Abstract
Using the graphical user interface GABEDIT and the computational chemistry software MOLPRO, the ab initio calculation method has been applied to explore 25 low-lying singlet and triplet electronic states, including the X1Σ+ ground state, of the calcium sulfide molecule CaS in the 2s+1Λ± representation. The multi-reference configuration interaction with Davidson correction (MRCI+Q) and the complete active space self-consistent field (CASSCF) calculations were performed to obtain the potential energy curves in terms of the internuclear distance R. The permanent dipole moments μe of these low-lying electronic states of CaS have been investigated, in addition to the corresponding spectroscopic constants (including the electronic energy with respect to the ground state Te, the equilibrium internuclear distance Re, the harmonic vibrational frequency ωe, the rotational constant Be, and the equilibrium dissociation energy De). In the present work, 19 new singlet and triplet CaS electronic states were investigated for the first time. In addition, it is noticeable that the current results and those already available in literature are in good agreement.
Highlights
The properties of metal sulfides have been of great interest among scientists in the domain of chemistry and other related fields due to their wide range of applications that are based on their unique metal-sulfur bond nature (Halfen, Apponi, Thompsen, & Ziurys, 2001)
As for the sites of avoided-crossing between the potential energy curves (PECs) of the electronic states, they appear in the following cases: X Σ with (2) Σ at R=5.1 Å, (2) Π with (3) Π at R=2.28 Å and R=3.72Å, (3) Π with (4) Π at R=3.75Å, (1) Π with (2) Π at R=5.67Å, (1) Π with (4) Π at R=5.31Å, (2) Δ with (3) Δ at R=3.9Å, (1) Σ with (2) Σ at R=5.07Å, (2) Σ with (3) Σ at R=2.16Å, (1) Σ with (2) Σ at R=2.91Å, (2) Π with (3) Π at R=2.22Å and R=3.09Å, (3) Π with (4) Π at R=3.69Å, (1) Π with (2) Π at R=4.11Å and (2) Π with (4) Π at R=5.43Å
Spectroscopic constants of the CaS molecule have been calculated for 18 singlet and triplet electronic states by fitting, into a polynomial in R, the potential energy values around the equilibrium internuclear distance
Summary
The properties of metal sulfides have been of great interest among scientists in the domain of chemistry and other related fields due to their wide range of applications that are based on their unique metal-sulfur bond nature (Halfen, Apponi, Thompsen, & Ziurys, 2001). In reactions involving proteins in biochemistry, metal sulfides play the role of sulfur-donating ligands (Holm, Kennepohl & Solomon, 1996) (Halfen, Apponi, Thompsen, & Ziurys, 2001) (Janczyk & Ziurys, 2006). Not many studies have been performed on the calcium sulfide molecule and they have been restricted to very few low-lying electronic states even though several theoretical and experimental investigations were done on various other alkaline-earth metal sulfides (Huber & Herzberg, 1979) (Hüttner, 2012). In 1987, a theoretical study (based on ab initio calculations) was conducted, by Partridge et al, on alkaline-earth mono-sulfides, including calcium mono-sulfide, in which 5 computational methods were used on the molecule CaS and 3 of its electronic states were reported (Partridge, Langhoff, & Bauschlicher, 1988). We were able to calculate, for 18 electronic states, the spectroscopic constants Te (the electronic energy with respect to the ground state), Re (the equilibrium internuclear distance), ωe (the harmonic vibrational frequency), Be (the rotational constant) and De (the equilibrium dissociation energy)
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