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Excited-State Triple-Proton Transfer in 7-Azaindole(H2O)2 and Reaction Path Studied by Electronic Spectroscopy in the Gas Phase and Quantum Chemical Calculations† Kenji Sakota,‡ Christophe Jouvet,*,§ Claude Dedonder,§ Masaaki Fujii,| and Hiroshi Sekiya*,‡ Department of Chemistry, Faculty of Sciences, and Department of Molecular Chemistry, Graduate School of Science, Kyushu UniVersity, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan, Institut des Sciences Mole´culaires d’Orsay and Centre Laser de L’uniVersite´ Paris Sud, UniVersite´ Paris-Sud 11, 91405 Orsay Cedex, France, and Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan ReceiVed: March 26, 2010; ReVised Manuscript ReceiVed: July 20, 2010
We have investigated the excited-state multiple-proton/hydrogen atom transfer reactions in the 7-azaindole water clusters, [7AI](H2O)n (n ) 2,3), in the gas phase by combining electronic spectroscopy and quantum chemical calculations. The fluorescence excitation (FE) spectrum of 7AI(H2O)2 has been observed by monitoring visible emission. In contrast, no vibronic bands are detected in the FE spectrum of 7AI(H2O)3 when the visible emission is monitored. The dispersed fluorescence spectra of 7AI(H2O)n (n ) 2,3) have been measured. The excitation of +180 cm-1 band from the electronic origin of 7AI(H2O)2 enhances the visible emission as compared with the 0-0 excitation. The +180 cm-1 band is assgined to an intermolecular mode (σ(1)) of the cyclic hydrogen-bonded ring structure. The calculated S1-S0 absorption spectrum for the cyclic hydrogenbonded structure is in agreement with the FE spectrum around the 0-0 region. The excitation of σ(1) significantly promotes the reaction and generates the tautomeric form of 7AI(H2O)2. These experimental results on 7AI(H2O)n (n ) 2,3) are very similar to those on 7AI(CH3OH)n (n ) 2,3) and 7AI(C2H5OH)n (n ) 2,3). We conclude that the excited-state triple proton/hydrogen atom transfer (ESTPT/HT) occurs in 7AI(H2O)2. Cuts of the potential energy surfaces along the proton/hydrogen atom transfer coordinates of 7AI(H2O)n (n ) 2,3) and 7AI(CH3OH)n (n ) 2,3) are comparatively calculated by quantum chemistry calculations (RI-CC2/ cc-pVDZ and TD-DFT(B3LYP)/cc-pVDZ) to explore the mechanism of the ESTPT/HT reaction. The calculated results suggest that concerted proton transfers occur in 7AI(H2O)2 as well as in 7AI(CH3OH)2, whereas the potential barrier for the excited-state quadruple proton transfer in 7AI(H2O)3 and 7AI(CH3OH)3 is higher than those for ESTPT. The theoretical results are consistent with the observation of ESTPT/HT in 7AI(H2O)2. 1. Introduction Proton and/or hydrogen atom transfer is one of the most important elementary reactions to understand complicated phenomena associated with the hydrogen bonds in the condensed phase at the molecular level, particularly in biological systems. For example, it is well accepted that the fluorescence ability of the green fluorescent protein is controlled by the multiple-proton transfer after photoexcitation.1 Spectroscopic study on relevant model systems is a promising way to reveal the mechanism of the multiple-proton/hydrogen atom transfer along the water network.2 However, it is difficult to obtain the characteristic features in the multiple-proton/hydrogen atom transfer mainly because of the thermal fluctuations in the reaction center in the condensed phase. Supersonically jet-cooled hydrogen-bonded clusters in the gas phase are good model systems to reveal the dynamics of the multiple-proton/hydrogen atom transfer, where the lowering of temperature significantly suppresses the thermal fluctuation and the cluster size can be controlled.3 7-Azaindole (7AI) and its hydrogen-bonded clusters have attracted much attention as a model system for DNA bases. In †
Part of the “Klaus Mu¨ller-Dethlefs Festschrift”. * Corresponding author. E-mail:
[email protected], sekiya@ chem.kyushu-univ.jp. ‡ Kyushu University. § Institut des Sciences Mole´culaires d’Orsay and Centre Laser de L’universite´ Paris Sud. | Tokyo Institute of Technology.
