Theoretical Study and NBO Analysis of the Decomposition Kinetics of Oxetane, 2-Methyloxetane and 2,2-Dimethyloxetane

T A theoretical study of the thermal decomposition kinetics of oxetane(1),2-methyloxetane(2), and 2,2-dimethyloxetane(3)has been carried out at the B3LYP/6-311+G<'**>,B3PW91/6-311+G<'**>, and MPWlPW91/6-311+G<'**> levels of theory. The MPW1PW91/6-311+G<'**> method was found to give a reasonable good agreement with the experimental kinetics and thermodynamic parameters. The decomposition reaction of compounds 1～3 yields formaldehyde and the corresponding substituted olefin. Based on the optimized ground state geometries using MPW1PW91/6-311+G<'**> method, the natural bond orbital(NBO)analysis of donor-acceptor (bond-antibond)interactions revealed that the stabilization energies associated with the electronic delocalization from σ<,c3-c4> bonding to σ<'*><,o1-c2> antibonding orbitals decrease from compounds 1 to 3. The σ<,c3-c4>→σ<'*> resonance energies for compounds 1～3 are 2.63, 2.59 and 2.45 kcal mol<'-1>,respectively. Further, the results showed that the energy gaps between σ<,c3-c4> bonding and σ<'*><,o1-c2> antibonding orbitals decrease from compounds 1 to 3. Also, the decomposition process in these compounds are controlled by σ→σ<'*> resonance energies. Moreover, the obtained order of energy barriers could be explained by the number of electron-releasing methyl groups substituted to the C<,sp3> atom(which is attached to oxygen atom). NBO analysis shows that the occupancies of σ<,Csp3-O> bonds decrease for compounds 1～3 as 3<2<1, and those of σ<'*><,Csp3-O> bonds increase in the opposite order(3>2>1). This fact illustrates a comparatively easier thermal decomposition of the σ<,Csp3-O> bond in compound 3 compared to compound 2, and in compound 2 compared to compound 1. NBO results indicate that these reactions are occurring through a concerted and asynchronous four-membered cyclic transition state type of mechanism.