The electronic structure of CH2 and the cycloaddition reaction of methylene with ethene

Abstract

Spin-coupled calculations are performed for the lowest ³B₁, ¹A₁ and ¹B₁ states of CH₂. The orbital picture for the ¹A₁ state is very different from that from molecular-orbital theory. This has important consequences for the stereospecificity of the cycloaddition reaction with ethene to form cyclopropane, for which we consider two paths on the potential surface. Spin-coupled valence-bond (non-orthogonal CI) calculations are also reported for the ³B₁ and ¹A₁ states. We use the same list of 202 spatial configurations for both states and obtain a singlet–triplet splitting of 12.11 kcal mol^–1, which compares favourably with a recent full CI value (11.97 kcal mol^–1) in the same basis set. The same list of structures with a more extended basis set gives a value of 9.6 kcal mol^–1, which is in very good agreement with experiment. The spin-coupled valence bond approach provides a clear and coherent description of the low-lying states of methylene and of its addition reaction with ethene.