Nuclear magnetic resonance spectroscopy. Carbon-13 spectra of five-membered aromatic heterocycles

Abstract

High-resolution ^(13)C nmr spectra have been obtained for the five-membered nitrogen heterocycles and for furan, thiophene, and selenophene. All possible carbon-proton coupling constants have been observed and assigned to the proper protons. The chemical shifts and coupling constants are discussed both empirically and with respect to extended Hückel calculations. The chemical shifts follow variations in the local charge densities and the long-range carbon-proton coupling constants may be correlated with geometrically corresponding carbon-proton and proton-proton coupling constants in substituted ethylenes. Extended Hückel calculations seem uniformly poor in correlating spin-spin coupling constants in these systems.