J . Phys. Chem. 1989, 93, 614-621
614
TABLE IV: Q, Spectral Shifts’ and Oscillator Strength Changes) AT, for Me-BPheo-a Oligomers in the Crystal Geometry” spectral spectral shift. cm-’ Ap shift, cm-I Ap dimer 349 0.020 hexamer 771 0.040 trimer 544 0.029 heptamer 798 0.041
tetramer
660
pentamer
729
0.035 0.038
octamer
814
0.042
Results are from Shipman-PCM-based exciton calculations which include monomer Soret band states. All shifts given are Fed shifts relative to the monomer Q, band. bGiven as Af = (1/M) floligomer) f(monomer), where M is the number of monomers in the oligomer. flmonomer) = 0.65.
aggregates studied here represent a somewhat limited model of a crystal, exploratory calculations on more extensively aggregated systems indicate that one-dimensional stacking is the major factor responsible for the observed spectral shifts, and only perhaps a 10% increase in shifts would be expected to arise from other modes of aggregation. Changes in oscillator strength are seen to increase steadily with aggregation, the oscillator strength of the octamer being approximately 6% larger than that of the monomer. As discussed previously, it is expected that these changes are underestimated by the present “limited” exciton calculations, and thus significant hyperchromism would be expected of the larger Me-BPheo-a aggregates. Conclusions
The present results as well as those obtained in the previous study of related dimer systemsz9reveal severe problems associated with application of the point-dipole approximation in exciton studies of aggregates of spatially-extended monomers. Unacceptable errors in both spectral shifts and oscillator strengths are produced by this method, regardless of whether experimentally or theoretically determined dipoles are employed. The method appears totally unreliable and should be avoided.
The present results also demonstrate the importance of including non-nearest-neighbor interactions in exciton calculations of aggregated systems. Computations in which only nearest-neighbor interactions were considered seriously underestimated the magnitude of spectral shifts, although oscillator strengths were largely unaffected. Neglect of interactions between molecules with center-to-center distances >20 8, appear to produce acceptable errors in spectral shifts. There are insufficient experimental data to determine the accuracy with which Shipman-PCM-based calculations can predict spectral shifts, but it is clear that red-shifts are underestimated by this method. It is possible that larger spectral shifts would be obtained from exciton calculations in which higher quality monomer wave functions were employed. The present FSGObased wave functions contain basis functions which do not extend into space as much as do larger atomic basis sets; consequently, the present exciton calculations might be expected to underestimate the intermolecular interactions responsible for spectral shifts. Beyond this, it has been suggested” that charge-transfer effects may be significant, and consideration of this as well as monomeric charge redistribution may be necessary in order to obtain accurate spectral shift values. Exciton calculations on the present aggregates using the Shipman-PCM appear to be useful for qualitative characterization of the spectroscopic properties of aggregates, such as distinguishing between conformations which exhibit blue-shifted spectra from those with red-shifted spectra, or identifying large or small spectral shifts, respectively, with extensive or limited aggregation. Due to the high speed with which such calculations may be carried out, the Shipman-PCM-based exciton procedure remains an attractive method for investigating the general qualitative spectral features of aggregates of larger molecules. Acknowledgment. This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences of the United States Department of Energy. Registry No. Me-BPheo-a polymer, 117309-61-2; BC polymer, 117309-62-3.
Dynamics of Double-Proton-Transfer Reaction in the Excited-State Model Hydrogen-Bonded Base Pairs Kiyokazu Fuket and Koji Kaya* Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14- 1 Hiyoshi, Kohoku-Ku, Yokohama 223, Japan (Received: May 9, 1988; In Final Form: August 10, 1988)
The absorption behavior and emission behavior of model hydrogen-bonded base pairs such as 7-azaindoleand 1-azacarbazole dimers and their heterodimer were studied in a supersonic jet by using a laser induced fluorescence method. These dimers were found to exist as the two conformational isomers, and one of the isomers, which has a nearly coplanar structure, selectively undergoes the double-proton-transfer reaction (DPTR) in the excited state. In these dimers, DPTR occurred even from the zero vibrational level of the SI state and the rates of DPTR were affected substantially by deuterium substitution. We found that the symmetricstretching vibration in the hydrogen bond promotes pronouncedly the DPTR of these dimers. Moreover, the fluorescence excitation and dispersed fluorescence spectra of the jet-cooled tautomer of 7-azaindole were measured by a two-step laser excitation, and the double minimum potential of the dimer was experimentally elucidated. On the basis of these results, the dynamics of DPTR was discussed in terms of a dynamic coupling of proton motion with the intermolecular vibration.
Introduction
The double-proton-transfer reaction (DPTR) both in the ground and excited has been the subject of studies since it is the simplest reaction in chemistry and biochemistry. Gen‘Present address: Institute for Molecular Science, Okazaki 444, Japan.
0022-3654/89/2093-0614$01.50/0
erally, the double hydrogen bonded system has two localized equilibrium structures as shown in Figure 1. The interconversion between these two nuclear configurations should involve a simultaneous proton transfer across a symmetric transition structure shown in the middle. An energy barrier that exists along the proton-transfer coordinate segregates the normal dimer and tautomer surfaces.
0 1989 American Chemical Society
DPTR Dynamics in Hydrogen-Bonded Base Pairs
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989 615
Figure 1. Double-proton-transfer scheme in the cyclic dimer. Left and right conformers correspond to the normal dimer and tautomer, respectively.
