Indicator spectral changes

When acetone is treated with hydroxylamine in aqueous solution near neutral pH, the carbonyl UV absorption intensity decreases very rapidly this fast spectral change is followed by a much slower absorption increase that is due to the appearance of the oxime product. This suggests that, at such pH values, the initial addition is very rapid and the second step, dehydration of the carbinolamine, is the rds. Figure 5-12 is a plot of the apparent first-order rate constant against pH for this reaction. As the pH is decreased from neutrality, the rate increases, indicating that the rds... 

The spectral changes which occur in increasingly acid solutions of polyaza-heterocycles may indicate a second ionization. This event, however, can readily be distinguished from dehydration by measuring the spectra in anhydrous dichloroacetic acid, provided that the pKa value for the anhydrous species is above 1. Anhydrous dichloroacetic acid has a Hammett acidity function (Hq) of — 0.9 (as determined using o-nitroaniline as the solute), and the ultraviolet spectrum of a base with a p > 1 would be that of the anhydrous cation in this 2 A. Albert and W. L. F. Armarego, J. Chem. Soc. 4237 (1963). 

By correlating the observed spectral changes with the concentrations of added cycloamylose, dissociation constants of the cycloamylose-substrate adducts may be calculated (Rossotti and Rossotti, 1961). Values of the dissociation constants determined in this manner for a variety of complexes are presented in Table II. In most cases, stoichiometries of the complexes have been shown to be 1 1 from the presence of distinct isosbestic points in the spectrophotometric titrations. In a few cases, additional spectral perturbations are observed as the cycloamylose concentration is increased, indicating more complex modes of association. Methyl orange, for example,... 

Spectral changes (Fig. 2) obtained on addition of pyridine to an aqueous solution of la indicate stepwise substitution of both aqueous ligands (Eq. (1)). 

All these data show that the ionic strength can induce a change in pKa up to 0.5-0.6 unit. Because the ionic strength effect causes spectral changes indistinguishable from those caused by pH, a correction method is desirable. A method based on a double pH indicator system, i.e. pyranine and 4-methylumbellifer-one, was proposed by Opitz and Lubbers 1. 

Figure 4. UV-visible spectral changes during 366-nm photolysis of a CHSCN solution of [PtgHs-(dppm)i PF6. Arrows indicate the direction of the spectral changes after beginning photolysis and the spectra were successively recorded after 0, 0.5, 1, 1.5, 2, 3, and 5 min total irradiation time.

These spectral changes are consistent with either (a) freezing out a stable non-classical ion, or (b) the presence of the classical ion in which the 6,2-hydride shift has been slowed down, but the Wagner-Meerwein rearrangement is still exceedingly rapid. If the ion is non-classical, the spectrum indicates that it must be a comer-protonated nortricyclene rather than the edge-protonated ion because the latter would have split into a 1 2 1 pattern. 

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