Rotational barriers from neutron scattering

In this section we will discuss several examples of the study of large amplitude (primarily torsional) vibrations by neutron scattering. We will try to present an instructive evaluation of these results and their comparison to relevent optical data. We will also briefly discuss the derivation of barriers to internal rotation from the observed torsional peaks. 

The H2 ligand undergoes rapid two-dimensional hindered rotation about the M-H2 axis, that is it spins (librates) in propeller-like fashion with little or no wobbling. This phenomenon has been extensively studied by neutron scattering methods and computationally [19-22], Significantly, there is always at least a small barrier to rotation, AE, brought about by M—>H2 cr backdonation (BD) (Scheme 20.8). The cr-donation from H2 to M cannot give rise to a rotational barrier since it is completely isotropic about the M-H2 bond. The barrier actually... 

The barrier to rotation of CH3 groups, V 5.0 kJ/mol, was first estimated from a correlation between barrier contributions of other groups and the minimum internuclear distance for various organic compounds (using d(Ge-C) = 1.98 A) [3]. Very low values based on H NMR relaxation measurements (2.7 kJ/mol) [34] and experimental and calculated entropies (3.1 kJ/ mol) [57] cannot be correct see also [90, p. 19]. Consistent values for solid Ge(CH3)4 have been obtained from far-infrared studies [52], neutron scattering [80,90,91], and proton Zeeman spin-lattice relaxation experiments [94] (barrier height V and torsional ground level Eq in... 

NHT The barriers to rotation for the NH4 ion in the ordered and disordered phases of NH4C1 and NH4Br have been calculated from their torsional frequencies and the results compared with the activation energies determined from n.m.r. results.46 Neutron quasielastic scattering has been used to probe the reorientations of NH4 ion in a single crystal of ammonium bromide.47 A Raman study of the phase transitions in NH4Br has been undertaken over a range of temperature and pressure. It was found that the ordered phase On is cubic, and that in this phase the NHJ ions are ordered.48... 

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