Quantum mechanical conformational analysis of β-alanine zwitterion in aqueous solution

The conformational behavior of beta-alanine zwitterion in aqueous solution has been investigated by post-Hartree-Fock and density functional methods using the polarizable continuum model (PCM) for the description of the solvent. Our results show that the PCM is able to reproduce the delicate balance between intramolecular hydrogen bridges and preferential solvation of isolated charged groups in determining the overall conformational preferences without any need of explicit solvent molecules. From a methodological point of view, the Hartree-Fock method is not sufficiently reliable for quantitative predictions of conformer equilibria, whereas approaches including electron correlation (DFT, MP2, MP4) perform remarkably well when using medium size basis sets including diffuse functions. Vibrational averaging arising from the torsional motion around the central CC bond leads to vicinal NMR coupling constants (J) in remarkable agreement with experiments when using an extended Karplus-type equation for the torsional dependence of J.