Theoretical approaches of the isomerization mechanism of GaSO isomers in doublet states at density functional theory levels

Abstract

The geometries and the harmonic vibrational frequencies for several GaSO doublet state isomers have been predicted at density functional theory levels with a 6-311+G* basis set. Results have indicated that there are five doublet states (two cyclic, two linear and one bent), but only four of them keep S–O bond linkage. The ground state corresponds to a cyclic 2 A ″ structure. The state–state energy separations of the low-lying excited states with a significant S–O linkage relative to the 2 A ″ ground state are 3.2 (linear GaOS( 2Σ )), 12.9 (linear SGaO( 2Σ )), 87.9 (cyclic 2 A ′) and 21.9 (bent 2 A ′) kcal/mol above, respectively. The GaSO species in the ground state ( 2 A ″) should be classified as the thiosuperoxide, while some other isomers exhibit different thiosuperoxide character. But they are not as ionic as the alkali metal superoxides and thiosuperoxides. The excited state without the S–O linkage possesses a linear structure and should be classified as the thio-oxide. The state–state correlation and the isomerization mechanism are also predicted by searching the transition states and the intrinsic reaction coordinate analysis. Results have indicated that the ground state 2 A ″ may directly correlate with two linear structures (GaOS( 2Σ ) and SGaO( 2Σ )) by experiencing a TS3 with a barrier of 4–6 kcal/mol and a TS5 with a barrier of ∼56 kcal/mol. Due to the unstability of the cyclic 2 A ′ state, its isomerization transition state (TS1) to the linear GaOS( 2Σ ) is not found. The isomerization between the linear GaOS and SGaO via a 2 A ′ pathway may be proceeded by experiencing a TS4 with a barrier of 45–48 kcal/mol. These height barriers should be attributed to the S–O bond rupture in proceeding from the linear GaOS( 2Σ ) or the cyclic 2 A ″ ground state to the linear SGaO( 2Σ ) structure. The bent 2 A ′ state may correlates with the cyclic 2 A ′ state (or linear GaOS( 2Σ ) state) and the linear SGaO( 2Σ ) state by undergoing TS2 and TS6 transition states, respectively, but these two isomerization pathways of the bent GaSO( 2 A ′) are energetically unfavorable due to the very high activation barriers being ∼85 kcal/mol (TS2) or 56–60 kcal/mol (TS6).