Long-Range Migration of a Water Molecule To Catalyze a

Oct 19, 2010 - Toyota Physical and Chemical Research Institute, Nagakute, Aichi 480-1192, Japan and Department of Chemistry, Graduate School of Scienc...
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J. Phys. Chem. A 2010, 114, 11896–11899

Long-Range Migration of a Water Molecule To Catalyze a Tautomerization in Photoionization of the Hydrated Formamide Cluster Satoshi Maeda,†,‡ Yoshiyuki Matsuda,*,‡ Shinichi Mizutani,‡ Asuka Fujii,‡ and Koichi Ohno*,†,‡ Toyota Physical and Chemical Research Institute, Nagakute, Aichi 480-1192, Japan and Department of Chemistry, Graduate School of Science, Tohoku UniVersity, Aoba-ku, Sendai 980-8578, Japan ReceiVed: July 28, 2010; ReVised Manuscript ReceiVed: September 24, 2010

The dynamics on the vacuum-ultraviolet one-photon ionization of a formamide-water cluster is investigated by a combination of theoretical reaction-path search and infrared spectroscopic methods. A keto-enol tautomerization of the formamide moiety occurs after photoionization by a catalytic action of the water molecule accompanied with its long-distance migration; the water molecule in the cluster migrates almost one turn around the formamide moiety. During the migration, the water molecule abstracts the proton of CH in the formamide moiety and carries it to the O atom side in the carbonyl group through a “catch and release”-type catalytic action. I. Introduction Water is the most fundamental solvent in chemistry and biology.1,2 Various chemical phenomena and biological functions occur in water. Because of this importance of water, the microscopic roles of water in chemical reactions have been of great interest. It has been proposed that water acts not only as polar solvent but also as a molecular catalyst for chemical reactions of solute molecules.3-16 Catalytic action of water has been implied in a variety of proton-transfer reactions such as acid-base reactions, prototropic tautomerization, and proton pump in biological systems. Proton-transfer dynamics in water and catalytic roles of water, however, have not easily been identified in the bulk systems, because the inhomogeneity of the system as well as other simultaneous processes obscure precise information of chemical reactions. Clusters generated in a supersonic jet are free from thermal inhomogeneity and environmental perturbations. Moreover, the systems are considerably simplified. Therefore, they are ideal to study solute-solvent interactions at the molecular level. Proton-relay mechanisms in electronic excited-state protontransfer reactions17,18 and long-distance displacements of water molecules through vibrational excitation on the electronic ground-tate potential19 have been demonstrated for solvated clusters in the supersonic jets. Recently, we reported an infrared (IR) spectroscopic study on neutral hydrated formamide (FA) clusters, FA-(H2O)n, n ) 1-4.20 The hydrated structures of neutral FA have been determined by comparisons of their observed IR spectra and harmonic vibrational calculations. In the same way, structures of (FA-H2O)+ produced by a 118 nm photoionization of the neutral FA-H2O cluster has been studied, and we proposed one structure where protonated formamide and OH radical are hydrogen-bonded. Though this structure is not the most stable among the calculated isomers, all the observed bands are well explained by this structure. Moreover, no IR spectral signature was observed for other more stable isomers. Ionization dynamics * To whom correspondence should be addressed. E-mail: matsuda@ m.tohoku.ac.jp (Y.M.); [email protected] (K.O.). † Toyota Physical and Chemical Research Institute. ‡ Tohoku University.

