Abstract
This article comprises the comparative study of bisulfate ion binding behaviour of three analogous naturally occurring plant alkaloids harmine (HM), harmane (HR), and norharmane (NHM), each containing the same basic skeleton but different substituents. A detailed density functional theoretical investigation including binding energy analysis, bond length variation, charge transfer, molecular electrostatic potential (MEP) study, computational IR and 1H NMR study, potential energy scan, HOMO-LUMO energy gap and electronic transitions in vacuum as well as in three different solvents of different polarity: n-heptane (non-polar, aprotic, ε = 1.9113), acetonitrile (polar, aprotic, ε = 35.688) and water (polar, protic, ε = 78.3553) reveal a gradual increase in anion binding affinity from NHM to HR and HM. Thereafter, the potential energy surface (PES) study indisputably digs out and establishes the excited state proton transfer (ESPT) process as the binding mechanism. Finally, the sensitivity and mechanism are entrenched through significant changes in the UV-Vis and steady-state fluorescence spectra of HM upon the gradual addition of bisulfate anion in an acetonitrile-water (5:1) solvent mixture.
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