fluorescence while another vibration of nearly the same frequency,
Figure 4. Comparison of the fluorescence and phosphorescence vibronic structure for 6-DBA. Bands apparently correlated in the two spectra are connected with dashed lines. However, note the discrepancies in frequency for the 321, 329 pair and for the 413,420 pair.
321 cm-’, only showed up in fluorescence, it would not be impossible. However, other peculiarities lead us to speculate further. For example, the fluorescence origin undergoes a large blue shift when the hydroxy proton is isotopically replaced. This blue shift of ca. 125 cm-’ is fairly typical of the HPO class of compounds and intramolecularly hydrogen-bonded systems in general. In more simply substituted aromatic compounds, e.g., benzene, naphthalene, and phenanthrene, blue shifts are generally the rule as well; the standard explanation is that overall bonding character and, hence, zero point energy are reduced in the excited electronic state. The 28-cm-‘ red shift of the phosphorescence origin upon isotopic substitution of the hydroxy proton is therefore quite striking. Moreover there is the very large deuterium isotope effect on the phosphorescence lifetime, 300 ms in 6-DBA vs. 19 ms in 6-HBA. Could it be that the asymmetry in the TI state proton-transfer function is opposite that in So or SI? In the terminology advanced by Rossetti, Haddon, and Brus the relationship between So and TI would be one of “reversed asymmetry”. Consider the schematic potential functions illustrated in Figure 5. The low-temperature fluorescence transitions terminate in one well of the ground state while the phosphorescence transitions terminate in the other. Of course, the bulk of the luminescence intensities arise from transitions involving normal modes other than that of the protontransfer/tunneling mode. However, these spectral features are not totally insensitive to which well of the double minimum potential function is selected. Since there are relatively small shifts in the fluorescence or phosphorescence spectra brought about by deuterium substitution of the hydrogen-bonded proton, one may conclude that the PT/Tmode is more or less decoupled from the other vibrational modes. Still there will be some interaction describable in terms of anharmonicity corrections to the vibrational energy expressions. In the simplest case we consider two vibrational modes with harmonic frequencies wt and wb where “t” denotes the tunneling mode and “b” denotes one of the other or bath modes. The total vibrational energy for this two-mode approximation can be written
where ut and ub are quantum numbers and Xtb is an anharmonic coupling constant. Diagonal anharmonicities add nothing to the
J . Phys. Chem. 1986, 90, 2600-2608
2600
I
energy expression of eq 1 assumes a near harmonic spectrum for the tunneling coordinate and this is decidely not the case. However, our point is that anharmonicity corrections arising from coupling between the tunneling mode and the other modes can have an effect on the apparent fundamental frequencies and this idea is still valid. As mentioned in our earlier Letter, a key experiment would be to probe the TI vibrational manifold via the phosphorescence excitation spectrum with direct excitation into the triplet state. We have still not attempted this experiment, although it seems to be within the limits of feasibility. The prediction drawn from So the above analysis is that the first strong band in the T, phosphorescence excitation spectrum will occur to higher energy than the 0-0 band of the phosphorescence. Prompted by the recent report of phosphorescence in 9-DPO by Bondybey et a L 3 we have begun to look at that molecule in Shpol'skii matrices as well. The fluorescence spectra of 9-HPO and 9-DPO in hexane are vibronically the same as those reported for the neon matrix. The other workers stated that their apparatus was not optimized for long-lived emissions but they were able to discem a 0 4 band for 9-DPO near 17 350 cm-' and some vibronic structure. Although the vibronic bands were characterized as having a "rather broad" appearance, we find the phosphorescence spectrum of 9-DPO to be as vibronically sharp as the fluorescence. Work is continuing at this time but we have determined that the phosphorescence/fluorescence intensity ratio for 9-HPO is only 2-3% that for 9-DPO. Moreover, just as in 6-HBA, deuterium substitution of the hydrogen-bonded proton blue-shifts the fluorescence by roughly 125 cm-I but red-shifts phosphorescence by almost 50 cm-I. Thus chances are good that similar factors are operative in 6-HBA and 9-HPO. It has been suggested to us that the discrepancies in the fluorescence and phosphorescence vibronic intervals arise because the TI state has time to relax into a different guest-host configuration before emission, compared to the S1guest-host structure. We cannot rule out this possibility although one generally assumes that intramolecular vibrational frequencies are for the most part insensitive to the host environment. The exception, of course, is when large amplitude motion is involved. Quite possibly proton transferltunneling falls into this category but then this interpretation becomes much the same as the one we presented above. Registry No. 6-HBA, 43099-11-2; 6-DBA, 83335-53-9.
-
Figure 5. Schematic potential energy curves along the proton-tunneling coordinate for 6-HBA. The asymmetry in the double minimum potential functions for So and TI is assumed to be sufficiently great to localize wave functions more or less completely in one well or the other.
discussion and are therefore ignored. We assign ut = 0 to the wave function along the tunneling coordinate localized in the more stable well and ut = 1 to the wave function localized in the other well. Now we have assumed a " m o n asymmetry for the fluorescence so the apparent frequency of the fundamental for mode b is given by E(vt=O, vb=l)
- E(vt=O, v b = o ) = wb - j/2X'b
In contrast, the TI state has been assumed to have reversed asymmetry relative to So so transitions terminate on the ut = 1 level of the tunneling coordinate and the apparent fundamental frequency of mode b in the phosphorescence is
E ( u , = l , Ub-1) - E ( v , = l , ub=o) = a b - 3/2xtb Now this analysis is admittedly highly oversimplified since the
FTIR Spectra of Halobenzene Complexes with Hydrogen Fluoride in Solid Argon Steven R. Davis and Lester Andrews* Chemistry Department, University of Virginia, Charlottesville, Virginia 22901 (Received: December 2, 1985)
Hydrogen-bonded complexes of halobenzenes and HF have been prepared by freezing argon-diluted reagents at IO K. Infrared spectra of the products characterized include a planar C6H5-F- -HF complex with HF hydrogen bonded to fluorine as well as two different C6H3-X- -HF complexes (X = C1, Br) involving hydrogen bonding to the aromatic ring or to the halogen atom. The two different complexes for chlorc- and bromobenzeneare identified by comparison to benzene- -HF and CH3X--HF spectra. Warming the matrix produced complexes of the type C6H5-X- -(HF), where X = F, CI, and Br. The strength of the hydrogen bond to the halogen atom increased with increasing atomic number, and the basicity of the ring in C6H5-CI and C6H5-Br was approximately equal to that of benzene when H-F stretching fundamentals in the complexes are used as a guide.
Introduction Intermolecular hydrogen bonding is very important in dictating the physical properties of a vast array of molecules, and matrix isolation studies of small hydrogen-bonded complexes can provide useful models for more complicated systems. Benzene and alkyl halides have been shown to undergo hydrogen bonding with HF and are of comparable basicity as measured by the H-F stretching frequency in the complexes.I4 The site of hydrogen bonding in 0022-3654/86/2090-2600$01 S O / O
a molecule with more than one base moiety is very important in chemistry and biology and the purpose of this work is to char(1) Baiocchi, F. A.; Williams, J. H.; Klemperer, W. J . Phys. Chem. 1983, 87, 2079. ,706. (2) Andrews, L.; Johnson, G. L.; Davis, S.R. J . Phys. Chem. 1985, 89, (3) Johnson, G . L.; Andrews, L. J . Am. Chem. SOC.1980, 102, 5736. (4) Arlinghaus, R. T.; Andrews, L. J . Phys. Chem. 1984, 88, 4032.
0 1986 American Chemical Society
