Applications of potential energy calculations to organic chemistry. Part 17. Calculations of highly strained cyclopropane derivatives. Evaluation of several solar energy storage systems involving valence isomerization of cyclopropane rings

Abstract

Available computational techniques were critically evaluated for their ability to reproduce and predict heats of valence isomerization of highly strained cyclopropane derivatives. Allinger's MM2 cyclopropane parameters were examined for diverse structures including bicyclo[n.1.0]alkanes, spiropentanes, and bicyclopropyls. For the bicyclopropyls, a new parameter set was introduced. Experimental structures and energies of these cyclopropane derivatives were well reproduced by MM2 calculation with these parameters. The standard deviation of the enthalpy calculation for 12 compounds was 0.63 kcal mol^–1. Quadricyclane (1) turned out to be the most difficult to calculate. Neither MM2, MNDO, nor ab initio calculations with a minimal basis set and gradient geometry optimization reproduced the heat of isomerization of (1) to norbornadiene (20). A homologous isomerization, tetracyclo[4.2.0^2,8.0^5,7]octane (23) into bicyclo[2.2.2]octadiene (24), is predicted to produce an enthalpy of 30 kcal mol^–1, 10 kcal mol^–1 larger than that of the familiar reaction of (1) to (20). Two other isomerizations, (1R,3R)-tricyclo[3.2.1.0^1,3]oct-6-ene (25) to 5-methylenenorbornene (26), and (1R,2R)-tricyclo[4.2.1.0^1,3]non-7-ene (27) to 5-methylenebicyclo[2.2.2]oct-2-ene (28), are predicted to give ΔH^‡ 26 kcal mol^–1. Mm2 is the most convenient method to calculate isomerization of cyclopropane derivative excepting quadricyclane.