Hydrogen disorder in potassium hydrogen carbonate

Abstract

¹⁷ O Quadrupole double resonance frequencies from the hydrogen-bonded C¹⁷OH and C ¹⁷O⋯H groups in the (HCO₃)^2–₂ dimer in crystalline KHCO₃ have been recorded in natural abundance between 77 and 360 K. The two sets of transitions corresponding to the 1/2 → 3/2 and 3/2 → 5/2 transitions for each group collapse to a single set at 341 K. It has been shown that this behaviour can only be explained by fast concerted two-proton flips between two positions available for the hydrogen atoms within the (HCO₃)^2–₂ dimer and not by 180° flips of the ion as a whole about its C⋯C axis. The two positions differ in energy, this energy difference being itself a function of temperature and hence of the degree of order (s), varying from ca. 6 kJ mol^–1 at 77 K (s= 1 ) to 0.2 kJ mol^–1 at 360 K (s= 0). There appears to be a hydrogen–deuterium isotope effect, the energy difference increasing with the degree of deuteration.