| Isothiocyanic acid (HNCS) is a well known interstellar molecule that always draws the attention of experimentalists and theoreticians because of its various structure, stability, and chemical reactivity. As four-atomic closed-shell molecules, HNCS and its isomers show the similar chemical activity to halides. These chemical characteristics are widely used in fields such as organic substituents and inorganic pseudo-halide ligands. In addition, HNCS and its isomers are of great use acting as important intermediates of cycloaddition reactions. Due to their abundant existence in the universe, the accurate characterization data of HNCS and its isomers, such as microwave (MV) and infrared (IR) spectra, would afford an important way to explore the unknown universe.Complete active space self-consistent–field (CASSCF) and multiconfigurational second order perturbation theory (CASPT2) calculations in conjunction with the ANO-L basis set were performed to investigate systematically the low-lying electronic states of HNCS and its ions in Cs symmetry. The ANO-L basis set consists of 116 contracted basis functions with a contracted scheme of H (8s4p3d/3s2p1d), C and N (14s9p4d3f/5s3p2d1f), and S (17s12p5d4f/6s4p3d1f). HNCS and its ions are adopted by Cs symmetry which had been proved by previous experimental and theoretical studies. In order to acquire accurate configuration, the active space consists of all of 13 valence orbitals (10 a′orbitals and 3 a″orbitals), leading to 16/13 CASSCF/CASPT2 calculations (16 electrons/ 13 orbitals). The same active space was chosen for cationic states and anionic states, but the corresponding electrons decreased and rose by one (17/13 and 15/13) respectively. During the CASPT2 calculations the weight values in the first-order wave functions were all larger than 0.84. The CAS state interaction (CASSI) method was used to compute the transition dipole moments of the various excited states in the Frank-Condon region.The equilibrium structure of ground state X1A′for HNCS was predicted to be RNH = 1.009 (?), RNC = 1.206 (?), RCS = 1.591 (?),∠HNC = 134.0°, and∠NCS = 173.6°, respectively. The ground state X1A′has a dominant electronic configuration (1a′)2(2a′)2(3a′)2(4a′)2(5a′)2(6a′)2(1a″)2(7a′)2(8a′)2(9a′)2(10a′)2(11a′)2(2a″)2(12a′)2(3a″)2(13a′)0(4a″)0. The theoretical predictions of harmonic vibrational frequencies for HNCS are v1(a′)(H-N) = 3658.3 cm-1, v2(a′)(C-N) = 2015.2 cm-1, v3(a′)(C-S) = 847.8 cm-1,v4(a′)(HNC) = 581.5 cm-1, v5(a′) (NCS) = 455.3 cm-1and v6(a″) (HNC) = 486.5 cm-1, respectively. All other states show multiconfiguration character except the states 13A″, 13A′, 11A″, 21A″31A′. Our highly accurate calculation indicated that theoretically determined geometric parameters and harmonic vibrational frequencies for the ground state X1A′are in good agreement with observed experimental data. The geometry of triplet HNCS is clearly favored C1 symmetry, and the relative energy is predicted to be 3.000 eV (69.2 kcal/mol). The vertical transition energies for the selected excited states of HNCS were calculated at CASSCF/CASPT2/ANO-L level of theory based on CASSCF optimized geometry. Except for a few linear states of X2Ⅱ(12A′, 12A″), 14Σ- (14A″), and 12Σ+ (32A′) states of HNCS+, our results confirmed that the majority of excited states are twisted trans-bend structures. The existence of bound excited anion states has been found for the first time in HNCS-. A more elaborate examination of ionization potential of HNCS (AIP, VIP) than previous reports has been presented.
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