particular, the excited-state double-proton transfer (ESDPT) in the 7AI dimer4-18 and the multiple-proton/hydrogen atom transfer in the hydrogen-bonded 7AI clusters with polar solvent molecules19-28 have been investigated extensively in the gas phase as well as in the condensed phase. Very recently, we reported the excited-state triple-proton/hydrogen atom transfer (ESTPT/HT) in 7AI(CH3OH)2 and 7AI(C2H5OH)2.22-24 It was found that the in-phase intermolecular stretching vibration selectively promotes ESTPT/HT. We also found another characteristic feature of ESTPT/HT in 7AI(CH3OH)2; the reaction mechanism changes from vibrational-mode specific to statistical fashion with increasing internal energy in the S1 state.23 The investigation of the multiple-proton/hydrogen atom transfer in the hydrated 7AI is fascinating because of the unrivaled importance of water in nature. Several groups reported the excited-state proton/hydrogen atom transfer in the gas phase and the condensed phase.5-25,28,29 In the condensed phase, ESDPT was observed in 7AI(H2O)1.20,21 However, in the gas phase, no evidence of ESDPT in 7AI(H2O)1 has been obtained. In the gas phase, the time profiles of the femtosecond pump-probe transients of 7AI(H2O)n (n ) 2-4) were observed by Folmer et al.25 The decay profiles were fitted with biexponential functions with time constants of 360 ( 50 fs and 6 ( 0.5 ps for 7AI(H2O)2 and 420 ( 80 fs and 6.1 ( 1 ps for 7AI(H2O)3. The authors ascribed the ultrafast decays to the excited-state multiple-proton transfer. However, the time constants for the faster decay component contradict those
10.1021/jp102733c 2010 American Chemical Society Published on Web 08/10/2010
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predicted from narrow bandwidths (fwhm ≈ 0.02 cm-1) obtained from the high-resolution electronic spectra measured by Nakajima et al.26 Previously, we reported the dispersed fluorescence (DF) spectra of 7AI(H2O)n (n ) 1-3), where the spectra due to the tautomer emission in the visible region were not detected.28 Therefore, there are discrepancies in the results between the groups. In the previous studies of 7AI(CH3OH)n and 7AI(C2H5OH)n,22-24 we successfully applied fluorescence excitation (FE), DF, resonance-enhanced two-photon ionization (RE2PI), and UV-UV hole-burning (HB) spectroscopic techniques to observe the ESTPT/HT reactions unambiguously. Characterisc features of the ESTPT/HT reactions in 7AI(CH3OH)n and 7AI(C2H5OH)n are the cluster-size selectivity and the vibrational-mode selectivity. The ESTPT/HT reaction in the 1:2 cluster is much faster than the ESDPT reaction in the 1:1 cluster and the excitedstate quadruple-proton/hydrogen atom transfer (ESQPT/HT) reaction in the 1:3 cluster. Therefore, FE spectrum of only the 1:2 cluster was observed when visible emission from the tautomeric form was observed, whereas FE spectra were observed for all 1:1, 1:2, and 1:3 clusters by monitoring UV emission. In addition, a specific intermolecular mode σ1 promotes the ESTPT/HT reaction. As a result, the relative intensity of the σ1 state against the origin band in the RE2PI spectrum is weaker than those of the other vibronic bands22,23 when we compared the vibronic distribution in the RE2PI spectrum with that in the FE spectrum measured by monitoring the visible emission. On the basis of the IR-dip spectrum and the rotationally resolved electronic spectrum, the geometry of 7AI(H2O)2 is very similar to the 7AI(CH3OH)2 geometry; the solvent molecules bridge the heteroaromatic N atom and the NH hydrogen by intermolecular hydrogen bonds, forming a cyclic structure.26,27 By analogy to 7AI(CH3OH)2, ESTPT/HT may occur in 7AI(H2O)2, although the DF spectrum did not provide evidence of the reaction because of low sensitivity of the DF spectrum.28 Therefore, we reinvestigated ESTPT/HT in 7AI(H2O)2 by combining FE, DF, and RE2PI spectroscopy. It has been found that ESTPT/HT occurs in 7AI(H2O)2, but the reaction rate must be much slower than that reported by Folmer et al.25 No evidence of the ESQPT/HT reaction in 7AI(H2O)3 has been obtained. On the basis of the experimental results on 7AI(H2O)n (n ) 2,3) and 7AI(CH3OH)n (n ) 2,3), we calculated the excited-state potential energy functions for 7AI(H2O)n (n ) 2,3) and 7AI(CH3OH)n (n ) 2,3) by quantum chemistry calculations to investigate the reaction mechanisms in these two systems. We suggest that the ESTPT reactions occur via a concerted mechanism in 7AI(H2O)2 as well as in 7AI(CH3OH)2. 