Why does formation of 1,3-dimethylenecyclobutane-2,4-diyl from 1,3-dimethylenecyclobutane show a large positive deviation from bond enthalpy additivity?

Abstract

B3LYP/6-31G∗ calculations have been performed in order to better understand the reasons for the large positive deviation from bond enthalpy additivity (BEA), found by Hill and Squires, for formation of triplet 1,3-dimethylenecyclobutane-2,4-diyl (2) from 1,3-dimethylenecyclobutane (3). The calculations find that, in contrast to the formation of diradical 2 from diene 3, formation of 2 from 1-methyl-3-methylenecyclobutene (5) has a negative deviation from BEA of −0.5kcal/mol. Our analysis of the differences between these two pathways to 2 shows that about a third of the large positive deviation from BEA in the formation of diradical 2 from diene 3 comes from the fact that the double bonds in 3 are not conjugated. The other two thirds of the deviation is due to the fact that, although “antiaromatic” resonance structures can be written for diradical 2, the symmetry of one of the singly occupied MOs prevents such structures from making any contribution to the ground state of 2.