ACIDITY FUNCTIONS FROM C-13-NMR

A method of generating a thermodynamic acidity function from C-13-NMR measurements is presented. An indicator base is sought in which two carbon atoms change their charge density very differently upon conversion to the corresponding conjugate acid. The difference between the chemical shifts of the signals for these atoms (DELTAdelta) is an acidity-dependent parameter from which other medium effects upon the chemical shifts have been canceled out. Unsaturated ketones like mesityl oxide (1a) and 4-hexen-3-one (1b) are sensitive probes for this approach because the signal for Calpha changes very little, whereas the signal for Cbeta undergoes a large shift upon hydronation to 2a and 2b. The DELTAdelta parameter varies linearly with the concentration of indicator, at least below 1.5 M. A short extrapolation gives the chemical shift difference at infinite dilution and eliminates the problem of noncancelability of the activity coefficient term in the construction of an acidity function. Rather than establish still another acidity function, the DELTAdelta0 parameters were correlated with H0 values for sulfuric acid solutions. This correlation was then used to determine H0 for other acids. The method was tested on two acids for which good literature values are available, HClO4 and CH3SO3H, and then used to determine H0 of H3PO4 and BF3.xH2O, for which the literature data needed verification and correction, and of samples of industrial catalysts. The slope s of the correlation of DELTAdelta with concentration of indicator varies with H0, being the most negative at the acidity where the indicator is half-protonated. The pK(BH+) value for the indicator can then be easily determined (pK(BH+) = H0,50%) from the position of the minimum in the s vs H0 plot.