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

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-dimethylene cyclobutane-2,4-diyl (2) from 1,3-dimethylenecyclobutane (3), The calculati ons find that, in contrast to the formation of diradical 2 from diene 3, fo rmation of 2 from 1-methyl-3-methylenecyclobutene (5) has a negative deviat ion from BEA of -0.5 kcal/mol, Our analysis of the differences between thes e two pathways to 2 shows that about a third of the large positive deviatio n 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 struc tures can be written for diradical 2, the symmetry of one of the singly occ upied MOs prevents such structures from making any contribution to the grou nd state of 2. (C) 2000 Elsevier Science B.V. All rights reserved.