The anisotropic effect of functional groups in 1H NMR spectra is the molecular response property of spatial NICS—the frozen conformational equilibria of 9-arylfluorenes

Abstract

Rotation about the single bond adjoining the aryl and fluorene moieties in 9-arylfluorenes can be frozen out on the NMR timescale if methyl groups are located at either one or both of the ortho positions of the aryl substituent. In the ground-state of these rotamers, the planes of the aryl and fluorene moieties are perpendicular to each other and the methyl substituents are consequently positioned either above the fluorene moiety or in-plane with it; thus, the methyl protons are either shielded or deshielded, respectively, due to the ring current effect of the fluorene moiety. This anisotropic effect on the 1H chemical shifts of the methyl protons has been quantified on the basis of through-space NMR shieldings (TSNMRS) and subsequently Δ δ calcd compared with the experimentally observed chemical shift differences, Δ δ exp. In this context, the experimental anisotropic effects of functional groups in the 1H NMR have proven to quantitatively be the molecular response property of theoretical spatial nucleus independent chemical shieldings (NICS). Differences between Δ δ calcd and Δ δ exp were, for the first time, also quantified as arising from steric compression.