Mitsuhiko Miyazaki

Excited state hydrogen transfer dynamics in substituted phenols and their complexes with ammonia: ππ*-πσ* energy gap propensity and ortho-substitution effect.

Lifetimes of the first electronic excited state (S(1)) of fluorine and methyl (o-, m-, and p-) substituted phenols and their complexes with one ammonia molecule have been measured for the 0(0) transition and for the intermolecular stretching σ(1) levels in complexes using picosecond pump-probe spectroscopy. Excitation energies to the S(1) (ππ*) and S(2) (πσ*) states are obtained by quantum chemical calculations at the MP2 and CC2 level using the aug-cc-pVDZ basis set for the ground-state and the S(1) optimized geometries. The observed lifetimes and the energy gaps between the ππ* and πσ* states show a good correlation, the lifetime being shorter for a smaller energy gap. This propensity suggests that the major dynamics in the excited state concerns an excited state hydrogen detachment or transfer (ESHD/T) promoted directly by a S(1)/S(2) conical intersection, rather than via internal conversion to the ground-state. A specific shortening of lifetime is found in the o-fluorophenol-ammonia complex and explained in terms of the vibronic coupling between the ππ* and πσ* states occurring through the out-of-plane distortion of the C-F bond.

Watching water migration around a peptide bond.

Life is believed to have its origin in aqueous environments, and 70% of our body consists of water. The essential components of biological systems have to interact in aqueous solutions with water molecules by intermolecular forces, such as hydrogen bonds, dispersion forces, and hydrophilic/hydrophobic interactions. Proteins are one of the most important biological supramolecules and offer at the CO and NH sites of the -CONHlinkages of the peptide chain attractive hydrogen-bonding sites, in which H2O can act either as a proton donor or a proton acceptor, respectively. The solvation of a protein has a strong effect on its molecular shape, and as a consequence the fluctuations of the water network on the surface have important influence on its folding properties and catalytic function. Most fundamentally, when a protein starts its folding motion, the water network hydrogen-bonded to the protein has to rearrange and thus affects the dynamics. Therefore, up-to-date quantum chemical simulations on protein folding and its functions include water molecules explicitly. A deeper understanding of these phenomena at the molecular level requires the characterization of the dynamical processes of individual water molecules interacting with the protein. However, most experiments yield only indirect dynamical information averaged over water molecules in the first hydration layer and thus only a tentative and often controversial interpretation of the underlying mechanisms. Measurements visualizing the motion of a specific water molecule in a real biological environment are challenging, and so far no experimental data have been reported yet. Such dynamical experiments need to distinguish between each single water molecule, which can bind to numerous different binding sites of the protein and readily exchange their role with other H2O molecules in the same or higher hydration solvation layers. This inherent complexity of the hydrated protein has so far prevented measurements of the migration of individual water molecules in solution, and therefore nearly all information about such processes relies on theoretical approaches. Although quantum chemical simulations for such complex systems have substantially progressed in recent years because of rapid computer developments, their accuracy is still rather limited and experimental benchmark data for model systems are highly requested for calibration purposes. To this end, we have developed in the past decade an experimental strategy for the investigation of dynamical intermolecular processes, which typically occur on the picosecond (ps) time scale. This approach involves the generation of molecular clusters isolated in supersonic beams and the characterization of their dynamics using ps time-resolved IR spectroscopy. The fruitful combination of spectroscopy and quantum chemistry currently provides the most direct and most detailed access to intermolecular interactions. IR spectroscopy is particularly sensitive to structural motifs. In initial benchmark experiments, we developed a three-color UV-UV-IR tunable picosecond pump–probe laser spectrometer and measured the ionization-induced p!H site switching dynamics of rare gas ligands attached to phenol. 5] In this case, the position of the ligand was monitored by the structure-sensitive frequency of the phenolic OH stretching vibration. Although for a very limited number of water complexes with aromatic molecules the laser-induced migration of the water ligand has recently been inferred from “static” spectroscopy using nanosecond lasers, 6] no time-resolved studies about the dynamics of this fundamental process have been reported yet. We have applied our ps pump–probe approach to the trans-acetanilide–H2O (AA-H2O) cluster to monitor the water migration dynamics around a peptide linkage by the time evolution of the IR spectra measured at a delay time Dt after the ionization event (Figure 1). AA is one of the smallest aromatic molecules with a -CONHpeptide bond (Figure 1) and thus serves as a suitable model for proteins and related biomolecules. It can form two different types of hydrogen bonds with water, resulting in NHand CO-bound isomers, which can readily be distinguished by their different IR and electronic spectra 7] (see the Supporting Information). The “static” IR spectrum of the CO-bound isomer (reactand R) in the neutral ground state (S0) measured by nanosecond lasers (Dt = 50 ns, Figure 2) reveals a sharp band at 3473 cm 1 and a broader one at 3496 cm 1 assigned to the free NH stretching vibration (nNH ) and the hydrogen-bonded OH stretching mode (nOH ) of the water ligand, respectively. The corresponding picosecond IR spectra before photoionization [*] K. Tanabe, Dr. M. Miyazaki, Prof. Dr. M. Sakai, Prof. Dr. M. Fujii Chemical Resources Laboratory, Tokyo Institute of Technology Yokohama 226-8503 (Japan) E-mail: mfujii@res.titech.ac.jp

