Methyl rotational tunnelling

An alkyne which has been studied is 2-butyne (dimethylacetylene, CH3-CSC-CH3) [28]. The crystal contains two molecules in the unit cell and the focus of the work was on the interactions between the methyl groups and how their influence on the methyl rotational tunnelling spectrum and the librational and torsional modes. 

Tetramethylplumbane mixtures with tetramethylstannane were studied at 2 K by high resolution INS (inelastic neutron scattering), revealing rotational tunnelling transitions for the methyl groups146. 

M. Plazanet, M.R. Johnson, A. Cousson, J. Meinnel H.P. Trommsdorff (2002). Chem. Phys., 285, 299-308. Molecular deformations of halogeno-mesitylenes in the crystal structure, methyl group rotational tunneling, and numerical modeling. 

There exist now many examples of PT reactions in molecular solids that take place by tunneling at low temperature. Relatively few of these reactions occur in sufficiently simple and well defined systems that are amenable to numerical modeling, so that observations can be confronted with theoretical predictions. However, the rapid increase and availability of computer power, together with the development of efficient algorithms for molecular dynamics calculations, is certain to extend predictive numerical studies to more complex systems for which often quite detailed experimental information is already available. As shown here, this interplay of numerical and classical experimentation is already very successful in tackling the rotational tunneling of methyl groups in crystals. 

Our discussion has focused on barriers in the range available via the usual DNMR methods. There are specialized approaches that allow determination of lower barriers usually found in most single bond rotations, such as methyl rotation.Note that investigations of very low barriers involving groups with hydrogen nuclei may need to consider quantum tunneling effects. 

Colmenero. J. Mudkhopadhyay. R. Alegria, A. Frick. B. Quantum rotational tunneling of methyl groups in polymers. Phys. Rev. Lett. 1998. 93(11), 2350-2353. 

Latanowicz investigated the concepts of correlation time, autocorrelation function and spectral densities in the context of the tunnelling jumps through the potential barrier, superimposed on another type of motion. Two specific case were discussed hindered methyl rotation combined with isotropic overall reorientation and jumps within a double-well hydrogen bond potential in connection with librations of the whole molecule. 

Figure 13 Hole burning via nuclear spin conversion in rotational tunnelling states of methyl groups. The sample is di-methyl-s-tetrazine in n-octane at 1.4 K. The central hole at the laser frequency is accompanied by two antiholes symmetrically shifted by 37 GHz.

Methyl-substituted malonaldehyde (a-methyl-/3-hydroxyacrolein) provides an opportunity to study the role of asymmetry of the potential profile in the proton exchange. In the initial and final states, one of the C-H bonds of the methyl group is in the molecular plane and directed toward the proton position. The double well potential becomes symmetric only due to methyl group rotation over tt/6, when the C-H bond lies in the plane perpendicular to the molecular one. As a result, proton tunneling occurs in combination with CH3 hindered rotation and the... 

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