Experimental and Theoretical Determination of Dissociation Energies of Dispersion-Dominated Aromatic Molecular Complexes

The dissociation energy (D-0) of an isolated and cold molecular complex in the gas-phase is a fundamental measure of the strength of the intermolecular interactions between its constituent moieties. Accurate D-0 values are important for the understanding of intermolecular bonding, for benchmarking high-level theoretical calculations, and for the parametrization of force-field models used in fields ranging from crystallography to biochemistry. We review experimental and theoretical methods for determining gas-phase D-0 values of M.S complexes, where M is a (hetero)aromatic molecule and S is a closed-shell solvent atom or molecule. The experimental methods discussed involve M-centered (S-0 -> S-1) electronic excitation, which is often followed by ionization to the M+.S ion. The D-0 is measured by depositing a defined amount of vibrational energy in the neutral ground state, giving M double dagger.S, the neutral S-1 excited state, giving M*.S, or the M+.S ion ground state. The experimental methods and their relative advantages and disadvantages are discussed. Based on the electronic structure of M and S, we classify the M.S complexes as Type I, II, or III, and discuss characteristic properties of their respective potential energy surfaces that affect or hinder the determination of D-0. Current theoretical approaches are reviewed, which comprise methods based on a Kohn-Sham reference determinant as well as wave function-based methods based on coupled-cluster theory.