Collision-induced dissociation of HS-(HCN):: Unsymmetrical hydrogen bonding in a proton-bound dimer anion

The energy-resolved competitive collision-induced dissociation of the proton-bound complex [HS center dot H center dot CN](-) is studied in a guided ion beam tandem mass spectrometer. H2S and HCN have nearly identical gas-phase acidities, and therefore, the HS- + HCN and the CN- + H2S product channels exhibit nearly the same threshold energies, as expected. However, the HS- + HCN channel has a cross section up to a factor of 50 larger than CN- + H2S at higher energies. The cross sections are modeled using RRKM theory and phase space theory. The complex dissociates to HS- + HCN via a loose transition state, and it dissociates to CN- + H2S via a tight transition state. Theoretical calculations show that the proton-transfer potential energy surface has a single minimum and that the hydrogen bonding in the complex is strongly unsymmetrical, with an ion-molecule complex of the form HS-center dot center dot HCN rather than CN-center dot center dot H2S or an intermediate structure. The requirement for proton transfer before dissociation and curvature along the reaction path impedes the CN- + H2S product channel.