DFT Study of Proton Dynamics in the Potassium Hydrogen Maleate Crystal: the Infrared Versus the Inelastic Neutron Scattering Spectra

The structure. harmonic frequencies and infrared (IR) intensities of the fundamental transitions in potassium hydrogen maleate (KHM) crystal have been computed using the BLYP/6-31G** method with periodic boundary conditions. The inelastic neutron scattering (INS) intensities of the fundamental transitions were evaluated and compared with the experimental INS spectra of hydrogenated sample and isotopic analogues of the KHM crystal: the agreement is reasonable good. Spectroscopic manifestations of the proton dynamics in the strong intramolecular H-bond were clarified by comparison of the computed frequencies of the IR and INS-active fundamental transitions. Due to several reasons, the number of the bands and their relative intensity were found to differ between the INS and IR spectra of the KHM crystal. Reduced masses for the asymmetric O...H...O stretching vibrations of the hydrogen maleate ion appear to he larger 1 a.m.u. because of the strong coupling between the bridging proton motion and the various intramolecular vibrations.