Investigation by the MNDO MO Method about the Paths for the Reactions between Hydrogen Sulfide and Hydrogen Atom

Abstract

The parallel paths (1) and (2) observed for the initial reaction between thiols and hydrogen atom have been investigated theoretically by the MNDO MO method.
RSH+H→R+H₂S(1)
→RS+H₂ (2)
Calculations were performed on the simplified reaction system H₂S + H. Pyramidal complex model ( I ) was assumed. Linear complex model (II) was also taken into account for reaction ( 2 ) (Figs.1 and 2). H-S-H-H(I)H-S-H-H (II)
Three characteristic features of the potential energy surface emerge in I as the attacking hydrogen atom approaches the sulfur atom. Firstly, the reaction system crosses a low energy barrier (4.6 kJ/mol), then, goes down to a deep potential well, and again goes over a high energy barrier (68.6 kilmol). The first barrier corresponds to the initial approach of the attacking hydrogen atom to the sulfur atom (Figs.3 and 4). The deep potential well corresponds to trivalent intermediate H₃S, which has Csymmetry and is 48.5 kJ/mol more stable than ELS+ H (Fig.5). The stability of H₃S is estimated to be comparable to that of H₄S (Fig.6). The potential energy change due to the abstraction of hydrogen atom from H₃S is the same as that induced by the approach of hydrogen atom to H₂S (Fig.7). The second barrier corresponds to the molecular hydrogen abstraction from H₃S (Figs.8 and 9). The potential energy surface via II is found to be more simple than that of I. The hydrogen atom of hydrogen sulfide is abstracted by the attacking hydrogen atom with the barrier of 19.7 kJ/mol (Figs.10, 11 and 12). The linear complex was found to be the most stable conformation (Figs.13 and 14). Reactions ( 1 ) and ( 2 ) are concluded to take place via I and II, respectively (Fig.15).