Electrostatic and relaxation effects in ionization of molecular dimers and intermolecular complexes

The shifts in ionization energies which occur when a molecule is incorporated as an asymmetric dimer or in an intermolecular complex are analyzed theoretically. MO ? SCF calculations with 4–31G basis sets were performed on closed- and open-shell states of (HF)₂, H₂O·HF, and their valence–hole ions, as well as on the heterodimers incorporating the higher homologues CH₃F, CH₃OH, and (CH₃)₂O. The analysis concerns the influence of electrostatic, polarization, and charge transfer effects associated with complexation on the initial molecular state of each monomer system, as well as monomer–dimer differences in the electronic relaxation mechanism considered as a final state effect in the ionization process. The calculated ionization energy shifts which agree well with the experimental data available for (CH₃)₂O·HF, show that the shifts are dominated by electrostatic effects, but some effects arising from differences in molecular size and electric polarizability of the monomers can be discerned.