of the cluster at the 118 nm photoionization and the following isomerization process have not been fully examined, and the reason for the missing more stable cluster cations has remained puzzling. In this study, we revisit the dynamics in the 118 nm photoionization of the FA-H2O cluster by a combination of newly developed theoretical and spectroscopic techniques. We propose a new interpretation of the IR spectrum, which shows the keto-enol tautomerization of the FA moiety in the photoionization. The structure of the photoionized (FA-H2O)+ is finally determined to the most stable structure, where the enol cation of FA is hydrogen-bonded with water. This most stable structure is newly found in the present theoretical approach using an automatic reaction path search method. The tautomerization upon photoionization is catalyzed by the “catch and release” action of the single water molecule in the proton-translocation process. Moreover, this catalytic action is accompanied by the long-distance migration of the water molecule and a proton. This is the first demonstration of this type of catalytic action of a water molecule. II. Calculation and Experiment II.1. Reaction Path Search. The global reaction route mapping (GRRM) method is a method for exploring potential energy surfaces (PESs) automatically and systematically on the basis of the anharmonic downward distortion following the ADDF approach.21 The GRRM method can find all reaction pathways on PESs of given chemical formula.22 Nevertheless, a full search is very expensive in systems with more than 10 atoms. Hence, an approximated large-ADDF (l-ADDF) treatment is available, limiting the exploration area to low-energy parts of PES.23 In this study, a GRRM/l-ADDF search was applied to the PES of the FA-water cluster cation. First, the PES at the PBE1PBE/6-31+G* level was explored by the GRRM/l-ADDF method, where the five largest ADDs were treated in the l-ADDF. Then, the key structures shown in Figure 1 were reoptimized by the MP2(full)/6-311+G(2d,2p) method. Single-point energy values were evaluated by the CCSD(T)/6311++G(3df,2p) method, and all relative energy values presented in Figure 1 are the CCSD(T) single-point energies. Energy, gradient, and Hessian were calculated by the Gaussian

10.1021/jp107034y  2010 American Chemical Society Published on Web 10/19/2010

Photoionization of the Hydrated Formamide Cluster

J. Phys. Chem. A, Vol. 114, No. 44, 2010 11897 monitoring the ion intensity of (FA-H2O)+, the IR spectrum of (FA-H2O)+ was obtained as an IR dip spectrum. The 118 nm light, which was used for IRPDS-VUV-PI, was generated by third-harmonic generation of the third-harmonic output (355 nm) of a Nd:YAG laser (Continuum Surelite-III) with a Xe-Ar mixture. The IR light is generated by difference frequency generation of the second-harmonic output (532 nm) of a Nd:YAG laser (Continuum Powerlite 8010) and an output of a dye laser (Continuum ND 6000). Results and Discussions

Figure 1. Reaction profile of the (FA-H2O)+ cluster cation produced by 118 nm photoionization of the neutral cluster. Energy values (in kJ/mol) were obtained by the CCSD(T)/6-311++G(3df,2p)//MP2(full)/ 6-311+G(2d,2p) calculations. The structure of neutral FA-H2O is shown at the top-left, which is the starting (Franck-Condon) point of the ionization dynamics. Long-distance migration of the water molecule and a water-assisted tautomerization generating the most stable minimum V can occur at the vertical ionization energy.

