The Methyl Torsion in Unsaturated Compounds.

Andrea Zachariou,Stewart F Parker,Russell F Howe,David Lennon,Alexander P Hawkins,Paul Collier

doi:10.1021/acsomega.9b03351

Andrea Zachariou, Stewart F Parker + Show 4 more

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https://doi.org/10.1021/acsomega.9b03351

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Abstract

How the methyl torsion transition energy in unsaturated systems is affected by its environment is investigated. It is strongly influenced by both its immediate neighborhood, (the number of methyl groups present in the molecule) and the intermolecular interactions. It is clear that the intermolecular interactions have a major influence on the torsion transition energy, as demonstrated unambiguously previously for mesitylene and also seen here for other systems. In part, this may be caused by the fact that the methyl torsion is rarely a pure mode (unless enforced by symmetry). Where the crystal structure is available, the assignments have been supported by CASTEP calculations of the unit cell. The agreement between the observed and calculated spectra is generally good, although not perfect, toluene being a case in point, and highlights just how demanding it is to obtain accurate transition energies for low energy modes. The disagreement between observed and calculated inelastic neutron scattering spectra for meta-xylene and 9,10 dimethylanthracene is so severe that it would suggest that there are additional phases to those presently known. Comparison between the full periodic calculations and those for the isolated molecule shows that intermolecular interactions raise the methyl torsion transition energy by at least 8% and in some cases by more than 50%. The presence of more than one methyl group in the molecule generally raises the average torsion energy from the <100 cm–1 seen for single methyl groups to 150–200 cm–1.

Highlights

Methyl groups are ubiquitous in organic molecules
Modes above this energy must be related to the presence of the methyl group; note that in addition to the torsion, in-plane and out-of-plane bending modes of the methyl group will occur below 600 cm−1. (These are not the C−C−H bending modes which occur around 1400 cm−1, but C−C− Me, where the methyl ( Me) group behaves approximately as a point mass)
We have investigated how the methyl torsion transition energy in unsaturated systems is affected by its environment

Summary

Introduction

Methyl groups are ubiquitous in organic molecules. their vibrational spectra have been studied since the early days of infrared spectroscopy.[1]. This arises because of several reasons: neutrons are scattered by nuclei, not electrons, so there are no selection rules, the intensity is determined by the scattering cross section of the atom, of which 1H has the largest magnitude and the amplitude of vibration, which is large for the methyl torsion This has enabled comprehensive studies of the in-plane[6,7] and out-of-plane[8] C−C−C bending modes of the n-alkanes (the longitudinal and transverse acoustic modes, LAM and TAM, respectively), the latter of which includes the methyl torsion. It is found at: 289 cm−1 for ethane, 233/277 cm−1 for propane, 234/265 cm−1 for butane, 244/257 cm−1 for hexane, and 250 ± 5 cm−1 for n ≥ 11.5,7,8 In the longer nalkanes the coupling between the ends of the molecule is essentially zero and the in-phase and out-of-phase torsions are accidently degenerate, so only a single mode is seen

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