Reorientation mechanism

However, the PDMS molecules demonstrate a remarkable flexibility when compared to hydrocarbon or fluorocarbon polymers. The rotation about siloxane bonds is virtually free, since the energy necessary is almost zero. Therefore the rotation of the side hydrophilic groups to the material 

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The Debye phenomenology is consistent with both gas-like and solidlike model representations of the reorientation mechanism. Reorientation may result either from free rotation paths or from jumps over libration barriers [86]. Primary importance is attached to the resulting angle of reorientation, which should be small in an elementary step. If it is... 

The Hubbard relation is indifferent not only to the model of collision but to molecular reorientation mechanism as well. In particular, it holds for a jump mechanism of reorientation as shown in Fig. 1.22, provided that rotation over the barrier proceeds within a finite time t°. To be convinced of this, let us take the rate of jump reorientation as it was given in [11], namely... 

Reorientation of Coordinate Frames and Vectors by Arbitrary Reorientation Mechanisms... 

We present here some very general exact results, which hold for arbitrary reorientation mechanisms of any molecule in an equilibrium isotropic fluid (but not a liquid crystal). A coordinate frame (R) is rigidly attached to the molecule of interest. Its orientation in the laboratory frame (L) is defined by the Euler rotation = (affy) that carries a coordinate frame from coincidence with the laboratory frame L to coincidence with the molecular frame R/ The conditional probability per unit Euler volume [( (0r at time t must depend only on the Euler rotation A = 1 (i.e., rotate first by < 0 then... 

In Section V the reorientation mechanism (A) was investigated in terms of the only (hat curved) potential well. Correspondingly, the only stochastic process characterized by the Debye relaxation time rD was discussed there. This restriction has led to a poor description of the submillimeter (10-100 cm-1) spectrum of water, since it is the second stochastic process which determines the frequency dependence (v) in this frequency range. The specific vibration mechanism (B) is applied for investigation of the submillimetre and the far-infrared spectrum in water. Here we shall demonstrate that if the harmonic oscillator model is applied, the small isotope shift of the R-band could be interpreted as a result of a small difference of the masses of the water isotopes. 

This is one of the simplest of a number of reorientation mechanisms that are allowed by the potential rotational mobility in the side chain of lysine. In this mechanism the function of the covalent intermediate is both to maintain the bond energy to the AMP group and to provide for structural mobility within the active site. 

The fact that the dielectric relaxation of ice is accurately described by a simple Debye curve implies that only a single mechanism is involved. Bjerrum (1951) discussed the possible reorientation mechanisms in ice—either ion-state motion or orientational defect motion—and concluded that the latter was the relevant process. As we shall see later, his conclusion is correct for pure ice, where the greater concentration of L- and D-defects more than makes up for their lower mobility, compared with ion... 

This is illustrated in Fig. 9. The 2D spectra refer to quadrupole echo sequences and characterize two possible reorientation mechanisms of a methyl group (three-site jumps vs continuous diffusion). Drastic spectral differences are observed. Ajqjarratly, these 2D relaxation spectra sensitively indicate the type of motion. The same is true for the corresponding normalized contour dots (see Fig. 9). We note that similar 2D spectra can be obtained from inversion recovery or Jeener-Broekaert sequences (see Fig. 6) [68]. Thus, by applying this 2D technique to different pulse sequoic, the various motions can be differentiated over an extremely wide dynamic range, extending from the fast-rotational to the ultraslow motional re me. Sin<% the different motions (see Fig. 4) modulate different kinds of molecular order (see Fig 3) these orders can be differentiated, likewise. 

Sekkat Z, Wood J, Knoll W. 1995b. Reorientation mechanism of azobenzenes within the trans cis photoisomerization. J Phys Chem 99(47) 17226 17234. 

Eight possible isomerizations could occur for the pinned tg + tg- conformation. Four of these isomerizations involved reorientation angles of about 67° or 113° and were consistent with the NMR data. However, each of the four motions implied a different electric dipole moment and only one of these tg + tg- to g-tg + ) was consistent with dielectric data. Therefore, from a combination of NMR data and dielectric data it was possible to uniquely determine the particular mechanism for reorientation of PVF2 although neither of these methods would determine the reorientation mechanism alone. 

Examination of the reorientation angle distribution W(6,t) obtained at Tg exhibits a secondary peak at around 6 = W that does not change its angular position with time. This is interpreted to indicate the presence of a second reorientation mechanism by which the bond undergoes a large angle jump. Such a revelation of the presence of the second mechanism would not have been possible if only the time-correlation functions, and not the full distributions, had been evaluated. This provides an example in which the molecular modeling offers information that cannot easily be obtained from experiment. 

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