Wen-Shyan Sheu

Iodine effect on the relaxation pathway of photoexcited I-(H2O)n clusters

Abstract The iodine effect on the photoexcited I − (H 2 O) n relaxation pathway is studied using ab initio calculations. Critical behavior of the vertical binding energy (VBE) of the excited I − (H 2 O) n is seen at short distances between the iodine and the remaining (H 2 O) n , which is correlated to the rapid experimental VBE increase within 1 ps (L. Lehr, M.T. Zanni, C. Frischkorn, R. Weinkauf, D.M. Neumark, Science 284 (1999) 635). Further comparisons of computed results with experimental data reveal that iodine detachment is a crucial step in the cluster relaxation dynamics, intrinsically different from the electron-detachment pathway of photoexcited aqueous iodide.

Dipole-bound anion of water dimer: Theoretical ab-initio study

The dipole-bound anion of the water dimer is studied via an ab-initio approach. Two nuclear stable configurations, trans-(H2O)2− and cis-(H2O)2−, are both identified on the potential energy surface of the anionic water dimer. In addition, the transition state connecting the stable trans-(H2O)2− and cis-(H2O)2− is also located. The transition barrier, relative stability and vertical detachment energies (VDEs) of these two configurations are evaluated at high electron-correlated levels of theory. Consequently, the trans-(H2O)2− is found to be unstable against electron autodetachment. The only electronic and nuclear stable species of the anionic water dimer is the cis-(H2O)2−. The experimental implications of these findings are also discussed.

Charge-transfer-to-solvent (CTTS) precursor states of X−(H2O)n clusters (X=Cl, Br, I)

Abstract The CTTS precursor states in X − (H 2 O) n clusters are studied by the ab initio molecular orbital method. We systematically compare the effects of the solvent structure and anion sizes on the excitation energy from the ground state to the first CTTS precursor state ( E CTTS ) and the vertical binding energy (VBE) from the first CTTS precursor state. VBE is found to be more sensitive to solvent environmental changes than E CTTS . At n =3–5, VBE for a surface ion configuration is ordered as Cl − − − , whereas E CTTS follows the opposite trend. More complex VBE variation is observed for the internal configuration, when n and anions are varied.

Theoretical ab initio study of the water trimer anion: Ground and excited state

The ground and excited state of the water trimer anion, (H2O)3−, are studied via high-level ab initio calculations. A systematic search for the stable (H2O)3− configuration is carried out, based on the configuration formed by the stable (H2O)2− plus one additional water molecule. Four isomers, including three surface structures and one interior structure, are located on the (H2O)3− potential energy surface. While the linear chainlike structure is determined to be the major species detected in supersonic expansion experiments, consistent with previous theoretical studies, the other three isomers may also exist in molecular beams as evidenced by the matches of their vertical detachment energies with high-energy shoulders observed in photoelectron spectra. A significant geometric distortion of water molecules directly interacting with the excess electron indicates that the Franck–Condon effects are the major cause for the experimental observation of the excitation of water vibrational modes upon the photodet...

First-passage-time-distribution approach to gated trapping problems

Two-state gated trapping systems with a static trap are solved by a first-passage-time-distribution (FPTD) approach. The intrinsic microscopic rate for either state need not be zero, an important condition toward developing a theory for gated trapping problems of multiple or continuous states. A one-state trap with a slower intrinsic rate is shown to be physically equivalent to a partially open trap with a faster intrinsic rate. This result is applied to explicitly express the two-state gated FPTD in terms of their corresponding ungated FPTD. Consequently, the gated trapping rates of the systems are calculated and discussed. An approach to experimentally determine gating mechanisms is also proposed.

Kinetic theory of ligand recombination of myoglobin: a model for a combination of entropic and enthalpic effects

A kinetic theory of ligand recombination of myoglobin is obtained through a microscopic model. The macroscopic time dependent rate constant is obtained by the first passage time distribution random walk method. When the ligand is outside the haem pocket, it diffuses in a continuum space. In this process, this rate corresponds to a Smoluchowski rate constant times the concentration of myoglobin. After penetrating through the hydration shell, the ligand waits in front of the gate or diffuses on the myoglobin surface for entering the gate. This waiting time refers to a large scale fluctuation of protein to open the gate. When the ligand is inside the pocket, the motion of the ligand ranges from a ballistic to a diffusive limit. To cover the whole range of friction, it is necessary to solve exactly a finite area random walk model with periodic gating in one- and two-dimensional finite lattices with slippery boundary conditions. Protein dynamics influence the ligand motion indirectly through the collision betw...

Survival of a walker in gated trapping systems

A first-passage-time-distribution (FPTD) approach is developed to investigate the survival and derived properties of a random walker in discrete lattices with a static trap gauged by a general gating mechanism. This approach is effective since the FPTD is directly related to the survival probability distribution of the walker. The random walk is allowed to be undertaken under any potential fields, such as an electric field. We find the gated FPTD can be exactly expressed in terms of its corresponding ungated FPTD in any dimension. Hence, the survival statistics can be calculated. Two gating mechanisms, Poisson and periodic gating, are explicitly considered to calculate their FPTDs, respectively. From the distributions, their mean first passage times (MFPTs) or mean survival times, and mean numbers of visits (MNVs) needed for the walker to become trapped are calculated. Based on the results of these two gating mechanisms, we conclude that the gated MFPT is equal to the sum of the ungated MFPT to the trap s...

Particle distribution of a one-dimensional imperfect annihilation reaction in the gas phase

A one-dimensional (1D) imperfect annihilation reaction in the gas phase is studied via computer simulation. Particular attention is paid to the effects of the spatial distribution of particles on the reaction kinetics. Compared with classical kinetics, the imperfect reaction shows anomalous kinetics by the slowdown of the reaction rate and the anomaly is alleviated when the reaction probability upon contact is lower. The classical rate law is approached when the reaction probability approaches zero. The inspection of the spatial structure of particles shows that the slowdown of the reaction rate is caused by two major factors; the aggregation of the same velocity-species and the segregation of the different velocity-species. When the reaction probability is reduced, these two effects are weakened, and consequently the classical rate law is approached. Other features of the spatial distribution of particles and their effects on the kinetics are also discussed.