Computational study of CO2 reduction by amines.

Calculations at the MP2/aug-cc-pVDZ//MPWB1K/aug-cc-pVDZ level are reported for the reduction of CO2 by amines--primarily triethylamine. A polarizable continuum model is used to represent acetonitrile solvent for the reaction. Starting from a photochemically generated radical ion pair state, the mechanism of reduction is deduced to be one in which the CO2(.-) begins to use one of its oxygens to abstract a hydrogen from an alpha carbon of the amine radical cation. During this event, and before the transition state for H transfer is reached, the system encounters a surface crossing, which provides pathways for unproductive back electron transfer and for productive reduction, with the latter involving attachment of the hydrogen to the carbon of CO2. The result of the reduction is a closed-shell iminium formate ion pair, which completes the reaction by proton transfer between the ions, to give an eneamine and formic acid. On the basis of the calculations, approaches for improving the efficiency of the reduction and increasing the wavelength of the light used to drive the reaction are discussed. One of these modifications involves the use of a bicyclic amine as reductant.