Solvent Effects on Nuclear Shieldings: Continuum or Discrete Solvation Models To Treat Hydrogen Bond and Polarity Effects?

Abstract

This paper presents a study on the effects of solvents on nuclear magnetic shielding parameters derived from NMR spectroscopy. In particular, the study focuses on a specific nucleus, nitrogen, in two molecular solutes, acetonitrile and pyridine, immersed in different solvents. Among the solvents, particular attention is devoted to chloroform; its specific characteristics (low polarity and proticity), in fact, make it a very challenging application for theoretical solvation models. Here, we exploit a coupling scheme of solute−solvent cluster structures generated through MD simulations and high-level quantum chemical calculations in which a continuum solvation model is also introduced. This scheme permits the study of the competitive effects due to short-range and highly directional H-bonds and to long-range electrostatic forces and of the way these two effects are taken into account through a discrete, a continuum, or a coupled description of the solvent. Natural bond analysis of computed results has been used to provide insight into the role of solvent-induced modifications of electronic distribution charge in the observed gas-to-solvent shift.