Exemplary model scenarios for molecular reorientation processes with and without restrictions. (a) Unrestricted isotropic rotational diffusion of more or less spherical molecules bearing two dipoles I and S. The cones symbolize precession about the external flux density B⃑0. A typical example is rotational diffusion of the hydration complexes of electron-paramagnetic ions in aqueous solutions (see, e.g., ref. 35 and 36). (b) Fast but restricted polymer segment reorientations by fluctuating rotational isomerism superimposed by slow Rouse chain modes (see Chapters 8 and 13 and ref. 18). (c) RMTD process: adsorbate molecules diffuse along a more or less rough surface of a solid adsorbent (see ref. 13 and 37). While being adsorbed at the surface, molecules are subject to fast and restricted rotational diffusion. This can be superimposed by a (slow) displacement process along the surface via excursions to the bulk fluid. After readsorption, the initial orientation u⃑i will be converted to the final orientation u⃑f controlled by the local surface topology. In a sequence of numerous ‘desorption/(diffusive bulk excursion)/readsorption’ cycles, adsorbate molecules can intermittently probe surfaces in this way over relatively long distances while – as per surface topology – they retain orientation correlations orders of magnitude longer than the actual surface residence times during the sporadic adsorption events. Reproduced from ref. 12 with permission. Copyright 2017 Elsevier BV. (d) Order-director fluctuations collectively reorient molecules on a time scale much longer than restricted rotational diffusion about the long-axis of the molecules (see Chapter 11 and ref. 5).
