Origin of the 31 P NMR Chemical Shift in Lewis Acid Adducts of Triethylphosphine Oxide. Does the Gutmann–Beckett Method Relate to Lewis Acid Strength?

The Gutmann–Beckett method involves the reaction of a phosphine oxide with a Lewis acid, followed by measurement of the change in 31P NMR chemical shift (Δδ) relative to the free phosphine oxide. This is the most commonly used experimental method to assess Lewis acid strength in solution and on solid materials containing Lewis acid sites. This study describes the origin of the 31P NMR Δδ deshielding that occurs in triethylphosphine oxide (TEPO) adducts of Lewis acids. 57 Lewis acid adducts were studied using DFT methods. These models span typical three-, four-, and five-coordinate Lewis acids as well as models that approximate the coordination sphere of Lewis acid sites proposed to be present in heterogeneous materials. When a TEPO···Lewis acid adduct forms, electron density from the oxygen is transferred to the Lewis acid, which reduces the negative hyperconjugation from the oxygen to the σ*P–C that weakens the P═O bond. Experimental and DFT studies show that the 31P NMR chemical shift deshields in TEPO···Lewis adducts because the most shielded δ33 component of the chemical shift tensor shifts dramatically downfield. This deshielding is correlated with the weakening of the P═O bond. Natural chemical shift (NCS) analysis shows that δ33 deshielding in Lewis acid adducts is due to coupling of the filled σP–C with the empty π*P═O, the LUMO of the TEPO fragment. This study connects the 31P NMR chemical shift, in particular the experimentally observable Δδ33, to P═O bond weakening. Thus, the Gutmann–Beckett method does not provide information on adduct formation energy, the more typically sought measure of Lewis acidity, but rather provides a different thermodynamic descriptor of Lewis acid strength in the weakening of the P═O bond.