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The Methyl Torsion in Unsaturated Compounds.

Andrea Zachariou^1,2, Alexander P Hawkins^1,2, Paul Collier³

¹School of Chemistry, University of Glasgow, Joseph Black Building, Glasgow G12 8QQ, U.K.

|February 26, 2020

Summary

Methyl torsion transition energy is significantly impacted by molecular environment and intermolecular forces. These interactions increase torsion energy, with multiple methyl groups further elevating it.

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Area of Science:

Physical Chemistry
Solid-State Chemistry

Background:

Methyl torsion transition energy is crucial for understanding molecular dynamics.
Environmental factors, including intermolecular interactions and neighboring groups, are known to influence this energy.

Purpose of the Study:

To investigate how the methyl torsion transition energy in unsaturated systems is affected by its environment.
To quantify the influence of intermolecular interactions and the number of methyl groups on torsion transition energy.

Main Methods:

Inelastic neutron scattering (INS) experiments were performed.
CASTEP calculations of unit cells were used to support spectral assignments where crystal structures were available.
Comparison of periodic calculations with isolated molecule calculations was conducted.

Main Results:

Intermolecular interactions were found to significantly raise the methyl torsion transition energy by at least 8% and up to over 50%.
The presence of multiple methyl groups generally increases the average torsion energy from <100 cm^-1 to 150-200 cm^-1.
Discrepancies in spectra for meta-xylene and 9,10 dimethylanthracene suggest potential additional phases.

Conclusions:

The methyl torsion transition energy is strongly influenced by both the immediate molecular neighborhood and intermolecular interactions.
Accurate transition energy calculations for low-energy modes are computationally demanding.
Intermolecular interactions play a major role in determining the methyl torsion transition energy in these systems.