Mechanism off the hydrogen atom abstraction from H 2 S by triplet CrO 3 , MoO 3 and WO 3 . †

We have investigated mechanism off the hydrogen atom abstraction from H2S by triplet CrO3, MoO3 and WO3 by quantum chemical ub3lyp/lanl2dz method. Activation energy of the CrO3 + H2S, MoO3+H2S, WO3+H2S reactions were calculated as: 2.45 kcal/mol, 2.41 kcal/mol, 2.16 kcal/mol. These values were greater than in the corresponding ones in the singlet state. Beside we have investigated the spin density distribution in the transition states all them calculated. Allocated spin density has said about spin density transition from one oxygen centre to sulfur atom. Transition have been uncompleted (because value sulfur’s spin density was equal 0.429, at the same time spin density on oxygen atom was of 0.631 in the case of the CrO3 + H2S reaction). Transition in the transition state has not led to great change in difference between HOMO and LUMO for all system studied. The mechanism of the reaction have been proposed.


Introduction
Exhaust sulfur components in atmosphere lead bad ly influence to our life.Desulfurization is one of the most important task to avoid ecological risks.[1][2][3] An usual way of the diesel fuel desulfurization is hydrodesulfurization.This method requires big amount of energy for reaching high temperature and it has also to be realize in the presence of hydrogen.Alternative method is to use a photochemical reaction of oxidation agent such as polyoxometalates.It has advantages like low temperature, light energy, green technology nature, etc.
The goal of our article is a quantum chemistry research of the photooxidation mechanism of hydrogen sulfur with the triplet state polyoxometalates.Our main hypothesis is that the oxidation mechanism includes a step of a hydrogen atom abstraction from H2S by metal oxides such as CrO3, MoO3, WO3 in triplet state.

Methodological part
We used ub3lyp/lanl2dz quantum chemical method.For compare probability of reactions involved we have determined their activation energies.It has been calculated like a difference between full electron energy of a transition state with correction for the zero-point vibrations added and total energy of the initial product correction for the zero-point vibrations added.For assign initial condition molecule was done preliminary search conform to minimum.

Results and Discussion
We have shown geometries of the initiate and transition states for the reactions in Figure 1-3.The corresponding geometry parameters are shown in Table 1.Activation energy of these transfers are sufficiently small ( See Table 2).The latter say that excitation of MetO3 to the first singlet level and next intersystem crossing triplet state could increase probability of the oxidation.Usually suggest that reaction CrO3 , MoO3, WO3 base on transfer electron density from atom O to atom Met.This mechanism is alternative to intersystem crossing.One of mechanism was suggested with charge-transfer [4] and this shown on Scheme 1.Our study of electron density distribution seems to reveal that there is no appreciate change in electron density (see Table 3).Despite for Table 3 we can say about mechanism of the reaction as off hydrogen from atom S to atom O (see Figure 1-3).

Table 1
has shown that there is large difference between the initiate and transition state in the O-H bond length, in the Met-O-H angle and the S-H-O angle values.One could note that (1) change has place near the reaction center, (2) there has place a transfer of hydrogen atom from H2S to one of atom of MetO3 in the triplet state.

Table 4 .
Milliken atomic spin densities distributionThere is allocation of spin density on metal atom Met, atom O, and atom S. So, we have proved our hypothesis about an advantage of hydrogen atom transfer between S and O atoms.