Polyene radicals

Jones et al. [144,214] used direct dynamics with semiempirical electronic wave functions to study electron transfer in cyclic polyene radical cations. Semiempirical methods have the advantage that they are cheap, and so a number of trajectories can be run for up to 50 atoms. Accuracy is of course sacrificed in comparison to CASSCF techniques, but for many organic molecules semiempirical methods are known to perform adequately. 

Evidence indicates [28,29] that in most cases, for organic materials, the predominant intermediate in radiation chemistry is the free radical. It is only the highly localized concentrations of radicals formed by radiation, compared to those formed by other means, that can make recombination more favored compared with other possible radical reactions involving other species present in the polymer [30]. Also, the mobility of the radicals in solid polymers is much less than that of radicals in the liquid or gas phase with the result that the radical lifetimes in polymers can be very long (i.e., minutes, days, weeks, or longer at room temperature). The fate of long-lived radicals in irradiated polymers has been extensively studied by electron-spin resonance and UV spectroscopy, especially in the case of allyl or polyene radicals [30-32]. 

Electronic structure of diene and polyene radical cations... 

We are aware that our review is by no means complete, the topic of diene and polyene radical cations having ramifications into such diverse fields as biology or interstellar chemistry. In the following we shall first discuss the photoelectron spectra of dienes... 

It should be mentioned that the present review does not cover in detail the ground-state electronic and/or molecular structure of diene and polyene radical cations as revealed, for example, by electron spin resonance (ESR) spectroscopy or variants thereof. 

III. EXCITED STATES OF POLYENE RADICAL CATIONS BY OTHER METHODS A. Introduction... 

Finally, before turning to a brief review of methodological and theoretical aspects, we mention that one of the distinguishing features of planar conjugated polyene radical cations (cf Section II.D) is that their EA spectra reveal a breakdown of the single-configuration picture for ionic excited states which had been used so successfully in interpreting their PE spectra (cf. Section II). 

The first systematic studies of polyene radical cations were carried out by Shida and coworkers230 using the above methods. In this connection an important feature of these species was discovered, namely that they exist in the form of multiple rotamers which do not interconvert easily due to the partial double-bond character which all polyene C—C bonds attain upon ionization (see below). 

This method proved quite generally applicable, in particular for the study of the electronic structure of several polyene radical cations and, more recently, for investigations of... 

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