As for the ground-state DPTR, several experimental efforts have been paid to determine the double-minimum potential, especially for cyclic dimers of carboxylic acids.'-3 For formic acid dimer, experimental results led to an estimate of the potential barrier of 80 kJ/mol from infrared measurement' and indicated IOL2s-' as an upper limit for the double-minimum tunnel ~ p l i t t i n g . ~ Recent studies for benzoic acid' and p-toluic acid6 dimers indicated an activation energy as about 4 kJ/mol and a tunneling rate not larger than IO9 s-l. These results for a reduced barrier and a reduced tunneling rate cannot be interpreted within the usual model for the motion of two protons in a symmetric double-well potential of a rigid frame. With the aid of ab initio quantum chemical calculations, Graf et al? explained the reduced barrier height in terms of a structural relaxation in the process of proton t r a n ~ f e r . ~They ? ~ suggested that coupling of a nuclear motion of proton with the frame of heavy nuclei leads to the increment of the effective mass of proton in order to interpret a reduced tunneling rate. Babamov and MarcusEand Carrington and Millerg studied theoretically the dynamics of intramolecular-protontransfer reaction in consideration of the similar coupling of the motion of proton and surrounding heavy nuclei. Under these circumstances, it is crucial to elucidate the role of intermolecular vibration of the hydrogen bond in the dynamics of DPTR. The excited-state DPTR has been also studied extensively because of its importance in photochemistry and photobiology. In these studies, much effort has been paid to understand the mechanism of this reaction in the model hydrogen-bonded system of DNA related with a photomutagenic phenomenon.'*12 Recently, these studies have been accelerated because a photochemical hole burning induced by the photoinduced proton transfer in hydrogen-bonded systems such as qunizarine, and free-base phthalocyanine is expected to be a possible candidate for ultrafast optical information storage in computer technology." As for the excited-state DPTR, the emission behavior of the model hydrogen-bonded base pairs of DNA such as 7-azaindole (7-AI) and 1-azacarbazole (1-AC) dimers (shown in Figure 2) has been studied extensively by numerous a ~ t h o r s . ' ~ ' ~ JIn "~~
(1) Morita, H.; Nagakura, S. J . Mol. Spectrosc. 1972, 42, 536. (2) Nagaoka, S.; Terao, T.; Imashiro, F.; Saika, A.; Hirota, N.; Hayashi, S. Chem. Phys. Lett. 1981, 80, 580. Nagaoka, S.;Terao, T.; Imashiro, F.; Saika, A,; Hirota, N. J. Chem. Phys. 1983, 79, 4694. (3) Meier, B. H.; Graf, F.; Ernst, R. R. J . Chem. Phys. 1984, 76, 767. (4) Rothschild, W. G.J . Chem. Phys. 1974, 61, 3422. (5) Clemens, J. M.; Hochstrasser, R. M.; Trommosdorff, H. P. J. Chem. Phys. 1984,80, 177. (6) Graf, F.; Meyer, R.; Ha, T.-K.; Ernst, R. R. J . Chem. Phys. 1981, 75, 29 14. (7) Nagaoka, S.; Hirota, N.; Matsushita, T.; Nishimoto, K. Chem. Phys. Lett. 1982, 92, 498. (8) Babamov, V . K.; Marcus, R. A. J . Chem. Phys. 1981, 74, 1790. (9) Carrington, Jr., T.; Miller, W. H. J . Chem. Phys. 1986, 84, 4364. (10) Taylor, C. A.; El-Bayoumi, M.A,; Kasha, M. Proc. Natl. Acad. Sci. U.S.A. 1969, 63, 253. (11) Ingham, K. C.; El-Bayoumi, M. A. J . Am. Chem. SOC.1974, 96, 1674. (12) Chang, C.; Shabestary, N.; El-Bayoumi, M. A. Chem. Phys. Lett. 1980, 75, 107. (1 3) Gutierrez, A. R.; Friedrich, J.; Haarer, D.; Wolfrum, H. ZBM J. Res. Dev. 1982, 26, 198. (14) El-Bayoumi, M. A.; Avouris, P.; Ware, W. R. J . Chem. Phys. 1975, 62. 2499. (15) Heterington, W. M.; Micheels, R. H.; Eisenthal, K. B. Chem. Phys. Lett. 1979, 66, 230.
Figure 2. Isomerization of the model hydrogen-bonded base pairs by DPTR: (a) 7-AI dimer, (b) 1-AC dimer, and (c) 7-AI-1-AC heterodimer.
solution, the excited-state dimers of these molecules have been known to emit a visible fluorescence having a large Stokes shift in addition to the UV fluorescence from the normal dimer. From the absorption and emission studies, the excited-state DPTR to form a tautomer has been confirmed and a double-minimum potential was proposed for the excited state of these dimers. However, the electronic spectrum in solution has no fine structure and provides us with no information on the detail of the dynamical aspect of DPTR. On the other hand, recently the supersonically cooled dimers of these molecules have been found to emit the visible tautomer emission and undergo DPTR in the first excited states.22 In a jet, the dimer is free from collision and is cooled to an extremely low temperature. The resulting spectrum has a well-resolved vibrational structure that may give microscopic information on the dynamics of proton-transfer reaction. The brief summaries of DPTR for 7-AI dimerz2 and the preliminary results for 7AI-1-AC heterodimer and 1-AC dimers23have been reported, previously. In the present studies, we reexamined the vibrational level dependence of DPTR for these dimers which are shown in Figure 2 and established the assignment of promoting mode in DPTR to a symmetric intermolecular N-H--N stretching vibration in the hydrogen bond. The contribution of tunneling effect in DPTR was also examined by substituting the hydrogen atom in the hydrogen bond with deuterium atom. In order to characterize the potential surface of DPTR further, we also succeeded in the measurement of the fluorescence excitation and dispersed fluorescence spectra of the jet-cooled 7-AI tautomer using a two-step excitation technique. On the basis of these results, a model for the dynamical coupling of proton motion and the frame of heavy nuclei was proposed to interpret the role of the promoting vibration in DPTR process. (16) Catalan, J.; Perez, P. J . Theor. Biol. 1979, 81, 213. (17) Sepiol, J.; Wild, U. P. Chem. Phys. Lett. 1982, 93, 204. (18) Waluk, J.; Grabowska, A.; Pakula, B.; Sepiol, J. J . Phys. Chem. 1984, 88, 1160. (19) Waluk, J.; Pakula, B. J . Mol. Struct. 1984, 114, 359. (20) Bulska, M.; Grabowska, A.; Pakula, B.; Sepiol, J.; Waluk, J.; Wild, U. P. J. Lumin. 1984, 29, 65. (21) Tokumura, K.; Watanabe, Y.; Itoh, M. J . Phys. Chem. 1986, 90, 2362. (22) Fuke, K.; Yoshiuchi, H.; Kaya, K. J . Phys. Chem. 1984, 88, 5840. (23) Fuke, K.; Yabe, T.; Chiba, N.; Kohida, T.; Kaya, K. J . Phys. Chem. 1986, 90, 2309.
