Band analysis, excited-state structure

Back electron-transfer rate, cyanometallates, 116 Band analysis, excited-state structure, 211,25 Base, effect on doublet excited... 

Unfortunately, the above analysis can never be widely applicable to the determination of excited-state geometries since so few molecules and ions exhibit vibronically structured absorption bands and excitation profiles, even at low temperatures. Moreover, some questions arise as to the possible breakdown of the Condon approximation. Other types of molecule for which similar analyses have been carried out include 3-carotene, carotenoids (9) and certain carotenoproteins such as ovorubin (10). In these cases the excitation profiles of three skeletal a bands are monitored, and estimates for the change in C-C and C=C bonds lengths ( 0.02 A) have been made. 

B. Vibrational Structure of Electronic Transitions 1. Normal vibrations and their symmetry classification An electronic band system belonging to a polyatomic molecule normally contains a large number and variety of transitions in which vibrational quantum changes are superimposed on the electronic jump. The analysis, besides supplementing infrared and Raman evidence of the ground state frequencies, yields values for the fundamental frequencies of the excited state and is one of the principal sources of information as to its structure. 

Rotational analysis of the ultraviolet bands has been carried out in different degrees by several authors (Brand, 1956 Callomon and Innes, 1962 Dieke and Kistiakowsky, 1934 Parkin et al., 1962). A negative inertial defect A (= Ic — Ia — Ib) in the vibrationless excited state proves conclusively that the structure is pyramidal at equilibrium (Robinson and DiGiorgio, 1958). The structural parameters are summarized in Table 7. 

The intensity distributions of well resolved vibronic spectra recorded in absorption and emission at low temperature are used to determine the geometric distortions of the electronically excited states of coordination compounds. In particular for complexes of lower symmetry, band analysis is necessary leading to results with which bond distance changes can be calculated. For spectra exhibiting no vibrational fine structure, a new technique is proposed which uses time resolved methods, considering deviations from the Poisson distribution of photons by recording time intervals between two successively emitted photons. 

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