Theoretical investigation of vibronic and spin-orbit effects in the ground X 2Πu electronic state of the dicyanoacetylene cation

The aim of the present study is to predict by means of ab initio calculations the vibronic and spin-orbit structure in the X (2)Pi(u) electronic state of NC4N+ and to elucidate some details in an observed laser-induced fluorescence spectrum of this species, particularly those interpreted in terms of the bending overtones and the spin-orbit splitting. The ground electronic state of NC4N+ was investigated by density functional (B3LYP) and complete active space self-consistent field-multi-reference configuration interaction (CASSCF-MRCI) methods. The bending vibrational frequencies omega(T1) = 558 cm(-1), omega(T2) = 266 cm(-1), omega(C1) = 459 cm(-1), and omega(C2) = 113 cm(-1) were obtained. The spin-orbit coupling constant was calculated using state-average CASSCF wave functions in the framework of the MRCI method, and the value of A(SO) = -44 cm(-1) was determined. This quantity and the data for the bending frequencies and Renner parameters were employed for handling a combined effect of the vibronic and spin-orbit coupling, according to a model developed in our earlier studies. (C) 2011 American Institute of Physics. [doi:10.1063/1.3608913]