Isomers of OCS and their reaction with H₂O on singlet potential energy surface

Abstract

The isomers of the carbonyl sulfide (OCS) molecule are investigated in detail at CCSD(T)/cc-pVTZ//MP2/6-311þþG(2d,2p) level of theory. One cyclic isomer was identified along with three different linear minima of the OCS molecule. Three interconversion transition states were also located between cyclic and linear forms of OCS. Among these four isomers, the singlet potential energy surface (PES) for the molecule-molecule reaction between the three most energetically favoured isomers of OCS and H 2 O has been explored theoretically at the CCSD(T)/ cc-pVTZ//MP2/6-311þþG(2d,2p) level. This singlet PES comprises of three paths. Path 1 is the reaction of linear OCS molecule with water producing the major product P1 (CO 2 þ H 2 S), minor product P2 (S þ HCOOH) and two isomers via 14 minima and 15 transition states. The Path 2 is an isomerization process in which cyclic isomer of OCS reacts with water molecule via another initial barrierless aduct producing five isomers of the OCS-H 2 O system through five interconversion transition states. The reaction of linear COS isomer with water is shown in Path 3. This path produces the radicals SH and COOH from another COS-H 2 O complex via a transition state. Among these three products, the product P1 is energetically most favoured. The overall exothermicity of the product channels for the formation of major product P1 on PES is calculated to be about 10.60 kcal/mol possessing initial high entrance barriers of 45.48 and 55.47 kcal/mol in two possible pathways. As the process is favoured thermodynamically but not kinetically, the reaction is expected to be very slow.

First infrared investigations of OCS–H2O, OCS–(H2O)2, and (OCS)2–H2O complexes isolated in solid neon: Highlighting the presence of two isomers for OCS–H2O

“…Several theoretical studies have been carried out on the 1:1 complex. [3][4][5][6][7][8][9][10] However, a disagreement exists on the different possible isomers (structures F1, F2, and F3 of Fig. 8).…”

“…3 Tatamitani and Ogata 10 Ghosh et al found the F1 and F2 structures [CCSD(T)/cc-pvTZ single point at MP2/6-311++G(2d,2p) geometries]. 6 In agreement with the work of Hartt et al 4 our MP2/AVTZ calculations lead to the three structures with D e values of 8.6, 8.9, and 8.7 kJ/mol and D 0 values of 6.6, 5.5, and 5.1 kJ/mol for F1, F2, and F3, respectively. However, the F3 structure seems to be an artefact of MP2 because all optimisations of this geometry at the higher level of theory CCSD(T)-F12a/AVTZ lead to the nearest stable structure F2.…”

“…The calculated frequencies are comparable with those given by the literature. [4][5][6][7][8] The assignments of the different vibrations to F1 and F2 isomers are also summarized in Table IV.…”

“…10 The most stable calculated structure has a C 2v symmetry, which is consistent with the experimental evidences, 10 the sulfur atom being bonded to the water oxygen; the calculated binding energy is of 5.5 kJ/mol. 4,8,9 Besides, other structures are predicted 3,4,6 with slightly weaker binding energies. For the 1:2 complex, there is no experimental data, and this species has been the subject of only two theoretical studies.…”

“…1 Several linear triatomic molecules (CO 2 , OCS, N 2 O, SeCO) complexed with water have been studied. [2][3][4][5][6][7][8][9][10][11] For the OCS-H 2 O complex, few theoretical works [3][4][5][6][7][8][9][10] have been published, and only one experimental study in the gas phase exists in the microwave domain. 10 The most stable calculated structure has a C 2v symmetry, which is consistent with the experimental evidences, 10 the sulfur atom being bonded to the water oxygen; the calculated binding energy is of 5.5 kJ/mol.…”