1H and 15N chemical shifts of adsorbed acetonitrile as measures to probe the Brønsted acid strength of solid acids: A DFT study

Isotropic chemical shift, a simple and uninterrupted NMR parameter, has been demonstrated to be a viable spectral scale to measure acid strength of solid acids. Here, the chemical shifts of acetonitrile interacting with modeled Brønsted acid sites were predicted by means of density functional theory (DFT) calculations. Results reveal linear correlations between 1H (acidic proton) or 15N chemical shift and proton affinity of Brønsted acid site, suggesting that the chemical shifts can be used to probe intrinsic Brønsted acid strength of solid acids. Steric effects of microporous solid acids on 1H and 15N chemical shifts were simulated by varying H⋯N distance or orientation of the CN bond in the CH3CN⋯H+ adsorption complexes. Linear correlation is obtained between 1H chemical shift and apparent acid strength regardless of the nature of the steric effect, while significant deviation occurs to 15N chemical shift when the orientation of the CN bond is sharply deviated from its optimal orientation. The calculation results suggest that combination of 1H and 15N chemical shift can provide additional information on the structure of zeolite framework.

Graphical abstractCorrelations between 1H or 15N chemical shift of acetonitrile/Brønsted acid adsorption complexes and acid strength of Brønsted acid site have been derived by DFT calculation.Figure optionsDownload full-size imageDownload high-quality image (78 K)Download as PowerPoint slide