The mechanism of the photochemical oxidation of substrates of different nature by nitro compounds

Using quantum-chemical methods we studied the mechanism of the photochemical oxidation of epoxy olefins by nitro compounds. It was first found the structure of the corresponding transition states. DFT / 6-31g + (d) and DFT / 6311 + g (d) methods were used to study the reaction of sulfur oxides (II, IV) with nitro compounds in the ground and excited states. It was first made an assumption that the transition states containing nitroso oxide fragment were formed in the course of the photochemical oxidation of various substrates by nitro compounds in the excited states. The nitroso compounds photochemical oxidation by nitro compounds in the excited states were studied by experimental and theoretical methods. By means of CASSCF method we determined the opportunity of the singlet triplet transition involvement in the reactions with nitroso oxides participation. It was shown proximity of the S0 and T2 levels.


Introduction
Nitro compounds are the basis of explosives which recycling or disposal is an important task at the moment.Nitro compounds possess strong oxidizing properties and are capable to oxidize a great majority of organic compounds.They have a great potential as photochemical oxidants, but the mechanisms of oxidation with their participation have not been studied enough.Preparation of propylene oxide and ethylene oxide by photochemical oxidation of propylene and ethylene by nitro compounds may be useful in industry.Currently available methods for the preparation of epoxides lead to high yields of by-products, and the methods proposed in our work are free from the above-mentioned drawbacks.In addition, it is of interest to obtain nitroso oxides in the course of the photochemical isomerization of nitro compounds into nitroso oxides or in the course of the nitro compound interaction with nitroso compounds in the triplet and singlet states.
They react with double bonds of olefin to form epoxides, with nitroso compounds readily oxidizing them into nitro compounds, with sulfoxides to transform them into sulfones, with divalent sulfur derivatives oxidizing them into sulfoxides and so on.For fundamental science it may be important to reveal mechanism oxidation by nitro compounds which includes the in situ formation of nitroso oxides.

Isomerization of nitroso oxides in the nitro compound
As model used nitromethane and different nitroso compounds.Table 1 shows the activation energies of the nitromethane isomerization into the corresponding nitroso oxides in the singlet and triplet states.We studied both forward and backward reactions.It follows from table 1 that the reaction could easily proceed for participants being in the triplet state in the direction of obtaining nitro compounds from nitroso oxides.Consider the mechanism of the reaction in more detail for of cis -isomer of nitroso oxides, based the atomic spin density and geometry changes for the initial compounds, products and the transition state in the course of the reaction (Table 2).Table 2 gives us an opportunity to conclude that distribution of spin density in cis-RNOO and in the transition state for the reaction are very similar and there is very long N-O bonds both in the nitroso oxides and the corresponding transition states.So we could say about early transition state for isomerization of nitroso oxide into nitro compounds.More clearly, this process is shown in Fig. 1 where the reaction coordinate diagram for the reaction is shown.A reaction similar to reaction 1 in which the starting materials and products are in the singlet state can not occur to form in situ nitroso oxide.Apparently, it is caused the fact that the cyclic conformation is more advantageous in energy than propylene oxide.In the triplet state a cyclic compound is not formed.The activation energies for reaction 1 are shown in Table 3 and Fig. 2. Thus, the formation of ethylene oxide or propylene occurs only for participants that are in the triplet state.The corresponding transition states are shown on Fig. 3 and 4.
Fig. 3 The TS of reaction 1 with imaginary frequency at 527i cm -1 (T 1 ) Fig. 4 The TS of reaction 2 with imaginary frequency at 209i cm -1 (T 1 ) A detailed examination of the spin density distribution for the reaction 1 and 2 in the photooxidation of propylene by nitro compound can give us possibility to conclude the following.Initially, there has place excitation of the nitro compound into the S 1 state.Then it undergoes intersystem crossing turning into the T 1 state.
After that there is an interaction with propylene to form the triplet biradical.Values of the spin density are presented in Table .4 There are two possible reaction pathways.The first way is a deactivation of the triplet biradical to form a singlet product followed by the formation of propylene oxide and nitroso compounds.However, as it was shown by the calculations of the activation energy this process is impossible.The second way is the destruction of the resulting biradical into propylene oxide and nitroso compounds.There has place a spin density redistribution to form in the transition state a nitroso compound-like fragment (in the triplet state) with the maxima of the spin density on nitrogen atom and one of the oxygen atom.Further redistribution could form nitroso compound in the triplet state and the propylene oxide precursor.
As a result, as shown in Table 5 there had place formation of triplet nitroso compound and propylene oxide.

