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Updated: Dec 5, 2025

Characterizing Lewis Pairs Using Titration Coupled with In Situ Infrared Spectroscopy
Published on: February 20, 2020
Study of interactions between Brønsted acids and triethylphosphine oxide in solution by 31P NMR: evidence for 2 : 1
Elisabet Pires1, José M Fraile1
1Instituto de Síntesis Química y Catálisis Homogénea (ISQCH), Facultad de Ciencias, CSIC-Universidad de Zaragoza, Pedro Cerbuna 12, E-50009 Zaragoza, Spain. josem.fraile@csic.es.
Abstract:
The variation of the 31P chemical shift of triethylphosphine oxide in CDCl3 solution with a series of Brønsted acids at different molar ratios allows the determination of the value for the 1 : 1 species (δ1 : 1), which is much lower than the reported value at infinite dilution. This value correlates with the pKa of the acid in two zones, for acids stronger and weaker than TEPO-H+. The acid strength also controls the exchange rate in solution. The evolution of the chemical shift at high acid/TEPO molar ratios indicates the existence of a second TEPO-acid interaction, which is also dependent on the acid strength. This interaction is much more favorable in the case of a diacid, which shows chemical shift higher than expected for its pKa1 value.
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