Computational Study of Thermolysis Reaction of Tetroxane and Methyl derivarives. Axial/equatorial methyl position substituent effect.?

ALEXANDER G. BORDÓN; ANDREA N. PILA; MARIELA I. PROFETA; NELLY L. JORGE; JORGE M. ROMERO; ANDRE GRAND

Montevideo

Congreso; XLII Congreso Internacional de Químicos Teóricos de Expresión Latina; 2016

Cyclic organic peroxides belong to a broad and highly demanded class of peroxide compounds that present many interesting capabilities, including high reactivity and even explosive character. The cyclic diperoxides are a very interesting substituted compound series, wheretetroxane is the leader compound. The main aim of the work is to ascertain the influence of substituent in the thermolysisreaction.The calculations were performed by using DFT method, with the 6-311+G** basis set, and the BHANDHLYP correlationexchangefunctional. The critical points of the PES?s of the ground singlet (S) and first triplet (T) states of the reaction were determined. Calculations were made with the GAUSSIAN09 program. Critical points of the PES were optimized by the Berny method, and TS were located with help of the Eigenvalue Following method. Minima and TS were characterized with harmonic vibrational frequencies. The IRC method was used to determine the connections of TS with their reactants and products. CASSCF was used at (10,10) 6-311 + G ** level to determine the crossing point.The two first compounds follow stepwise reaction paths following the S-PES (Table 1), however, the reaction is not clearly exothermic as a diperoxide compound should be. However, there is a non-adiabatic path by a crossing point at a T diradical open structure,intermediate product of the first step of the reaction, as a consequence of a spin-orbit coupling, and the reactions follows the T-PES reaching, by a stepwise mechanisms products (formaldehyde/acetaldehyde molecules plus O2in the T state), being highly exothermic. However, the dimethyl derivative does not follow the stepwise S-PES, reacting in an endothermic single step reaction from the diradical open structure (Scheme 1), however, if the reaction follows the stepwise T-PES (Scheme 2), reaction goes on also highly exothermic. One methyl substitution becomes S-PES [b →p (Scheme1)] in exothermic, two methyl, reaction does not reach this step. In tetroxane and methyl derivative, the highest activation energy (ΔE) of S-PES corresponds to the Tsob?, however, in two-methyl derivatives, the highest ΔE corresponds to the the first TS, being the eq-eq the highest, ax-ax shows the lowest ΔE. In T-PES, the ethylsubstitution decreases ΔE in the first step, being eq-eq the largest.A crossing point between the single and triplet PES appears in the first step of the reaction, where one of the peroxide bonds is open, giving a diradical open structure in all derivatives of tetroxane. A spin orbit coupling is found at the crossing point between the singlet and triplet state open diradicals. So, a non-adiabatic process occurs with passage between the two distinct multiplicity PES. Reaction start at a singlet state, at the first step, the diradical structure is reached, crossing from the singlet to the triplet state, and the next steps go on in the triplet state PES, giving formaldehyde/acetaldehyde and molecular oxygen as final products. The same products are obtained in all isomers. Reaction in the triplet state PES turns out to be an exothermic reaction. Eq-eqisomer in dimethyl derivatives yields the highest activation energy.