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The dependence of the photon (H)-atom tunneling in Debye approximation of the expression of the rate constant on temperature and pressure is discussed. The oscillatory barrier preparation and media reorganization are the two mechanisms that affect the temperature dependence of this phenomenon.

Ab initio electronic structure theory calculations on model systems are used for the comparison of tunneling through covalently bonded contacts, hydrogen bonds, and van der Waals contacts. The results have shown that a modest difference is observed between tunneling mediated by H-bonds and tunneling mediated by van der Waals contacts at typical distances for each type of interaction.

The unimolecular dissociation of isopropyl chloride cation using mass- analyzed ion kinetic energy spectrometry is investigated. The dissociation mechanism for the loss of hydrogen chloride (HCL) from isopropyl chloride cation is described.

The time-dependent characteristics of quantum mechanical tunneling, fast tunneling in the early stage and slow and long-lasting tunneling in the later stage are studied. The results show that slow tunneling in the long time region is important, because it should dominate the time-dependent feature of tunneling.

A new algorithm is presented for carrying out large-curvature tunneling (LCT) calculations. It is tested on five hydrogen-atom transfer reactions and it is found that more than an order of magnitude can reduce the cost of LCT calculations.

Research into the role of quantum tunneling effects in H2 reactions is presented. Tunneling has a significant impact on the computed activation energy.

Issues concerning hydrogen-atom tunneling from and to linear alkenes in organic matrices at 77 K are discussed. It has been established that matrix molecules retard the abstraction of hydrogen atoms by methyl radicals from the allylic carbon of linear alkenes.

A study was conducted to investigate the consequences of changes in the potential of the scattered electron experiences when tunneling occurs. It has been hypothesized that the modulation of the scattering cross-section can be traced as a function of electron-electron interactions. Experiments involved the use of scanning tunneling microscopy. Findings have revealed that the exclusion principle and the triplet-singlet transition in O2 reduce the barrier parameters and increase the tunneling current.

Research was conducted to examine alkyl radicals using selectively deuterated alkane molecules. The energy of the deformation of alkane molecules during the reaction and the potential energy barriers for the hydrogen atom transfer were calculated to gain an insight into the impact of the steric hindrance on the hydrogen atom tunneling. Results suggest that the rate of hydrogen atom tunneling from alkane molecules depends on the steric hindrance to the motion of the molecules.

A study on intermolecular proton transfer investigated quantum mechanical tunneling effects and rates using a new, dynamical, ab initio, semiclassical formalism. This new semiclassical method was applied to proton transfer in malonaldehyde. Proceeding entirely from first principles, the results obtained were in excellent agreement with the experimentally measured tunneling splitting. The new technique turned out to be very accurate in the test case used and is a very promising approach to examining proton transfer problems.

The bromium loss of the propargyl bromide molecular ion was investigated through mass-analyzed ion kinetic energy spectrometry. The kinetic energy release distribution in the unimolecular dissociation was obtained and the potential energy surface for the mechanistic pathway was studied by quantum chemical computations on the B3LYP/6-311G** density functional theory level. Computation of the rate constants with statistical rate models indicated that the product C3H3+ ion has only the cyclopropenium structure and that the observed unimolecular reaction occurs via tunneling through an isomerization barrier for the H-atom transfer.

Research was conducted to examine the effects of the oxygen at the C8 position on the intramolecular proton transfer between the 6-hydroxy and 6-oxo forms of C8-oxidative guanine. High-level quantum chemical measurement approaches were used to analyze the geometric parameters, proton-transfer process, energies and vibrational modes of the C8-oxidative guanine. Results indicate the significance of the tunneling effect and its possible dominance in the tautomeric process in guanine and its derivatives at room temperature.

A study was conducted to examine the pressure effect on the tunneling rate of deuterium in the acridine-doped fluorene crystal at 77, 150 and 200 K. Results showed that the pressure coefficient is more dependent for deuterium than for hydrogen tunneling. The ratio of the pressure coefficients for hydrogen and deuterium was found to be at 2.6 at 77 K and gradually decreases with increasing temperature until it reaches unity at room temperature.

A study was conducted to extend an investigation into, as well as test, a semiclassical approach for tunneling in multi-dimensional systems by applying the method to calculate energy level splittings in the isomerization of tropolene. All 39 vibrational degrees of freedom in the study were explicitly treated since the method includes tunneling calculations in standard trajectory simulations.

