Relationship between 1H chemical shifts of deuterated pyridinium ions and bronsted acid strength of solid acids

Deuterated pyridine (pyridine-d(5)) is one of the NMR probe molecules widely used for determination of acid strength of solid catalysts. However, the correlation between the H-1 chemical shift of adsorbed pyridine-d(5) and the Bronsted acid strength of solid acids has rarely been investigated. Here, an 8T zeolite model with different Si-H bond lengths is used to represent the Bronsted acid sites with different strengths (from weak, strong, to superacid) and to predict the pyridine adsorption structure as well as the H-1 chemical shift. The theoretical calculation suggests that a smaller H-1 chemical shift of the pyridinium ions on the solid acids indicates a stronger acid strength. On the basis of the results of theoretical calculations, a linear correlation between the pyridine-d(5) H-1 chemical shift and the proton affinity (PA) of the Bronsted acid site has been derived. In combination with the available H-1 MAS NMR experimental data, we conclude that pyridine-d(5) can be used as a scale to characterize the solid acid strength.