Formation and isomerization mechanisms of M +CO 2− ( M = Li, Na) ion-pair complexes. An ab initio MO and RRKM study

Abstract

Structures and electronic states of the M +CO 2 − ( M = Li and Na) ion-pairs have been investigated by means of ab initio MO calculations. The potential energy surfaces (PESs) for the ion-pair formation reaction ( M + CO 2 → M + CO 2 −) show that the ground state MCO 2 complex has a van der Waals (vdW) character (M · CO 2) at the entrance region in the reaction and an ionic one (M +CO 2 −) at short MC distances and that the activation barrier is found at the intermediate region between the vdW and ion-pair states. The wavefunctions of the ion-pair for the low-lying excited states are obtained by the multireference single and double excitation configuration interaction (MR-SD-CI) method. The first and second electronic excitations are characterized by charge-transfer transition from CO 2 − to Na + and local excitation within the CO 2 − moiety in M +CO 2 −, respectively. In order to elucidate the isomerization mechanism of the Li +CO 2 − ion-pair from C s to C 2 v structures, which was observed by Kafafi et al. (J. Am. Chem. Soc., 105 (1983) 3886), the Rice Ramsperger-Kassel and Marcus (RRKM) theory is applied to calculate the microcanonical reaction rate along the reaction coordinate. The calculations strongly support their experimental results in which the reaction Li + CO 2 −( C s ) → Li + CO 2 − ( C 2 v ) occurs preferentially on IR irradiation.