03 programs,24 and all geometry displacements were treated by the GRRM program.21 II.2. Anharmonic Vibrational Analysis. In anharmonic vibrational analyses, the most time-consuming part is extensive calculations of potential energy values over a wide area of PES for accurate integration of the vibrational Hamiltonian. In this study, the PES was constructed of sixth-order functions in terms of f normal modes and the integrations were performed analytically. Although obtaining all (∼Of 6) coefficient values for the sixth-order functions is computationally very expensive in general (typically ∼Of 6 potential data are necessary), we developed a very efficient algorithm to get all the coefficient values through a sort of preconditioning technique on the basis of the GRRM/ADDF search.25 The initial GRRM/ADDF searches were applied to the PESs of the B3LYP/6-31+G** method. Then, the coefficient values in sixth-order functions were determined by higher computation levels, where harmonic and anharmonic coefficients were evaluated by the G326 and MP2(full)/6-311+G(2d,2p) methods, respectively, on the basis of the multilevel (ML) approach.27 Large-amplitude vibrational modes were treated by the second-order approximation as discussed previously.25 Dipole moment surfaces for infrared intensity calculations were constructed in third-order functions by numerical differentiations of dipole derivatives at the MP2(full)/6-311+G(2d,2p) level. The vibrational Schro¨dinger equations were solved by the QDPT[1+2] method28 to obtain vibrational wave functions and eigenvalues. Details of our implementation of the QDPT[1+2] method are shown in a previous paper.29 Energy, gradient, and Hessian were calculated by the Gaussian 03 programs, and GRRM/ADDF and coefficient fitting were treated by the GRRM program.21 II.3. Experimental procedures. An IR spectrum of the formamide (FA)-water cluster cation was observed with IR predissociation spectroscopy based on the vacuum-ultraviolet (VUV) one-photon ionization detection, which is called IRPDSVUV-PI (IR predissociation spectroscopy of VUV-pumped ion).30 Details of the spectroscopy have been described elsewhere.30 Briefly, (FA-H2O)+ was generated with the VUV photoionization of jet-cooled neutral FA-H2O at 118 nm. The ion intensity of (FA-H2O)+ was monitored with a time-of-flight mass spectrometer. The IR light was introduced after the VUV photoionization process. When the IR absorption of (FA-H2O)+ causes its fragmentation, the monitored ion intensity of (FA-H2O)+ decreases. By scanning the IR frequency while

First, we search the ionization dynamics of FA-H2O by use of the GRRM method.21-23 The GRRM search gave more than 100 structures of the cluster cation including local minima and first-order saddle points. Many of them were related to very high energy channels inaccessible with the 118 nm photon energy, which is experimentally used for ionization of FA-H2O. We pick up accessible reaction channels, and they are summarized in Figure 1. Relative energies of the minima and transition states in Figure 1 are calculated at the CCSD(T)/6311++G(3df, 2p) and MP2(full)/6-311+G(2d,2p) levels. Their zero-point vibrational energies are also computed at the MP2(full)/6-311+G(2d,2p) level. These energy evaluations are summarized in the Supporting Information. The search started with the structure of the neutral FA-H2O to examine the vertical ionization by the 118 nm photon. The structure of the neutral cluster has previously been determined to be the ring-type formed by two hydrogen-bonds (H-bonds), as depicted in Figure 1. Minimum I is the nearest local minimum to the vertically ionized structure. Minimum I can isomerize into minima II (reaction path 1) and III (reaction path 2) via water migration and proton transfer, respectively. The main channel should be reaction path 1, leading to minimum II because of the much lower energy barrier in the path. Further water migration can occur from minimum II to the most stable minimum V. During this second migration, the water molecule catches the proton on the C atom and carries it to the carbonyl oxygen of the FA moiety. Thus, the GRRM search predicts that minimum V is dominantly formed through reaction path 1, where the water molecule migrates around the FA moiety and catalyzes the keto-enol tautomerization of FA in the cationic state. In the previous study, minimum V was not found because its formation requests the unexpected long-range migration of the water and proton. Minimum VI was previously concluded for (FA-H2O)+ by comparisons of the observed IR spectrum and the harmonic vibrational simulation. However, the present GRRM search indicates that the reaction path to minimum VI would be minor because of the high energy barrier between minima I and III. The CCSD(T)/6-311++G(3df,2p) level calculation shows that minimum VI is less stable by 66.9 kJ/mol than the most stable minimum V. When we proposed minimum VI structure for (FA-H2O)+, we had a problem to explain the absence of lower energy isomers. The new structure based on the GRRM calculations solves this puzzle. In the following, the validity of the assignment to minimum V will be confirmed with the IR spectroscopic investigation. Figure 2 compares (A) the observed IR spectrum of the FA-H2O cation and (B-F) the simulated IR spectra with the GRRM program. The (FA-H2O)+ is experimentally generated by the 118 nm one-photon ionization of the corresponding neutral cluster of the ring-type structure.20 The spectrum of (FA-H2O)+ has been reported in our previous work.20 However, the present spectrum is newly measured again with great care to normalization with the IR laser power. The spectral assign-

