IR low-temperature matrix, X-ray and ab initio study on l-isoserine conformations

Abstract

The IR low-temperature Ar and Kr matrix spectra of L-isoserine were registered for the first time and interpreted by means of the anharmonic DFT frequencies calculated at the B3LYP/aug-cc-pVTZ and B3LYP/aug-cc-pVDZ levels. 54 L-isoserine conformers were predicted to be stable at the B3LYP/aug-cc-pVDZ level. Population of the 8 most stable conformers was based on the QCISD/aug-cc-pVDZ energies, corrected for thermal anharmonic factors obtained at the B3LYP/aug-cc-pVDZ level. We found several conformers to be present in the measured matrices and conformer 1 to be dominating. Presence of the conformer 2 is well confirmed by the ν(CO) band at 1790 cm⁻¹ and two bands at 1380 and 1350 cm⁻¹. Presence of the conformer 4 is quite well confirmed by the ν(C–O) bands at 1120 and 1095 cm⁻¹. Slightly weaker arguments are found for the observation of conformers 6 and 3. Calculations on 54 neutral and 5 zwitterionic conformers in water at the IEF-PCM/B3LYP/aug-cc-pVDZ level suggest that one neutral and one zwitterionic conformer co-exist in the aqueous environment. The crystal structure of L-isoserine was solved by X-ray diffraction analysis. The compound crystallizes without solvent in the chiral P2₁2₁2 space group. The asymmetric unit contains a single molecule. The molecule is in its zwitterionic form with the CH₂–NH₃ side chain in the gauche conformation with respect to the hydroxyl group and in the anti conformation with respect to the carboxylate group. The structure of L-isoserine is dominated by a set of intermolecular hydrogen bonds. The strongest one appears between the OH and COOH groups of two neighbouring molecules: the O⋯H contact is of 1.66(2) Å, which is amongst the shortest H-bonds of this kind observed in amino acid crystal structures.