Infrared spectroscopy of the benzene–H2O cluster cation: experimental study on the drastic structural change upon photoionization

Abstract Infrared photodissociation (IRPD) spectra of ( benzene ( Bz )– H 2 O ) + was measured in the 3 μm region. The cluster cations produced by two different methods, resonant photoionization of the neutral cluster and collisions of the bare benzene cation with water molecules, gave the same IR spectrum. This fact indicates that the most stable isomer cluster cation was generated in both methods. The cluster cation was found to have the side structure bound by the charge–dipole interaction. These results demonstrate that the water molecule on the benzene C6 axis in the neutral state is flipped to the side of the benzene ring upon the photoionization.

Infrared spectroscopy of the phenol-N2 cluster in S0 and D0: Direct evidence of the in-plane structure of the cluster

The OH stretching vibration of jet-cooled phenol-N2 in the neural and cationic ground states was observed by using infrared–ultraviolet double resonance spectroscopy and infrared photodissociation spectroscopy, respectively. The OH vibration showed a small but significant low-frequency shift of 5 cm−1 upon the cluster formation in the neutral, while the shift drastically increased up to 159 cm−1 in the cation. These results represent the direct evidence of the in-plane cluster structure, in which phenolic OH is hydrogen bonded to N2, as was proposed in the zero kinetic energy photoelectron study [S. R. Haines et al., J. Chem. Phys. 109, 9244 (1998)].The OH stretching vibration of jet-cooled phenol-N2 in the neural and cationic ground states was observed by using infrared–ultraviolet double resonance spectroscopy and infrared photodissociation spectroscopy, respectively. The OH vibration showed a small but significant low-frequency shift of 5 cm−1 upon the cluster formation in the neutral, while the shift drastically increased up to 159 cm−1 in the cation. These results represent the direct evidence of the in-plane cluster structure, in which phenolic OH is hydrogen bonded to N2, as was proposed in the zero kinetic energy photoelectron study [S. R. Haines et al., J. Chem. Phys. 109, 9244 (1998)].

Infrared spectroscopy of hydrated benzene cluster cations, [C6H6-(H2O)n]+(n = 1–6): Structural changes upon photoionization and proton transfer reactions

Infrared (IR) spectra of benzene–(water)n cluster cations (Bz–Wn)+ (n=1–6) in the OH and CH stretching vibrational region were observed in order to investigate their structure and reactivity. The cluster cations were prepared by two different production methods: one is due to collision between bare benzene cations and water clusters; and the other utilizes resonance enhanced multiphoton ionization (REMPI) of neutral clusters. The former method prefers the production of the most stable isomer cluster cations, while the latter would reflect the Franck-Condon restriction in the ionization process. The structures of the n=1 and n=2 clusters were determined on the basis of the comparison between the IR spectra and density functional theory (DFT) calculations. In the n=1 cluster cation, the oxygen atom of the water molecule is located in the benzene ring plane and coordinates to the benzene moiety by two identical CH–O hydrogen bonds. The IR spectra of the n=2 cluster cation showed absorption bands arising from two different types of isomers: one has a hydrogen-bonded water dimer interacting with the benzene cation; in the other isomer two water molecules are independently bound to the benzene cation. The production ratio between the isomers was found to strongly depend on the cluster ion preparation methods. Except for the case of the n=2 cluster, the cluster cations prepared by the two different methods gave identical IR spectra. This means that quite extensive rearrangements of the cluster structure occur upon ionization of the neutral clusters, leading to the most stable form of the cluster cations. The spectral features of the n=3 cluster cation are very similar to the n=2 cluster, suggesting similar structures among these clusters. Higher clusters larger than the n=3 cluster showed quite different IR spectra from those of the n≤3 clusters, but their spectral features are very similar to those of hydrated clusters of protonated species, X–H+–(H2O)n, indicating that proton transfer reactions from the benzene cation to the water moiety occur in the larger clusters than those with n=3.

High-energy, broadly tunable, narrow-bandwidth mid-infrared optical parametric system pumped by quasi-phase-matched devices.

We have developed a tunable, narrow-bandwidth (<2 cm(-1)) mid-infrared (MIR) optical parametric system with a large-aperture periodically poled Mg-doped LiNbO(3) (LA-PPMgLN)-based high-energy pump source. The system has a continuously tunable tuning range from 4.6 to 11.2 mum and produces a maximum output energy of 2.0 mJ at 5.1 mum. Practical use of the MIR source is demonstrated by MIR-UV double-resonance spectroscopy of jet-cooled acetanilide.

