Molecular relaxation processes

Special attention is always paid to the questions of estimation of molecular mobility of polymer chains [54-56]. The reasons are obvious thermodynamically non-equilibrium solid-like media, particularly relaxation media, and their physical properties are defined by passing relaxation molecular processes in them, which in turn depend on features of the chemical constitution of the molecular chains and the structural organisation of the polymers [56]. As for parameters, there exist different points of view in describing these processes. So, for example, it is assumed that fast relaxations are defined by the mobility of free chains placed between densely packed domains, which are at the same time nodes of macromolecules physical entanglements network. Such treatment corresponds to the main postulates of the cluster model of the structure of polymers in the amorphous state [13], with the aid of which structure elements can be quantitatively described. 

The dielectric permittivity as a function of frequency may show resonance behavior in the case of gas molecules as studied in microwave spectroscopy (25) or more likely relaxation phenomena in soUds associated with the dissipative processes of polarization of molecules, be they nonpolar, dipolar, etc. There are exceptional circumstances of ferromagnetic resonance, electron magnetic resonance, or nmr. In most microwave treatments, the power dissipation or absorption process is described phenomenologically by equation 5, whatever the detailed molecular processes. 

A plastic s molecular orientation can be developed during processing. Initially the melted molecules are relaxed. During processing the molecules tend to be more... 

For spin-f nuclei, dipolar interactions may be modulated by intramolecular (DF, reorientation etc.) and/or intermolecular (TD) processes. In general, the intra- and inter-molecular processes can produce quite different Tj frequency dispersion curves. In practice, NMR field cycling experiments are often needed to extend the frequency domain from those employed in conventional spectrometers to a lower frequency range (i.e., the kHz regime) for unambiguous separation (and identification) of different relaxation mechanisms. The proton spin relaxation by anisotropic TD in various mesophases has been considered by Zumer and Vilfan.131 133,159 In the nematic phase, Zumer and Vilfan found the following expression for T ... 

Since different molecular processes may simultaneously contribute to the spin relaxation in LC, the relaxation rates due to various relaxation mechanisms are additive if the motions can be decoupled on the basis of sufficiently different correlation times 30... 

The Linewidtli option in the Analysis pull-down menu allows you to measure the linewidths at half height. The ability to recognize different linewidths in your spectrum is important because it may indicate additional molecular processes going on in solution. Broadening of some of the resonances may be indicative of additional non-resolved couplings, dynamic processes or different types of relaxation mechanisms selectively affecting a particular observed nucleus. This option will also be used to estimate linewidths for use as input data for WIN-DAISY as described in detail in Modem Spectral Analysis, volume 3 of this series. 

There is good reason to believe that the pseudo Jahn-Teller effect commonly assumes an important role in a number of molecular processes including electronic relaxation. Large classes of heterocyclic molecules whose zero-order BO electronic states may be very close to one another are ideal candidates for these types of vibronic interaction.127 Should the lowest state 0X of a given multiplicity be strongly coupled to a nearby electronic state 02 the following spectral effects can be expected ... 

The investigation of pure PET and PET/additive blends by combining dynamic mechanical analysis, dielectric relaxation and solid-state NMR techniques, leads to a clear attribution of the molecular processes involved in the ft relaxation of PET, as well as an understanding of the effect of an antiplasticiser additive ... 

Both intra- and inter-molecular processes, which occur in liquids, affect the relaxation rates of the spin systems involved. Our considerations are concerned with reactions and rearrangements which are characterized by life-times that are appreciably longer than typical correlation times for molecular rotations. The latter are included in the derivation of the equation (35) of motion. It seems, therefore, that the equation should be valid for the description of a spin system during the period between reactions. 

PTHPMA is a good example of the analysis of relaxational processes in polymers containing saturated and substituted side rings, and, allow to an understanding the origin of the molecular motions responsible of the fast relaxational complex processes in these systems. 

In summary, it is expected that the bulk attenuation function for ordinary molecular fluids is reasonably well represented by relaxation-type processes in the microwave region. At high frequencies, in the region of the extreme far-infrared, deviations from Eq. (4) will occur, even for a process with a single relaxation time. Phenomenologically, as to- , the efficiency with which a representative collision interrupts the absorption or emission of radiation in a molecular fluid must decrease toward zero, and the relaxation time in Eq. (4) must become frequency-dependent. 

What is the mechanism of spins dropping down from the state to the a state and fanning out around the two cones, and what determines the rates (R = HT and/ 2 = 1/72) of NMR relaxation These processes are intimately tied to the motion of molecules as they tumble ( reorient ) in solution in their rapid Brownian motion, and measurement of the NMR relaxation parameters T and T2 can even give us detailed information about molecular dynamics (motion) from the point of view of each spin in the molecule. A simplified model... 

Media molecular dynamics is important to the formation of the energetic profile of the electron transfer. When ET occurs faster then the medium relaxation, the process is governed by the medium dynamics with the medium relaxation time xs. In such a case the pre-exponential factor in non-adiabatic equation is described by equation (Bixon, 1992)... 

J. D. Lambert, Relaxation in Gases, in Atomic and Molecular Processes, edited by D. R. Bates, Academic, New York, 1962. 

The term relaxation, as applied in physical chemistry (c/. Section 4.6.8), refers to molecular processes occurring after the imposition of a stress on a system. Thus, one can have a system at equilibrium to which a new constraint is applied (e.g., an electric field switched on suddenly onto a dipole-containing liquid). The system is then... 

Dielectric and Related Molecular Processes Two relaxation rates. Let the sites be equally... 

For example, MD simulations are practically the only theoretical tool to give information about various molecular processes behind intermolecular NMR relaxation. MD can also be used to separate the different intramolecular relaxation mechanisms from each other- typically a challenging problem to the experimentalists. In addition, it can be used to evaluate motional models, assumed to be valid in interpretation of NMR results. The topics covered in this chapter will demonstrate how MD simulations can be used as an ideal partner to the NMR relaxation experiment - at the same time as the experimental results can be used to refine the used theoretical models to describe liquids and solutions. It is clear that the both parts, theoreticians and experimentalists, will find a close collaboration beneficial. 

The molecular processes, causing the relaxation in molecular liquids, are on... 

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