Mid- and Near-IR Electronic Transitions of P⁺: New Probes of Resonance Interactions and Structural Asymmetry in Reaction Centers

Abstract

Semiempirical molecular orbital treatments provide a useful framework for relating the reaction center’s structure to its unusual spectroscopic properties and to the interaction matrix elements that govern the rates of electron transfer. Such treatments encounter a number of obstacles, but perhaps the most difficult of these is to evaluate the interactions of atoms that are relatively far apart, where interatomic resonance integrals are not well calibrated. Alternative approaches to this problem have led to diverging opinions concerning the resonance interactions that mix intramolecular transitions with charge-transfer transitions of the special pair of BChls.* Many of the RC’s spectroscopic properties can be rationalized either by strong interactions with a charge-transfer state that lies considerably higher in energy than the Q_y transitions of the BChls, or by weaker interactions with a state that lies closer in energy.^1–3