XRD structure and vibrational analysis of DL-β-Leucine, as aided by DFT tetramer model and characterized by NBO, AIM and NCI calculations

Abstract

Vibrational spectra due to molecular vibrations of amino acids are influenced by their zwitterionicity and oligomerization as defined by H-bonds. When these two influencing factors are incorporated into computational modeling, a very good agreement between theory and experiment is achieved. In the present work we show this good agreement for the unnatural amino acid, DL-β-Leucine. From a single-crystal X-ray diffraction analysis, DL-β-Leucine has been found to be a zwitterion in the triclinic space group P-1, having three distinct N–H⋯O bonds from a cationic –NH3+ and anionic –CO2‒ moieties. This structure has been utilized as a model for building a tetramer at B3LYP/6-311G (d,p) level. It is shown that each of the three N–H bonds in the –NH3+ of a central S-enantiomer in the tetramer is H-bonded with the oxygen atoms in the –CO2‒ moiety belonging to its three neighbour molecules. Experimentally rich vibrational spectra of IR and Raman bands, especially belonging to the NH3+ and –CO2‒ moieties agree very well with the tetramer modes, including Raman bands below 200 cm−1 generically called ‘lattice modes’. As for the electronic characterization of N–H⋯O bonds, AIM calculation gives bond energies of −6.44, −9.67 and −11.24 kcal/mol at bond critical points (BCP: 3, −1), with relative changes that are consistent with the geometrical parameters as well as IR spectral shifts of the N–H⋯O bonds. This is further supported by NBO analysis as well. Further characterization of N–H⋯O bonds as noncovalent interaction by reduced density gradient analysis in relation to van der Waals and steric interactions has been shown to be consistent with AIM and NBO results.