Structuring and destructuring effects along a pathway toward formation of zwitterionic glycine···(H2O)2 complex: many body analysis of clusters and molecular electrostatic potential investigations



We investigate molecular cooperativity in the process of conversion of neutral “canonical” form of glycine···(H2O)2 to its zwitterionic form by an intramolecular proton transfer induced by displacement of the oxygen of the water molecule proximal to the amino group, toward the hydrogen bonded H of the latter. In one of the possible pathways considered here, the carboxyl proton is seen to migrate to the amino group resulting into a stable, energetically locally minimal albeit slightly higher, zwitterionic state. Second-order Møller-Plesset perturbation theory (MP2) is used for this study; also, the Density Functional Theory (DFT) that includes popular density functionals, viz. B3LYP, B3PW91, as well as a new-generation functional M06-L; in conjunction with the basis set 6-311++G(2d,2p). The proton transfer concomitantly brings in tangible changes in the molecular electrostatic potential profiles as well as repercussions in the salient infra-red spectra. It is gratifying that DFT provides a consistently competent description of the transfer process. The proton transfer is examined through a “many-body interaction-energy analysis of clusters” scheme whence it is perceived that glycine primarily is neither a structure maker nor a breaker with respect to water···water interactions in the cluster. © 2012 Wiley Periodicals, Inc.