The structures and proton tunneling in the S1 states of jet cooled tropolone-(N2)n (n = 1,2) and tropolone-(CO)1 van der Waals complexes and their deuterated species were investigated. The hole-burning and fluorescence excitation spectra in the S1-S0 region were measured. The results showed that the decreases in the tunneling splittings of tropolone van der Waals complexes were due to the coupling of the intermolecular vibrations with intramolecular vibrations. These resulted in an increase in the height of the potential energy barrier and distortion of the potential energy surface in the S1 state.

Variational transition state calculations and tunneling calculations for a new class of reactions with high-frequency vibrations strongly linked to the reaction coordinate, (Cl-)(CH3NH3+) -> ClCH3(NH3) and CICH3 + NH3, and CH3Cl(H2O) + NH3(H2O) -> (CH3NH3+)(Cl-)(H2O)2. The calculations used the new RODS algorithm based on optimizing the orientation of the dividing surface at each point along the reaction path. It has proven useful in performing stable calculations of the reactions and for speeding up convergence on several other problems.

The selective reaction of hydrogen atoms as a function of the local motion of the solute alkane was investigated using electron spin resonance and electron spin echo spectroscopy. Experimental results showed that temperature exerts significant effects on the reaction, an indication that the hydrogen atoms are thermal atoms rather than hot atoms. The hydrogen atoms react with the solute alkane by quantum tunneling at low temperature. Furthermore, local motions of the solute i-C4H10 molecules occur above 40 K.

The dissociation dynamics of energy-selected 2-bromobutane ions was investigated using the photoelectron photoion coincidence method. The goal was to determine whether the slow rate constant correlated with HBr loss is caused by tunneling through an H atom transfer barrier. Experimental results showed that the 2-bromobutane ion dissociates via Br and HBr loss within 0.3 eV of the ionization limit. The Br loss channel yields the sec-butyl ion C4H9+ via a simple bond cleavage and proceeds at a fast rate.

The calculated results of canonical variational transition state theory/small-curvature tunneling (ICVT/SCT) rate constants for the reaction HFCO + OH is presented. The reaction rate constant is significantly influenced by the tunneling effect in the lower temperature range.

Accurate quantum-mechanical results are presented for thermodynamic data, cumulative reaction probabilities thermal rate constants and kinetic isotope effects for the isotopic reactions [H.sub.2]/HD/[D.sub.2] + C[H.sub.3]. The transition state theory has predicted the kinetic isotope effect when the mass of the transferred atom is unchanged, but has failed in the low-temperature regime due to the neglect of the tunneling effect.

The hybrid quantum-classical approach, which has combined the quantum hydrodynamic and classical Liouvillian representations, is demonstrated for the dynamics of a quantum double well coupled to a classical harmonic coordinate. The studies have shown the tunneling dynamics in both the Eulerian and Lagrangian representations.

The one-dimensional Eckart and Gaussian barrier scattering problems are analyzed using proper quantum trajectories. The computation and properties of isochromes as well as the behavior of the initial wave packet in the complex plane are examined.

Since electron-tunneling interactions are exponentially sensitive to geometry changes, thermal fluctuations are expected to have a large influence on room temperature tunneling currents and scanning tunneling microscope images. The influence of conformational freedom on tunneling currents in a simple model for tunneling mediated by a single small molecule that bridges between a model tip and substrate is explored.

Dynamical tunneling and tunneling between two Born_oppenheimer potential energy surfaces are examined.

A study on the free radical association reaction H + C2H4 calculated 21 kinetic isotope effects for the addition of the hydrogen atom to ethylene using canonical variational-state theory with multidimensional tunneling contributions. Results of the experiment demonstrate the significance of the tunneling effect for the primary kinetic isotope effects and reaction rate increases in the series deuterium-protium-muonium. Experimental methods, calculations and findings are discussed.

Electronic structure calculations and canonical variational transition state theory were used to study the gas-phase HO + H2O to H2O + OH reaction and three associated isotope exchange reactions. Results indicated that the reactions occur through several dynamical bottlenecks. Moreover, it was observed that tunneling is very important in the hydrogen transfer processes at and over 300 degrees Kelvin.

The dynamics of electron tunneling in quasi-one-dimensional resonant donor-bridge-acceptor systems in which the bridge consists of a sequence of atoms arranged in a straight line linking donor and acceptor complexes were investigated using the method of tunneling currents. The method involves an ab initio electronic structure measurement of the spatial distribution of tunneling currents that take place during the tunneling transition in the system. All electrons in the molecular system were analyzed at the Hartree-Fock level.