Theoretical study on stability and nonlinear optical properties of tetrahydropyrrole diradical and its isoelectronic systems in different electronic states

The polarizabilities and hyperpolarizabilities of the tetrahydropyrrole diradical in different electronic states have been investigated using ab initio and density functional theory (DFT) methods combined with the finite field (FF) approach. The polarizability average value alpha (s) is a maximum for the singlet state, while that for the closed-shell is a minimum. The trend in second hyperpolarizability average value gamma is in good agreement with that for alpha (s). The gamma values of the singlet and triplet states are, respectively, about 3 and 2 times larger than that of the closed-shell. The order of the first hyperpolarizability total effective value beta (tot) is beta (tot (closed-shell)) > beta (tot (singlet)) > beta (tot (triplet)). The alpha (s), beta (tot), and gamma values of different electronic states obtained using the B3LYP and MP4SDQ methods are close to those obtained using the reliable CCSD method. The nonlinear optical (NLO) properties of two systems isoelectronic with the tetrahydropyrrole diradical - cyclopentane and tetrahydrofuran diradicals - show that the polarizabilities and hyperpolarizabilities of these systems are all smaller than those of the tetrahydropyrrole diradical in the three electronic states.