616
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The Journal of Physical Chemistry, Vol. 93, No. 2, 1989
I
i\
(a) P~,=300 Torr
i I J U , 32300
I
I
32800
,
(b) PHe=3 atrn
,
33300
0
WAVENUMBER
Figure 3. Fluorescence excitation spectra of jet-cooled 7-AI dimers obtained by monitoring UV (a) and visible (b) emissions, respectively.
Experimental Section The experimental setup of the LIF experiment was the same as reported in the previous paper^.^^,^^ Briefly, the supercooled dimers were produced by expanding the heated sample with 3 atm of helium through a 400-pm pinhole into an evacuated chamber. To elucidate an existence of isomers in the dimer, the internal temperature of supersonic beam was controlled by changing the stagnation pressure of expansion form 300 Torr to 3 atm. As is known from the solution studies, the excited tautomer emits visible fluorescence while the normal dimer exhibits UV fluorescence. So, an excitation spectrum monitoring the visible fluorescence, which give information on the excess vibrational energy dependence of DPTR rate, was obtained by the use of a UV cutoff filter (Toshiba Y-48). On the other hand, the excitation spectrum of the S1-So transition of the nonreacted dimer was obtained by monitoring the UV fluorescence through a UVtransmitting filter (Toshiba UV-D36C). In order to improve the experimental accuracy, the UV and visible monitoring spectra were measured simultaneously; two photomultipliers to monitor the UV and visible emissions were placed a t opposite sides with respect to the exciting laser beam. To examine a tunneling mechanism, similar experiments were carried out for the deuteriated dimers. Supersonically cooled 7-AI tautomer was produced by the following method. The output of a KrF excimer laser (Lambda Physics EMG 103) was focused into the expansion chamber with a 1-m lens and crossed the expanded free jet as closely as possible to the nozzle exit. The fluorescence excitation and dispersed fluorescence spectra of the tautomer were measured at a point 15 mm downstream of the nozzle exit by using the YAG pumped dye laser. 7-AI (Aldrich, GR) was purified by repeated sublimations in vacuo. I-AC was synthesized at the organic chemistry laboratory (Prof. Yamamura) of our department and was purified by repeated sublimations and finally by a zone melting method under an argon atmosphere. Deuteriated 7-AI and 1-AC were prepared by mixing the sample with deuteriated methanol in vacuo. Results 7-AI Normal Dimer. Figure 3 shows the fluorescence excitation
spectra of the SI-Sotransition of the 7-AI dimer obtained by monitoring the UV (a) and visible (b) emission, respectively. Although the features of two spectra are completely different, the assignment of both spectra to (7-AI)z dimer has been firmly established by a mass-selected multiphoton ionization (MS-MPI) method as described in the previous paper.2z On the basis of the result of two-photon resonant four-photon ionizttion spestrum, we assigned the visible monitoring spectrum to the A(2Ag)-X( 1A,) tramition and the UV monitoring spectrum to the transition to the B B, state. However, as shown in Figure 4, the spectral intensity of these transitions drastically changes with the thermal equilibrium condition. For instance, the relative intensity of the
20
60
40
80
100
clli'
Figure 4. Fluorescence excitation spectra of 7-AI dimers obtained at stagnation pressure of 300 Torr (a) and 3 atm (b) of He. The abscissa indicates the relative frequency with respect to the origin band of the
reactive isomer. The different temperature dependence of the origin and 38-cm-' bands indicates that these bands are ascribed to the origins of two isomers.
0
200
cm'
400
Figure 5. Fluorescence excitation spectrum of jet-cooled 7-AI dimer obtained by monitoring visible emission. The numbers inserted with
arrow are the bandwidths (fwhm). band at 32290 cm-' decrease significantly by decreasing He stagnation pressure from 3 atm to 300 Torr. This result and the MS-MPI spectra reported previouslyZZcompel us to revise the assignment of these two transitions to those of two different conformational isomers; the origin bands of the SI-Sotransitions are located at 32 252 and 32 290 cm-' for the reactive and nonreactive isomers, respectively. Dispersed fluorescence spectrum obtained by the excitation of the 32 252-cm-l band exhibits a broad emission peaked at 480 nm, which is similar to the tautomer emission observed in solution, while that for the 32 290-cm-l band excitation shows a normal UV fluorescence as shown in ref 22. These results lead us to the conclusion that one of the isomers selectively undergoes the excited-state DPTR even under a collision-free condition. As shown in Figure 3, a difference exists in the spectral features of two isomers. Higher vibrational bands appear in the fluorescence excitation spectrum of reactive isomer; starting at the 740-cm-' band, vibrational bands, similar to those associated with the origin are observed. On the other hand, the spectrum of the nonreactive isomer disappears drastically in the energy region higher than 700 cm-' above the origin. These results indicate that the deexcitation process of the nonreactive isomer in the higher vibrational levels is an intramolecular nonradiative relaxation to a nonemissive level, while that of the reactive isomer is predominantly DPTR. Figure 5 shows the fluorescence excitation spectrum by monitoring the visible tautomer emission in an expanded scale, which corresponds to the action spectrum of the proton-transfer reaction. The spectrum consists of the progressions and combinations of 98- and 120-cm-' vibrations which are assigned to the intermolecular vibrational modes. As is shown in Figure 4, the bandwidth
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989 617
DPTR Dynamics in Hydrogen-Bonded Base Pairs
0
2
1
107
3
5
4
G
7
8
9
105
106
Jb 500
0
I
0
- .
