The kinetic method reveals secondary deuterium isotope effects on the proton affinity and gas-phase basicity of glycine and alanine methyl esters

The kinetic method for measuring proton affinities (PA) and gas-phase basicities (GB) was applied to the methyl esters of simple amino acids. The experiments show that the GB and PA values for deuterium labeled glycine methyl ester are indeed greater than that of the corresponding unlabelled glycine methyl ester. The PA of l -Ala-OCD3 is also slightly greater than that of the unlabeled alanine methyl ester. The secondary isotope effects originate, as shown by density functional theory, in differences in zero-point energies and thermal-energy corrections between H and D-bearing molecules.