Pure and Applied Chemistry

Abstract: We review recent studies of the interconversion mechanism of OH···O hydrogen-bonded centrosymmetric dimers through proton transfer in the prototype crystals of potassium hydrogen carbonate (KHCO₃) and benzoic acid (C₆H₅COOH). The point at issue is whether the proton distributions at various temperatures arise from classical statistical mixtures of tautomers or quantum mechanical superposition states. A related issue is whether it is possible to probe a quantum superposition without inducing decoherence and classicality. We show that neutron diffraction can realize decoherence-free measurements for strictly defined scattering geometries and thus evidence macroscopic quantum correlations. We present a theoretical framework for decoherence-free macroscopically entangled states of the sublattice of protons. The neutron diffraction cross-section of protons is enhanced by a factor of ~45, compared to regular Bragg diffraction, and quantum correlations are observed with remarkable contrast. At elevated temperatures, up to 300 K, quantum correlations are unaffected by proton transfer. The crystal is a coherent superposition of macroscopic tunnelling states, like Schrödinger's cat in a superposition of dead and alive states.