Análise de parâmetros de RMN em agregados de glicina

Abstract

High-level Density Functional Theory (DFT) calculations have been performed to study the effect of the hydrogen bond formation on the nuclear magnetic resonance (NMR) parameters of clusters of glycine molecules in gas-phase. Clusters containing up to four glycine molecules besides the isolated glycine have been considered for the present work. DFT predicted isotropic chemical shifts of H, C, N and O of the isolated glycine with respect to standard reference materials are in reasonable agreement with available experimental data. The variations of isotropic and anisotropic chemical shifts for all atoms constituting these clusters containing up to four glycine molecules have been investigated systematically employing gradient corrected hybrid B3LYP functional with three different types of extended basis set : 6-31++G(2d,2p) of Pople, aug-cc-pVDZ of Dunning and aug-pc1 of Jensen. All three models show very consistent results. The glycine clusters are mainly stabilized by a network of hydrogen bond formation among the carboxylic (COOH) groups of glycine monomers. The formation of hydrogen bonds influences significantly the molecular structure of the clusters which, on the other hand, gets reflected in the variation of NMR properties. The bridging hydrogen (H) of the proton-donor O-H bond, the carbon (C) atom of the –COOH group and the oxygen (O) atom of the proton-acceptor C=O bond suffer downfield shift due to formation of hydrogen bond. The length of hydrogen bond formed between the glycine monomers and the complexity of the structure are found to vary with the number of monomers present in the cluster. A direct correlation between the hydrogen bond length and isotropic chemical shift of the bridging hydrogen is observed in all cases.