Relaxation rotation-translation

The rotational relaxation times of these nitrocompounds have not been measured. Comparison with the studies of perylene by Klein and Haar [253] suggests that most of these nitrocompounds have rotational times 10—20 ps in cyclohexane. For rotational effects to modify chemical reaction rates, significant reaction must occur during 10ps. This requires that electron oxidant separations should be <(6 x 10-7x 10-11)J/2 2 nm. Admittedly, with the electron—dipole interaction, both the rotational relaxation and translational diffusion will be enhanced, but to approximately comparable degrees. If electrons and oxidant have to be separated by < 2 nm, this requires a concentration of > 0.1 mol dm-3 of the nitrocompound. With rate coefficients 5 x 1012 dm3 mol-1 s 1, this implies solvated electron decay times of a few picoseconds. Certainly, rotational effects could be important on chemical reaction rates, but extremely fast resolution would be required and only mode-locked lasers currently provide < 10 ps resolution. Alternatively, careful selection of a much more viscous solvent could enable reactions to show both translational and rotational diffusion sufficiently to allow the use of more conventional techniques. 

A second difference from the continuum model is that large stresses near the reaction center should undergo thermally activated relaxation. According to the molecular mechanism of stress relaxation proposed above, such irreversible, or plastic, deformations occur in UP when the two decyl radicals back away from the reaction center by rotational translation along their long axes. In the process of making more room for the two new C02 molecules, each radical chain is driven into the adjacent interface between two layers of peroxide molecules. Introduction of a defect or a hole at the end of the peroxide chain should facilitate this motion and allow efficient relaxation of the stress. 

First, collisional relaxation of rotation is fast. One to ten collisions are sufficient for rotation-translation energy transfer. At 1 torr at least one collision per microsecond will occur for both 02 and HC1. In contrast, rotational relaxation by radiation, when allowed, is very slow, of the order of 102 seconds. The absence of... 

Rotational quanta are much smaller than vibrational quanta, and rotational energy is therefore much more easily degraded to translational energy. For most molecules the collision number for rotation-translation transfer is less than 10, corresponding to relaxation times smaller than 10 9 sec at one atmosphere pressure. 

The issue of the relationship between rotational dynamics and the distribution of charge and mass in a molecular ion has seen some attention in studies of high temperature fused salts, most notably cyanide-containing species. Experimental [184] and theoretical [185] studies note complex rotational relaxation dynamics for these systems. These phenomena have been interpreted with the framework of rotational-translational coupling [186], a more detailed but less transparent description than the charge arm framework described above. This may be a useful approach to understanding IL dynamics, but has not to our knowledge been applied to these systems. 

In view of these results for binary glasses composed of molecules with different masses respectively different Tg, one may speculate that the isotropic reorientation of the small molecules results from translational diffusion in an essentially rigid glassy matrix formed by the large molecules, i.e., the diffusional process may be probed by 2H NMR via rotational-translational coupling.183 Finally, we note that the stimulated-echo technique was also applied to study the main relaxation in... 

Miklavc, A. and Smith, I.W.M. (1988) Vibrational relaxation of C2H2 and C2D2 by vibration-rotation, translation (V-R,T) energy transfer. J. Chem. Soc., Faraday Trans. 2 84,227-238. 

The vibrational excitation of COF by collision with argon atoms (1000-3000 K) has been treated bending modes were found to be more easily excited than stretching modes [2233]. The vibrational relaxation of COFj both vibrational — (rotational, translational)... 

It has recently become more widely appreciated that the presence of rotational diffusional anisotropy in proteins and other macromolecules can have a significant affect on the interpretation of NMR relaxation data in terms of molecular motion. Andrec et al. used a Bayesian statistical method for the detection and quantification of rotational diffusion anisotropy from NMR relaxation data. Sturz and Dolle examined the reorientational motion of toluene in neat liquid by using relaxation measurements. The relaxation rates were analyzed by rotational diffusion models. Chen et al measured self-diffusion coefficients for fluid hydrogen and fluid deuterium at pressures up to 200 MPa and in the temperature range 171-372 K by the spin echo method. The diffusion coefficients D were described by the rough sphere (RHS) model invoking the rotation translational coupling parameter A = 1. 

Relaxation within each degree of freedom but still nonequilibrium between vibration and rotation/translation... 

Rotational-rotational (RR) and rotational-translational (RT) energy transfer processes are usually non-adiabatic and fast, because rotational quanta and, therefore, the Massey parameter are small. As a result, the collision of a rotator with an atom or another rotator can be considered a classical collision accompanied by essential energy transfer. The Parker formula for calculation of number of collisions, Zrot, required for RT relaxation was proposed by Parker (1959) and Bray and Jonkman (1970) ... 

The characteristic parameters generally considered are the temperature and electron density which are of primary importance under equilibrium coiulitions For example, with a D.C. plasma generator, two different zones have been pointed out in a nitrogen plasma jet at atmospheric pressure The first zone is such that 10 < n < lO cm and 9000 K < T < 15000 K, and the different criteria show that LTE is achieved. The second zone has a low electronic density 10 < n, < < 10 cm , a quite low temperature 3000 K < T < 7000 K, and uilibrium is not realized, due in part to diffusion. Nevertheless in both cases, the relaxation times of rotation-rotation, and rotation-translation exchanges are sufficiently low to consider the rotational and the translational temperatures to be equal, b) Spectroscopic measurement methods... 

The extremely narrowband emission of a laser allows the specific excitation of molecular states. The non-Boltzmann distribution produced by the excitation process is quickly destroyed by radiation processes and collisional deactivation. The relative contribution of these different deactivation channels depends on the nature of the level excited as shown in Fig. 3. In the microwave region where rotational levels are excited, the radiative life time is very long compared to the very efficient rotational relaxation processes (R—R rotation—rotation transfer and R—T rotation—translation transfer). Therefore, the absorbed radiation energy is transformed within a few gas kinetic collisions into translational energy. The situation is similar for... 

Molecular motions (rotation, translation, and vibration) of a water molecule also turn out to be quite different from those of other common liquids. Here all the six unique features of an individual water molecule outlined in Chapter 1 manifest themselves in diverse ways. As we discuss below, not only is the mechanism of displacements of individual water molecules different, but the collective dynamics and dynamical response of bulk water are also different. For example, the rotational motion of an individual water molecule contains a surprising jump component and vibrational energy relaxation of the O—H mode involves a cascading effect mediated by anharmonicity of the bond. These motions are reflected in many important processes such as electrical conductivity, solvation dynamics, and chemical reactions in aqueous medium. 

Considering these figures, we can estimate that once a CO-molecule has reached a higher vibrational state, it will bring the rotational distribution of this state into equilibrium due to the rotation-translation relaxation (RT) and will be ready for further vibration-vibration (VV-) transfer, e.g. going from v=2 to v=3 etc. In each step, the anharmonicity difference is contributed to the translational energy. This process can go both ways since in the case of two molecules in different vibrational states, an exothermic and endothermic path is possible (Fig.3.3b) ... 

---