A theoretical study of the unimolecular decomposition of N-chloro-α-amino acids in aqueous solution

Abstract

The substituent effects on the barrier heights for the unimolecular decomposition of the anionic form of different N-chloro-α-amino acids have been analyzed. The positions of the transition structures on the potential energy surfaces in the singlet and triplet electronic states have been analyzed by means of a More O'Ferrall–Jencks diagram. The selected model systems were: N-chloroglycine, N-chloroalanine, N-chloro-2-amino-butyric acid, N-chlorosarcosine, N-chloro-N-ethylglycine, N-chloro-2-amino-isobutyric acid and N-chloro-N-methylalanine. The present study has been carried out taking into account the solvent effects by means of the self-consistent reaction field method. The calculations has been carried out with 6-31G* basis set and the electron correlation energy has been included at CISD level. The results complement our earlier works (J. Phys. Chem. 100 (1996) 3561; J. Phys. Org. Chem. 9 (1996) 371), in which the unimolecular decomposition mechanism of the N-chloroglycine and other N-chloroamino acids was described as an asynchronous concerted process with an antiperiplanar conformation for the corresponding transition structures. The main change with respect to the gas phase results is an enhancement of the asynchronicity of the decomposition process. In addition, an increase of the size and number of substituents on the Cα atom, and to a lesser extent on N atom, decreases the barrier height. N-substitution effects control the perpendicular effects along the reaction pathway.