75 of the ground state molecules that must be considered Schulman et al

have suggested that, since the ground state thermodynamic reaction parameters are defined with respect to the ground state molecules in the ground state equilibrium solvent cage configuration, it would be most meaningful to define excited state thermodynamic parameters with

134

solvent cage configuration. In this scheme it was proposed that the *

best Fdrster Cycle calculated pK value would be obtained using

fluorescence spectra alone. In fact, in some cases this was found to *

give a pK value similar to that obtained by fluorescence titration *

but in others much better calculated pK values were obtained using methods (a) and (c). It has been argued that only the average of the

absorption and fluorescence provides a "best value" of the energy gap between the ground and excited state. In any case, there would appear to be no one valid approach to the Fdrster Cycle and the validity of employing absorption spectra alone, fluorescence spectra alone, or an average of the absorption and fluorescence maxima depends upon the relative importance of vibronic and solvent effects in the electronic

*

spectra of the base and its conjugate acid. The pK values derived

using (a), (b), or (c) in a particular case can differ significantly | and the "best" method must be chosen on the experimental evidence for j

that case. With benzophenones the 0-0 frequency of the B form could be | determined directly from the structured phosphorescence excitation j

+ I

spectrum while for BH the method of averaging the fluorescence and j

phosphorescence maxima was used, j

Another consideration mentioned by Jaffe was the effect of the j

nature of the electronic transition of the two species which according to Jaffe, must be of the same type for the Fdrster Cycle to be applicable.

He suggested that if the absorption bands of the acid and base pair j

correspond to excitation to states of different electronic configuration, j j the proton transfer is necessarily accompanied by an electronic i

i

transition. The low probability of this phenomenon and the modification of the solvent shell likely to accompany the change of electronic I

^ I

state may, according to Jaffe, be expressed in terras of a large entropy effect. Since the electronic transitions of most substituted aromatic Î molecules do not correspond to a well-defined classification, but are

a combination of many transitions mixed through configuration interaction, the strict application of the above rule leads to the conclusion

that the Fürster Cycle can only be strictly applied in very few cases. This factor may be important in the consideration of the

calculations on carbonyl protonation since the base form generally

* + *

has a n-K transition while that of BH is of tc-tc character. Table 5,1

shows that there is always a difference of more than one pK unit between the Fdrster Cycle calculated and the directly measured pK(S^) for

carbonyl protonation. Before considering the possible breakdown of the equal entropy assumption, another effect important to a consideration of carbonyl compounds must be discussed. There is always the

possibility that for weak bases the pK(S^) value used is not based on the correct acidity function and, even if the acidity function is applicable to the ground state molecule, it may still be incorrect for the excited species. Contradictory evidence has been reported as to the acidity function followed by carbonyl protonation. Thus

193

Greig and Johnson reported that aromatic carbonyl group protonations do not follow the amide acidity function H^j conversely Zalewski and

194

Dunn have concluded that the protonation behaviour of aromatic carbonyl compounds does follow H^, The pK(S^) values given in Table 5,1 are based on the H ^ ‘ scale and conversion to the H^ or H^ scale would give more positive pK(S^) values thus increasing the pK(S^) discrepancy, while the pK(S^) values derived by Grieg and Johnson are generally more negative thus decreasing the pK(8 ^) differences.

In the xanthone case, for example, the difference between the Ftirster Cycle calculated pK(8 ^)(F) and the directly derermined value pK(S^)(D) of 2,5 units has three possible sources; it may be due to (a) unreliable estimates of the 0 - 0 energies, (b) basing the pK values on the wrong acidity function or (c) the invalidity of the equal entropy asumption of the Fdrster Cycle in this case. Without experimentally determining an acidity function for xanthone it is not

136

possible to estimate accurately the effect of (b). Since both and acidity scales (see FIG. 1,7) would give more positive pK(S^)

(pK(S^) = -4.1 of xanthone is based on the H^' scale) and thus increase the pK discrepancy, it seems unlikely at first sight that acidity function effects will explain the differences.

Assuming that pK(S^) = -4.1 holds for xanthone then we have

calculated pK(S^) values using two independent sets of results. Through a normal Fdrster Cycle calculation using the spectroscopically

determined energy levels pK(S^)(F) has been found to have a value of 3,6, The fact that a slightly better estimate is obtained in the

xanthone case using the fluorescence maxima alone (see Iküe 3.6) may be due to the fact that the fluorescence spectra of both B and BH^ forms were obtained in aqueous solutions of low acidity where the medium

effects will not be large. In a similar Fdrster Cycle type calculation, but this time using the experimentally determined

A

A

values, a pK(8 ^)(H) of -0.2 ^ 0,5 is obtained (see Chapter 3.1(c)), This is a better estimate of the actual pK(S^) than that obtained from the spectral data and the negative deviation is more easily explicable in terms of an acidity function effect. Both pK(8 ^)(F) and pK(8 ^)(H) are calculated assuming equal entropy changes for the reaction in the ground and the excited state, but now using pK(S^) = -4.2, pK(S^) = 1.0,

A

^

^

A

^

A

A

= 2.303RT pK andZlS* =

T