I
1
100
&I-'
200
Figure 6. Fluorescence excitation spectra of deuteriated 7-AI dimers (b) obtained by monitoring visible emission. For comparison, the spectra of the undeuteriated dimer (a) is also presented. The origin bands of the mono- and dideuteriated dimers are located at 41 and 61 cm-' above that of the undeuteriated dimer, respectively. The numbers inserted with arrow are the bandwidths (fwhm).
of the 32 252-cm-' band ( 5 cm-') of the reactive isomer is much wider than that of the nonreactive isomer at 32 290 cm-I (2 cm-l). The v' = 1 and v' = 2 bands of the 120-cm-I vibration exhibit the bandwidths of 10 and 30 cm-' (fwhm), from which the excited-state lifetimes are estimated to be 0.5 and 0.2 ps, respectively. As was described in ref 22, the SI-Soabsorption spectrum of the reactive isomer could not be obtained from the one-photon resonant two-photon ionization spectrum, while that of the nonreactive isomer was observed in the MPI spectrum exhibiting sharp bandwidth. Moreover, the reactive isomer emits no UV fluorescence from the S1 state. This implies that the lifetime of the SI state of the reactive isomer is short enough to explain the observed broad linewidth of the SI-Soabsorption. On the other hand, the bandwidth of the v' = 1 band of the 98-cm-' vibration is 3 cm-l, while that of the combination band with the 120-cm-I vibration at 215 cm-' above the 0-0 band again increases more than twice (7 cm-I), indicating the increment of the rate of DPTR. Thus it is concluded that the 120-cm-' vibration is the promoting mode of DPTR in the 7-AI dimer. It is worthwhile to note that the anharmonicity of this vibration is quite small as seen in Figure 5. In order to assign the 120-cm-I vibration and to examine a tunneling mechanism in the DPTR of the excited-state 7-AI dimer, a similar experiment was carried out for a deuteriated dimer as shown in Figure 6. In this figure, the visible monitoring spectrum of deuteriated dimer (b) is shown with that of undeuteriated dimer (a) for comparison. Owing to a small amount of water which remains in the sample reservoir, the spectrum of undeuteriated dimer is superimposed on those of deuteriated dimers. The bands at 41 and 61 cm-l above the origin of the undeuteriated dimer correspond to those of mono- and dideuteriated dimers, respectively. The 120-cm-I vibrational bands of the deuteriated isomers are also indicated in Figure 6b. From this figure, it is clear that the bandwidths of the 120-cm-l vibration of deuteriated dimers decrease by half of that of the undeuteriated dimer, indicating the suppression of the DPTR rate by deuteriation. The other significant feature in Figure 6b is that the vibrational frequency of the 120-cm-I mode is not affected by deuteriation. This result supports the assignment of the 120-cm-l vibration as an intermolecular N-H-aN symmetric stretching vibration. This assignment will be confirmed further by the discussion given in the next section. 7-AI Tautomer. Figure 7 shows the fluorescence excitation spectrum obtained by irradiating a weakly focused K r F laser (typically 50 mJ) to an expanded 7-AI-He beam ilt a nozzle exit. The intensities of these spectral signals show the first-order dependence on the intensity of the excimer laser. The fluorescence lifetimes of individual vibrational bands are 3-4 ns, which agree with that of the visible emission of 7-AI tautomer (2 ns) reported previously.22 Moreover, the excitation spectrum shown in Figure
CM-'
Figure 7. Fluorescence excitation spectrum of the SI-So transition of jet-cooled 7-AI tautomer obtained by two-step excitation. The origin band was observed at 23 071 cm-I. ,,
0
+129
500
loo0
1500
2000
C M-l
Figure 8. Dispersed fluorescence spectra of jet-cooled 7-AI tautomer obtained by excitation of u' = 0 (a), 1 (b), 2 (c), and 3 (d) vibrational levels of the symmetric stretching mode, respectively.
7 is observed in the same wavelength region as the spectrum of 7-AI tautomer in solution (435 nm) obtained by a transient absorption These facts confirm that the observed spectrum corresponds to the S1-So transition of the jet-cooled tautomer as a result of DPTR. In consideration of efficient DPTR of the excited-state dimer and feasibility of H-bonded dimer formation of 7-AI, the mechanism of a tautomer formation in a jet may be explained by the process hv
D-D*
PTR
-T*
hu'
-T
In this mechanism, the ground-state normal dimer formed already at the nozzle exit undergoes tautomerization by the irradiation of KrF laser photon. Then, the excited-state tautomer relaxes to the ground-state tautomer radiatively within a few nanoseconds. Finally, the ground-state tautomer is cooled down by an expansion process to form a cold stable tautomer. The origin band of the SI-So of the tautomer is located at 23 071 cm-l (433.4 nm), and the spectrum consists of the progressions and combinations of the 72-, 80-, and 107-cm-I vibrations. These vibrations are readily attributed to the intermolecular vibrational modes in comparison with those of the dimer. Especially, the 107-cm-I vibration, which is assigned to the symmetric N-He-N stretching mode, exhibits a long progresion up to v' = 9. This indicates fairly large change of bond distance of two 7-AI moieties between the ground and excited states.
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The Journal of Physical Chemistry, Vol. 93, No. 2, 1989
0:
r
I/
400
Figure 9. Fluorescence excitation spectra of 1-AC dimer obtained by monitoring UV (a) and visible (b) emissions, respectively. The abscissa indicates the relative frequency with respect to the origin band of the visible monitoring spectrum.
