Crystals methyl group tunneling

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. 

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. 

Three crystal structures of ACS have been reported, two of CODH/ACS from M. thermoacetica [90,91] and one of the monomeric ACS from C. hydrogeno-formans [104]. ACS consists of three globular domains with the catalytic A-cluster bound to domain 3 at its interface with domain 1. As mentioned above, CODH/ACS has a hydrophobic tunnel network that allows CO, the product of Eq. 2 catalyzed by the C-cluster, to diffuse to the A-cluster where it combines with the methyl group donated by CFeSP to form an acetyl group that, in turn, binds to coenzyme A (Eq. 4). The presence of a tunnel connecting the two active sites was predicted before the structure was determined because, under normal turnover conditions, no CO was detected in the reaction medium [9,10]. 

What is the relevance of the NiFeC species What can be deduced, based on the closed and open ACS conformations observed in the crystal structure, is that CO is more likely to bind in the closed form, prior to methyl group binding (in the open form), than the other way round [91,105]. This is because the tunnel is blocked in the open form and apparently there is not enough space in the closed form for the binding of a methyl group at the E2 site. 

The structure of manganese diacetate tetrahydrate determined with X-ray diffraction reveals the orientations of the rotational axes of the methyl groups (see Figure 8.15) [73-75]. Then, with properly oriented single crystals, the INS intensity of the tunnelling transitions can be probed as a function of the orientation of the momentum transfer vector Q with respect to the rotational axes. The intensity is a maximum when Q is perpendicular to the axis of rotation and it vanishes if Q is parallel. According to such measurements, the tunnelling frequencies at 1.2,50 and 137 p,eV are attributed to sites C, B and A, respectively. 

The different rotational dynamics of the three methyl groups in the manganese diacetate tetrahydrate crystal at 14 K are immediately recognized by visual inspection of the Fourier difference maps (Figure 8.16). The highest potential barrier (lowest tunnelling frequency) occurs for the methyl group C whose protons are quite localized. 

Early measurements of the proton spin-lattice relaxation revealed almost free rotation of the methyl groups [83]. However, the tunnelling bands observed in the 300 xeV range (see Figure 8.21) are quite below the frequency anticipated for almost free rotation ( 675 xeV). Moreover, since all methyl groups in the crystal experience the same effective potential the rather complex spectrum must be interpreted in terms of dynamical correlation between indistinguishable quantum rotors. 

Fig. 8. The torsional potential and energy levels of a methyl-like rotor. The feasible group is isomorphic with C3. The three minima of the potential correspond to the three equilibrium orientations of the rotor in its molecular/crystal surrounding. The torsional levels come in triplets whose individual components transform according to the irreps A, Ea, and E, of C3. The E sublevels come in perfectly degenerate Kramers pairs those of A symmetry are shifted in energy from the Kramers sublevels by the tunnelling quanta the magnitudes of which rapidly grow with...

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