Effect of Substituents on Methylenecyclobutane / 1-Methylcyclobutene System

A theoretical study was performed on the methylenecyclobutane and 1-methylcyclobutene in the gas and aqueous phases using PM3 and DFT calculations. Methylenecyclobutane was found to be more stable than 1-methylcyclobutene in the gas and aqueous phases according to PM3 calculations, and only slightly more stable than 1-methylcyclobutene in the case of DFT calculations. Also the effect of substituents (X = F, CH3, NH2, CN, NO2, CHO and CF3) was studied on the relative stabilities of these two tautomers. It was found that all the substituents increase the stability of 1-methylcyclobutene in the gas and aqueous phases. Heats of formation, and electron densities, were reported. The stability effect of X- substituents on the methylenecyclobutane and 1-methylcyclobutene was explained by Gibbs free energies calculations and confirmed by isodesmic reactions.