Energy Exchange in Molecular Collisions

As stated in Section III.8, the derivation of kinetic equations for non-equilibrium reactions requires knowledge of microscopic rate constants both for elementary chemical reactions and for relaxation processes. The relaxation rate constants must be allowed for explicitly only for processes occurring at a rate lower than or comparable with the reaction rates, i.e. for relaxations that can be considered to be incomplete in microscopic conversions (see III.8). 

Taking into account the usual relation between the times of translational (Ttr), rotational (vrot) a-nd vibrational (v ib) relaxation 

In more complicated cases, the perturbations of equilibrium distributions over vibrational, rotational, and sometimes translational degrees of freedom must be taken into account. It must be borne in mind that the relation between macroscopic relaxation and the reaction times is far from always defining the extent of non-equilibrium. The true criterion for the reaction-induced perturbation of equilibrium distribution is formulated in terms of the microscopic relaxation and of the reaction rate constants which define the relation between the rates of changes in the population of the given quantum states of reactants caused by these two processes. For this reason, the study of elementary rates of relaxation processes is of essential kinetic interest. 

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E.E.Nikitin, Band shapes of induced rotational and vibrational spectra of diatomic molecules, in Adv. Molec. Spectroscopy, Pergamon Press, p.298 (1962) E.E.Nikitin, Resonance and nonresonance intermoleeular energy exchange in molecular collisions, Disc.Faraday Soc. 33, 14 (1962)... 

E.E.Nikitin, Resonance and nomesonance intermolecular energy exchange in molecular collisions, Disc.Faraday Soc. 33, 14 (1962)... 

The physical mechanism of thermal-energy conduction in liquids is qualitatively the same as in gases however, the situation is considerably more complex because the molecules are more closely spaced and molecular force fields exert a strong influence on the energy exchange in the collision process. Thermal conductivities of some typical liquids are shown in Fig. 1-5. 

For example, the study of global warming is hindered, in his view, because the intermolecular interactions are not well known. "To properly model the processes, we have to understand the exchange of energy during molecular collisions. If we know them, we can predict an immense amount."... 

At present it is universally acknowledged that TTA as triplet-triplet energy transfer is caused by exchange interaction of electrons in bimolecular complexes which takes place during molecular diffusion encounters in solution (in gas phase -molecular collisions are examined in crystals - triplet exciton diffusion is the responsible annihilation process (8-10)). No doubt, interaction of molecular partners in a diffusion complex may lead to the change of probabilities of fluorescent state radiative and nonradiative deactivation. Nevertheless, it is normally considered that as a result of TTA the energy of two triplet partners is accumulated in one molecule which emits the ADF (11). Interaction with the second deactivated partner is not taken into account, i.e. it is assumed that the ADF is of monomer nature and its spectrum coincides with the PF spectrum. Apparently the latter may be true when the ADF takes place from Si state the lifetime of which ( Tst 10-8 - 10-9 s) is much longer than the lifetime of diffusion encounter complex ( 10-10 - lO-H s in liquid solutions). As a matter of fact we have not observed considerable ADF and PF spectral difference when Sj metal lo-... 

Since the products are the same chemical species as the reactants, the over-all reaction is substantially thermoneutral except for activation energy, the problem of energetics is thus side-stepped. The apparent ter-molecular reaction required by Eq. (43) is also no problem, as the dissolved molecules are essentially in constant collision with water molecules. Wilmarth, Dayton, and Flournoy questioned the adequacy of this concerted attack mechanism, however, as they believed that it would predict acid catalysis of the exchange reactions, as well as base catalysis. 

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