Ab Initio Study of Photochemical Reactions of Ammonia Dimer Systems

Potential energy surfaces of ammonia dimer systems corresponding to the proton (hydrogen) transfer reaction from the reactant (NH3 + NH3) to the product (NH4 + NH2) have been calculated with ab initio HF, MP2, and SECI methods using a TZP basis set including diffuse Rydberg basis functions. The reaction path surfaces of the neutral, excited, and cationic states each have one local minimum corresponding to complexes such as (NH3···NH3) for a neutral one, (H3N*−H···NH2) for an excited one, and (H3N+−H···NH2) for a cation, respectively. Dimer complexes such as (H3N−H···NH2) and (H3N···H−N+H2) do not exist. The photoionization and -dissociation reactions take place along the potential energy surfaces of the neutral, excited, and cationic states. The binding energies of the complexes relative to the asymptote are small. Particularly, the binding energies of H3N*−H···NH2 (1A‘‘) and (H3N···NH3) are only 0.24 and 0.16 eV, respectively. And the head-to-head type [(H3N···NH3)+] with a binding energy of 1.52 eV does not exist on the proton-transfer reaction surfaces.