2. Methods 2.1. Experimental Section. 7AI was purchased from TCI and was used without further purification. The experimental setup was similar to that previously used.22,23,28 The sample introduced in a stainless tube was heated to 353 K by a coiled heater and expanded into the vacuum chamber with Ne as a carrier gas. The backing pressure was 2 to 3 atm. The carrier gas passed through a reservoir containing water and expanded into a vacuum chamber with a pulsed valve (General Valve, series 9, 0.5 mm diameter) operated at 10 Hz. The RE2PI spectra were measured with a differentially pumped linear time-of-flight mass spectrometer. A frequency-doubled dye-laser (Lumonics HD 300 and HT 1000) pumped by the second harmonic of an Nd3+:YAG laser (Spectra Physics GCR 230) was used for the RE2PI experiment. The UV-UV HB spectra were measured
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Figure 1. FE spectra measured by detecting (a) UV and (b) visible fluorescence and RE2PI spectra of (c) 7AI(H2O)2 and (d) 7AI(H2O)3, respectively. The relative fluorescence intensity is shown for parts a and b, whereas the relative intensity of ion signal is shown for parts c and d. The circles and the triangles indicate vibronic bands of 7AI(H2O)2 and 7AI(H2O)3, respectively. Assignments of the 0-0 band, σ(1), and 2σ(1) are given for 7AI(H2O)2 to discuss the difference in the relative intensities between RE2PI and FE spectra. An asterisk is the band of 7AI2.
by using a frequency-doubled dye-laser (Spectra Physics PDL-3 and Inrad Autotracker III) pumped by a second harmonic of the Nd3+:YAG laser (Spectra Physics GCR 150) as a probe laser. For FE spectra, UV or visible emission was selectively detected by a photomultiplier (Hamamatsu 1P28A) with Toshiba UV35+UV-D33S and Toshiba O57 glass filters for the detection of UV emission and visible emission, respectively. We used a monochromator (Horiba Jobin Ybon MicroHR) equipped with a photomultiplier (Hamamatsu R955) to record the DF spectra. 2.2. Theoretical Calculations. Ab initio and DFT (density functional theory) calculations have been performed with the TURBOMOLE30 program package, making use of the resolution-of-the-identity (RI) approximation31 for the evaluation of the electron-repulsion integrals. The equilibrium geometry of the clusters in their ground electronic state (S0) has been determined at both the MP2 and DFT levels using the B3LYP hybrid functional. Excitation energy and equilibrium geometry of the lowest excited singlet state (S1) have been determined at both RI-CC2 and TD-DFT levels. Calculations were performed with the correlation-consistent polarized valence double-ζ basis set (cc-pVDZ). 3. Results and Discussion 3.1. Electronic Spectra of 7AI(H2O)n (n ) 2,3). Figure 1a,b shows the FE spectra measured by monitoring (a) UV and (b) visible (>570 nm) fluorescence with glass filters, respectively. Figure 1c,d shows the RE2PI spectra of 7AI(H2O)2 and 7AI(H2O)3, respectively. In Figure 1a, the intensity of the FE spectrum obtained by monitoring the UV emission is much stronger than that obtained by monitoring the visible emission. The observed vibronic bands in Figure 1a could be assigned to either 7AI(H2O)2 or 7AI(H2O)3 based on the previous studies.26-28 However, in Figure 1b, only the vibronic bands assignable to 7AI(H2O)2 are observed in the spectral region below ∼33 100 cm-1. It is worth noting that the visible fluorescence is detected only from the tautomeric form of 7AI(H2O)2.20,21 Therefore, the observation of visible fluorescence clearly indicates that the tautomeric form of 7AI(H2O)2 is produced by ESTPT/HT. The DF spectra of 7AI(H2O)n (n ) 2,3) have been reexamined. Figure 2a shows the DF spectra of 7AI(H2O)n (n )
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Figure 2. DF spectra of 7AI(H2O)n (n ) 2,3) measured by exciting (a) the 0-0 band of each clusters and (b) the 0-0 band of 7AI(H2O)3 and the +180 cm-1 band of 7AI(H2O)2.
