Ab Initio Studies for Geometrical Structures of Ammonia Cluster Cations

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 104; no. 21; pp. 5093 - 5100
• Main Author: Park, Jong Keun
• Format: Journal Article
• Language: English
• Published: American Chemical Society 01.06.2000
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp994052q

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 Møller−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, protonated, and proton-transferred cations, a core has only the hydrogen-bonded interaction. In unprotonated tetramer and protonated cations, isomers do not exist, while, in the proton-transferred cations, some types of isomers are optimized at the MP2 level. With increasing cluster cation size, the bond lengths [R NN] between two nitrogen atoms and the distances [R N···H] of the hydrogen bond increase regularly. The binding energies (E n , n - 1) of ammonia cluster cations decrease with size. For the dissociation reaction of unprotonated heptamer (NH3)7 + into protonated tetramers NH4 +(NH3)3 and NH2(NH3)2, the proton-transferred heptamer (NH4 +−NH2)(NH3)5 is the most stable one.