Theoretical study of cyclopropenones and cyclopropenethiones: decomposition via intermediates

The mechanism of the decomposition of cyclopropenones and cyclopropenethion es giving acetylenes (R-1-C=C- R-2 with R-1=R-2=H, F) plus carbon monoxide and carbon monothioxide, respectively (C=X, X=O, S) has been probed using D FT (B3LYP/6- 311G(d,p)) and MO (CCSD(T)/6-31G(d)) calculations. It turns ou t that the decomposition is stepwise, forming first an intermediate which h as the properties of a semi-carbene, semi-zwitterion, whose structure is be st described in both cases as a resonance hybrid between a carbene and a zw itterion. Using the PCM model, study of the solvent effect on the reactions in three continua (water, acetonitrile and benzene) shows that solvents do not affect the shape of the potential energy surfaces but tend to stabiliz e all the isomers. Estimation of the first vertical excited states by CIS a nd TD-B3LYP methods suggests that the photochemical reactions are likely to take place in the ground state rather than in an excited state. Hardness a nd polarizability profiles along the reaction paths show that there is a ma ximum in the polarizability profile besides an inverse relationship between hardness and polarizability. Fluorine substitution tends to stabilize the intermediate making it a genuinely detectable moiety.