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Prediction by means of the Forster cycle

Assuming that the ionization entropies AS and AS of AH and AH , respectively, are equal, the difference in enthalpies AH — AH can be replaced by the difference in standard free energies AG — AG , each of them being given by [Pg.104]

This equation shows that if the emission band of the basic form is located at higher wavelengths than that of the acidic form, the excited-state pK is lower than the ground-state pK, and AH is a stronger acid than AH. Conversely, if the emission band of the basic form is blue-shifted with respect to the emission band of the acidic form, the acid in the excited state is weaker than in the ground state. [Pg.104]

In practice, the wavenumbers vah and va- corresponding to the 0-0 transitions of AH and A are often difficult to determine accurately. They are usually estimated by means of the average between the wavenumbers corresponding to the maxima of absorption and emission  [Pg.104]

However, the best approximation is found by using the intersection point of the mutually normalized absorption and emission spectra (as was the practice in Forster s laboratory). In the case of large Stokes shift, it may be difhcult to determine an intersection point it is then preferable to take the average of the wave-numbers corresponding to half-heights of the absorption and emission bands, which is a better approximation than the average of the wavenumbers corresponding to the maxima. [Pg.104]

8) The approximation AS AS is reasonable in most cases because the major contribution to entropy variation is the change in the number of molecules and in the number of charges (and of the corresponding solvation). Both these contributions are the same in the ground and excited states. The changes in the internal degrees of freedom can be shown to [Pg.104]


See other pages where Prediction by means of the Forster cycle is mentioned: [Pg.103]    [Pg.103]    [Pg.103]    [Pg.103]    [Pg.103]    [Pg.103]    [Pg.103]    [Pg.103]    [Pg.396]   


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