Theoretical evaluation of the organocatalytic behavior of the negatively charged carbon atom in a fused five-member ring in carbon dioxide transformation to methanol

In this article, the efficiency of five kinds of the carbon-centered organocatalysts in the CO2 transformation to methanol was investigated. Each organocatalyst has a negatively charged carbon atom in a five-member ring, whose nucleophilicity character plays an important role in the CO2 transformation. Theoretical results showed that the local nucleophilicity index (Nk) values of the carbon atoms, in the studied organocatalysts, are different and organocatalyst 5 having the highest Nk value, not only has the lowest activation Gibbs free energy in the CO2 activation step but also is more favorable, thermodynamically. The minimum value of the molecular electrostatic potential demonstrates that the negatively charged carbon atom is more potent than the other atoms in the nucleophilic attack. The contribution of the lone pair electrons of the carbon atom in the aromaticity character of the five-member ring is the main factor in the nucleophilicity of carbon atom. The nucleus-independent chemical shifts (NICS)-XY scan analysis illustrated that five-member ring of organocatalyst 5 has the lowest aromaticity character. Therefore, its lone pair electrons have a minimum participation in aromaticity, which makes it favorable in the reaction. Finally, organocatalyst 5 is proposed as a better catalyst, which is in agreement with the experiments.