ab initio Calculations on Organic Ion Structures

Abstract

The role of hydrogen bridging in liquid and crystal structures is a wellestablished fact in chemistry. There has been a multitude of theoretical studies of hydrogen-bonded complexes such as water dimers, hydrogen halide dimers as well as higher clusters of various composition. In a number of cases, good agreements with experimental binding energies have been achieved.[1] However, only a few examples are presently known of hydrogen bridged structures in gas phase ion chemistry. Recent experimental studies, for example, were able to verify the existence of stable H-bridged [CH₂ = CHOH/ CH₃OH]+· and [CH₂ = CHOH/H₂O]+· complexes in the mass spectrometer.[2] Their remarkable thermochemical stability towards decomposition leads to the tempting suggestion that these species may play a more important role in gas phase ion chemistry than anticipated in the past. Thus, the determination of stabilities, equilibrium geometries, and the chemistry of hydrogen-bridged ion-dipole complexes provide a challenging field for both experimentalists and theoreticians. In particular, there remains some debate as to what level of theory is adequate for the description of these unconventional structures and their energetics. Basis set effects on equilibrium geometries, the role of electron correlation effects, and the need of corrections for basis set superposition errors have been discussed on a workshop conducted at the NATO ASI conference in Les Arcs, 1986.