Determination of acidity functions and acid strengths by 13C NMR

Correlation between Chemical Reactivity and the Hammett Acidity Function in Amorphous Solids Using Inversion of Sucrose as a Model Reaction

“…The definition of acid strength becomes complicated when the solvent is nonionizing or aprotic. It has been pointed out that defining acid strength by acid dissociation alone can be flawed as the nondissociated acidic species and general acid catalysis might also contribute to the overall acidity and rate of reactivity 32, 33. However, if the acid strength is defined and measured relative to an acid–base indicator, then the relative strength of two acids can be determined by comparing the extent to which they protonate the same indicator.…”

Quantitative NMR spectroscopy of biologically active substances and excipients

“…This is due to a corresponding variation of the pH value of the test solution. It is well known that the proton chemical shifts of an analyte containing acidic and/ or basic groups are sensitive to the pH value of the sample since the chemical shift of an atom is related to the amount of charge present in the molecule [15]. The pH-dependency of chemical shifts complicates both the identification and the quantitative analysis of the potential impurities in Ala when utilizing D 2 O as solvent.…”

“…Using qNMR spectroscopy it is sufficient to find, out of the multitude of NMR signals, a single baseline separated resonance for each component of interest. Besides the application of elaborative pulse sequences and line shape fitting techniques [7] an optimal separation of overlapping resonances can be achieved by changing the kind of solvent [8][9][10][11][12][13][14] or the pH value of the test solution [15][16][17][18][19], by applying different sample concentrations [20][21][22], or by adding auxiliary reagents such as cyclodextrins [23][24][25] or lanthanide shift reagents [23,26]. In some cases, the temperature also affects the signal separation [27,28].…”

The origin and development of the acidity function

“…If the ratio of the activity coefficients is close to one, or at least is relatively constant throughout the extensive measurements, H o reflects -loga H þ . This is likely true because the researchers were able to work at low concentrations of base (and protonated base) for which the activity coefficients are close to unity (Fȃrcaşiu and Ghenciu 1996), and they mostly used bases of the same structural type (substituted anilines) for which one might expect the activity coefficients of the various B's and BH ? 's to be similar in value.…”

“…NMR spectroscopy has become the method of choice in the determination of acidity functions in recent years, especially for highly acidic media. NMR is particularly useful for highly colored liquids where spetrophotometry is ineffective and for solid catalysts which inevitably are highly colored (Fȃrcaşiu and Ghenciu 1996). Kinetic measurements (Olah et al 1985) and sophisticated molecular orbital calculations (Koppel et al 2000) are also useful in estimating H o .…”