The study of the photochemical oxidation of sulfur compounds by nitro compounds
The third part is devoted to the study of the oxidation of sulfur (IV) by nitro compounds.Reaction schemes (3) and (4) can serve as a way to explain the obtained.
Reaction of 3 is a photo-oxidation of sulfur oxide (IV) by the nitro compound molecule in the triplet state.During the reaction the separation of the oxygen atom and the formation of nitroso oxide which is then converted into the nitro compound.The reaction takes place, apparently, through the five-member transition state.The yield of reaction products depends upon the relative magnitudes of the energy for the transition state.Reaction 4 is the separation of the oxygen atom of the triplet sulfur trioxide and its transfer to a molecule of the nitroso compound.In the course of it there may also be formed nitroso oxide in the triplet state followed by rearrangement of it into nitro compound.In other words, the reaction of SO 2 with HNO 2 does not lead to the formation of SO 3 and HNO, and the mechanism is similar to the mechanism discussed in reaction 3. Table 6 shows characteristics of the transition states and energies of the corresponding reactions in the singlet and triplet states.

Possibility of formation of nitroso oxides during oxidation reactions of nitro compounds
In this part of our work the mechanism of formation of nitroso oxides on the example of the reaction of nitro compounds with nitroso compound HNO 2 * + HNO * ↔ HNO + HNO 2 ** was suggested.
When modeling the aforementioned reaction, as well as similar to it, we can note that the transition state is formed as a compound having the geometry similar to nitroso oxide.Optimal geometry of nitroso oxides corresponding to the minimum energy is shown in Fig. 5 and 6.During further study of the geometry, as well as the calculated IR spectra of the transition states we identified similarities in the structure and vibrations of the atoms of the corresponding compounds.Geometry of nitroso oxides transients compared with the geometry of nitroso oxides.The calculation was performed by uB3LYP / 6-311 + g (d) *.The results are shown in Table 7.
Table 7.The geometric parameters of nitroso oxides (S 0 and T 1 ) Let us consider the interaction of nitro compound with nitroso compound in the triplet state.Ability to flow this reaction is described.In the course of calculations we obtained the transition state which is we can see easily the nitroso oxide fragment particiupation.Comparison of its geometrical parameters calculated for a nitroso oxide molecule is given in Table 8. * The experimental data were taken from [1] From Table 8 it can be noted that the geometry of nitroso oxide in a vacuum is almost identical to the geometry of the nitroso oxide-like fragment in the transition states.Such a coincidence is observed for the vibration frequencies of groups of atoms NO 829.42 and 893.16 cm -1 , respectively.For groups of atoms OO coincidence of oscillation is observed.This is caused by, apparently, the fact that nitroso oxide-like fragment affects a group of atoms in a transient state.Calculated IR spectra are shown on Fig. 7 and 8.
Рис. 8 The IR spectrum of the transition state in the T 1 state.
Thus, when modeling reactions R1NO 2 + R 2 NO ↔ R 1 NO * + R 2 NO 2 , where R 1 = H, CH 3 , t-Bu, R 2 = H, CH 3 , Ph, and SO 2 + HNO 2 ** ↔ SO 2 * + HNO 2 * we observed that in the transition state there is formed a fragment of a compound having the geometry similar to nitroso oxide.

The energy levels of nitroso oxides
It is known that nitro compounds rather easily undergo the intersystem crossing (isc) [2].Since nitroso oxides and nitro compounds are isomers, it is interesting to explore the possibility of isc and for niroso oxides.Fig. 8 shows the location of the energy levels for HNOO.It can easily be seen that the main level is T 1 .Then there are disposed S 0 , T 2 and S 1 levels.The proximity of T 2 and S 0 levels is estimated as 5 kcal/mol.Thus, isc can occur for both nitro compounds and nitroso oxide.3. A common feature of the mechanisms of studied reactions is the transfer of an oxygen atom from the nitro compound to oxidizing substances.
4. It is shown the proximity of the energy levels of the triplet and singlet states in nitro and nitroso compounds.
5. It is suggested the formation of in situ nitroso oxide -like fragments in the transition states in the photochemical oxidation of nitroso compounds and sulfurcontaining compounds in the presence of nitro compounds.

Fig. 1
Fig. 1 Recovering of the reaction coordinate IRC method

Fig. 2
Fig. 2 Energy diagram of the oxidation of propylene by HNO 2 in the T 1 state

Table 1 :
Activation Energy (E a ) nitromethane isomerization to the corresponding N nitroso oxides

Table 3 :
Activation energies of the propylene photooxidation by nitromethane.

Table 4 .
The values of the spin densities for the participants in reaction 1

Table 5 .
Spin densities for the participants in the reaction 2

Table 6 :
Transition states in these reactions

Table 8 .
Comparison of geometric parameters and IR spectra in the transition state of the reaction and nitrosooxide molecule in the triplet states.

Table 9 :
Energy levels in HNO 2 и HNOO The mechanism of the propylene and ethylene photochemical oxidation into epoxide by nitro compounds in the singlet and triplet states was studied.The oxidation takes place in two stages: a) formation of the biradical complex; b) collapse of the complex into the epoxy and nitroso compounds.It is shown that all three mechanisms of the reactions are of the same type.2.The mechanisms of the reactions of nitro compounds with sulfur-containing compounds on the example of reactions SO 2 + RNO 2 ** ↔ SO 2 * + RNO 2 * и SO 3 + RNO ↔ SO 2 + RNOO → SO 2 + RNO 2 by uB3LYP/6-31+G(d) was also studied.It was established that the reactions occur in the triplet state.