Spin-coupling properties of 2,2’-(1,2-acetylene-4,1-phenyl)bis[4,4’,5,5’-tetrahydro]imidazole- oxide radical and their isomers were investigated by using the DFT UB3LYP method at the level of 6-31G(d). Nonlinear optical (NLO) coefficients were studied at the same level combined with finite field approach. We discussed the relationship bewteen spin-coupling and nonlinear optical properties of imidazole-oxide radicals. The calculated results show that ground state spin accords with spin coupling rules. The polarizability decreased with increasing spin multiplicity. Due to molecular symmetry effects, the first hyperpolarizability changed when molecular symmetry changed. The second hyperpolarizability increased with increasing spin multiplicity. We probe into relationship between spin coupling rules and NLO activity, which provides theoretical guide for molecular design and experimental synthesis of organic radical NLO materials.

Key words: radical, spin coupling, NLO coefficients, UB3LYP method