35Cl nuclear quadrupole resonance and infrared studies of hydrogen-bonded adducts of 2-chloro-4-nitrobenzoic acid

35 Cl nuclear quadrupole resonance and infrared spectra of solid hydrogen-bonded adducts of 2-chloro-4-nitrobenzoic acid have been studied in relation to the variation of the ΔpKa value from –2.22 to 9.04. Both methods yield the same critical value of ΔpKa equal to 3.6–3.7, which corresponds to the lowest value of the position of the centre of gravity of the i.r. protonic vibrational band and the stepwise change of 35Cl n.q.r. frequency.