Theoretical study on the elimination kinetics in the gas phase of allyl methyl compounds

The thermal decomposition kinetics of allyl methyl amine, allyl methyl ether, and allyl methyl sulfide in the gas phase has been studied theoretically using the M06-2x/aug-cc-pVTZ quantum chemical approach. The observed activation parameters are consistent with a concerted unimolecular mechanism involving a non-planar cyclic six-membered transition state. Based on the optimized ground state geometries, a natural bond orbital analysis of donor–acceptor interactions reveals that the stabilization energies corresponding to the electronic delocalization from the lone-pair (LP) non-bonding orbitals on the heteroatom to the neighboring $$\sigma_{{{\text{C2}} - {\text{C3}}}}^{*}$$ antibonding orbitals decrease from allyl methyl amine to allyl methyl sulfide. This delocalization fairly explains the increase of occupancies of LP orbitals on the heteroatom from allyl methyl sulfide to allyl methyl amine. The results also suggest that the kinetics of the thermolysis of the studied compounds are dominated by $${\text{LP}}\, \to \,\sigma^{*}$$ electronic delocalization effects. Analysis of bond order, bond indices, and synchronicity parameters demonstrates that these reactions proceed through a concerted and slightly asynchronous mechanism.