Correlation between proton transfer and ³⁵Cl NQR frequency as well as molecular geometry of chloranilic acid in co-crystals with some organic bases

Proton transfer in hydrogen-bonded organic co-crystals of chloranilic acid with some organic bases was investigated by nuclear quadrupole resonance (NQR) spectroscopy. The ³⁵ClNQRfrequencies of chloranilic acid molecule as well as ¹⁴NNQRfrequencies of the organic base molecule were measured with the conventional pulse methods as well as double-resonance methods, respectively. The extent of proton transfer in the O⋯H⋯N hydrogen bondwas estimated from Townes-Dailey analysis of the ¹⁴N NQR parameters. The ³⁵Cl NQR frequency andmolecular geometry of chloranilic acid are correlated to the extent of proton transfer in the protonation process of the organic base molecule. It is shown that the hydrogen bond affects the p-electron system of chloranilic acid. Geometry dependence of the O⋯H⋯N hydrogen bond, i.e. the H-N valence bond order versus the hydrogen-bond geometry correlation is also discussed.