ISIS Neutron and Muon Source Data Journal

Rotational recoil of ammonia and methyl groups in the inelastic neutronic scattering spectra from TOSCA-like instruments: a quantitative study

Abstract: Methyl rotational tunnelling is a well-known phenomenon. In methanol, this group undergoes a rotational torsion around the C-O axis driven by a shallow potential, while, as the temperature increases, the rotational torsion disappears transforming into a hindered rotation. This behaviour is also achieved via neutron scattering with E>50 meV. In such an event, -CH_3 spins around the C-O axis while the other vibrations can get simultaneously excited. One can call this effect Rotational Recoil since it was described via the Sachs-Teller mass tensor. Recently, the vibrational dynamics of solid methanol has been investigated by INS on TOSCA. A good quantitative match between INS- and DFT-simulated spectra was possible only adding a large background unrelated to either multiple scattering or multiphonons. It was interpreted as an effect of the methyl rotational recoil. However, some doubts still remain, since the INS spectra of formic acid, which contain no -CH_3, show the existence of some background, the origin of which is unclear. For this reason, a new quantitative study of INS from two similar molecular crystals, one with and the other without -CH_3, is mandatory for a rigorous assessment of the effect of the methyl rotational recoil on TOSCA-like instruments. So CD_3OD and HCOOH have been selected, since CH_3OH has been already investigated. After the experiment, INS spectra will be simulated using DFT lattice dynamics for an accurate description of the multiphonons.

Principal Investigator: Dr Daniele Colognesi
Experimenter: Dr Ferdinando Formisano
Experimenter: Dr Miguel Angel Gonzalez
Experimenter: Dr Simone De Panfilis
Local Contact: Dr Svemir Rudic

DOI: 10.5286/ISIS.E.RB2610015

ISIS Experiment Number: RB2610015

Publisher: STFC ISIS Neutron and Muon Source

Data format: RAW/Nexus
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Data Citation

The recommended format for citing this dataset in a research publication is as:
[author], [date], [title], [publisher], [doi]

For Example:
Dr Daniele Colognesi et al; (2026): Rotational recoil of ammonia and methyl groups in the inelastic neutronic scattering spectra from TOSCA-like instruments: a quantitative study, STFC ISIS Neutron and Muon Source, https://doi.org/10.5286/ISIS.E.RB2610015

Data is released under the CC-BY-4.0 license.