Structure and property of glycine's derivatives bound by multications (H+, Li+, and Na+):: A theoretical study

The calculations using several different methods (B3P86, MP2, MP3, MP4SDQ, and CCSD) and basis sets [6-31G*, 6-31+G*, and 6-311+G(2df,2pd)] have been first performed for 15 glycine derivatives (one Gly-2H(+), one Gly-3H(+), five Gly-H+Li+ isomers, three Gly-H+Na+ isomers, three Gly-Li+Na+ isomers, and two Gly-2Na(+) isomers) formed by multications (H+, Li+ or Na+) and different active sites of a glycine molecule. These calculations yield accurate geometric structures, relative energies, bond energies, vibrational frequencies, infrared intensities, and charge distributions. The comparisons of relative energy for each isomer show that both Gly-2H(+) and Gly-3H(+) derived from the most stable neutral glycine hold the lowest energies among their respective corresponding isomers. For the Gly-2H(+), two protons are, respectively, bound to the amino nitrogen and the syn carbonyl oxygen of the most stable neutral glycine molecule. On the basis of the Gly-2H(+), the derivative Gly-3H(+) can be generated when the third proton binds to the hydroxyl oxygen. For five Gly-H+Li+ isomers, three Gly-H+Na+ isomers, three Gly-Li+Na+ isomers, and two Gly-2Na(+) isomers, each of their corresponding ground state possesses the structure with the heavier cation coordinated to carbonyl oxygen and the lighter one to the anti-amino nitrogen of another kinds of neutral glycine molecule. The bond energies first reveal that some of these derivatives must surmount an activation energy barrier in the course of some cation (proton) dissociating from it. The origin of these barriers are investigated and discussed. Finally characteristic frequency calculations imply that the study is very important in the search of the glycine derivatives by rotational spectroscopy, or for the identification of their isomers by their infrared bands. (C) 2002 American Institute of Physics.