Conventional strain energy in dimethyl-substituted cyclobutane and the gem-dimethyl effect.

The gem-dimethyl effect is the acceleration of cyclization by substituents in the chain and is often used in organic synthesis as a ring-closing effect. Calculations on cyclobutane, methylcyclobutane, and 1,1-dimethylcyclobutane are performed. 1,1-Dimethylcyclobutane is a four-membered carbon ring with gem-dimethyl substituents. Optimum equilibrium geometries, harmonic vibrational frequencies, and corresponding electronic energies are computed for all pertinent molecular systems using SCF theory, density functional theory (DFT), and second-order perturbation theory (MP2) with two triple-zeta quality basis sets, 6-311G(d,p) and 6-311G+(2df,2pd). Additional single-point calculations are performed using the optimized MP2/6-311G+(2df,2pd) geometries and coupled-cluster theory including single and double excitations and noniterative, linear triple excitations (CCSD(T)). Calculations indicate that 1,1-dimethylcyclobutane is more than 8 kcal mol-1 less strained than cyclobutane, that is, there is at least some thermodynamic component to the gem-dimethyl effect.