2,3) obtained by exciting the 0-0 band of each cluster. In Figure 2a, the DF spectrum of 7AI(H2O)3 does not show distinct emission in the λ g 530 nm region. We clearly observed emission in the λ g 530 nm region by exciting the 0-0 band of 7AI(H2O)2. In the DF spectrum of 7AI(CH3OH)2, a peak was observed around 500 nm, which was ascribed to the tautomer emission.22 No peak appears in the DF spectrum of 7AI(H2O)2 in the visible region. This may be due to an overlap of very weak visible emission with the UV emission because the contribution of the visible emission of 7AI(H2O)3 in the same wavelength region is insignificant. Figure 2b displays the DF spectrum of 7AI(H2O)2 measured by exciting the +180 cm-1 band together with the DF spectrum of 7AI(H2O)3 recorded by exciting the 0-0 band for comparison. It is clear that the visible emission in the λ g 530 nm region is strongly observed for the excitation of the +180 cm-1 band. This observation is consistent with the enhancement of +180 cm-1 band (σ(1)) as compared with the origin band in the FE spectrum measured by monitoring the visible emission (Figure 1b). These results indicate that the visible emission is due to the electronic transition from the excited state of the tautomeric form to the ground state of 7AI(H2O)2. In the previous study, we could not observe the DF spectra of 7AI(H2O)n (n ) 2,3) in the visible region.28 In this study, we used a bright monochromator, and the fluorescence-collection optical system was improved, which allowed us to observe the DF spectrum of 7AI(H2O)2 in the visible region. Figures 3 and 4 show the UV-UV HB and RE2PI spectra of 7AI(H2O)2 and 7AI(H2O)3, respectively. The spectral features in Figures 3 and 4 are similar to those previously reported.28 The S1-S0 origin bands are observed at 32 632 and 32 554 cm-1, which are red-shifted by 1996 and 2074 cm-1, respectively. The UV-UV hole-burning spectra in Figures 3a and 4a clearly show that there is only a single isomer in these spectral regions under our experimental conditions for 7AI(H2O)2 and 7AI(H2O)3. The cyclic hydrogen-bonded structures with two or three water molecules bridging the NH group and the heteroaromatic N atom of 7AI were obtained for 7AI(H2O)2 and 7AI(H2O)3 by analyzing the rotationally resolved electronic spectra and the IR-dip spectra.26,27 Therefore, the initial state of ESTPT/HT in 7AI(H2O)2 has the cyclic hydrogen-bonded structure. Just after ESTPT/HT, the tautomeric form of 7AI(H2O)2 should have a cyclic hydrogen-bonded structure because the reaction proceeds by a concerted mechanism. (See Section 3.2.) However,
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Figure 3. (a) UV-UV hole burning (HB) spectrum measured by probing the origin band of 7AI(H2O)2. (b) RE2PI spectrum of 7AI(H2O)2. The S1rS0 origin band is observed at 32 632 cm-1, which is red-shifted by 1996 cm-1. The power of the pump laser in HB spectra was attenuated to avoid saturating the absorption. RE2PI and HB spectra were calibrated against the laser power. Notation νintra indicates the intramolecular vibration of 7AI. Superscript on νintra indicates the vibrational wavenumber of each mode. Notations σ(1) and σ(2) indicate the intermolecular stretching vibrations. Vibrational modes of σ(1) and σ(2) are shown in Figure SI of the Supporting Information. The values in parentheses indicate the observed wavenumber from the origin band. The calculated structures in S1 (TD/DFT(cc-pVDZ)) are shown.
Figure 4. (a) UV-UV hole burning (HB) spectrum measured by probing the origin band of 7AI(H2O)3. (b) RE2PI spectrum of 7AI(H2O)3. The S1rS0 origin band is observed at 32 554 cm-1, which is red-shifted by 2074 cm-1. The power of the pump laser in HB spectra was attenuated to avoid saturating the absorptions. RE2PI and HB spectra were calibrated against the laser power. Notation νintra indicates the intramolecular vibration of 7AI. Superscript on νintra indicates the vibrational wavenumber of each mode. The band labeled by X is unassigned. The values in parentheses indicate the observed wavenumber from the origin band. The calculated structures (CASSCF(10,9)/ 6-31++G(d,p)) in S1 are shown.
