Abstract
Phosphonic acids, aminophosphonic acids, and phosphonocarboxylic acids are characterized by an advanced hyphenated technique, combining potentiometric titration with NMR spectroscopy. Automated measurements involving 13C, 19F and 31P nuclei lead to “pseudo 2D NMR” spectra, where chemical shifts or coupling constants are correlated with analytical parameters. Dissociation constants, stability constants, dynamic and specific chemical shifts are determined. Macroscopic and microscopic dissociation equilibria are discussed.
Highlights
NMR-controlled titration, known as NMR titration, a useful tool combining NMR and analytical aspects, is based on fundamental observations in the dawn of NMR spectroscopy: “Early phosphorus NMR studies of condensed phosphates showed that raising the acidity of phosphate solutions increased the shielding of the phosphorus nucleus, causing a shift of the 31 P resonances to higher fields by several ppm” [1]
A first titration curve of H3 PO4 shown as δP vs. pH was derived in this paper [2]. 13 C-NMR measurements on linear aliphatic acids revealed that COOH groups in Cn H2n+1 COOH (n = 0 to 4) exhibit higher chemical shifts δC than COO− groups of corresponding anions Cn H2n+1 COO−
The TXO-HPLC probe head improved the signal-to-noise ratio of “pseudo 2D NMR” spectra and reduced the concentration of titrand required by this procedure: The following concentrations for sensor nuclei are recommended: 1 H: 0.25–0.01 mol/L, 13 C: 0.50–0.005 mol/L, 19 F: 0.01–0.005 mol/L, 31 P: 0.01–0.001 mol/L, and 113 Cd: 0.25–0.1 mol/L. 113 Cd NMR was used when studying protolytic and complex formation equilibria of (H2 O3 P-CH2 )2 NCH2 CH2 N(CH2 PO3 H2 )2 (EDTMP)
Summary
NMR-controlled titration, known as NMR titration, a useful tool combining NMR and analytical aspects, is based on fundamental observations in the dawn of NMR spectroscopy: “Early phosphorus NMR studies of condensed phosphates showed that raising the acidity of phosphate solutions increased the shielding of the phosphorus nucleus, causing a shift of the 31 P resonances to higher fields by several ppm” [1]. A characteristic downfield shift of δC ranging from 5.1 to 4.7 ppm was observed for deprotonation by addition of tetramethylammonium hydroxide to carboxylic acids [3]. In subsequent years, those phenomena attracted the attention of numerous studies dealing with inorganic and organic phosphorus chemistry. A key paper in understanding the NMR titration of geminal bisphosphonate structures described three asymmetric esters of chlodronic acid (HO) (O)P-CCl2 -P(O)(OiPr)OH,. Those sophisticated and detailed instructions should be followed for accurate analytical and NMR measurements, data evaluation and subsequent publications [22]
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