Gas-phase spectroscopy of synephrine by laser desorption supersonic jet technique.

In our previous work, we found that synephrine has six conformers in the gas phase, while adrenaline, which is a catecholamine and has the same side chain as synephrine, has been reported to have only two conformers. To determine the conformational geometries of synephrine, we measured resonance enhanced multiphoton ionization, ultraviolet-ultraviolet hole burning, and infrared dip spectra by utilizing the laser desorption supersonic jet technique. By comparing the observed infrared spectra with theoretical ones, we assigned geometries except for the orientations of the phenolic OH group. Comparison between the determined structures of synephrine and those of 2-methylaminno-1-phenylethanol, which has the same side chain as synephrine but no phenol OH group, leads to the conclusion that the phenolic OH group in synephrine does not affect the conformational flexibility of the side chain. In the case of adrenaline, which is expected to have 12 conformers if there are no interactions between the catecholic OH groups and the side chain, some interactions possibly exist between them because only two conformations are observed. By estimation of the dipole-dipole interaction energy between partial dipole moments of the catecholic OH groups and the side chain, it was concluded that the dipole-dipole interaction stabilizes specific conformers which are actually observed.

Microhydrated aromatic cluster cations: Binding motifs of 4-aminobenzonitrile-(H2O)n cluster cations with n ≤ 4

Infrared photodissociation (IRPD) spectra of mass-selected 4-aminobenzonitrile-(water)n cluster cations, ABN(+)-(H2O)n with n ≤ 4, recorded in the N-H and O-H stretch ranges are analyzed by quantum chemical calculations at the M06-2X/aug-cc-pVTZ level to determine the evolution of the initial microhydration process of this bifunctional aromatic cation in its ground electronic state. IRPD spectra of cold clusters tagged with Ar and N2 display higher resolution and allow for a clear-cut structural assignment. The clusters are generated in an electron impact source, which generates predominantly the most stable isomers. The IRPD spectra are assigned to single isomers for n = 1-3. The preferred cluster growth begins with sequential hydration of the two acidic NH protons of the amino group (n = 1-2), which is followed by attachment of secondary H2O ligands hydrogen-bonded to the first-shell ligands (n = 3-4). These symmetric and branched structures are more stable than those with a cyclic H-bonded solvent network. Moreover, in the size range n ≤ 4 the formation of a solvent network stabilized by strong cooperative effects is favored over interior ion hydration which is destabilized by noncooperative effects. The potential of the ABN(+)-H2O dimer is characterized in detail and supports the cluster growth derived from the IRPD spectra. Although the N-H bonds are destabilized by stepwise microhydration, which is accompanied by increasing charge transfer from ABN(+) to the solvent cluster, no proton transfer to the solvent is observed for n ≤ 4.

Solvation Dynamics of a Single Water Molecule Probed by Infrared Spectra—Theory Meets Experiment

The dynamics and energetics of water at interfaces or in biological systems plays a fundamental role in all solvation and biological phenomena in aqueous solution. In particular, the migration of water molecules is the first step that controls the overall process in the time domain. Experimentally, the dynamics of individual water molecules is nearly impossible to follow in solution, because signals from molecules in heterogeneous environments overlap. Although molecular dynamics simulations do not have this restriction, there is a lack of experimental data to validate the calculated dynamics. Here, we demonstrate a new strategy, in which the calculated dynamics are verified by measured time-resolved infrared spectra. The coexistence of fast and slow migrations of water molecules around a CONH peptide linkage is revealed for a model system representative of a hydrate peptide.

IR spectroscopy of monohydrated tryptamine cation: Rearrangement of the intermolecular hydrogen bond induced by photoionization

Rearrangement of intermolecular hydrogen bond in a monohydrated tryptamine cation, [TRA(H(2)O)(1)](+), has been investigated in the gas phase by IR spectroscopy and quantum chemical calculations. In the S(0) state of TRA(H(2)O)(1), a water molecule is hydrogen-bonded to the N atom of the amino group of a flexible ethylamine side chain [T. S. Zwier, J. Phys. Chem. A 105, 8827 (2001)]. A remarkable change in the hydrogen-bonding motif of [TRA(H(2)O)](+) occurs upon photoionization. In the D(0) state of [TRA(H(2)O)(1)](+), the water molecule is hydrogen-bonded to the NH group of the indole ring of TRA(+), indicating that the water molecule transfers from the amino group to NH group. Quantum chemical calculations are performed to investigate the pathway of the water transfer. Two potential energy barriers emerge in [TRA(H(2)O)(1)](+) along the intrinsic reaction coordinate of the water transfer. The water transfer event observed in [TRA(H(2)O)(1)](+) is not an elementary but a complex process.