Why are methylenecyclopropane and 1-methylcylopropene more ''strained'' than methylcyclopropane?

Ab initio calculations have been performed in order to investigate why the introduction of each trigonal center into a cyclopropane ring results in an increase in strain energy of 12-14 kcal/mol. Our calculations find that, as is commonly believed, introduction of a trigonal center into a three-membered ring does create some additional angle strain. However, our computational results show that the major source of the additional ''strain'' that results from the introduction of each trigonal center into cyclopropane is not an increase in angle strain but the loss of a very strong cyclopropane C-H bond.