Stereochemistry, recoil

Mossbauer spectroscopy is the study of recoilless resonant fluorescence " Sn Mossbauer spectroscopy has been found to be a most usefifl method for studying the bonding and stereochemistry of tin compounds in the solid state. The two most important parameters are the isomer shift (5, mm s ) and the quadrupole sphtting (A q, nuns ), although the recoil-free fractions and temperature coefficients can also supply useful structural indications. 

This chapter summarizes many of the contributions that the recoil technique of generating excited radiotracer atoms in the presence of a thermal environment is making to the field of chemical dynamics. Specific topics discussed critically include characterization of the generation and behavior of excited molecules including fragmentation kinetics and energy transfer, measurement of thermal and hot kinetic parameters, and studies of reaction mechanisms and stereochemistry as a function of reaction energy. Distinctive features that provide unique approaches to dynamical problems are evaluated in detail and the complementarity with more conventional techniques is addressed. Prospects for future applications are also presented. 

In this review a summary of some of the recent progress in applying recoil radiotracers to three areas of dynamics will be outlined. No attempt has been made to be encyclopedic but rather selected examples have been chosen, often from the authors own work. The areas into which this review is divided are excited molecule dynamics kinetic parameter characterization and reaction mechanisms and stereochemistry. 

Early interest in stereochemistry of recoil reactions focused on developing a model for hot processes. The conventional view was that such reactions were direct in nature in the gas phase, occurring on a time scale that promoted retention of configuration. In condensed phases the picture was complicated by the expectation that the local environment caged fragments generated in the recoil process, and thus the stereochemical outcome of the reaction was either random or dependent on specific features of the nearest neighbors. 

The most careful study to date of the stereochemistry of high-energy substitution reactions involving a heavy recoil atom characterized the chlorine-for-chlorine replacement reactions in 2-chloropropionyl chloride and 2-chloro-4-methylvaleryl chloride (49). Definitive evidence for a predominance of inversion of the chiral center in the 2(S) and 2(R)-chloropropionyl chlorides during reaction in the gas phase was obtained as illustrated in Equation 10. 

The stereochemistry of certain thermal reactions involving atom addition processes and resultant radical intermediates can also be studied conveniently by recoil techniques. In this type of experiment the recoil atom used to initiate the process must be thermalized before reaction, by diluting the system with a large concentration of nonreactive bath molecules. Two recently reported results on geometric isomerization serve as examples of the type of processes that can be studied (55,56). Both involve recoil chlorine atom reactions with unsaturated substrates. 

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