Figure 8 shows the dispersed fluorescence spectra obtained by the excitations of u' = 0-3 levels (Figure 8a-d) of the symmetric N-H-N stretching vibration of the tautomer. The most distinguished feature of these spectra is again the long progression of the 129-cm-' vibration in the ground state, which corresponds to the intermolecular symmetric stretching vibration (107 cm-I in the excited state). The other intermolecular mode seen in Figure 8 is the 93-cm-' vibration. By careful analysis of the bandwidths of these fluorescence spectra, it was concluded that there still exists a discrete vibrational level in the energy region of more than 2000 cm-' above the zero vibrational level of the ground-state tautomer. These facts and the survival of the tautomer at 15 mm downstream from the nozzle exit where the tautomer was first generated by the K r F laser irradiation indicate that the lifetime of the ground-state tautomer is fairly long (more than 10 11s) and the rate of the back proton transfer to the dimer in the ground state is negligibly small in the present experimental time scale. This result agrees qualitatively with the lifetime of tautomer obtained in the solution experiment (183 11s at 172 K).z' 1 -AC Dimer. Figure 9 shows the fluorescence excitation spectra of 1-AC dimer by monitoring the UV (a) and visible (b) emissions. From the MS-MPI experiments, these spectra were confirmed to be due to the 1-AC dimer.23 As in the case of 7-AI, we also measured the excitation spectra at various stagnation pressure, and the results are shown in Figure 10. It is clear from this figure that the band at 80 cm-' above the 28 556-cm-' band decreases dramatically with decreasing stagnation pressure of helium from 2 atm to 290 Torr. Thus, similar to the case of the 7-AI dimer, there exist two conformational isomers in the l-AC dimer. From the previous study, one of the isomers having the origin at 80 cm-' above the 28 556-cm-' band was found to give a fluorescence only in the UV region and does not undergo the DPTR.23 On the other hand, the isomer having the origin a t 28 556 cm-' emits a weak visible emission in addition to the strong normal UV fluorescence. Therefore, the latter isomer was concluded to selectively undergo the excited-state DPTR even under the collision-free condition. In contrast to the 7-AI dimer, the DPTR rate (k,) of the 1-AC dimer is considerably smaller and is comparable to the radiative decay rate (kf) and the rate (k,) of the other nonradiative decay channels such as internal conversion and intramolecular vibrational relaxation. Thus, one can estimate an excess energy dependence of the DPTR rate from the comparison of the intensities of individual peaks in the visible and UV monitoring spectra. The time-integrated intensity of the UV monitoring spectrum of the reactive isomer is proportional to k f / ( k f+ k, + k,). Similarly, the intensity of the visible monitoring spectrum is proportional to k,/(kf + k, + kn). Then, the intensity ratio of the visible and UV monitoring spectra for each vibrational band gives k,/kr. In Figure 11, the
1
0
100
C M-' Figure 10. Pressure dependence of excitation spectrum of 1-AC dimers. (a) and (b) were obtained at the stagnation pressure of 290 Torr and 2 atm of He, respectively.
0
100
300
200
400
500
CM-I
Figure 11. Visible monitoring spectrum of 1-AC dimer and the plots of the ratio k , / k f , normalized to that of the origin band. 0, A, 0,A, and are for the 109-cm-' vibration, the overtones of the 55- and 67-cm-' vibrations, and the combination bands with the 109-cm-I vibration, respectively. The excitation of 109-cm-' vibration enhances the DPTR rate, while the excitations of the 55- and 67-cm-' vibrations rather suppress
the reaction. ratios k , / k f , normalized to that of the origin band, are plotted. Since the electronic transition considered here is the optically allowed the radiative rate, kf,can be reasonably assumed to be constant in the present spectral range. Thus, the plots shown in Figure 11 give the vibrational level dependence of the DPTR rate (k,), which clearly implies that the excitation of the 109-cm-' vibration enhances the DPTR, while the excitation of the 55- and 67-cm-' vibrations rather suppresses the rate. Actually, the relative rates for the latter vibraions and their overtones with respect to the origin band are estimated to be 0.8 and 0.7, respectively. However, as seen in Figure 11, the DPTR rate again increases in the combination bands of these vibrations with the 109-cm-' vibration. Therefore, similar to the 120-cm-' vibration in the 7-AI dimer, the 109-cm-' vibration of the 1-AC dimer plays the role of the promoting mode in the excited-state DPTR. Figure 12 shows the fluorescence excitation spectra of deuteriated 1-AC dimer by monitoring the visible (a) and UV (b) emissions. For comparison, the UV monitoring spectrum (c) of undeuteriated dimer is also shown in this figure. Because of a small amount of water remaining in the sample reservoir, the spectrum includes the absorption bands of dideuteriated, monodeuteriated, and undeuteriated dimers. The 0bands of monoand dideuteriated 1-AC dimers are located 22 and 40 cm-' above the origin of the undeuteriated dimer as shown in Figure 12b. By
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989 619
DPTR Dynamics in Hydrogen-Bonded Base Pairs
114
0
114
100
113
300
200
400
500
CM-l
Figure 14. Visible monitroing spectrum of heterodimer and plots of the ratio k,/kf, normalized to that of the origin band. 0, A, A, and 0 are for the 114-cm-I vibration, the overtone of the 61-cm-' vibration, and the combination bands of the 61- and 90-cm-' vibrations with the 109-cm-' vibration, respectively. 0
100 cm-l
Figure 12. Fluorescence excitation spectra of deuteriated 1-AC dimers obtained by monitoring visible (a) and UV (b) emissions, respectively. For comparison, the spectrum of the undeuteriated dimer (c) is also
presented.
