Spectroscopic and theoretical studies of CH_3~+-Rg_n clusters (Rg = He, Ne, Ar): from weak intermolecular forces to chemical reaction mechanisms

摘要

This review summarizes the recent combined experimental and theoretical effort of high-resolution spectroscopy, mass spectrometry and quantum chemical calculations to characterize isolated CH_3~+-Rg_n clusters (Rg + He, Ne, Ar; n ≤8). These complexesserve as a model system for the solvation of a fundamental reactive carbocation by non-polar ligands. The results provide unprecedented and detailed information about important properties of the interaction potential as a function of the interaction strength and the degree of microseolvation of the methylcation. These include the geometries and binding energies of minima and transition states, the structure of solvation (sub) shells, the competition between various types of intermolecular bonding (p bonds versus H bonds), the change in the origin of the interaction as a function of the size of the Rg atom and the degree of solvation (induction versus charge transfer), the importance of monomer relaxation, the large angular-radial coupling and zero-point effects of the potential of these prototype disk-and-ball dimers, and the significance of non-cooperative three-body effects. The studies of CH_3~+-Rg and related dimers demonstrate that the inert Rg ligands can be used as a sensitive probe of th chemical reactivity of specific orbitals in molecular ions. In addition, the Rg-CH_3~+-Rg trimers represent simple prototype intermediates in degenerate cationic nucleophilic substitution (S_N2) reactions, and the results of Ar-CH_3~+-Ar provide the first spectroscopic evidence that such reactions proceed via a double minimum potential in the gas phase. The CH_3~+-Rg_n results show that the fruitful combination of modern state-of-the-art spectroscopic and theoretical approaches provides a powerful route to the understanding of the physical and chemical properties of ion-solvent interactions at the molecular level.