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Theoretical calculations on the mechanism of the elimination kinetics of allyl cyclohexyl-, -amine, -sulfide, -ether, and allyl ethyl ether in the gas phase

Abstract:

The mechanism of the gas-phase elimination allyl cyclohexyl amine, allyl cyclohexyl sulfide, allyl cyclohexyl ether, and allyl ethyl ether has been studied by using ab initio combined methods CBS-QB3, and Density Functional Theory CAM-B3LYP, MPW1PW91, PBE1PBE, M06, and M062X. Products formation is described below: Theoretical calculations of these reactions support the unimolecular process of these gas-phase eliminations. These thermal decompositions undergo a retro-ene type of mechanism and proceed through a non-planar concerted six-membered cyclic transition structure. The energy of activation follow the order allyl ethyl ether (187.0 kJ/mol) > N-allyl cyclohexyl amine (171.7 kJ/mol) > allyl cyclohexyl ether (170.5 kJ/mol) > allyl cyclohexyl sulfide (137.9 kJ/mol). The polarization of C5-Z6 bond, and the electronegativity of the heteroatom (O, N) increases the reaction rate compared to allyl ethyl ether. Conversely, the S atom is positively charged and its electronic effect causes a high dissymmetry in the TS geometry, which appear to accelerate the decomposition. The rate determining step of these substrates is the change in hybridization from sp3 to sp2 reflected in the bond order in C3-C4. Calculated kinetic and thermodynamic parameters from the M06/6-311++G(d,p) level of theory are in reasonable good agreement with the experimental values.