Heavier alkali-metal monosulfides (KS, RbS, CsS, and FrS) and their cations

Abstract

The heavier alkali-metal monosulfides ͑KS, RbS, CsS, and FrS͒ have been studied by high-level ab initio calculations. The RCCSD͑T͒ method has been employed, combined with large flexible valence basis sets. All-electron basis sets are used for potassium and sulfur, with effective core potentials being used for the other metals, describing the core electrons. Potential-energy curves are calculated for the lowest two neutral and cationic states: all neutral monosulfide species have a 2 ⌸ ground state, in contrast with the alkali-metal monoxide species, which undergo a change in the electronic ground state from 2 ⌸ to 2 ⌺ + as the group is descended. In the cases of KS, RbS, and CsS, spin-orbit curves are also calculated. We also calculate potential-energy curves for the lowest 3 ⌺ − and 3 ⌸ states of the cations. From the potential-energy curves, spectroscopic constants are derived, and for KS the spectroscopic results are compared to experimental spectroscopic values. Ionization energies, dissociation energies, and heats of formation are also calculated; for KS, we explore the effects of relativity and basis set extrapolation on these values.

Electronic and Vibrational Spectroscopy of CsS

“…For instance, the harmonic frequency of 1 2 Σ + jumps from ω e ∼ 213 cm –1 up to an unreasonable value for 2Ω 1/2 (of ω e ∼ 406 cm –1 , not shown in Table ), which is associated with an unphysical very large value of the anharmonic term (ω e x e ∼ 57 cm –1 , not shown in Table ). These findings are in line with the analysis of Lee and Wright . Since there are no experimental measurements for these states, we list in Table the pattern of vibrational levels for v ≤ 12 deduced from variational calculations.…”

“…For instance, alkali-metal monoxides exhibit a change in the electronic ground state as they descend the alkali column: LiO, , NaO, and KO have a 2 Π ground state, while RbO and CsO possess a 2 Σ + ground state. For alkali-metal monosulfides, Lee and Wright established a 2 Π ground state whatever the nature of the alkali . Our results on the two lowest 2 Σ + and 2 Π states of CsS confirm these findings, as shown in Figure by the MRCI+Q/aug-cc-pV5Z potential energy curves along the Cs–S bond length ( R CsS ).…”

“…Since alkali-metal monoxides are commonly found in many high-temperature systems, they have been the subject of many studies, both experimental and theoretical. − However, their sulfur isovalent analogues, i.e., alkali-metal monosulfides (MS), were investigated, , but not to the same extent; consequently, there is a lack of information on these molecules. For instance, spectroscopic data on CsS, of interest in the present study, are limited to the determination of the nature of its lowest lying electronic states by Partridge et al and by Lee and Wright . They deduced spectroscopic constants for the ground state, of 2 Π symmetry, and for the first close-lying excited state of 2 Σ + symmetry.…”

“…For instance, spectroscopic data on CsS, of interest in the present study, are limited to the determination of the nature of its lowest lying electronic states by Partridge et al 17 and by Lee and Wright. 18 They deduced spectroscopic constants for the ground state, of 2 Π symmetry, and for the first close-lying excited state of 2 Σ + symmetry. Both states are coupled by spin−orbit interactions.…”

Study of strong Σ–Π and spin–orbit vibronic coupling effects in linear triatomic molecules

“…This combination of parameters leads to the interesting situation where the nondegenerate 2 R state lies between the two SO components of the 2 P state. This situation may arise for excited states of molecules containing relatively heavy atoms, as is well known for diatomic molecules [46]. Fig.…”