Study of intramolecular electron transfer with a two-state model based on the orbital deletion procedure

Abstract

Carbocations H2C-bridge-CH2+ are often used as models for donor-bridge-acceptor complexes to study the role of bridges in the electron transport process. In an attempt to delineate the electron delocalization effect from the bridge to the positively charge terminal in the unrestricted Hartree–Fock (UHF) wave functions which are often used for diabatic states to compute the electronic coupling energy, we propose to employ an orbital deletion procedure (ODP) to generate the strictly localized wave functions for the initial (A) and final (B) diabatic states in the electron transfer process in the carbocations of H2C-bridge-CH2+. The electronic coupling energy VAB can be subsequently computed with the two diabatic states by solving a 2×2 secular equation. The comparison of our results with previous theoretical studies based on the widely adopted charge-localized UHF wave functions and Koopmans’ theorem in the case of positively charged 1,3-dimethylenebicyclo[1.1.1]pentane reveals that charge-localized UHF wave functions overestimates the electronic coupling VAB compared with our method and the Koopmans’ theorem. A further study incorporating four water molecules suggests that the aqueous solution has very limited effect on VAB in the positively charged 1,3-dimethylenebicyclo[1.1.1]pentane. To demonstrate the applicability of the current two-state model based on the ODP strategy, we also examined the electron transport across strain-free linear alkyl chains (CH2)n (n=1–8) and linear π-conjugated bridges (CH=CH)n (n=1–5).