Unimolecular resonance compound state

Figure A3.12.8. Possible absorption spectrum for a molecule which dissociates via isolated compound-state resonances. Eq is the unimolecular threshold. (Adapted from [4].)...

Compound-state resonances are important in quantal theories of unimolecular decomposition. They are prepared in low-energy atom (molecule)-molecule collisions when part of the relative kinetic energy of the motion becomes temporarily converted into excitation of the internal (rotational and/or vibrational) degrees of freedom of either partner. When this excitation occurs, the molecular system has insufficient energy in its relative motion to separate. One can also prepare compound-state resonances by using electromagnetic radiation (e.g., a laser) to excite the molecule. Thus, it is proper to view these resonances as the natural extension of the bound vibration-al/rotational eigenstates into the dissociative continuum. 

The theory of isolated resonances is well understood and is discussed below. Some initial work has been done on the theory of overlapping resonances (Remade et al., 1989 Desouter-Lecomte and Culot, 1993 Someda et al., 1994a,b) and its relation to experiment (Reid et al., 1994). Much of the research of overlapping resonances has its origins in nuclear physics, where the dissociation of a compound nucleus is treated (Ericson, 1960, 1963 Satchler, 1990 Rotter, 1991). For example, fluctuations in product state populations, called Ericson fluctuations (Satchler, 1990 Rotter, 1991), may arise from coherent excitation of overlapping resonances. However, more work needs to be done to develop a complete theory of overlapping resonances and this topic is not discussed here. Mies and Krauss (1966, 1969) and Rice (1971) were pioneers in treating unimolecular rate theory in terms of the decomposition of isolated Feshbach resonances. 

As stated in section I, the termination mode of the particular monomer determines the number of functionalities per macromolecular chain. Most monomers undergo both unimolecular and bimolecular termination reactions. It is often observed that both respective monofunctional and bifunctional polymers are formed and well-defined functional polymers cannot be prepared. The use of allylmalonic acid diethylester in free-radical polymerization has been proposed to overcome the problems associated with the aforementioned functionality. In the presence of the allyl compound, the free-radical polymerization of monomers, regardless of their termination mode, proceeds entirely with the unimolecular termination mechanism, as shown in Scheme 9. Because allyl compounds lead to degradative chain transfer, the resulting allyl radical is quite stable due to the allyl resonance. Monofunctional polystyrene, polyvinylacetate, and poly(t-butyl methacrylate) were prepared by using this approach [33]. Subsequently, various macromonomers were derived from these polymers. 

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