Nonbonded 1,3-diaxial interaction energies are commonly used to approximate strain energy in cyclic molecules, as values for these interactions are available. By taking the difference in nonbonded interactions for each conformer, the equilibrium enthalpy can be estimated. The strain energy for methylidenecyclohexane has been calculated to be 4.5 kcalmol using estimations for 1,3-diaxial strain (0.9 kcalmol), methyl/hydrogen allylic strain (1.3kcalmol), and methyl/methyl allylic strain (7.6 kcalmol) values.
The strain energy in 1,8-dimethylnaphthalene was calculated to be 7.6 kcalmol and around 12-15 kcalmol for 4,5-dimethylphenanthrene. Allylic strain tends to be greater for cyclic molecules compared to olefins as strain energy increases with increasing rigidity of the system. An in depth summary of allylic strain in six membered rings has been presented in a review by Johnson, F.