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Figure 2. (A) Observed IR spectrum of (FA-H2O)+ and (B-G) simulated IR spectra for the minimum structures calculated in the GRRM reaction path search in Figure 1. The simulated IR spectra are computed by the VQDPT[1+2] theory28,29 applied to sixth-order potential energy functions constructed by the GRRM/ADDF method25 and the multiresolution27 [G326:MP2(full)/6-311+G(2d,2p)] approach. The simulated vibrational characters, vibrational frequencies, transition intensities of the combination band at 2718 cm-1 in spectrum H, and other simulated vibrational bands are summarized in a table in the Supporting Information.

ments are also totally changed on the basis of the GRRM calculations. The simulated spectra are based on the stable structures, which are on the accessible reaction paths 1 and 2 (Figure 1) in the 118 nm ionization process. In the observed IR spectrum, there are three intense bands in the 3300-3600 cm-1 region. No remarkable band is seen in the 3680-3780 cm-1 region, which is characteristic of the asymmetric free OH stretching vibration of water. This disagrees with the calculated spectra of minima I, II, and VII, all of which are expected to show a free OH stretch band in this region. No band is also seen in the lower frequency region other than 3220 cm-1, although intense bands are simulated for minima I-IV and VII in the 2700-3200 cm-1 region. Therefore, minima I-IV and VII are excluded from candidates for the spectral carrier of the observed IR spectrum. On the other hand, minimum V, the most stable isomer, shows no H-bonded band in the observed spectral range. Moreover, the 3680-3780 cm-1 region is also blank because its symmetric and antisymmetric OH stretch bands are predicted to be ∼100 m-1 lower than those of the bare water molecule. The simulated spectrum of this structure qualitatively reproduces the observed spectral features. The observed three bands at 3310, 3470, and 3590 cm-1 are assigned to the symmetric NH stretch, the

Maeda et al. asymmetric NH stretch, and the asymmetric OH stretch, respectively. These observed bands are broadened by thermal population and overlap with weak bands of overtones and combinations arising from the anharmonicity. The present GRRM calculation counts the effect of the anharmonicity, and the simulation qualitatively reproduces the broadening of the observed bands. It should be noted that thermal effects, i.e., hot bands, are neglected in the present anharmonic analyses, and this can be a significant source of some discrepancies between the observed and the simulated spectra of minimum V. In the simulated spectrum of minimum V, the free OH stretch bands show a remarkable low-frequency shift from the ordinarily expected region of the free OH stretch band. The calculated OH distances from the shared proton to the carbonyl oxygen and the oxygen of the water moiety are 1.133 and 1.269 Å, respectively, at the MP2(full)/6-311+G(2d,2p) level. These values indicate that the water moiety has the character of the hydronium ion (H3O+) by sharing the proton with the carbonyl oxygen. Thus, the two original OH bonds of the water moiety become weaker, and the asymmetric free OH stretch of minimum V appears at ∼3600 cm-1. In the previous report, we assigned minimum VI to the spectral carrier of the observed spectrum.20 The intensity of the previously observed IR spectrum was not normalized by the IR power, and therefore, its intensity profile was somewhat affected by the IR power, which was not constant in the observed frequency range. The calculated harmonic vibrational frequencies of minimum VI at the MP2 level were previously scaled to fit them with the observed ones. These led to accidental similarity between the previously observed spectrum and the harmonic vibrational spectrum of minimum VI. As shown in Figure 2, when the anharmonicity is counted in the IR simulation, the spectrum of minimum VI does not agree well with the observed spectrum as to both the intensity profile and the band positions. In addition, the simulation predicts a band at 3127 cm-1, but no corresponding band is seen in the observed spectrum. According to the reaction route search result shown in Figure 1, minimum VI is formed through more stable structures of minima I, II, and IV. If minimum VI is formed, they would coexist in thermal equilibrium. However, no spectral signature for minima I, II, and IV is seen in the observed IR spectrum, as described above. Therefore, because of the finding of a much more reasonable structure, minimum V, the previous conclusion should be withdrawn. Minimum VI is not formed in the photoionization of neutral FA-H2O at 118 nm. In conclusion, the comparison between the observed IR spectrum and the anharmonic vibrational calculation demonstrates that minimum V including the enol-FA cation moiety is formed through reaction channel 1 (Figure 1) in the 118 nm photoionization of FA-H2O. This is clear evidence for the prediction by the GRRM search. In the tautomerization reaction process found in the present study, the water molecule migrates at anomalously long distance, i.e., almost one turn around the FA moiety starting from the Franck-Condon structure to the most stable minimum V. During migration, the water molecule acts as the “catch and release”-type catalyst to abstract the proton from the CH group, transfer it, and finally release the proton to the carbonyl oxygen. Such a “catch and release”-type catalytic action is theoretically known to be particularly favorable when the proton affinity (PA) of the water molecule lies between those of the two sites involved in the tautomerization.8-12 In the present case, PAs of the water molecule, the proton-donating site (C atom in NH2CO radical), and the proton-accepting site (O atom in NH2CO