n
Figure 13. Dispersed fluorescence spectra of jet-cooled 7-AI-1-AC heterodimer obtained by excitation of the origin (a) and the band at 26 cm-' above this band (b).
deuteriation, the intensity of the origin band in the visible monitoring spectrum of deuteriated dimers decreases substantially, especially for the monodeuteriated dimer. From these results, the DPTR rates of mono- and dideuteriated dimers are estimated to be reduced by about one-hundredth and one-seventh, respectively, in comparison with the undeuteriated dimer. The vibrational frequency of the promoting mode of deuteriated dimer is 108 cm-l and not affected by deuterium substitution as the case of 7-AI. These results also confirm the assignment of the promoting mode of the intermolecular symmetric stretching vibration in the hydrogen bond. 7-AI-1-AC Heterodimer. As was shown in the previous paper:3 the heterodimer of 7-AI and 1-AC is readily formed and the two isomers exist. Similar to other dimers, one of the dimers is reactive in the excited state, while another isomer is nonreactive as seen in the dispersed fluorescence spectra shown in Figure 13. In this figure, two spectra were obtained by the excitation of the origin bands of the S1-So transitions of the reactive (a) and nonreactive dimers (b) at 28 513 and 28 539 cm-', respectively. The former isomer exhibits a weak visible emission peaked at 500 nm, which
is ascribed to the tautomer emission. This result suggests that the reactive heterodimer emits both UV and visible emissions and the DPTR process competes with the other decay process as in the case of the 1-AC dimer. Figure 14 shows the plots of the excess energy dependence of the relative DPTR rate (k,/k,) obtained by the analysis of the simultaneously measured UV and visible monitoring spectra. Among the three intermolecular vibrational modes (61, 90, and 114 cm-I), the excitation of the 114-cm-' vibration enhances the DPTR rate dramatically as is clearly seen in this figure. The DTPR rate from the vibrational levels of the combination bands (61 cm-' + 114 cm-' X n ) also successively increases with the excitation of the 114-cm-I vibration. Therefore, in the case of heterodimer, the 114-cm-I vibration plays the role of the promoting mode in the excited-state DPTR.
Discussion The temperature dependence of the excitation spectrum shows that all three dimers examined in the present study form two isomers. One isomer selectively undergoes the DPTR, while the other isomer is nonreactive. The spectral feature of the nonreactive dimer common to all three dimers is congested due to the appearance of many more intermolecular vibrational modes than those in the visible monitoring spectra of the reactive isomers. By the dimer formation, six intermolecular vibrational modes are expected to arise in addition to the intramolecular vibrational modes. From a simple symmetry consideration, three of them are optically allowed for the coplanar dimer, while all of them will be observed if the structure of the dimer distorts considerably from the coplanarity. Actually, in the nonreactive isomer of 7-AI dimer, five or six intermolecular vibrational modes are observed as shown in Figure 3. The same trend is also found in the 1-AC dimer and 7-AI-1 -AC heterodimer. Therefore, the nonreactive isomers seem to have the structures distorted considerably from a coplanar form. This structural distortion may give rise to an appreciable barrier height along the reaction coordinate and prohibits the excited-state DPTR. This conclusion is supported by the evidence that only two or three intermolecular modes are observed in the spectra of the reactive isomers in contrast to the nonreactive ones as seen in Figures 3 and 9. From the above arguments, we can conclude that the isomers having the coplanar structure undergo the excited-state DPTR. At present, we cannot discuss further the relation between the structure and reactivity of these isomers without a precise knowledge of the structures of monomers. Nevertheless, we can conclude that the reactive isomers have structures close to the coplanar structure in view of the fact that the number of the intermolecular modes observed in the spectra of the reactive isomers is limited in comparison with those of the nonreactive isomers. In the excitation spectra of the reactive isomers, two or three intermolecular vibrational modes were observed as shown in Table
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Fuke and Kaya
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989
TABLE I: Rates of DPTR (s-l) and Vibrational Frequencies of the Hvdronen-Bonded Modes (ern-') for the Model Base Pairs
&end
120 98
p-lp
0.130
kDmR
10'2
Vsym str
107 (129)' 80 (93)" 72
109 67 55 0.109 1096
114 90 61 0.120
I I
Lz
109b
OThe values in parentheses are for the ground state. bEstimated from the reduction of monomer fluorescence lifetime (15 ns) by the dimer formation (2 ns).
