Density functional theory (DFT) and natural bond orbital (NBO) study of vibrational spectra and intramolecular hydrogen bond interaction of l-ornithine–l-aspartate

• NBO analysis was used to explain intramolecular hydrogen bonding.
• The LOLA molecule exists in the form of salt by intramolecular H-bonds and ionic bonding.
• Computed X-ray powder diffraction pattern and vibrational wavenumbers were seen to be in good agreement with experimental findings.
• Potential energy distribution of the normal modes of the vibrations has been done.

In this study, exhaustive conformations of (S)-4-amino-4-carboxybutan-1-aminium (S)-3-amino-3-carboxypropanoate (LOLA) have been scanned. Experimental and theoretical studies on the structure and vibrations of the title compound are presented. The optimized molecular structure, vibrational wavenumbers, Mulliken atomic charges, natural bond orbital (NBO) and molecular electrostatic potential studies have been performed by density functional theory (DFT) using B3LYP method with the 6-311++G(d,p) basis set. Computed X-ray powder diffraction (XRPD) data has been carried out by DFT calculations and ab initio from measured XRPD finding. The LOLA molecular geometry has been determined which exists in the form of salt by intramolecular H-bonds and ionic bonding. Moreover, calculated vibrational frequencies were applied to simulate IR and Raman spectra of the title compound which showed excellent agreement with observed spectra. Reliable vibrational assignments have been made on the basis of potential energy distribution (PED) and 0.992 has been obtained by least squares method which is the uniform scaled factor for theoretical frequencies at 6-311++G(d,p) basis set. In addition, the hydrogen bonding in LOLA molecule has been explored by calculation of the hyperconjugative charge transfer interaction on [LP X-σ*(Y-H)], under NBO analysis, Mulliken atomic charge analysis, molecular electrostatic potential map (MEP) and vibrational spectra. Finally, HOMO–LUMO of the title compound has been plotted for predicting reactive sites.

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