Multireference Ab Initio Study On The Low-lying Excited States Of Isoselenocyanic Acid, HNCSe

The chemistry of selenium continues to receive much attention, due to their role inatmospheric, environment, and biological chemistry.[H, C, N, Se] are expected to bekey intermediates in producing selenium containing heterocyclic compound. Amongthe [H, C, N, Se] system, HNCSe is the most stable molecule and is now relative wellknown. The gaseous HNCSe was first synthesized by heating up the mixture ofAgNCSe and HBr. Isoselenocyanic acid (HNCSe) and its isomers [H, C, N, Se] showthe similar chemical activity to halides. These chemical characteristics are widelyused in many fields such as organic substituent and inorganic pseudohalide ligands.Multireference ab initio methods are used to study the ground and the low-lyingelectronic states of HNCSe and its cation and anion in the article. This study is helpfulto the understanding of the photochemistry properties of HNCSe. The main contentsare summarized as follows:In Cssymmetry, we research the the geometric parameters, harmonic vibrationalfrequencies, vertical transition energies, oscillator strengths and spin-orbit interactionsof the ground and excited states of HNCSe at CASPT2/ANO-RCC-VTZP level. Thebonding characteristics in the low-lying excited states were analyzed on the basis ofthe valence molecular orbitals. The active space is consisted of16active electrons and13active orbitals. Comparing the geometries and harmonic vibrational frequencies ofHNCSe at different levels (CASPT2/ANO-RCC-VTZP andCASSCF/ANO-RCC-VTZP), the calculated results at CASPT2/ANO-RCC-VTZP level are consistent of experimental values. The CASPT2calculations show that theground state of HNCSe in the gas phase is the singlet state X1A’, which has atrans-bent planar structure. We determined the spin-free states of HNCSe whenvertical excitation energy is lower than6.85eV. The lower excitation energetic regionof HNCSe is formed by transferring electronic density from the selenium atom to themixture of antibonding orbitals, π*(Se-C-N). In the Frank-Condon (FC) region, below5.00eV there are six states, that is, X1A’,13A’,13A",11A",21A’, and23A’. Therefore,the characters of these electronic states were considered in the present study. Thevertical excitation energies of the singlet excited states computed by us are297.6,263.1,230.6,207.0,200.6,182.1nm, respectively, which are in agreement with thevalues (307.0,272.7,231.7,205.8,196.1,185.1nm) reported by V r s. Bycomparing the oscillator strengths, the41A’←X1A’ transition is the most intenseamong these transitions. According to the Mulliken population analysis of the13A,21A’,13A", and11A" states, the3-center4-electron/3-center5-electron systems pullselectron density to HNC moiety, creating nearly zero charge on the Se atom.The ground state of HNCSe+ion and the ground state and excited states of HNCSe-are investigated with the same basis set and methods as HNCSe. The calculatedresults show that the ground-state of HNCSe+, and the existence of bound excitedanion states have been found for the first time in HNCSe-. The extra electron in the14A’(0.7393H) and14A"(0.8458H) states resides mainly on the positive end of the dipoleof the HNCSe, hydrogen atom. Electron affinity of HNCSe is predicted for the firsttime and its value is-0.06eV.