Hindered motion

Probing highly hindered motion by 2H NMR theoretical background and... 

We show that NMR is well suited to study such highly hindered motion when the spatial resolution of the standard echo techniques is improved. The NMR results are discussed in the framework of findings from other experimental techniques so as to provide a comprehensive picture of secondary relaxation processes in glasses. 

A study of the photochemical reactivity of salts of the amino ketone (44) with enantiomerically pure carboxylates has been reported. The irradiations involved the crystalline materials using A, > 290 nm and the reactions are fairly selective which is proposed to be the result of hindered motion within the crystalline environment. Some of the many results, using (S)-(—)-malic acid, R-(+)-malic acid and (2R,3R)-(+)-tartaric acid, are shown in Scheme 1. The principal reaction in all of the examples is a Norrish Type II hydrogen abstraction and the formation of a 1,4-biradical. This leads mainly to the cis-cyclobutanol (45) by bond formation or the keto alkene (46) by fission within the biradical. A very minor path for the malate example is cyclization to the trn 5-cyclobutanol (47). A detailed examination of the photochemical behaviour of a series of large ring diketones (48) has been carried out. Irradiation in both the solid phase and solution were compared. Norrish Type II reactivity dominates and affords two cyclobutanols (49), (50) and a ring-opened product (51) via the conventional 1,4-biradical. Only the diketone (48a) is unreactive... 

Again, in the case of a dissociation reaction in which the transition state was not loose (e.g., the reverse reaction of methyl radical addition to an olefin had a steric factor), PST did not adequately describe the translational energy distribution of the products, and resort was made to a more general treatment that made some allowance for hindered motions.9... 

We assume that the transport of ions across the membrane occurs by passage through pores large enough to accommodate small hydrated ions (e.g., Na+ and Cl-). However, the presence of ions within small pores requires that the Bom energy (44) and hindered motion both be considered. The bulk electrolyte conductivity ae is a function of the concentrations, ci and of the mobilities, t, of its ions ... 

The particle motion in a liquid where the interactions with adjacent particles are important is known as hindered motion. For a monodisperse system the effect of constraint should be taken into account starting from the volume concen-... 

In the case of non-hindered motion, when the drops are relatively far apart... 

The coefficient of turbulent diffusion without consideration of the hindered motion of drops is given by [2] ... 

This is an important aspect since tiny oxide particles may exhibit different catalytic properties with respect to those of large particles (59). The presence of such particles may also hinder motion of molecules within the channels or cavities and thus increase shape selectivity properties. Moreover their presence in addition to isomorphous substitution may greatly favor bifimctional-type reactions. 

Dielectric data revealed secondary transitions in all of the polymers studied. Secondary relaxations were all much broader than those of the a transitions. A representative plot of e" versus temperature for the P relaxation in PHFiPA is shown in Figure 8. It is well known that poly(alkyl methacrylate)s exhibit both P and y transitions. The P transition has been interpreted as due to the hindered motion of the -COOR group about the carbon-carbon bond which links the side group to the main chain. The y transition is thought to be due to local molecular motion of alkyl groups in the side chain (5). The P transitions in PEMA and PTFEMA overlapped the a, Tg, transitions. An attempt to resolve the P transition with the PeakFit program resulted in tan(5) and e versus temperature curves which exhibited anomalous fi-equency effects. 

Randles model are used to describe the frequency dependence of diffusion and the capacitive impedance observed in the intermediate and low frequency ranges. A dual transmission line model has been proposed by including ionic and electronic resistance rails connected in parallel with a capacitance Cp (Fig. 6.5). The model has been used to define the electrochemical behavior of polyaniline, and the capacitance was explained as a result of oxidation and reduction of the pol5mier. Ionic (i i) and electronic (Rg) resistances are used to describe hindered motion of ions and electrons in the system, respectively. The impedance behavior has been found to be dependent on the ratio of the two resistances. 

Contact angle hysteresis is one mechanism that hinders motion of fluid drops in a porous medium. Find an expression for the pressure drop... 

In hydrophobic environments such as activated carbon powder, the vibrational density of states for confined water has been determined by inelastic neutron scattering as a function of temperature and compared with bulk water. For the lowest level of hydration, the translational peak around 6meV and the vibrational peak at 70meV, are less affected than in the Vycor case. However, an up-shift of the librational peak at 70meV, characteristic of hindered motions, is observed [50]. 

This is shown in the figure as a solid line. The general behavior of the curve is in agreement with the data obtained, supporting that stacked side-chains do not contribute the dispersion at room temperature. Thus, it is concluded that the stacking of benzene rings markedly hinders motion of side chain. 

In unfilled polymers the crystallites generally seem to be present in spherulitic aggregates [86]. Because of hindered motion, thermoplastics have a marked tendency to snper cool before crystallising. The degree of super cooling will itself determine the nnmber of... 

Our concern in this section has been the determination of the growth rate of individual particles from dynamic measurement of particle size distributions. Insofar as growth refers to particle convection along the size coordinate, the problem of determining hindered motion of particles differs only in the coordinate (spatial) along which convection occurs. Although we had excluded the mechanics of dispersions from the scope of this book, the reader will note that the analysis of hindered motion, as presented next, eminently serves the purposes of this section. 

Informations on the chain mobility and thus on the micellar structure can further be obtained by NMR. In fact, a decreased mobility of protons in polymer chains with hindered motion causes broadening of the respective NMR lines and even disappearance of the corresponding signal when the polymer is in the glassy state as observed already in the early 1980s by Spevacek [132] and more recently by Riess and Hurtrez [129] onPMMA-b-PAA diblock copolymers and by Eisenberg and co-workers [133]. 

Recent refined studies on CO in Ca ion-containing zeolites A give an estimate of 35 Nm for the force constant between C and the Ca ion. But, for the assignment of the satellites 180 cm distant from the main band it is still unclear whether they are due to coupled translational and rotational motions including those of the cation or due to ihe hindered motions themselves [79]. 

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