Electronic absorption spectroscopy bonding, excited-state spectroscopic

The physical chemist of today has a wide variety of methods at his disposal for the experimental investigation of electronic structure and all of them have been used in attempts at obtaining evidence of the participation of outer d-orbitals in bonding. One such group of methods is constituted by the various techniques of radiofrequency spectroscopy, which have the advantage that they yield information about the molecule in its ground state. In this they have a distinct superiority over, say, electronic absorption spectra where it is necessary to consider both ground and excited states. Moreover much of the data derived from radiofrequency spectroscopic methods concerns essentially just one part of the molecule so that attention can be concentrated on those atoms of interest in whatever study happens to be under way. 

The radical pairs in the diradical and dicarbene reaction intermediates have been extensively studied spectroscopically and lead to a consistent picture of the reaction mechanisms and of the ir-bond and radical electron structure of the intermediates. A new field of research has been opened by time resolved ESR spectroscopy of transient triplet states on the polymer chains. Spectroscopic worlc concerning triplet excited states is just at the very beginning. The correlation of the optical spectra (Figure 8), the transient absorption data well... 

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