Mechanism of α-Amino Acids decomposition in the gas phase. Experimental and theoretical study of the elimination kinetics of N-Benzyl Glycine Ethyl Ester

The gas-phase elimination kinetics of N-benzylglycine ethyl ester was examined in a static system, seasoned with allyl bromide, and in the presence of the free chain radical suppressor toluene. The working temperature and pressure range were 386.4-426.7 °C and 16.7-40.0 torr, respectively. The reaction showed to be homogeneous, unimolecular, and obeys a first-order rate law. The elimination products are benzylglycine and ethylene. However, the intermediate benzylglycine is unstable under the reaction conditions decomposing into benzyl methylamine and CO₂ gas. The variation of the rate coefficients with temperature is expressed by the following Arrhenius equation: log k₁ (s^-1) = (11.83 ± 0.52) - (190.3 ± 6.9) kJ mol ^-1 (2.303RT)^-1. The theoretical calculation of the kinetic parameters and mechanism of elimination of this ester were performed at B3LYP/6-31G*, B3LYP/6-31+G**, MPW1PW91/6-31G*, and MPW1PW91/6-31+G** levels of theory. The calculation results suggest a molecular mechanism of a concerted nonsynchronous six-membered cyclic transition state process. The analysis of bond order and natural bond orbital charges implies that the bond polarization of C(=O)O-C, in the sense of C(=O)O^δ-⋯C^δ+, is rate determining. The experimental and theoretical parameters have been found to be in reasonable agreement.