Diffusion in molecular crystals

Studies of nano-structured liquids in confined geometries and at surfaces

“…This gives rise to a significant increase in T 2 at the onset of rotational motion, followed by a plateau in T 2 (T ) as T increases [94]. If this plateau is fitted with a constant amplitude, and this value is then subtracted from the T 2 relaxation data on the Arrhenius graph, a further straight line is obtained.…”

“…In plastic crystals, rotation is well known to give rise to a form of motional averaging such that the actual proton separations must be replaced by the separation of the molecular centres. This gives rise to a significant increase in T 2 at the onset of rotational motion, followed by a plateau in T 2 (T ) as T increases [94].…”

“…At higher temperatures there is the plateau that is believed to relate to the dynamical region where rotational motion is probably still increasing, but where, for the purposes of NMR relaxation, the effective proton separations just average to a constant value equal to the molecular-centre separations 35 [94]. There are various possible causes that need to be considered for this factor of 10 increase in the T 2 at around 200K to 220K, for the defective ice, over that for brittle ice as discussed previously [2,83,20].…”

Nuclear magnetic resonance cryoporometry

“…There is also some evidence for the existence of metastable states (Dore et al, 2004a;Mu and Malhotra, 1992) although this has not been observed in NMR measurements due to problems associated with rapidly cooling the relatively large thermal mass of the samples. Further, there is now evidence (Awschalom and Warnock, 1987;Liu et al, 2006;Webber et al, 2007b) suggesting that, even for water, some of the ice formed in a pore may be in a plastic or rotationally mobile phase (Chezeau and Strange, 1979), with a T 2 longer than in normal brittle ice.…”

Rapid measurements of diffusion using PFG: Developments and applications of the Difftrain pulse sequence

“…In the earliest days of NMR experimentation it was recognized that longitudinal T 1 and transverse T 2 relaxation time measurements were sensitive to molecular reorientations in solids (1). A good example of such measurements can be found in the review of diffusion in molecular crystals by Chezeau and Strange (2). It was also noted that transverse relaxation measurements of liquids via analysis of spin echo decays were sensitive to translational motion (3,4) and could be used to estimate the self-diffusion coefficients of liquids.…”