7AI(H2O)2 containing the tautomeric form may isomerizes to another cluster structures after ESTPT/HT because the internal energy of the tautomeric form of 7AI(H2O)2 must be much larger than that of the initial state. In the present stage, we cannot distinguish the structural isomers of the 7AI(H2O)2 tautomer produced by ESTPT/HT. The vibronic bands in Figures 3 and 4 are assigned by calculating harmonic vibrational frequencies in the S1 state at the TD/DFT(cc-pVDZ) level. For 7AI(H2O)2 (Figure 3), the vibronic bands observed at +180 and +201 cm-1 from the origin band are assigned to intermolecular vibration σ(1) and σ(2), respectively. The other vibronic bands in Figure 3 at +595
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and +745 cm-1 are the fundamentals of the intramolecular vibrations in 7AI labeled by νintra. It should be noted that the spectral features of the RE2PI spectrum in Figure 3b are very similar to those of 7AI(CH3OH)2. The S1-S0 electronic spectrum of the cyclic 7AI(H2O)2 in the ∆ν ) 0-500 cm-1 region has been simulated using a Franck-Condon analysis with PGOPHER software.32 The calculated spectrum and two vibrational modes σ(1) and σ(2) prominently observed are shown in Figure SI (Supporting Information). The calculated spectrum is in agreement with the observed one, supporting the assignment of the cyclic hydrogen-bonded structure of 7AI(H2O)2. For 7AI(H2O)3 (Figure 4), the vibronic bands at +163 and +323 cm-1 from the origin band are assigned to the fundamental and the overtone of the in-phase intermolecular stretching vibration, 1σ and 2σ, respectively. The vibrational energy of the σ mode of 7AI(H2O)3 is smaller than that in 7AI(H2O)2 mainly because of the mass difference between the water dimer and the water trimer. The other vibronic bands in Figure 4 are assigned to the fundamentals of intramolecular vibrations in 7AI and combination bands with the σ mode. In the previous studies of 7AI(CH3OH)2 and 7AI(C2H5OH)2,22-24 we have demonstrated that the comparison of the band intensities between the FE spectra obtained by monitoring the visible emission and the RE2PI spectrum provides information on the vibrational-mode selective ESTPT reaction. In the case of 7AI(CH3OH)2, the relative intensities of the vibronic bands that enhance ESTPT/HT against the origin band are smaller in the RE2PI spectrum as compared with those in the FE spectrum, obtained by detecting the visible fluorescence. These observations provide experimental evidence of the vibrational mode-selective ESTPT/HT.22,23 The vibrational-mode selective ESTPT/HT in 7AI(CH3OH)2 and 7AI(C2H5OH)2 has been consistently explained by shorter lifetimes of the vibronic bands that enhance ESTPT/HT; the ionization efficiency becomes smaller for the vibronic bands having shorter lifetimes. A prominent feature in the electronic spectrum of 7AI(H2O)2 is that the intensities of the σ(1) and 2σ(1) bands relative to the origin band are much larger in the FE spectrum than those of the corresponding bands in the RE2PI spectrum. This difference is clear by comparing the intensity distributions in the FE spectrum (Figure 1b) obtained by monitoring the visible emission with the RE2PI spectrum in Figure 1c. This observation is very similar to the cases for 7AI(CH3OH)2 and 7AI(C2H5OH)2. The results show that the σ(1) of 7AI(H2O)2 is a promoting mode of ESTPT/HT. Another prominent feature is that the FE spectrum of 7AI(H2O)2 measured by monitoring UV fluorescence is distinctly observed in Figure 1a, but the corresponding FE spectra were not observed for 7AI(CH3OH)2 and 7AI(C2H5OH)2.22-24 This may suggest that the ESTPT/HT rate for 7AI(H2O)2 is much smaller than those for 7AI(CH3OH)2 and 7AI(C2H5OH)2. No evidence of ESQPT/HT reaction has been obtained from the observed electronic spectra of 7AI(H2O)3. 3.2. TD-DFT/RI-CC2 Calculations of the Potential Energy Function along the Tautomerization Coordinate for 7AI(H2O)n (n ) 2,3). To check the possibility of a concerted excited state proton/hydrogen atom transfer, we have done the following calculations. The excited-state optimization is giving the initial geometry. Then, all X-H distances involved in the concerted tautomerization reaction are stretched by steps of 0.1 Å (N1H1, O1H2, and O2H3). At each step, all other degrees of freedom are optimized before going on with the next step. (See Figure 5.) Two theoretical methods, RI-CC2 and TD-DFT (B3LYP), have
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Figure 5. Scheme of the concerted mechanism and labeling of the atoms in the 7AI(H2O)2 case.
