Mechanism of α-Amino Acids Decomposition in the Gas Phase. Experimental and Theoretical Study of the Elimination Kinetics of N-Benzyl Glycine Ethyl Ester

摘要

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 CO_2 gas. The variation of the rate coefficients with temperature is expressed by the following Arrhenius equation: log k_1 (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.