Nitrogen‐15 nuclear magnetic resonance studies of hydrogen bonding and proton transfer in some pyridine trifluoroacetates in dichloromethane

The 15N NMR chemical shifts of ten substituted pyridines (B) and their complexes with trifluoroacetic acid (AHB) were measured at the natural abundance level in dichloromethane. The plot of the relative chemical shifts [Δδ(15N)δ(AHB)δ(B)] against ΔpKa gives a titration curve which reflects a protometric equilibrium AH…B⇌A−…HB+. These data were used to determine the constants of the overall proton transfer reaction (Kexp). A linear relationship holds between log Kexp and ΔpKa. Δδ(15N) values can be treated as a good hydrogen bond parameter.