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Multiconfigurational ab initio methods are used in this study to examine two initial reactions that take place during the OH radical attack of the DNA base guanine: a ring opening reaction and a hydrogen transfer reaction. The same reactions are also studied in the presence of a single water molecule. The ring opening reaction has a moderate barrier height of ∼20-25 kcal/mol that is relatively insensitive to the presence of water. The barrier of the H-transfer reaction, on the other hand, is lowered from ∼50 to ∼22 kcal/mol when one water molecule is added, thus becoming comparable to the barrier height of the ring opening reaction.

An eight-dimensional time-dependent quantum dynamics calculation is reported to study the isotopic reaction, HD+C(2)H, on a new modified potential energy surface. Initial-state-selected reaction probability, integral cross section, and rate constants are presented in this isotopic reaction study. Initial-state-selected reaction probability is obtained by summing over all the possible product's arrangements in this isotopic reaction study. This study shows that vibrational excitations of HD enhance the reactivity, whereas stretching excitations of C(2)H only have a small effect on the reactivity. Furthermore, the bending excitations of C(2)H, compared to the ground-state reaction probability, hinder the reactivity. The present results are consistent with those obtained for the H(2)+C(2)H reaction. The comparison of these two reactions also shows the isotopic effect in the initial-state-selected reaction probability, integral cross section, and rate constants. The rate constant comparison shows that the HD+C(2)H reaction has a smaller reactivity than the H(2)+C(2)H reaction.

An eight-dimensional time-dependent quantum dynamics wave packet approach is performed for the study of the H2+C2H-->H+C2H2 reaction system on a new modified potential energy surface (PES) [L.-P. Ju et al., Chem. Phys. Lett. 409, 249 (2005)]. This new potential energy surface is obtained by modifying Wang and Bowman's old PES [J. Chem. Phys. 101, 8646 (1994)] based on the new ab initio calculation. This new modified PES has a much lower transition state barrier height at 2.29 kcal/mol than Wang and Bowman's old PES at 4.3 kcal/mol. This study shows that the reactivity for this diatom-triatom reaction system is enhanced by vibrational excitations of H2, whereas the vibrational excitations of C2H only have a small effect on the reactivity. Furthermore, the bending excitations of C2H, compared to the ground state reaction probability, hinder the reactivity. The comparison of the rate constant between this calculation and experimental results agrees with each other very well. This comparison indicates that the new modified PES corrects the large barrier height problem in Wang and Bowman's old PES.

We report a detailed three-dimensional time-dependent quantum dynamics study of the state-to-state N+N(2) exchange scattering in the 2.1-3.2 eV range using a recently developed ab initio potential energy surface (PES). The reactive flux arrives at the dividing surface in the asymptotic product region in a series of six packets, instead of a single packet. Further study shows that these features arise from the "Lake Eyring" region of the PES, a region with a shallow well between two transition states. Trappings due to Feshbach resonances are found to be the major cause of the time delay. A detailed analysis of the Feshbach resonance features is carried out using an L(2) calculation of the metastable states in the "Lake Eyring" region. Strong resonance features are found in the state-to-state and initial state selected reaction probabilities. The metastable states with bending motions and/or bending coupled with stretching motions are found to be the predominant source of the resonance structure. Initial state selected reaction probabilities further indicate that the lifetimes of the metastable states with bending motions in the "Lake Eyring" region are longer than those of states with stretching motions and thus dominate the reactive resonances. Resonance structures are also visible in some of the integral cross sections and should provide a means for future experimental observation of the resonance behavior. A study of the final rotational distributions shows that, for the energy range studied here, the final products are distributed toward high-rotational states. Final vibrational distributions at the temperatures 2000 and 10,000 K are also reported.