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 b een studied at 300, 14 and 5 K using three-dimensional single-crystal neutr on diffraction data. There is no evidence for any phase transition. The cry stal 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 neu tron scattering (INS) spectra, from 16 to 4000 cm(-1), of the fully hydroge nated sample and partially deuterated analogues, KH(OOC-CD=CD-COO) and KD(O OC-CH=CH-COO), have been recorded for powdered samples and aligned single c rystals 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-C OO), 20%, and KD(OOC-CD=CD-COO), 80%, reveals that intermolecular coupling is negligible. Frequencies and intensities are amenable to symmetrical pote ntial functions with a central minimum between secondary minima at higher e nergy. The INS and infrared band profiles suggest that the planar conformat ion with C-2v symmetry of the maleate ring is unstable in excited vibration al states. The quantum nature of the hydrogen bond is emphasised with the c oncept of 'hydrogen bonding/antibonding' states. (C) 1999 Elsevier Science B.V. All rights reserved.