| Abstract
| - The effect of substitutents on the strain energy (SE) of cyclic molecules is examined at the CBS,G2, and G2(MP2) levels of theory. Alkyl substitutents have a meaningful effect upon the SE ofsmall ring compounds. gem-Dimethyl substitution lowers the strain energy of cyclopropanes,cyclobutanes, epoxides, and dimethyldioxirane (DMDO) by 6−10 kcal/mol relative to an unbranchedacyclic reference molecule. The choice of the reference compound is especially important for geminalelectronegative substitutents. The SE of 1,1-difluorocyclopropane is estimated to be 20.5 kcal/molrelative to acyclic reference molecule 1,3-difluoropropane but is 40.7 kcal/mol with respect to thethermodynamically more stable (ΔE = −20.2 kcal/mol) isomeric reference compound 2,2-difluoropropane. The SE of dioxirane (DO) is estimated to be ≈18 kcal/mol while the SE of DMDO ispredicted to be ≈11 kcal/mol by using homodesmotic reactions that maintain a balanced groupequivalency. The total energy (CBS-APNO) of DMDO is 2.6 kcal/mol lower than that of isomeric1,2-dioxacyclopentane that has an estimated SE of 5 kcal/mol. The thermodynamic stability ofDMDO is a consequence of its relatively strong C−H (BDE = 102.7 kcal/mol) and C−CH3 (BDE =98.9 kcal/mol) bonds. By comparison, the calculated sec-C−H and -C−CH3 G2 bond dissociationenergies in propane are 100.3 and 90.5 kcal/mol.
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