Neutron spectroscopy of deuterated substitutes and DFT modeling vibrational spectra of methanol clusters

Abstract

The vibrational density of states of the four differently deuterated substitutes of methanol in crystalline and amorphous phases were obtained from the INS spectra measured at 20 K in the frequency range up to 1600 cm −1. The experimental spectra are interpreted and assigned with help of the DFT ab initio modeling vibrational spectra of the chain and ring methanol clusters. With the increasing number of methanol molecules in clusters, the calculated vibrational spectra satisfactorily reproduced the experimental ones. The character of different bands observed in experimental spectra of solid methanol can be well interpreted by the harmonic dynamics of the 4-ring or 8-chain molecular clusters accepted as typical structural units of liquid methanol. The “boson peak” in the vibrational spectrum of glassy methanol below 50 cm −1 can be assigned as the low frequency oscillations of methyl groups around OH–O bonds. The lowest internal mode of free molecule (contra-balance twist of hydroxyl and methyl around C–O bond), splits into two different character modes of methanol clusters. The methyl torsions around C–O bond mixed with external vibrations in the frequency range 100–200 cm −1. The OH group torsions around O–C bond form a broad band of hydrogen bonds vibrations at ∼700–900 cm −1, while the OD group torsion form the band at 500–700 cm −1. These bands are quite well reproduced in the experimental spectra of differently deuterated substitutes of solid methanol at 20 K.