Adamantane diffusion

Large amplitude motions in molecular crystals such as phase transitions and self-diffusion of solid adamantane have also been treated by a MM method (322). However, the program used in this work is useful only for the calculation of intermolecular interactions. 

Eq. (3.21) discussed in Section 3.3.2 is only valid if the motion of the molecules under study has no preferential orientation, i.e. is not anisotropic. Strictly speaking, this applies only for approximately spherical bodies such as adamantane. Even an ellipsoidal molecule like trans-decalin performs anisotropic motion in solution it will preferentially undergo rotation and translation such that it displaces as few as possible of the other molecules present. This anisotropic rotation during translation is described by the three diagonal components Rlt R2, and R3 of the rotational diffusion tensor. If the principal axes of this tensor coincide with those of the moment of inertia - as can frequently be assumed in practice - then Rl, R2, and R3 indicate the speed at which the molecule rotates about its three principal axes. 

Fig. 85. Two-dimensional, 3C spin diffusion spectra of mixtures of adamantane and 2,2,3,3-tetramethylbutane at 75.4 MHz (424) (a) Mixture of powders (b) mixture by melt. Note the absence of cross-peaks between signals belonging to different species in the heterogeneous sample in (a).

Fig. 4.5. Ratio of cross-peak to CH diagonal-peak intensity for r.f.-driven spin diffusion beween the CH and CH2 resonances of adamantane (powder sample) as a function of the offset of the r.f.-carrier frequency from the center of the two resonance lines. The circles represent experimental values obtained with a 30 ms cw spin-lock. The squares represent experimental values obtained with a 30 ms WALTZ-17 spin lock using a spin-lock pulse of...

Fig. 4.7. R.f.-driven ZAS experiment schematic diagram and application to a homogeneous and heterogeneous mixture of adamantane (a) and hexamethyethane (h) [76]. The appearance of cross-peaks, due to intermolecular spin diffusion between the two components, is seen clearly in the homogeneously mixed sample, while it is absent in the heterogeneous mixture, (Figure adapted from Ref. [33]).

SELF-DIFFUSION IN THE MOLECULAR CRYSTAL ADAMANTANE, COMPARISON OF NMR AND PLASTIC-FLOW METHODS. 

Potential energy calculations have been employed in a description of the dynamic structure of adamantane and the nature of the order-disorder phase transition, as well as the mechanism of self-diffusion in the molecular solid. Theoretical calculations on the acid-catalysed hydrolysis of aliphatic esters lend support to the use of Taft , values as a measure of steric effects of alkyl groups correlation of with relative rate data (log k,., ) is discussed as well as that with steric energy parameters (from force-field calculations), which are found to parallel AAH values. " The same authors have outlined a new source of data of use in the estimations of van der Waals strain in hydrocarbons (including norbornane and adamantane). 

Relative reactivity studies of homolytic substitution of monosubstituted benzene derivatives reveal that the 1-adamantyl radical has more pronounced nucleophilic properties than have other, more strained, bridgehead radicals. With benzo-nitrile 21.1) almost exclusive para-substitution is observed, whereas with anisole 0.65) the three possible sites are attacked almost equally. Kinetic studies on the reaction of adamantanethione with adamantane-2-thiol indicate that the rate-controlling chain-propagation step is hydrogen abstraction from the thiol (AdHSH) by the carbon-centred radical AdHSSAd-, and that the main mode of termination involves the diffusion-controlled bimolecular self-reaction of these radicals. ... 

While the experimental results are suggestive, simulation results are conclusive in their support for Lieb and Stein s hypothesis. Further, the simulations suggest refinement of their hypothesis. MD simulations of the passive diffusion within the bilayer of molecules of a range of sizes, including methane, benzene, adamantane, and nifedipine clearly show that small molecule (methane and benzene) movement includes discrete rapid jumps between voids within the bilayer. These voids are often as much as the volume of benzene, but seldom much larger. These jumps have been observed to be moderated by the torsional isomerization of the lipid hydrocarbon chains, whose motion creates passages between existing voids [4, 5,7,21]. These jumps are occasional for benzene, however they are very frequent for the much smaller molecule, methane. The movement of adamantane and nifedipine is... 

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