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

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(11)) 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) Angstrom 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, nu(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/antibonding' states. (C) 1999 Elsevier Science B.V. All rights reserved.