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Proton Dynamics in the Strong Chelate Hydrogen Bond of Crystalline Picolinic Acid N-Oxide. A New Computational Approach and Infrared, Raman and INS Study
journal contribution
posted on 2008-02-21, 00:00 authored by Jernej Stare, Jarosław Panek, Juergen Eckert, Jože Grdadolnik, Janez Mavri, Dušan HadžiInfrared, Raman and INS spectra of picolinic acid N-oxide (PANO) were recorded and examined for the
location of the hydronic modes, particularly O−H stretching and COH bending. PANO is representative of
strong chelate hydrogen bonds (H-bonds) with its short O···O distance (2.425 Å). H-bonding is possibly
well-characterized by diffraction, NMR and NQR data and calculated potential energy functions. The analysis
of the spectra is assisted by DFT frequency calculations both in the gas phase and in the solid state. The
Car−Parrinello quantum mechanical solid-state method is also used for the proton dynamics simulation; it
shows the hydron to be located about 99% of time in the energy minimum near the carboxylic oxygen; jumps
to the N−O acceptor are rare. The infrared spectrum excels by an extended absorption (Zundel's continuum)
interrupted by numerous Evans transmissions. The model proton potential functions on which the theories of
continuum formation are based do not correspond to the experimental and computed characteristics of the
hydrogen bond in PANO, therefore a novel approach has been developed; it is based on crystal dynamics
driven hydronium potential fluctuation. The envelope of one hundred 0 → 1 OH stretching transitions generated
by molecular dynamics simulation exhibits a maximum at 1400 cm-1 and a minor hump at ∼1600 cm-1.
These positions square well with ones predicted for the COH bending and OH stretching frequencies derived
from various one- and two-dimensional model potentials. The coincidences with experimental features have
to be considered with caution because the CPMD transition envelope is based solely on the OH stretching
coordinate while the observed infrared bands correspond to heavily mixed modes as was previously shown
by the normal coordinate analysis of the IR spectrum of argon matrix isolated PANO, the present CPMD
frequency calculation and the empirical analysis of spectra. The experimental infrared spectra show some
unusual characteristics such as large temperature effects on the intensity of some bands, thus presenting a
challenge for theoretical band shape treatments. Our calculations clearly show that the present system is
characterized by an asymmetric single well potential with no large amplitudes in the hydronium motion,
which extends the existence of Zundel-type spectra beyond the established set of hydrogen bonds with large
hydronic vibrational amplitudes.
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NQR dataDFT frequency calculationspositions squareRamanProton Dynamicstemperature effectsNew Computational Approachargon matrixchelate hydrogen bondsOHproton dynamics simulationcharacteristicCPMD frequency calculationmodel potentialsIR spectrumCPMD transition envelopegas phasehydronic vibrational amplitudesdynamics simulation exhibits1400 cmenergy functionsCOHhydronium motionPANOspectra showNMRband shape treatmentshydronic modesmodel protoncarboxylic oxygencrystal dynamicsnovel approachINS StudyInfraredINS spectracontinuum formationhydrogen bondsEvans transmissionsCrystalline PicolinicStrong Chelate Hydrogen Bondanalysishydrogen bond
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