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

Abstract

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)Π(u) electronic state of NC(4)N(+) 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 NC(4)N(+) was investigated by density functional (B3LYP) and complete active space self-consistent field-multi-reference configuration interaction (CASSCF-MRCI) methods. The bending vibrational frequencies ω(T1) = 558 cm(-1), ω(T2) = 266 cm(-1), ω(C1) = 459 cm(-1), and ω(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.

Vibronic structure of the2Πuground electronic state of dicyanoacetylene cation revisited by PFI-ZEKE photoelectron spectroscopy andab initiocalculations

“…The bond distances are found to be converged within 10 −4Å for both VnZ and AVnZ basis set series. UB3LYP/AVTZ and MRCI/AVTZ values calculated for X + by Ranković et al [38] differ by up to 7.10 −3Å from our CASPT2/ AVTZ results, but the convergence is not yet achieved with the AVTZ basis set.…”

“…The splitting is found to be insensitive to the addition of diffuse [38]. In their paper, they obtained another value of −49.99 cm −1 by excluding the 3π u MO from the active space but, at this time, the former calculated value was in better agreement with the only available experimental value of Sinclair et al [29] (−45(10) cm −1 ), and it was thus preferred.…”

“…The latter quantity, estimated to be of −217.4 cm −1 , has been derived from the experimental values, from Refs. [13,15] for X and from this work for X + (the missing ν + 6 value has been replaced by the UB3LYP harmonic frequency [38], see below). The convergence is clearly not yet completely achieved with the largest basis sets (V6Z and AV5Z), but the extrapolations of both basis set series lead to CBS values (94,651 and 94, 655 cm −1 , respectively) which differ by only 4 cm −1 .…”

“…Thus, its vibronic structure is affected by Renner-Teller (RT) and spin-orbit (SO) interactions. To our knowledge, the only attempt to describe these interactions is the theoretical work of Ranković et al [38]. They used a second-order perturbation theory formulation specifically developed for describing the small-amplitude bending vibrations of electronic states of six-atomic linear molecules in the framework of the harmonic approximation [40].…”

“…Small hydrocarbon chains have been proposed as good candidates for DIBS [24] and several studies have been carried out to study the vibronic structure of C 4 N + 2 by fluorescence [25][26][27], IR and visible absorption spectroscopy [28,29], as well as by photoelectron spectroscopy (PES) [27,[30][31][32] and mass spectrometry [33]. Up to now, only few calculations are available about the electronic and vibrational structure of the C 4 N + 2 cation [34][35][36][37][38][39]. The ground electronic state of this cation is a 2 u state.…”