Methyl group rotational tunneling

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. 

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... 

A.J. Horsewih, Quantum tunnelling aspects of methyl group rotation studied by NMR, Prog. Nucl. Magn. Reson. Spectrosc. 35 (4) (1999) 359—389. http //dx.doi.org/10. 

V0 is the potential height, and describes the rotation of the methyl groups. Steiskal and Gutowsky (20) give the dependence of the tunneling frequency vT of the reciprocal absolute temperature as parameter for V0. Figure 5 gives these results. 

Figure 5. Comparison of the correlation frequencies calculated according Equation 1 with the averaged tunneling frequencies vr calculated by Stejskal and Gutowsky for the methyl groups assumed as quantum-mechanical rotators as a function of the temperature for PC (x) and PMST (o). The potential height in units of kcal./mole is the parameter...

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. 

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