I. Among these modes, the highest frequency vibration of each dimer was found to enhance the reaction and play the role of the promoting mode in DPTR. In the case of cyclic dimer, six intermolecular vibrational modes appear: those are a, (2), a, (2), b, and b, symmetries, and only g-type vibrations are expected to be observed in the electronic spectrum for near-coplanar conformation Among the g-type vibrations, the highest frequency vibration will be a symmetric stretching mode, a,, in the hydrogen bond as it is discussed for acetic acid dimer.24 From the fact that the deuterium substitution does not affect the frequencies of the highest frequency vibrations of 7-AI and 1-AC dimers, it is reasonable to assign the promoting mode to the symmetric NH-N stretching vibration in the hydrogen bond. If this assignment is correct, the vibrational frequencies of the promoting mode observed in the present study should show a good correlation with the inverse square root of the reduced mass of the monomer. Clearly, these values show good correlation with the frequencies of the promoting mode as shown in Table I. Therefore, it is concluded that the promoting mode in DPTR is the symmetric N-H-sN stretching vibration in the hydrogen bond. As it is shown in the previous section, the fast DPTR occurs for 7-AI dimer ( 10l2 s-') even from the zero vibrational level and this decay channel surpasses all other decay channels even in the energy region of 1000 cm-' above the zero vibrational level. On the other hand, in the case of other two isomers, the DPTR rates ( lo9 s-*) are by 3 orders of magnitude smaller than that of 7-AI dimer and this process competes with the radiative and other nonreactive decays. The successful detection of 7-AI tautomer in a jet indicates that there exist two potential minima both in the ground- and excited-state surfaces as proposed from the studies in solution. Although the experimental elucidation of tautomer for the other dimers is not yet successful, the same double-minimum potential may be expected along the DPTR coordinate. Since the pyridinic nitrogen and pyrrolic nitrogen in the 7-AI and 1-AC molecules are expected to be more basic and acidic in the excited state than in the ground state, the potential minima of the excited-state dimer are located much closer to each other along the hydrogen bond. This structural change brings about the observed long progression of the symmetric stretching vibration for these dimers. From the theoretical calculation of 7-AI dimer, Catalan and Peretzt6showed that the barrier height between the dimer and tautomer is reduced substantially by the decrease in the distance of hydrogen bond. The most important result obtained in the present study is the dramatic enhancement of the DPTR rate by the excitation of the symmetric N-H-N stretching vibration. The characteristic feature of this mode is the harmonic potential for three dimers. Especially, in the case of the 7-AI tautomer, the pronounced long progressions are observed in both the excitation and dispersed fluorescence spectra. In order to explain the mechanism of enhancement by the promoting vibration, we assumed the following model potential surface for the excited-state DPTR as seen in Figure 15. Although the real potential surface is multidimensional, for simplicity, two coordinates, which are the PTR coordinate rN-H and the vibrational coordinate R~+H-.N of the promoting mode, are adopted to interpret the observed behaviors of DPTR. Since these two coordinates are intrinsically collinear, N
(24) Lassegues, J. C.; Lascombe, J. Vib. Spectra Strucr. 1981, 1 1 , 51.
Figure 15. Schematic representation of the Sl-state potential energy surface of the model hydrogen-bonded base pair plotted as a function of the DPTR, r,-,, and intermolecular symmetric stretching coordinates, RN+..*. D and T indicate the dimer and tautomer, respectively.
the DPTR can be reasonably assumed as a collinear-type exchange reaction such as an atom-diatom molecular system. At the present stage, we do not know the exact barrier height and the effective mass for the DPTR. The excited-state potential surface presented in Figure 15 is schematic in nature. In this figure, the potential surface along the symmetric stretching coordinate is harmonic with a barrier height of less than 10 kcal/mol, which corresponds to the energy of double hydrogen bonds. Two potential minima on this surface correspond to the dimer (D) and tautomer (T), respectively. Along the minimum energy path, there should be a small energy barrier. Using the above model potential surface, the role of the promoting mode is explained as follows. As the result of strong double hydrogen bonds in these dimers, the frequency of the N-H stretching vibration in the monomer moiety becomes much lower and this vibration is assumed to couple efficiently with the symmetric stretching vibration of the hydrogen bonds. Thus, by the excitation of the latter vibration, the system will reach a deep point in the valley of the potential surface of the excited-state normal dimer, where the barrier height to the tautomer surface may be less than that from the zero vibrational level. At that point, the barrier width becomes also much narrower and the tunneling rate increases appreciably. In the visible monitoring spectra of 1-AC dimer and 7-AI-1-AC heterodimer, two other intermolecular vibrations have been observed. These vibrations can be assigned to the in-plane bending vibration (a,) and the out-of-plane vibration (b,), respectively, assuming that the dimer has nearly coplanar structure. The excitation of these vibrations rather suppresses the ratio of DPTR. This may be explicable in terms of the increase of the potential energy barrier of the reaction which arises from an increment of the N-H-N bond distance by the bending mode excitation. Recently, Sat0 and IwataZ5studied the proton-transfer reaction theoretically using the same two-dimensional model potential mentioned above. They analyzed the vibrational Schrijdinger equation and predicted the appearance of a new path, which they called the classical detour path, resulting from the dynamical coupling of a proton with surrounding heavy nuclei. Although, at the present stage, the number of the examples is limited, the promoting effect of the symmetric stretching intermolecular mode in the PTR seems to be a general phenomenon for the hydrogen-bonded system.
Conclusion In the present study, the DPTR in the excited state was studied extensively for model hydrogen-bonded systems. These results indicate that the DPTR rate is affected extraordinarily by a subtle change in the structure of dimer and only the isomer having a near coplanar structure selectively undergoes the reaction. The DPTR rates of the dimers also depend on the electronic structure (25) Sato, N.; Iwata, S. J . Chem. Phys. 1988, 89, 2932.
J . Phys. Chem. 1989, 93, 621-625 of the excited state of each monomer moiety as is clear from the observation that the DPTR rate of hetercdimer is by 3 orders of magnitude smaller than that of the 7-AI dimer. To explain this large difference, it seems to be necessary to carry out an elaborate calculation of the energy barrier in the excited-state surface. However, an ab initio calculation for these many electron system with high accuracy may be beyond our scope, at the present stage, and is an subject to be solved in the future. The most significantly feature of the DPTR for these dimers is that the rate is promoted dramatically by the excitation of the symmetric stretching vibration in the hydrogen bond. This finding unveiled the microscopic aspect of proton-transfer dynamics. The 7-AI tautomer, which is the product of DPTR, was also trapped and characterized in a jet. The fluorescence excitation and dispersed fluorescence spectra of tautomer exhibit an extensive
621
progression of the symmetric stretching mode, indicating that this vibration plays a key role in DPTR. In order to explain the present results, we considered a twodimensional model potential. Using this potential surface, the mechanism of the promotion is explained as the enhancement of the classical detour pathway overtaking the quantum mechanical (tunneling) pathway as the result of the dynamical coupling of proton motion with the intermolecular vibrational motion. Acknowledgment. We thank Professor S. Iwata, Dr. Y. Osamura, and Dr. N. Sato for many stimulating discussions and for providing us with the results of their calculations prior to publication. We are also very grateful for the measurement of a part of the present results carried out by H. Hashizume and K. Shirai. Registry No. 7-AI, 27 1-63-6; 1-AI, 244-76-8; deuterium, 7782-39-0.
