Unimolecular theory

The quasi-equilibrium theory (QET) is the most widely used theoretical framework for the discussion of the fragmentation pattern of the parent ion in a uni-molecular process. Although other unimolecular theories (see Levine, 1966) have been subsequently proposed, the QET has traditionally been applied for... 

It should be noted that his conclusion is based upon the assumption that the density of the oil in such films is the same as that of oil in bulk. If this were not the case, but if an oil film, for example, had only half the density of ordinary oil, the value calculated for the thickness would be doubled, and we should either have to abandon the unimolecular theory or make the further assumption that the change of density occurred entirely in the direction perpendicular to the surface. This question has hardly been rigorously decided, but the mass of evidence now available strongly supports the unimolecular character of insoluble films upon I water. Another assumption tacitly made is that the molecule is approximately symmetrical. 

This problem was resolved in 1922 when Lindemann and Christiansen proposed their hypothesis of time lags, and this mechanistic framework has been used in all the more sophisticated unimolecular theories. It is also common to the theoretical framework of bimolecular and termolecular reactions. The crucial argument is that molecules which are activated and have acquired the necessary critical minimum energy do not have to react immediately they receive this energy by collision. There is sufficient time after the final activating collision for the molecule to lose its critical energy by being deactivated in another collision, or to react in a unimolecular step. 

The crucial step in the development of unimolecular theory was the postulate of a time lag between the activation and reaction steps in the master mechanism for all elementary reactions given in Chapter 1. During this time an activated molecule can either be deactivated in a deactivating energy transfer collision, or it can alter configuration to reach the critical configuration and react. All elementary reactions involve three steps, two energy transfer steps and one reaction step, and for unimolecular reactions... 

This reaction is important in its own right, but it is also important in showing that all aspects of kinetics must be at one s fingertips when interpreting the kinetic data and proposing a mechanism. To understand the arguments leading to the interpretation of the data for this reaction it is essential that the basics of unimolecular theory, Section 4.5, are understood. It is also essential that, when a predicted rate expression... 

Internal excitation of the products of these reactions is limited by the small amount of energy which is available for redistribution, and, so far, no details of the energy partitioning have been reported. As Miller, Safron, and Herschbach point out, it should be possible to apply unimolecular theories [76] or statistical theories such as that invoking phase space, which has been developed by Light and his co-workers [77] and requires strong coupling in the reactive collisions. 

Modern unimolecular theory has its origins in the work of Rice, Ramsberger and Kassel [44] who investigated the rate of dissociation of a molecule as a function of energy. Marcus and Rice [44] subsequently extended the theory to take account of quantum mechanical features. This extended theory, referred to as RRKM theory, is currently the most widely used approach and is usually the point of departure for more sophisticated treatments of unimolecular reactions. The key result of RRKM theory is that the microcanonical rate coefficient can be expressed as... 

Looking at the dates associated with the development of bimolecular and unimolecular theories, respectively, reveals that it took at least two decades to go from two to one. It was so slow it was like molasses drippin through a bitty hole. —Old Southern Saying. 

The unimolecular theory largely rests on the basis of negative evidence which opposes other theories. An elimination mechanism involving a cyclic transition state has been proposed [6] ... 

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