Crystal and molecular structure of DL-serine hydrochloride studied by X-ray diffraction, low-temperature Fourier transform infrared spectroscopy and DFT(B3LYP) calculations

The structure of DL-serine center dot HCl was studied by three complementary techniques. Experimental Fourier transform infrared (FT-IR) spectra of pure NH/OH polycrystalline DL-seiine center dot HCl [HO-CH2-CH(NH3+)-COOH-Cl-] and the respective deuterated derivatives [ND/ODAlcohol/Acid (< 10% and ca. 60% D)] were recorded in the region 4000-400 cm(-1) in the temperature range 300-10 K and interpreted. The assignments were confirmed by comparison with the vibrational spectra of crystalline DL- and L-serine zwitterions [HO-CH2-CH(NE3+)-COO-]. Further insight into the structure of the title compound was provided by theoretical DFT(B3LYP)/6-311++G(d,p) calculations of the infrared spectra and energies of 13 different conformers. Potential energy distributions resulting from normal coordinate analysis were calculated for the most stable conformer (1) in its hydrogenated and deuterated modification. Frequencies of several vibrational modes were used in the estimation of enthalpies of individual H-bonds present in the crystal, using empirical correlations between enthalpy and the frequency shift that occurs as a result of the establishment of the H-bonds. X-ray crystallography data for DL-serine center dot HCl were recorded for the first time and, together with the experimental vibrational spectra and the theoretical calculations, allowed a detailed characterization of its molecular structure.