Vibrational spectra and natural bond orbital analysis of organic crystal l-prolinium picrate

Vibrational spectral analysis and quantum chemical computations based on density functional theory (DFT) have been performed on the organic crystal l-prolinium picrate (LPP). The equilibrium geometry, various bonding features and harmonic vibrational wavenumbers of LPP have been investigated using B3LYP method. The calculated molecular geometry has been compared with the experimental data. The detailed interpretation of the vibrational spectra has been carried out with the aid of VEDA 4 program. The various intramolecular interactions confirming the biological activity of the compound have been exposed by natural bond orbital analysis. The distribution of Mulliken atomic charges and bending of natural hybrid orbitals associated with hydrogen bonding also reflects the presence of intramolecular hydrogen bonding thereby enhancing bioactivity. The analysis of the electron density of HOMO and LUMO gives an idea of the delocalization and low value of energy gap indicates electron transport in the molecule and thereby bioactivity. Vibrational analysis reveals the presence of strong O–H⋯O and N–H⋯O interaction between l-prolinium and picrate ions providing evidence for the charge transfer interaction between the donor and acceptor groups and is responsible for its bioactivity.

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► DFT calculations have been performed on the organic crystal l-prolinium picrate.
► The IR and Raman spectra of the compound were recorded.
► Wavenumber shift of NH2+ asymmetric stretching confirms N–H⋯O bonding.
► Downshift of carbonyl stretching mode due to hydrogen bonding enhances bioactivity.
► NBO, Mulliken analyses and HOMO–LUMO plot suggests bioactivity.