Reaction spectroscopically linear independent

As in thermal kinetics, the changes in concentration are usually monitored in photokinetics by spectroscopic measurements. Under these conditions, a distinction between different mechanisms is not possible for many reactions. Just the number of spectroscopically linear independent steps of reaction can be determined (see Chapter 4). 

Redox potential data frequently correlate with parameters obtained by other spectroscopic measurements. The correlation of E° potentials with gas-phase ionization potentials has already been briefly discussed. Electronic transitions observed by UV-visible spectroscopy involve the promotion of an electron from one orbital to another and this can be viewed as an intramolecular redox reaction. If the promotion involves the displacement of an electron from the HOMO to the LUMO, then the redox potentials for the reduction of the compound, °REd, and for its oxidation, °ox, are of importance. For a closely related series of compounds, trends in oxidation and reduction potentials can be related to shifts in the absorption frequency, v. If the structural perturbation causes the HOMO and the LUMO to rise or fall in energy in tandem, then (E°RED — E°ox) will remain constant in such cases the HOMO—LUMO frequency (energy) will be essentially independent of the structural perturbation. Where there is a differential influence of the perturbation on the HOMO and the LUMO, then ( °red E°ox) will vary as will the energy of the electronic transition. In such cases a linear correlation of °red or E°0x may result. In the limit the energy of the HOMO, or more usually the LUMO, will be unaffected by structural perturbation where the acceptor orbital is pinned, direct linear correlation of E°Gx with v should be apparent. With E°ox and v in a common energy unit, the plot E°0x versus v should have a slope close to one.33-36... 

This is the simplest explanation for the observation that when L and M have come to an equilibrium which contains these species in comparable amounts, the concentration of L decreases to near zero even while M remains at its maximal accumulation. Recent measurements of the quasi-equilibrium which develops in asp96asn bacteriorhodopsin before the delayed reprotonation of the Schiff base confirm this kinetic paradox [115]. Two M states have been suggested also on the basis that the rise of N did not correlate with the decay of M [117]. In monomeric bacteriorhodopsin the two proposed M states in series have been distinguished spectroscopically as well [115]. It is well known, however, that kinetic data of the complexity exhibited by this system do not necessarily have a single mathematical solution. Thus, assurance that a numerically correct model represents the true behavior of the reaction must come from testing it for consistencies with physical principles. It is encouraging therefore that the model in Fig. 5 predicts spectra for the intermediates much as expected from other, independent measurements, and the rate constants produce linear Arrhenius plots and a self-consistent thermodynamic description [116]. 

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