A theoretical study on the size-dependence of ground-state proton transfer in phenol-ammonia clusters.

Geometries and infrared (IR) spectra in the mid-IR region of phenol-(ammonia)n (PhOH-(NH3)n) (n = 0-10) clusters have been studied using density functional theory (DFT) to investigate the critical number of solvent molecules necessary to promote ground-state proton transfer (GSPT). For n ≤ 8 clusters, the most stable isomer is a non-proton-transferred (non-PT) structure, and all isomers found within 1.5 kcal mol-1 from it are also non-PT structures. For n = 9, the most stable isomer is also a non-PT structure; however, the second stable isomer is a PT structure, whose relative energy is within the experimental criterion of population (0.7 kcal mol-1). For n = 10, the PT structure is the most stable one. We can therefore estimate that the critical size of GSPT is n = 9. This is confirmed by the fact that these calculated IR spectra are in good accordance with our previous experimental results of mid-IR spectra. It is demonstrated that characteristic changes of the ν9a and ν12 bands in the skeletal vibrational region provide clear information that the GSPT reaction has occurred. It was also found that the shortest distance between the π-ring and the solvent moiety is a good indicator of the PT reaction.

Electron-Proton Decoupling in Excited-State Hydrogen Atom Transfer in the Gas Phase.

Hydrogen-release by photoexcitation, excited-state-hydrogen-transfer (ESHT), is one of the important photochemical processes that occur in aromatic acids and is responsible for photoprotection of biomolecules. The mechanism is described by conversion of the initial state to a charge-separated state along the O(N)-H bond elongation, leading to dissociation. Thus ESHT is not a simple H-atom transfer in which a proton and a 1s electron move together. Here we show that the electron-transfer and the proton-motion are decoupled in gas-phase ESHT. We monitor electron and proton transfer independently by picosecond time-resolved near-infrared and infrared spectroscopy for isolated phenol-(ammonia)5 , a benchmark molecular cluster. Electron transfer from phenol to ammonia occurred in less than 3 picoseconds, while the overall H-atom transfer took 15 picoseconds. The observed electron-proton decoupling will allow for a deeper understanding and control of of photochemistry in biomolecules.

The mechanism of excited-state proton transfer in 1-naphthol-piperidine clusters.

The geometries of 1-naphthol-(piperidine)n (1-NpOH-(Pip)n) (n = 0-3) clusters have been calculated by using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods to investigate excited-state proton transfer (ESPT) in the low-lying singlet excited states, La and Lb. For the n = 1 cluster, no PT structure was found in Lb and La as well as the ground state, S0. For n = 2, optically accessible Lb from S0 shows the PT structure. We therefore concluded that the threshold size of ESPT is n = 2, which is consistent with previous experimental results. ESPT in 1-NpOH-(Pip)n is simply triggered by optical excitation to Lb. It is essentially different from the 1-NpOH-(NH3)n cluster in which an internal conversion process is required to promote ESPT. From the calculated structures, the importance of the solvation of the π-ring is strongly suggested rather than the proton affinity in ESPT.

Theoretical study on the size dependence of excited state proton transfer in 1-naphthol-ammonia clusters.

The geometries and energetics of the ground and lower-lying singlet excited states S0, La, and Lb of 1-naphthol (NpOH)-(NH3)n (n = 0-5) clusters have been computed using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. Cluster size dependence of the excited state proton transfer (ESPT) reaction was investigated by the vertical transitions from the geometries that can be populated in the molecular beam experiments. For the n = 3 and 4 clusters, the proton-transferred geometries cannot be accessible without significant geometrical rearrangement from the initially populated isomers. For the n = 5 clusters, the proton-transferred structure is found in the La excited state of the isomer that can be populated in the beam. Thus, ESPT is possible by the optically prepared Lb state via internal conversion to La. We concluded that the threshold cluster size of ESPT is n = 5 under the experimental condition with low excess energy.

Laser desorption ionization mass spectrometry of bioactive substances by using 2,4,6-trihydroxyacetophenone on cation-substituted zeolite.

By using 2,4,6-trihydroxyacetophenone adsorbed to cation-substituted zeolite, laser desorption/ionization mass spectrometry for six kinds of bioactive substances was carried out. The compounds, which were usually difficult to observe by conventional matrix-assisted laser desorption/ionization mass spectrometry, were ionized by cation adduction from the zeolite surface.

