Molecular motions relaxation characteristics

Anisotropy of molecular motion monosubstituted benzene rings, e.g. phenyl benzoate (44), show a very typical characteristic in the para position to the substituents the CH nuclei relax considerably more rapidly than in the ortho and meta positions. The reason for this is the anisotropy... 

NMR signals are highly sensitive to the unusual behavior of pore fluids because of the characteristic effect of pore confinement on surface adsorption and molecular motion. Increased surface adsorption leads to modifications of the spin-lattice (T,) and spin-spin (T2) relaxation times, enhances NMR signal intensities and produces distinct chemical shifts for gaseous versus adsorbed phases [17-22]. Changes in molecular motions due to molecular collision frequencies and altered adsorbate residence times again modify the relaxation times [26], and also result in a time-dependence of the NMR measured molecular diffusion coefficient [26-27]. 

Using the time-dependent aspect of state diagrams, Roos (2003) illustrated the effects of temperature, water activity, or water content on relaxation times and relative rates of mechanical changes in amorphous systems (Figure 36). This diagram can be considered as a type of mobility map, where mobility increases (relaxation time decreases) as temperature and/or water content/activity increases. Le Meste et al. (2002) suggested the establishment of mobility maps for food materials showing characteristic relaxation times for different types of molecular motions as a function of temperature and water content. 

When a chain has lost the memory of its initial state, rubbery flow sets in. The associated characteristic relaxation time is displayed in Fig. 1.3 in terms of the normal mode (polyisoprene displays an electric dipole moment in the direction of the chain) and thus dielectric spectroscopy is able to measure the relaxation of the end-to-end vector of a given chain. The rubbery flow passes over to liquid flow, which is characterized by the translational diffusion coefficient of the chain. Depending on the molecular weight, the characteristic length scales from the motion of a single bond to the overall chain diffusion may cover about three orders of magnitude, while the associated time scales easily may be stretched over ten or more orders. 

This type of molecular motion seems to occur less frequently than the preceding ones. The existing results indicate that it is probably more characteristic of polyacrylates127 than of polymethacrylates149. Fragmentary evidence of this relaxation motion obtained up to now is presented in Sect. 5.3. 

It is evident that simple theories of molecular motion are not adequate to explain experimental nmr relaxation parameters in certain polymer systems as well as in some highly associated small molecules. As field dependent nmr relaxation studies become more widespread, the observation of these relaxation characteristics will undoubtedly be found more general than is currently thought. 

Freezing of a dipolar liquid is accompanied by a rapid decrease in its electric permittivity [8-10]. Following solidification, dipole rotation ceases and the electric permittivity is almost equal to n, where n is refractive index, as it arises from deformation polarisation only. Investigation of the dynamics of a confined liquid is possible from the frequency dependences of dielectric properties, which allows both the determination of the phase transition temperature of the adsorbed substance and characteristic relaxation frequencies related to molecular motion in particular phases. 

The dynamic characteristics of adsorbed molecules can be determined in terms of temperature dependences of relaxation times [14-16] and by measurements of self-diffusion coefficients applying the pulsed-gradient spin-echo method [ 17-20]. Both methods enable one to estimate the mobility of molecules in adsorbent pores and the rotational mobility of separate molecular groups. The methods are based on the fact that the nuclear spin relaxation time of a molecule depends on the feasibility for adsorbed molecules to move in adsorbent pores. The lower the molecule s mobility, the more effective is the interaction between nuclear magnetic dipoles of adsorbed molecules and the shorter is the nuclear spin relaxation time. The results of measuring relaxation times at various temperatures may form the basis for calculations of activation characteristics of molecular motions of adsorbed molecules in an adsorption layer. These characteristics are of utmost importance for application of adsorbents as catalyst carriers. They determine the diffusion of reagent molecules towards the active sites of a catalyst and the rate of removal of reaction products. Sometimes the data on the temperature dependence of a diffusion coefficient allow one to ascertain subtle mechanisms of filling of micropores in activated carbons [17]. 

Anisotropy of molecular movement monosubstituted benzene rings, e.g. phenyl benzoate (44), show a very typical characteristic in the para position to the substituents the CH nuclei relax considerably more rapidly than in the ortho and meta positions. The reason for this is the anisotropy of the molecular motion the benzene rings rotate more easily around an axis which passes through the substituents and the para position, because this requires them to push aside the least number of neighbouring molecules. This rotation, which affects only the o-and m-CH units, is too rapid for an effective spin-lattice relaxation of the o- and m-C atoms. More efficient with respect to relaxation are the frequencies of ijiolecular rotations perpendicular to the preferred axis, and these affect the p-CH bond. If the phenyl rotation is impeded by bulky substituents, e.g. in 2,2, 6,6 -tetramethylbiphenyl (45), then the T, values of the CH atoms can be even less easily distinguished in the meta and para positions (3.0 and 2.7 s, respectively). 

The solid-state C-NMR spectra of the two polymorphs of furosemide revealed the existence of altered chemical shifts and peak splitting patterns indicative of differences in molecular conformations. In this work, studies of T-[p relaxation times were used to show the presence of more molecular mobility and disorder in form II, whereas the structure of form I was judged to be more rigid and uniformly ordered. During a solid-state spectroscopic study of the polymorphs of losartan, it was deduced that the spectral characteristics of form I implied the presence of multiple orientations for the w-butyl side chain and the imidazole ring. It was also concluded that form II was characterized by a large molecular motion of the w-butyl side chain. 

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