Density functional studies of the stepwise substitution of pyrrole, furan, and thiophene with BCO.

The structures, stabilities, and aromaticities of a series of (BCO) n (CH)₄-n NH (n = 0-4), (BCO) n (CH)₄-n O (n = 0-4), and (BCO) n (CH)₄-n S (n = 0-4) clusters were investigated at the B3LYP density functional level of theory. The most stable positional isomers of the individual clusters were obtained. All of the calculated CO binding energies were exothermic, suggesting that these BCO-substituted species are stable. Calculated differences in strain energy between the BCO-substituted structures and their corresponding hydrocarbon clusters were all exothermic, indicating that the BCO-substituted structures are less strained. The negative nucleus-independent chemical shift (NICS) values obtained show that these BCO-substituted clusters are aromatic compounds, in good agreement with the aromaticities of the corresponding hydrocarbon species. To aid further experimental investigations, CO-stretching frequencies were also computed.