Computational analysis of solvent effects in $^{19}F$ magnetic resonance spectra of fluoropyridines

Abstract

The effects of some solvents on the $^{19}F$ NMR shift of 2-fluoropyridine and 3fluoropyridine were first investigated by C.S. Giam and J.L. Lyle in the 1970’s (Journal of Organic Chemistry, 1973, 95 (10), 3235-3239), and their results showed that the solvent dependence of these fluorine chemical shifts could not be correlated with a simple function of the solvent dielectric constant. However, they do correlate with the solvent ET(30) value, based on the solvatochromism of Reichardt’s dye. Be that as it may, the chemical shifts of the fluoropyridines are referenced to fluorobenzene present in the same solvent. Therefore, the influence of fluorobenzene with the solvent is not taken into consideration, where the shifts may have been affected by the solvent induced dependence on the chemical shifts of fluorobenzene itself. Our goal is proper modelling of the solvent-dependence of chemical shifts, first using implicit solvent models. We are also working towards an approach using a combination of explicit and implicit solvent models. Our work has also focused on re-acquiring NMR data for 2- and 3- fluoropyridine and fluorobenzene using trifluoroacetic acid as an internal standard, so that the effects of solvent on fluorobenzene (used as the reference compound) for Giam and Lyle’s study on the fluoropyridines can be assessed. An acceptable modelling system is found for 2-fluoropyridine in SMD model system with def2-TZVP/M052X basis sets for geometry optimization and B3LYP/6-31+G(D) for NMR shielding calculations. A satisfactory correlation was seen between the calculated chemical shifts and experimental values of 2-fluoropyridine, validating our approximations in Gaussian. However, reliable methods are still lacking for 3-fluoropyridine.