DFT studies of the structure, vibrational and NMR spectra of 2-amino-pyridine betaine monohydrate

Four of the most stable conformers of 2-amino-pyridine betaine (1-carboxymethyl-2-amino-pyridinium inner salt) monohydrates, 2-NH2PB·H2O, and one anhydrous were analyzed by the B3LYP/6-31G(d,p) calculations and compared with the X-ray data. Two types of optimized conformers can be distinguished: (a) with NH2 and COO groups and (b) an imino tautomer with NH and COOH groups. A common feature of the optimized molecules are intramolecular hydrogen bonds between the COO− and H2N or COOH and HN groups. In the crystal both NH2 and COO groups participate in intermolecular hydrogen bonds. The probable assignments of the anharmonic experimental solid state vibrational frequencies of 2-NH2PB·H2O and 2-ND2PB·D2O (conformer 2) based on the calculated B3LYP/6-31G(d,p) harmonic frequencies have been made. Correlations between experimental chemical shifts for 2-NH2PB, its hydrochloride and 1-carboxyethyl-2-amino-pyridinium inner salt (13C and 1H in D2O) and GIAO/B3LYP/6-31G(d,p) calculated isotropic shielding constants, δexp=a+bσcalc, are reported. Good linear regression between experimental and theoretical results for 13C was obtained. Only in 2-NH2PB the hydrogen at α-position is outside the linear correlation.