Mechanisms of the deamination reactions of isoguanine with H2O, OH–, and OH–/H2O and of protonated isoguanine (isoGH+) with H2O have been investigated by theoretical calculations. Eight pathways, paths A–H, have been explored and the thermodynamic properties (ΔE, ΔH, and ΔG), activation energies, enthalpies, and Gibbs energies of activation were calculated for each reaction investigated. Compared with the deamination reaction of isoguanine or protonated isoguanine (isoGH+) with water, the deamination reaction of isoguanine with OH– shows a lower Gibbs energy of activation at the rate-determining step, indicating that the deamination reaction of isoguanine is favorably to take place for the deprotonated form isoG– with water. With the assistance of an extra water, the reaction of isoguanine with OH–/H2O, pathways F and H, are found to be the most feasible pathways in aqueous solution due to their lowest Gibbs energy of activation of 174.7 and 172.6 kJ mol–1, respectively, at the B3LYP/6-311++G(d,p) level of theory.

Investigations on the dissociation kinetics of hydrated protonium ions, (H2O)2H+ and their deuterated species (D2O)2D+, are reported based on the harmonic and anharmonic oscillator model using the transition state theory and ab initio calculations. We find that the dissociation of (H2O)2H+ and (D2O)2D+ exhibits a distinct threshold behavior due to the existence of activation energies. Moreover, the deviation between the harmonic and anharmonic dissociation rate constants becomes larger in the high energy or temperature range, with the rate constants becoming unreasonably large under the harmonic oscillator model. The isotope effect is found to become more distinct but only in the case of the anharmonic oscillator model. These results show that the anharmonic Rice−Ramsperger−Kassel−Marcus (RRKM) theory can provide a reasonably good description for the dissociation of (H2O)2H+ and (D2O)2D+. Furthermore, a theoretical model to demonstrate the principle of vibrational predissociation spectroscopy (VPS) is established from the viewpoint of RRKM theory and applied in determining the experimental conditions and understanding the role of the dissociation rate constant k(E) played in the VPS experiment, using (H2O)2H+ and (D2O)2D+ as examples.