Figure 6. Comparison between TD-DFT and ri-CC2 calculation for the concerted mechanism in 7AI(H2O)2.
been tested. Excited-state calculations using the TD-DFT method are known to fail when charge transfer states are involved, which is the case if the πσ* state is involved; TD-DFT finds the CT state at lower energy than the locally excited state. However, in all calculations presented here, the πσ* state is higher than the first excited state by at least 0.5 eV and does not play any role. Therefore, TD-DFT calculations should give reasonable results. For the 7AI(H2O)2 system, the Cs (planar) symmetry cannot be used; two hydrogen atoms are out of plane, in agreement with previous IR and ab initio studies.29 In Figure 6, the concerted proton transfer mechanism is presented for the two methods. The first point corresponds to the vertical excitation from the optimized ground-state geometry. The second point corresponds to the energy optimization of the S1 state, without constraints, starting from the ground-state geometry. From this excited-state optimized geometry, the X-H bonds are stretched, and the other degrees of freedom are optimized. The TD-DFT method gives slightly lower energies, as expected, but both calculations give similar barrier height around 0.2 eV, lower than the barrier calculated in the case of 7AI(NH3)2.29 In both methods, the barrier is found for an X-H distance between 1.1 and 1.2 Å. Another interesting aspect of this concerted mechanism is the behavior of the other atoms, in particular the variation of the O-N distances (between the water and 7AI) and the O-O distance (between the two water molecules) upon the X-H stretch. These coordinates are presented in Figure 7 for 7AI(H2O)2. The first point corresponds to the ground-state
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Figure 9. Comparison between TD-DFT and ri-CC2 calculation for the concerted mechanism for 7AI(CH3OH)2. The potential energy (in electronvolts) is plotted against the X-H distance in angstroms.
Figure 7. Variation of the N-O and O-O distances in 7AI(H2O)2 when stretching all X-H distances and optimizing the other degrees of freedom. These distances have been calculated at the TD-DFT level. Both axes correspond to the distances in units of angstroms.
Figure 10. Scheme of the concerted mechanism and labeling of the atoms in the 7AI(CH3OH)2 case.
Figure 8. Comparison of the potential energy functions for 7AI(H2O)2 and 7AI(H2O)3 in the concerted mechanism. These functions are obtained at the TD-DFT level. The potential energy (in electronvolts) is plotted against the X-H distance in angstroms.
geometry (vertical excitation), and the second point corresponds to the excited-state optimization that leads to a shortening of all intermolecular distances. When the X-H bonds (NH/OH) are stretched simultaneously, the optimization leads to a further shortening of the O-O and O-N distances. The shortest distance is observed when the X-H distance is between 1.2 and 1.3 Å, thus just after the barrier (Figure 7). This variation of the O-O and O-N distances as a function of the X-H distance shows that the mechanism is indeed totally concerted because all intermolecular degrees of freedom of the cluster are involved in the mechanism. In a very simplified view, the maximum of the barrier to a proton transfer reaction is expected when the proton is at mid distance between the two heavy atoms and before the barrier the potential energy increases as the X-H distance increases. Therefore, decreasing the distance between the heavy atoms is a good and simple way to decrease the reaction barrier. For the 7AI(H2O)3 cluster, the results are quite similar; the barrier to the reaction is somewhat higher than that for the 7AI(H2O)2 case (Figure 8). As mentioned above, this barrier is an upper value because all X-H distances increased by the same amount, and the barrier would probably be lower if small variations of the X-H distances are allowed. This would require more or less characterizing the entire potential energy surface,
which is not obvious owing the number of degrees of freedom involved in this reaction mechanism. The case of 7AI(CH3OH)2 is very similar to the case of 7AI(H2O)2. The difference in the barrier heights between the two clusters is
