Structure and dynamics of the symmetric hydrogen bond in potassium hydrogen maleate: a neutron scattering study

Abstract

The crystal structure of potassium hydrogen maleate KH(OOC–CHCH–COO) has been studied at 300, 14 and 5 K using three-dimensional single-crystal neutron diffraction data. There is no evidence for any phase transition. The crystal structure is orthorhombic, P bcm (D 2h ll), with four entities in the unit cell ( Z=4), in accord with previous X-ray diffraction work. The ion is nearly planar and deviation from C 2v symmetry is negligible. The short internal hydrogen bond parallel to the c axis, R O⋯O=2.427(1) Å at 5 K, is symmetrical with the proton located at the center. Inelastic neutron scattering (INS) spectra, from 16 to 4000 cm −1, of the fully hydrogenated sample and partially deuterated analogues, KH(OOC–CDCD–COO) and KD(OOC–CHCH–COO), have been recorded for powdered samples and aligned single crystals at 20 K. Complex spectral profiles are observed for the stretching mode of the hydrogen bond, ν a OHO, and for the totally symmetric out-of-plane deformation of the maleate ion. An isotopic mixture of KH(OOC–CDCD–COO), 20%, and KD(OOC–CDCD–COO), 80%, reveals that intermolecular coupling is negligible. Frequencies and intensities are amenable to symmetrical potential functions with a central minimum between secondary minima at higher energy. The INS and infrared band profiles suggest that the planar conformation with C 2v symmetry of the maleate ring is unstable in excited vibrational states. The quantum nature of the hydrogen bond is emphasised with the concept of `hydrogen bonding/anti-bonding' states.