Inelastic neutron scattering and DFT study of 1,6-anhydro-β-d-glucopyranose (levoglucosan)

We report an inelastic neutron scattering (INS) study of a monosaccharide, 1,6-anhydro-β-d-glucopyranose, C6O5H10 (levoglucosan). The assignment of the experimental vibrational spectrum measured in the range 30–4000 cm−1 on TOSCA spectrometer has been made by means of DFT calculations. To simulate crystal environment both molecular cluster and periodic models were used. The INS spectrum calculated with the periodic model is a better fit of the experiment than the cluster model. On the other hand the peaks between 630 and 730 cm−1 in the experimental INS assigned to OH torsional modes were reproduced better by the cluster calculations. The study has been completed by an alternative approach, molecular dynamics (MD) calculations, done at the same level of the DFT theory. The simulation trajectories were analyzed in both time and frequency space to determine the dynamical contributions to the observed spectra. Comparing the positions of the peaks in the power spectra and in the INS spectra, the former gave a better fit of the experimental ones in all the frequency ranges when compared to the spectra based on dynamical matrix. Our calculations, in general, underestimated the frequencies of the modes in the region between 500 and 1500 cm−1 by maximum of 30 cm−1. The power spectra of the velocity autocorrelation function for centre of mass of a molecule of levoglucosan show peaks below 160 cm−1.