FORMATION AND ISOMERIZATION MECHANISMS OF M(-) (M=LI, NA) ION-PAIR COMPLEXES - AN AB-INITIO MO AND RRKM STUDY()CO(2)()

Structures and electronic states of the M(+)CO(2)(-) (M = Li and Na) i on-pairs have been investigated by means of ab initio MO calculations. The potential energy surfaces (PESs) for the ion-pair formation react ion (M + CO2 --> M(+)CO(2)(-)) show that the ground state MCO(2) compl ex has a van der Waals (vdW) character (M . CO2) at the entrance regio n in the reaction and an ionic one (M(+)CO(2)(-)) at short M-C distanc es and that the activation barrier is found at the intermediate region between the vdW and ion-pair states. The wavefunctions of the ion-pai r for the low-lying excited states are obtained by the multireference single and double excitation configuration interaction (MR-SD-CI) meth od. The first and second electronic excitations are characterized by c harge-transfer transition from CO2- to Na+ and local excitation within the CO2- moiety in M(+)CO(2)(-) respectively. In order to elucidate t he isomerization mechanism of the Li+CO2- ion-pair from C-s to C-2v st ructures, which was observed by Kafafi et al. (J. Am. Chem. Sec., 105 (1983) 3886), the Rice-Ramsperger-Kassel and Marcus (RRKM) theory is a pplied to calculate the microcanonical reaction rate along the reactio n coordinate. The calculations strongly support their experimental res ults in which the reaction Li+CO2-(C-s) --> Li+CO2- (C-2v) occurs pref erentially on IR irradiation.