Temperature dependence of one-dimensional hydrogen bonding in morpholinium hydrogen chloranilate studied by ³⁵Cl nuclear quadrupole resonance and multi-temperature X-ray diffraction

The temperature dependence of ³⁵Cl NQR frequencies and the spin-lattice relaxation times T₁ has been measured in the wide temperature range of 4.2-420 K for morpholinium hydrogen chloranilate in which a one-dimensional O-H⋯O hydrogen-bonded molecular chain of hydrogen chloranilate ions is formed. An anomalous temperature dependence of the NQR frequencies was analyzed to deduce a drastic temperature variation of the electronic state of the hydrogen-bonded molecular chain. The hydrogen atom distribution in the O⋯H⋯O hydrogen bond is discussed from the results of NQR as well as multi-temperature X-ray diffraction. Above ca. 330 K, the T₁ showed a steep decrease with an activation energy of ca. 70 kJ mol^-1 and with an isotope ratio ³⁷Cl T₁/ ³⁵Cl T₁ = 0.97 ± 0.2. The orientational change of the z axis of electric field gradient tensor in conjunction with the hydrogen transfer between adjacent hydrogen chloranilate ions is suggested as a possible relaxation mechanism.