Proton Transfer Reaction Rates in Phenol-Ammonia Cluster Cation

Proton transfer (PT) in an interaction system of a hydroxyl-amino group (OH-NH) plays a crucial role in photoinduced DNA and enzyme damage. A phenol-ammonia cluster is a prototype of an OH-NH interaction and is sometimes used as a DNA model. In the present study, the reaction dynamics of phenol-ammonia cluster cations, [PhOH-(NH₃)]⁺ (n = 1-5), following ionization of the neutral parent clusters, were investigated using a direct ab initio molecular dynamics (AIMD) method. In all clusters, PTs from PhOH⁺ to (NH₃) were found postionization, the reaction of which is expressed as PhOH⁺-(NH₃) → PhO-H⁺(NH₃). The time of the PT was calculated as 43 (n = 1), 26 (n = 2), and 13 fs (n = 3-5), suggesting that the rate of PT increases with an increase in n and is saturated at n = 3-5. The difference in the PT rate originates strongly from the proton affinity of the (NH₃) cluster. In the case of n = 3-5, a second PT was found, the reaction of which is expressed as PhO-H⁺(NH₃) → PhO-NH₃-H⁺(NH₃), and a third PT occurred at n = 4 and 5. The time of the PT was calculated as 10-13 (first PT), 80-100 (second PT), and 150-200 fs (third PT) in the case of larger clusters (n = 4 and 5). The reaction mechanism based on the theoretical results is discussed herein.