Theoretical studies on structural properties and electron transfer reactivity of the C 4H 4O/C 4H 4O + furan coupling pair

Abstract

The geometries and vibrational frequencies of furan, furan cation and their three encounter complexes have been determined using density functional theory and/or ab initio methods employing 6-311+G ∗ basis set. The frequency analysis for furan and furan cation and the assignments of the vibrational modes with notable infra-red (IR) intensity have been performed. The natural bond orbital analysis has been carried out for the studying of the bonding property of the complexes. The two rings of the encounter complex 1 (EC.1) are connected by two atoms, C 3 and C 12, which belong to two different rings, separately. There exists weak interaction between the two rings of encounter complex 2 (EC.2), whose positions are specially interlaced. The positions of the two rings of encounter complex 3 (EC.3) are especially parallel. The frequency analysis has indicated that the former two encounter complexes are stable, but the third (EC.3: C 2v , 2A 2) has one small imaginary frequency. Their stabilization energies are calculated to be 18.63(EC.1: C 1 ), 19.06(EC.2: C 1 ) and 19.78(EC.3: C 2v , 2A 2) kcal mol −1 at UB3LYP/6-311+G ∗ level with the correction of basis set superposition error and zero-point vibrational energy. The activation energy, the electronic coupling matrix element and the electron transfer (ET) rate of these coupling systems at the equilibrium states have been determined using UB3LYP and UMP2 methods at 6-311+G ∗ basis set level in this paper. ET rates calculated by B3LYP method are bigger distinctly than the ET rates obtained using MP2 method. These two methods have been proven to be reliable one in calculating the ET mechanism for this system at the equilibrium state. It should be noted that it is not always true that the ET must takes place via the most stable encounter complex mechanism. For the C 4H 4O/C 4H 4O + system, while the stabilization energy of Complex 2 is bigger than that of Complex 1, the ET rate via Complex 2 is lower than that of Complex 1. ET rate via EC.3 is almost as 2.2/1.7 times as the rate via EC.2 obtained using UB3LYP/UMP2 method.