Theoretical studies on the contact distance dependence of the electron transfer reactivity of the ClO/ClO+ coupling system

Abstract

The structures and properties of ClO, ClO⁺ and their coupling system are studied with ab initio (HF and MP2) and density functional theory (DFT: B3LYP, B3P86, B3PW91) employing the 6-311+G(3df) basis set. Results indicate that there are five possible stable coupling complexes, which correspond to generous minima on the global potential energy surfaces (PES). The most stable coupling complex is EC.3 (C_s, ²A′) in which there is one O–O linkage and two anti-disposed Cl atoms. The stabilization energies are calculated to be 35.78 (EC.1: C_2h, ²B_g), 58.34 (EC.2: C_2v, ²A₂), 61.72 (EC.3: C_s, ²A′), 59.07 (EC.4: C_2h, ²B_g) and 25.21 (EC.5: C_2h, ²A_g) kcal·mol⁻¹ at the B3LYP/6-311+G(3df) level with correction of the basis set superposition error (BSSE); the stability order of these encounter complexes is EC.3 > EC.4 > EC.2 > EC.1 > EC.5. On the basis of the five encounter complexes, five coupling modes are designed for the electron transfer reactivity of this system, in which the contact distance between the two directly linked atoms is defined as the reaction coordinate. The dissociation energy curves at the activated states and the corresponding activation energies of these five coupling modes are obtained and also are compared at the B3LYP/6-311+G(3df) and MP2/6-311+G* levels. The inapplicability of DFT methods in predicting the energy curves, especially with long contact distances, is also discussed in this paper; the DFT methods give an abnormal behavior for the dissociation of the complexes due to the “inverse symmetry breaking” problem. On the basis of the golden rule of time-dependent perturbation theory, the electron transfer reactivity and the contact distance dependences of the various electron transfer kinetics parameters (the activation energy, the coupling matrix element, etc.) have been analyzed at the MP2(full)/6-311+G* level. The electron transfer can take place over a range of contact distances, but the most effective coupling distance perhaps corresponds to only a small range of distances. The coupling orientation analysis also indicates that the most favorable coupling mode for the electron transfer does not always correspond to the mechanism for the formation of the most stable encounter complex. Some highly energetic coupling modes perhaps favor the electron transfer reaction.