Photoionization of the Hydrated Formamide Cluster radical) are evaluated to be 716.7, 747.4, and 766.4 kJ/mol, respectively, at the CCSD(T)/6-311++G(3df,2p)//MP2(full)/ 6-311+G(2d,2p) level. Since the PA of the water molecule is slightly lower than the PA of the proton-donating site, this watercatalyzed proton-transfer reaction requires a small activation energy of 18.8 kJ/mol. Therefore, isomerization to minimum V is favorable even with the vertical ionization energy. The activation energy of the tautomerization is totally different in bare FA cation. The GRRM search estimates it to be 171.2 kJ/mol at the CCSD(T)/6-311++G(3df,2p)//MP2(full)/6311+G(2d,2p) level. The catalytic action of the single water molecule reduces ∼90% of the energy barrier in the tautomerization reaction. Conclusion In this study, the “catch and release”-type catalytic action and long-distance migration of the water molecule are elucidated for the keto-enol tautomerization of the FA cation following photoionization of the FA-H2O cluster. In the reaction, the water moiety migrates around the FA moiety to find the most stable structure, catalyzing the proton-transfer reaction. Chemical reactions such as proton transfer in the bulk system may undergo catalytic action as well as hydration effects of water. The present findings of the catalytic property of a single water molecule would contribute to further understanding the microscopic roles of water in chemical reactions. Acknowledgment. K.O. acknowledges the Grants-in-Aid for Scientific Research (nos. 21350007 and 21655002) from the Ministry of Education, Science, Sports, and Culture. S.M. is supported by a Research Fellowship of the Japan Society of Promotion of Science for Young Scientists. Y.M. and A.F. acknowledge the Grant-in-Aid for Young Scientists and Scientific Research (KAKENHI) (nos. 20750002 and 22350001) from the JSPS, the Grant-in-Aid for Scientific Research on Priority Areas Molecular Science for Supra Functional Systems [477] from MEXT, Japan, and a Research Grant from the Human Frontier Science Program (RGY82/2008). Supporting Information Available: Relative energies and structural parameters of minimum and transition-state structures in the simulated reaction paths at the CCSD(T)/6-311+ +G(3df,2p) and MP2(full)/6-311+G(2d,2p) levels as well as their simulated zero-point vibrational energies; vibrational characters, frequencies, and intensities of harmonic vibrational simulations at the MP2(full)/6-311+G(2d,2p) level and anharmonic vibration simulated results. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Koeller, K. M.; Wong, C.-H. Nature 2001, 409, 232. (2) Li, C.-J.; Chen, L. Chem. Soc. ReV. 2006, 35, 68. (3) Lledo´s, A.; Bertra´n, J. Tetrahedron Lett. 1981, 22, 775.

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