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

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 H-2, He, and N-2 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. (C) 2020 Published by Elsevier B.V.