Pi-Electron Charge Densities and Chemical Shifts in Nitrogen Heterocyclics

The relation Δσ = aΔq is examined in some detail, where Δσ is the chemical shift between a proton in an aromatic nitrogen heterocyclic and the corresponding proton in its aromatic polycyclic analog, and Δq is the excess π-electron charge density on the carbon to which the proton is bonded in the heterocyclic. It is found that the relation is quite satisfactory for positions ortho to a nitrogen, as long as only one nitrogen appears in that particular cycle.