Synthesis and physicochemical studies of some novel picrate derivatives

1H and 13C NMR spectra were recorded for some novel picrate derivatives derived from some 3,3-methyl-2,6-diarylpiperidin-4-ones and 3-benzyl-2,6-diarylpiperidin-4-one. The difference in the chemical shift of equatorial methylene proton and axial methylene proton at C(5) [Δ = δeq − δax] is highly negative which is in contrast to the value observed in the corresponding parent piperidin-4-ones and this is attributed to the syn 1,3-diaxial interaction between the axial N–H bond and axial hydrogen at C-5. The effect of protonation on the chemical shifts was studied in detail. The chemical shifts of the heterocyclic ring protons are influenced by the picrate anion. Solvatochromism of picrates were studied in detail. DFT calculations were carried out in order to find out the NBO analysis, HOMO–LUMO energies, MEP studies and hyperpolarisability behaviour. The electric dipole moment (μ) and the first-hyperpolarisability (β) value of the investigated molecules have been studied theoretically which reveal that the synthesized molecules have microscopic non-linear optical (NLO) behaviour with non-zero values.

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► Syn 1,3-diaxial interaction: δeq − δax is negative.
► Reichardt–Dimroth solvent ET parameters: Dielectric solute solvent interactions—solvatochromic shift.
► Marcus optical dielectric solvent function: Direction of excited dipole is opposite to that of the ground state-dipole.
► HOMO → LUMO transition: Implies intramolecular charge transfer takes place within the molecule.
► MEP: Picryl oxygen is more electronegative than other carbonyl and nitro group oxygen's.