The bond-forming reactions of atomic dications with neutral molecules:: Formation of ArNH+ and ArN+ from collisions of Ar2+ with NH3

An experimental and computational study has been performed to investigate the bond-forming reactivity between Ar2+ and NH3. Experimentally, we detect two previously unobserved bond-forming reactions between Ar2+ and NH3 forming ArN+ and ArNH+. This is the first experimental observation of a triatomic product ion (ArNH+) following a chemical reaction of a rare gas dication with a neutral. The intensity of ArNH+ was found to decrease with increasing collision energy, with a corresponding increase in the intensity of ArN+, indicating that ArN+ is formed by the dissociation of ArNH+. Key features on the potential energy surface for the reaction were calculated quantum chemically using CASSCIF and MRCI methods. The calculated reaction mechanism, which takes place on a singlet surface, involves the initial formation of an Ar-N bond to give Ar-NH32+. This complexation is followed by proton loss via a transition state, and then loss of the two remaining hydrogen atoms in two subsequent activationless steps to give the products (ArN+)-Ar-3 + H+ + 2H. This calculated pathway supports the sequential formation of ArN+ from ArNH+, as suggested by the experimental data. The calculations also indicate that no bond-forming pathway exists on the ground triplet surface for this system.