Theoretical investigations on bond enthalpy additivity deviations in the formation of triplet diradicals from hetero-substituted 1,3-dimethylenecyclobutanes

In our continuing effort to understand better the cause of the large positi ve deviation from bond enthalpy additivity (BEA) observed in the formation of triplet 1,3-dimethylenecyclobutane-2,4-diyl (1c) from 1,3-dimethylenecyc lobutane (1a), we performed B3LYP/6-31G* calculations on a series of 1,3-di substituted cyclobutanes, where the methylene groups in la are replaced by hetero groups, specifically O, NH, S, PH and SiH2. The size of the deviatio n from BEA (Delta BEA) is found to increase consistently with decreasing bo nd strength of the exocyclic pi bonds. DFT calculations were also carried o ut on related hydrocarbons and the Delta BEA value in each case was found t o be in close agreement with that predicted by simple Huckel theory. All of the computational results provide convincing evidence that antiaromaticity is absent in these diradicals, and the positive deviations from BEA, as la rge as 30.3 kcal mol(-1), are due to the more substantial increase in the p i electron delocalization in the monoradicals than that in the diradicals. Copyright (C) 2001 John Wiley & Sons, Ltd.