Heteronuclear editing method for detecting NOE between chemically equivalent protons: Application to polycyclic aromatic hydrocarbons

The complete characterization of polycyclic aromatic hydrocarbons (PAHs) and tetrasubstituted ethylenes is critical to an understanding of their reactivity, for which NMR is an important tool. Chemical shifts can provide a direct measure of charge distribution and aromaticity. Unfortunately, COSY, NOESY and heteronuclear correlation cannot provide a complete assignment of the NMR spectra for some carbon-rich PAHs with symmetrical bay regions. The protons in the bay regions would yield NOE signals if they were not symmetrical. Natural substitution of ¹²C with ¹³C can be used to break the symmetry and yield these useful NOE signals. Using gradient-assisted editing, unequivocal assignments have been achieved for some previously problematic molecules.