Structural evolution of (1-NpOH)(n) clusters studied by R2PI and IR dip spectroscopies.

A large-size 1-naphthol cluster, (1-NpOH)(n), with n ≤ 30 was prepared by using a high-pressure pulsed valve. The electronic and vibrational transitions of (1-NpOH)(n) with n = 3-9 were measured by resonant two-photon ionization (R2PI) and ion-detected IR dip spectroscopies. The S(1) ← S(0) R2PI spectrum shows partially resolved structures around the origin band in the (1-NpOH)(n) cluster with n = 3-8. The (1-NpOH)(3) and the (1-NpOH)(6) clusters show relatively sharp origin bands. The structure of (1-NpOH)(3) was determined by comparison of the IR dip spectrum with the simulated one by DFT calculation, while those of (1-NpOH)(n) (n ≥ 4) were discussed in terms of topological geometries of a hydrogen-bonded network. Those analyses suggest that (i) the (1-NpOH)(3) cluster has the cyclic structure where three 1-NpOH monomers are linked by both the hydrogen-bonding and the π···C-H interaction between naphthyl rings and (ii) the (1-NpOH)(n) cluster with n ≥ 4 is built up by attaching the 1-NpOH monomers to the (1-NpOH)(3) core.

Formation and localization of a solvated electron in ground and low-lying excited states of Li(NH3)n and Li(H2O)n clusters: a comparison with Na(NH3)n and Na(H2O)n.

A theoretical study of the ground and low-lying excited states of Li(NH(3))(n) and Li(H(2)O)(n) (n = 1-8) clusters is presented. Their structures, binding energies, vertical ionization energies and vertical transition energies were calculated using ab initio molecular orbital methods at correlated levels. Compared with Na(NH(3))(n) and Na(H(2)O)(n), the incremental binding energies and the spectroscopic energies are found to be almost metal-independent, but solvent-dependent after first-shell closure in both M(NH(3))(n) and M(H(2)O)(n) (M = Li and Na) clusters(.) Autoionization of the alkali atoms occurs via a spatial expansion of unpaired electron distribution extending to outside of the first solvation-shell, irrespective of the combinations of the metal and the solvent. The localization mode of the wave function of the solvated electron was investigated in both the ground and excited states. A change from a one-center diffuse state to a two-center localized state proceeds more quickly against n in M(H(2)O)(n) than in M(NH(3))(n), which is behind the solvent-dependence of the evaluated quantities.

Hydrogen transfer dynamics in a photoexcited phenol/ammonia (1:3) cluster studied by picosecond time-resolved UV-IR-UV ion dip spectroscopy.

The picosecond time-resolved IR spectra of phenol/ammonia (1:3) cluster were measured by UV-IR-UV ion dip spectroscopy. The time-resolved IR spectra of the reaction products of the excited state hydrogen transfer were observed. From the different time evolution of two vibrational bands at 3180 and 3250 cm(-1), it was found that two isomers of hydrogenated ammonia radical cluster .NH(4)(NH(3))(2) coexist in the reaction products. The time evolution was also measured in the near-IR region, which corresponds to 3p-3s Rydberg transition of .NH(4)(NH(3))(2); a clear wavelength dependence was found. From the observed results, we concluded that (1) there is a memory effect of the parent cluster, which initially forms a metastable product, .NH(4)-NH(3)-NH(3), and (2) the metastable product isomerizes successively to the most stable product, NH(3)-.NH(4)-NH(3). The time constant for OH cleaving, the isomerization, and its back reaction were determined by rate-equation analysis to be 24, 6, and 9 ps, respectively.

Hydration process of Na2- in small water clusters: photoelectron spectroscopy and theoretical study of Na2- (H2O)n.

