Observation of quantum signature in rivastigmine chemical bond break-up and quantum energetics, spectral studies of anti-Alzheimer inhibitors Abstract: Semi-empirical calculations on the torsion potential for dihedral angle of rivastigmine linking NAP andCarbamyle moieties consistently show regions of several discontinuities and a cusp indicating molecu-lar instability and eventual break-up of rivastigmine observed in the X-ray structure. The phenomena can be explained both by definition of large classical force or quantum nature of chemical bond break-up. Also, to better understand the molecular properties and quantum energetics of the inhibitormolecules, we have performed several ab initio based calculations on all four inhibitors at equilibrium geometry, in ground state and gas phase using the density functional theory level wB97X/6-31G
andHF/6-31G
. A number of properties like computational vibrational (IR), Raman and nuclear magneticresonance (NMR) spectra as well as HOMO and LUMO orbital energies at optimized geometries havebeen computed by SPARTAN16 and Gaussian16 utilities. Also, the thermodynamic and QSAR proper-ties of the inhibitors have been assessed and compared by a number of different semi-quantum, Hartree-Fock and density functional methods. The theoretical NMR and IR spectra have been bench-marked against experimental spectrum to compare and assess suitability of the computational methodologies and basis set levels for the calculations.