Hydrogen bonding in 1,2-diazine-chloranilic acid (2: 1111) studied by a ¹⁴N nuclear quadrupole coupling tensor and multi-temperature X-ray diffraction

Protons involved in the H-bond system in 1,2-diazine-chloranilic acid (2 : 1) are assumed to be in jumping motion in the double-minimum potential corresponding to the two extreme electronic states of O-H⋯N and O ^-⋯H-N⁺. ¹⁴N nuclear quadrupole coupling constants were determined by ¹H-¹⁴N nuclear quadrupole double resonance. Assuming that the observed coupling constants are result of a fast exchange of the two extreme electronic states, the coupling constants for each state were estimated by use of the equilibrium populations of the two extreme states determined from multi-temperature X-ray single-crystal diffraction. It was suggested that not only the population but also the electron distribution of the extreme electronic states itself changes with temperature.