Vibrational dynamics in solid methanol investigated through inelastic neutron scattering and molecular dynamics simulations

Abstract

The vibrational dynamics of crystalline $\alpha$-phase methanol and its three isotopic substitutes has been investigated by inelastic neutron scattering, classical molecular dynamics, and ab initio lattice dynamics. Experimental data were collected on the TOSCA neutron spectrometer at low temperature ($T\le 15$ K). Classical molecular dynamics trajectories were obtained using a modified OPLS-AA effective force field. Ab initio lattice dynamics simulations were performed in the DFT framework using a high-level hybrid functional. Methanol vibrational bands have been divided into two main intervals: “external” (i.e., including lattice phonons and molecular librations) and intramolecular. The former domain showed a large dispersion of the vibrational excitations and, in the case of CH${}_3$OH and CH${}_3$OD, high-resolution H-projected vibrational densities of states have been obtained. Such physical quantities favorably compared with the respective infrared/Raman data, while only a semi-quantitative agreement with simulated spectra has been achieved. Above 200 meV, large multiphonon components in the experimental data masked all the weak fundamental vibrational bands. Still, in spite of these difficulties, the reconstruction of the intramolecular methanol spectra has been attempted producing globally good results for CH${}_3$OH and CH${}_3$OD, while those pertaining to CD${}_3$OH and CD${}_3$OD included small discrepancies in the band positions and intensities. DFT lattice dynamics calculations compare reasonably well with the neutron scattering spectra, but still worse than what classical molecular dynamics simulations do. We underline the importance of the present results for interpreting spectral data of amorphous water-methanol mixtures.