Photoelectron spectroscopy (PES) of Na2- (H2O)n (n < or = 6) was investigated to examine the solvation of sodium aggregates in small water clusters. The PES bands for the transitions from the anion to the neutral ground and first excited states derived from Na2 (1(1)Sigmag+) and Na2 (1(3)Sigmau+) shifted gradually to the blue, and those to the higher-excited states correlated to the 3(2)S + 3(2)P asymptote dropped down rapidly to the red and almost degenerated on the 1(3)Sigmau+-type band at n = 4. Quantum chemical calculations for n up to 3 showed that the spectra can be ascribed to structures where one of the Na atoms is selectively hydrated. From the electron distributions, it is found that the Na- -Na+(H2O)n- -type electronic state grows with increasing cluster size, which can be regarded as a sign of the solvation of Na2- with ionization of the hydrated Na.

Electronic states of sodium dimer in ammonia clusters: theoretical study of photoelectron spectra for Na2-(NH3)n (n = 0-6).

The geometries, energetics, and vertical detachment energies of Na2-(NH3)n (n = 0-6) were examined by ab initio molecular orbital methods in connection with their photoelectron spectra. One of the Na atoms is selectively solvated in the most stable structures for each n. The solvated Na is spontaneously ionized and the formation of a solvated electron occurs with increasing n, giving rise to the Na-Na+(NH3)n(e-)-type state. The ground and two lowest-lying excited states derived from the 11Sigma g+, 13Sigma u+, and 13Pi u states of Na2, respectively, are of ion-pair character though the 13Sigma u+-type state has an intermediate nature slowly changing to the radical-pair state with increasing n. On the other hand, the higher states stemming from the 11Sigma u+, 13Sigma g+, and 11Pi u states of Na2 show a developing radical-pair nature as n increases. The size dependences of the photoelectron spectra such as the near parallel shifts of the first and second bands, as well as the rapid red shifts of the higher bands, are studied on the basis of the electronic change of the neutrals by solvation.

Theoretical study of O2-H2O: potential energy surface, molecular vibrations, and equilibrium constant at atmospheric temperatures.

The intermolecular potential energy surface of O(2)-H(2)O was investigated at ab initio MP2 and MRSDCI levels using the aug-cc-pVTZ basis set. The vibrational levels were evaluated by numerically solving the Schrödinger equations for the nuclear motions with the ab initio potential functions using one- to three-dimensional finite-element methods. On the basis of the calculated partition functions, the equilibrium constant of the complex, K(p), was studied. The K(p) values at atmospheric temperatures of 200-300 K were found to be 1-2 orders of magnitude less than previous estimates from the harmonic oscillator approximation.

Theoretical study of the electronic state and H-elimination reactions for solvated magnesium cluster ions.

The potential-energy curves of the ground and low-lying excited states for Mg(+)NH(3) along the N-H distance were examined by the ab initio configuration interaction method. The photoinduced hydrogen elimination reaction found by the recent experiment is considered to occur via the ground-state channel. The geometries, energetics, and electronic nature of the ground-state Mg(+)(NH(3))(n) and MgNH(2) (+)(NH(3))(n-1) (n=1-6) were also investigated by second-order Møller-Plesset perturbation theory and compared with those of the corresponding hydrated species. In contrast to Mg(+)(H(2)O)(n), the successive solvation energies of Mg(+)(NH(3))(n) become as large as those of MgNH(2) (+)(NH(3))(n-1) containing the Mg(2+)-NH(2) (-) core for n=5 and 6, because of the growing one-center ion-pair state with the Mg(2+) and the diffuse solvated electron. As a result, the solvation energies of the MgNH(2) (+)(NH(3))(n-1) are insufficient to overcome the huge endothermicity of Mg(+)(NH(3))-->MgNH(2) (+)+H, even at these sizes, which is responsible for no observation of the H-loss products, MgNH(2) (+)(NH(3))(n-1).

Four-color hole burning spectra of phenol/ammonia 1:3 and 1:4 clusters.

The hole burning spectra of phenol/ammonia (1:3 and 1:4) clusters were measured by a newly developed four-color (UV-near-IR-UV-UV) hole burning spectroscopy, which is a kind of population labeling spectroscopy. From the hole burning spectra, it was found that single species is observed in an n = 3 cluster, while three isomers are observed simultaneously for n = 4. A possibility was suggested that the reaction efficiency of the hydrogen transfer from the electronically excited phenol/ammonia clusters, which was measured by a comparison with the action spectra of the corresponding cluster, depends on the initial vibronic levels.