Ab Inltlo Energy and Structure of H-(H2)2 Rick A. Kendall,+vl*llJack Simons,**+Maciej Gutowski,+**Jand Grzegorz Chalasiiiskit Department of Chemistry, University of Utah, Salt Lake City, Utah 84112, and Department of Chemistry, University of Warsaw, ul. Pasteura 1.02-093 Warsaw, Poland (Received: May 10, 1988)
The structure and energy of the H-(H2), complex have been investigated by using the restricted Hartree-Fock (RHF) and Mdler-Plesset perturbation theory (MPPT) methods. The geometries were optimized at the RHF and second-order MPPT (MP2) levels of theory, and energies were calculated at the RHF through fourth-order MPPT (MP4) levels. The predicted equilibrium geometry of the complex changes drastically when correlated methods are used. The RHF results predict the complex to be linear with the hydride between the two H2 moieties and yields an interaction energy of -1.76 kcal/mol relative to H-, Hz, and H2. The correlated results show the complex to be V-shaped with the hydride at the base of the V and give interaction energia of -2.79 and -2.75 kcal/mol at the MP2 and MP4 levels of theory, respectively. The nonadditive contribution to the interaction energy is found to be large and to cause the geometry of the complex to be nonlinear. The in-phase bending motion of the two Hz species about the central hydride is found to be an extremely low frequency mode; in fact, the energy difference between the linear complex and the V-shape complex is -0.24 kcal/mol at the correlated level. The thermodynamic stability of the H-(H2)2anion cluster is predicted to be unlikely, but a few of the isotopically substituted complexes are predicted to be thermodynamically stable.
Introduction Recently, there has been considerably interest in lightly solvated New developments in various types of anion sources and anion spectroscopy have opened this field to be a wide variety of experimental avenues of study?e42 These techniques are used to probe isolated anions and those that are interacting with one or more solvent molecules. We believe it is important for these systems to be studied theoretically as well, because the interactions among the various species are unusual whenever anions are involved. Anions associate more intimately with solvent molecules than do neutral species due to both the charge and the diffuse electron distribution of the anion. In turn, the solvent molecules more strongly affect the loosely bound electrons of an anion. Thus, the anion can strongly polarize the solvent and the polarized solvent can back-polarize the anion. Because of these facts, it is likely that investigations of solvated anions will produce results that are somewhat different from observations of solvated neutral or cationic species. There has been much interest in hydride anions solvated by various species. In particular, solvation by H2molecules to form hydrogen anion clusters has generated substantial attention.1e28 This work is important to researchers interested in anion-molecule interactions and to atmospheric, combustion, and interstellar +University of Utah. 'University of Warsaw. 'University of Utah Graduate Research Fellow. Permanent address: University of Warsaw. 1 Present address: Argonne National Laboratory, Theoretical Chemistry Group, Chemistry Division, Argonne, IL 60439.
0022-3654/89/2093-0621$01.50/0
~ h e m i s t r y . ~ ~Most * ~ ~ of , ~ the * previous work has been confined to H3-, the smallest of the hydrogen anion clusters. To date, only (1) Chalasinski, G.; Kendall, R. A.; Simons, J. J. Chem. Phys. 1987, 87, 2965.
(2) Hirao, K.; Kawai, E. J. Mol. Struct. (THEOCHEM) 1987,149, 391. (3) Simons, J.; Jordan, K. D. Chem. Rev. 1987, 87, 535. (4) Wetzel, D. M.; Brauman, J. I. Chem. Rev. 1987, 87, 607. (5) Coe, J. V.; Snodgrass, J. T.; Freidhoff, C. B.; McHugh, K. M.; Bowen, K. H. J. Chem. Phys. 1987,87,4302. (6) Ortiz, J. V. J. Chem. Phys. 1987, 87, 3557. (7) Bowen, K. H.; Eaton, J. G., to be published in Proceedings of the International Workshop on the Structure of Small Molecules and Ions. (8) Griffiths, W. J.; Harris, F. M. Int. J. Mass Spectrom. Ion Processes 1987, 77, R7. (9) Griffiths, W. J.; Harris, F. M. Org. Mass Spectrom. 1987, 22, 812. (10) Crerner, D. C.; Kraka, E. J. Phys. Chem. 1986, 90, 33. (1 1) Rohlfing, C. M.; Allen, L. C.; Cook, C. M.; Schlegel, H. B. J. Chem. Phys. 1983, 78, 2498. (12) Dacre, P. D. J. Chem. Phys. 1984,80, 5677. (13) Rao, B. K.; Kester, N. R. J. Chem. Phys. 1984,80, 1587. (14) Kalcher, J.; Rosmus, P.; Quack, M. Can. J . Phys. 1984, 62, 1323. (15) Coe, J. V.; Snodgrass, J. T.; Freidhoff, C. B.; McHugh, K. M.; Bowen, K. H. J. Chem. Phys. 1985, 83, 3169. ( 16) Squires, R. R. In Ionic Processes in the Gas Phase; Amoster Ferreira, M. A., Ed.; Reidel: Dordrecht, 1984. (17) Paulson, J. F.; Henchman, M. J. In Ionic Processes in the Gas Phase; Amoster Ferreira, M. A., Ed.; Reidel: Dordrecht, 1984. (18) Viggiano, A. A,; Arnold, F. In Ionic Processes in the Gas Phase; Amoster Ferreira, M. A., Ed.; Reidel: Dordrecht, 1984. (19) Rayez, J. C.; Rayez-Meaume, M. T.; Mass, L. J. J. Chem. Phys. 1981, 75, 5393. (20) Sapse, A. M.; Rayez-Meaume, M. T.; Rayez, J. C.; Massa, J. L. Nature (London) 1979, 278, 332.
0 1989 American Chemical Society
