Substituent effects on the lowest singlet state of planar trimethylenemethane

Abstract

The effect of substituents on planar trimethylenemethane in its singlet ground state is discussed. Substituents can considerably lift the degeneracy between the ¹A₁ and ¹B₂ configuration. The energy difference between planar and bisected geometry can be reduced by (a) elongation of the π-chain (while maintaining the feature of an alternant hydrocarbon) and (b) cyclic substitution. According to route (a), the exchange integral between the degenerate molecular orbital will be reduced. In route (b) substituents depress the ¹A₁ configuration lowest in energy more in the planar geometry than the singlet configuration ¹B₁ for the orthogonal geometry. As a consequence, unsubstituted trimethylenemethane has an upper limit for the rotational barrier.