Gutmann–Beckett method

Background

The ³¹P chemical shift (δ) of Et₃PO is sensitive to chemical environment but can usually be found between +40 and +100 ppm. The O atom in Et₃PO is a Lewis base, and its interaction with Lewis acid sites causes deshielding of the adjacent P atom. Gutmann, a chemist renowned for his work on non-aqueous solvents, described an acceptor-number scale for solvent Lewis acidity with two reference points relating to the ³¹P NMR chemical shift of Et₃PO in the weakly Lewis acidic solvent hexane (δ = 41.0 ppm, AN 0) and in the strongly Lewis acidic solvent SbCl₅ (δ = 86.1 ppm, AN 100). Acceptor numbers can be calculated from AN = 2.21 × (δ_sample − 41.0) and higher AN values indicate greater Lewis acidity. It is generally known that there is no one universal order of Lewis acid strengths (or Lewis base strengths) and that two parameters (or two properties) are needed (see HSAB theory and ECW model) to define acid and base strengths and that single parameter or property scales are limited to a smaller range of acids (or bases). The Gutmann–Beckett method is based on a single parameter NMR chemical shift scale but is in commonly used due to its experimental convenience.