Type-I dyotropic reactions: understanding trends in barriers.

To understand the factors that control the activation barrier of type-I 1,2-dyotropic reactions (X-EH(2)-CH(2)-X*→X*-EH(2)-CH(2)-X, with E=C and Si, X=H, CH(3), SiH(3), F to I) and trends therein as a function of the migrating groups X, we have explored ten archetypal model reactions of this class using relativistic density functional theory (DFT) at ZORA-OLYP/TZ2P. The main trends in reactivity are rationalized using the activation strain model of chemical reactivity, which had to be extended from bimolecular to unimolecular reactions. Thus, the above type-I dyotropic reactions can be conceived as a relative rotation of the CH(2)CH(2) and [X···X] fragments in X-CH(2)-CH(2)-X. The picture that emerges from these analyses is that reduced C-X bonding in the transition state is the origin of the reaction barrier. Also the trends in reactivity on variation of X can be understood in terms of how sensitive the C-X interaction is towards adopting the transition-state geometry. A valence bond analysis complements the analyses and confirms the picture emerging from the activation strain model.