Structures and electronic states of ion-pair complexes formed between a carbonyl compound and a sodium atom. An ab initio molecular orbital and MRSDCI study

Abstract

The geometric structure and electronic state of ion pairs, formed by an electron transfer from an alkali metal atom to the CO group of a carbonyl compound, have been determined using ab initio molecular orbital (MO) and multireference single and double excitation configuration interaction (MRSDCI) methods. As model systems of the ion pair, acetone–Na (AT–Na) and formaldehyde–Na (FA–Na) systems were chosen. The geometry optimizations of the FA–Na ion pair gave two structures as the stable form. One is a linear form with a CO—Na angle of 168.7° and the other is the π-form with an angle of 87.6°. The total energies of both ion pairs are similar, although the energy calculation shows that the π form is slightly more stable than the linear form. On the other hand, the geometry optimization of the AT–Na ion pair gave only a linear form for the stable structure. The MRSDCI wavefunctions of the ion pairs indicate that the interaction between the alkali metal and the carbonyl group is composed of an attractive Coulomb force [CO^–⋯Na⁺] at the ground state, whereas the nature of the bonding is determined by the weakly attractive van der Waals force [CO⋯Na] at the first excited state. From an analysis of the excited-state wavefunctions, the first absorption band of the ion pairs is assigned to the charge-transfer (CT) transition expressed by π^*_CO→ Na(3s). The solvent effects on the spin density of the carbonyl oxygen in a 2-methyltetrahydrofuran (MTHF) matrix were also discussed on the basis of the fractional charge model.