Theory of electron localization and its application to blue-shifting hydrogen bonds

Abstract

We propose a theory of electron localization or stabilization by electron localization through the interactions between occupied (i) and vacant (j*) orbitals under certain conditions, which have been believed so far to cause only electron delocalization. Electrons localize when the electrons redistributed by the interaction are more stable in the i-th occupied orbital than in the overlap region: h_ij* > s_ij*h_ii for s_ij* > 0. Electron delocalization occurs when h_ij* < s_ij*h_ii for s_ij* > 0. The h_ij* and s_ij*h_ii terms represent the energy of the electrons in the overlap region and the energy of the redistributed electrons in the occupied orbital, respectively. The theory of electron localization is substantiated by the correlation of the C–H bond lengths of fluorinated methanes H_4–nCF_n (n = 1, 2, 3) to the electron population of the σ_CH bonding orbital, and successfully applied to understanding blue-shifting hydrogen bonds in F₃CH⋯X (X = CO, N₂, OC, Ne, OC(CH₃)₂) and designing some proton donors, HCO₂CH₃ and hypervalent molecules HPF₄ and HSF₅, for blue-shifting hydrogen bonds.