THEORETICAL CHARACTERIZATION OF FREE N-(METHOXYCARBONYL)GLYCINE AND ITS INTERACTION WITH WATER

N-(methoxycarbonyl)glycine (NMG) has been characterized by ab initio m olecular orbital methods. HF/6-31G geometry optimizations have been p erformed to mimic the C-5 and C-7 conformations of model dipeptides. T he C-7 conformation is characterized by an intramolecular hydrogen bon d between the OH group and the oxygen atom of the methane function. In the C-5 conformation, the NH group is not involved in an intramolecul ar hydrogen bond and the nitrogen atom is more pyramidal than in the d ipeptides (pseudo C-5 conformation). The C-7 conformer is about 8 kJ . mol(-1) higher in energy than the C-5 conformer (MP2/6-31G*). The di fferences between the normal modes of the two conformers and their OD and ND isotopomers are discussed. Interaction of NMG with a water mole cule induces only small changes in geometry. The most stable structure is obtained for a cyclic COOH ... OH2 system with a shorter O-W ... H O and a longer OWHW ... O=C hydrogen bond. A second stable monohydrate is obtained for a cyclic structure involving the NH bond and the carb oxylic oxygen atom. The energies of association are about -41 and -33 kJ . mol(-1), respectively. These results are discussed as a function of the experimental gas-phase acidities of biological NH and COOH prot on donors. Some infrared shifts induced by the association with water are discussed. A linear relationship is found between computed frequen cy shifts of the OH stretching mode and the elongations of the corresp onding bond. This relation is very similar to the empirical correlatio n for OH ... O hydrogen bonded systems in the solid state.