Collisional stabilization of ion-molecule association complexes in He, H2, or N2 buffer gases

Abstract The role of collisional energy transfer in ion-molecule association reactions is analyzed. Rate constants for the formation of adducts between HCl and NO3-(HNO3)x(H2SO4)y or HSO4-(HNO3)x(H2SO4)y (with x = 0 – 2 and y= 0 – 2) in the buffer gases H2, He, and N2 and at temperatures between 150 and 300 K are considered. Quantum-chemical calculations of molecular parameters and statistical unimolecular rate theory are combined to model low-pressure rate constants whereas ion-molecule capture theory provides high-pressure rate constants. The comparison with experimental results indicates that energy transfer is dominated by overall collision numbers while weak-collision effects are only of minor importance. On the other hand, often neglected falloff effects between termolecular and bimolecular reaction behavior have to be taken into account.