Ab Initio Studies for Geometrical Structures of Ammonia Cluster Cations

Abstract

Structures of unprotonated (NH3)n+ (n = 1−6), protonated NH4+(NH3)n-1 (n = 1−6), and proton-transferred (NH4+−NH2)(NH3)n-2 (n = 3−7)] ammonia cluster cations have been optimized with ab initio Hartree−Fock (HF) and second-order Moller−Plesset (MP2)/6-31+G** levels, and the harmonic vibrational frequencies have also been evaluated. In ammonia cluster cations, NH3+, NH4+, and NH4+−NH2 form as a central core of the first ammonia solvation shell, respectively. In unprotonated dimer cation, the totally symmetric structure with the head-to-head interaction is optimized to be stable. In the hydrogen-bonded dimer cation, the unprotoned NH3···NH3+ cation is optimized to the protonated-transferred NH4+···NH2 cation. In unprotonated trimer cation, there are two types of isomers (hydrogen-bonded and head-to-head). The hydrogen-bonded type of unprotonated trimer cation is more stable. In unprotonated pentamers and hexamers, a NH3+ core has both interactions in a complex. On the other hand, in unprotonated tetramer, pr...