Structure and dynamics of the symmetric hydrogen ... - ResearchGate

402( )F. Fillaux et al.rChemical Physics 244 1999 387–403driven by collisions with the molecules of the solvent.Momentum transferred to the proton parallel tothe hydrogen bond Ž Q .H 5 is needed for the excitationof the stretching mode whilst the rotation of thecarboxylic groups requires momentum transfer to theoxygen atoms Ž Q , Fig. 10 .O H . Therefore, energyand momentum must be transferred to the H and Oatoms independently and simultaneously, within thevery short life time of vibrational excited statesŽy11-10 s .. The interconversion rate at a giventemperature depends on the distribution of kineticmomentum in the solution and thus on the mass andshape of the molecules in the solvent.8. ConclusionNeutron diffraction and INS confirm that the hydrogenbond in potassium hydrogen maleate at lowtemperature is symmetrical with the proton located atthe center. The maleate ring is planar with moderatemean-square amplitudes for the out-of-plane deformations.The naOHO dynamics and the totallysymmetric out-of-plane ring deformation are largelydecoupled from other modes and from each other.They are compatible with very anharmonic potentialfunctions composed of three non-equivalent minima.These potentials account for infrared band activitiesand frequency shifts upon deuteration. All the datasupport C 2v symmetry for the hydrogen maleate.The potential function for the proton stretchingmodeis quite different from single or double wellsaccounted for by theoretical models. The narrowcentral well shows that the hydrogen bond is stableonly if the proton is localised at the center of theO PPP O bond. This can be related to the distributionof electrical charges in the symmetric planar ion.Upon rather small proton displacements away fromthe central position, the distribution of electriccharges and the structure change. We conclude thatthe proton ground state is hydrogen bonding whilstexcited vibrational states are hydrogen anti-bonding.This is a spectacular manifestation of the quantumnature of the strong symmetric hydrogen bond. Consequently,the binding energy, which is essentiallyelectrostatic in nature, is not simply related to thedissociation energy determined by the quantum dynamicsof the proton.In the ground state, the hydrogen maleate ion canbe effectively regarded as an intermediate state forproton transfer from one carboxylic group to theother. In excited vibrational states of the protonstretching-mode, the hydrogen bond is broken, theproton is transferred to an oxygen atom and thecarboxylic group rotates by a large angle of ;"458.The energy dissociation of ;500 cm y1 is compatiblewith thermal activation at room temperature. Theconcept of hydrogen bondingranti-bonding statescould be thus a key issue in further studies andmodelling of the proton transfer mechanism.AcknowledgementsHere, we should thank S.W. Parker for the helpfulassistance on the TFXA spectrometer and stimulatingdiscussions.Referenceswx 1 A.N. Baker, J. Chem. Phys. 22 Ž 1954.1625.wx 2 G.C. Pimentel, A.L. McClellan, The Hydrogen Bond, Freeman,San Francisco, 1960.wx 3 S.N. Vinogradov, R.H. Linell, Hydrogen Bonding, Van Nostrand-Reinhold,New York, 1971.wx 4 M.D. Joesten, L.J. Schaad, Hydrogen Bonding, MarcelDekker, New York, 1974.wx 5 R. Janoschek, in: P. Schuster, G. Zundel, C. Sandorfy Ž Eds. .,The Hydrogen Bond, Recent Developments in Theory andExperiments, Vol. 1, North-Holland, 1976, p. 165 and referencescited therein.wx 6 E. Matsushita, T. Matsubara, Prog. Theor. Phys. 67 Ž 1982.1.wx 7 K. Shibata, S. Ikeda, J. Phys. Soc. Jpn. 61 Ž 1992.411.wx 8 S. Ikeda et al., J. Phys. Soc. Jpn. 61 Ž 1992.2619.wx 9 E. Matsushita, J. Phys. Soc. Jpn. 62 Ž 1993.2079.w10x S. Ikeda et al., J. Phys. Soc. Jpn. 63 Ž 1994.1001.w11x W.W. Cleland, Biochemistry 31 Ž 1992.317.w12x W.W. Cleland, M.M. Kreevoy, Science 264 Ž 1994.1887.w13x J.A. Gerlt, P.G. Gassman, Biochemistry 32 Ž 1993.11943.w14x S.F. Darlow, W. Cochran, Acta Crystallogr. 14 Ž 1961.1250.w15x S.F. Darlow, Acta Crystallogr. 14 Ž 1961.1257.w16xH.M.E. Cardwell, J.D. Dunitz, L.E. Orgel, J. Chem. Soc.,1953, p. 3740.w17xK. Nakamoto, Y.A. Sarma, G.T. Behnke, J. Chem. Phys. 42Ž 1965.1662.w18xJ. Maillols, L. Bardet, R. Marignan, J. Chim. Phys., 66,1969, pp. 522 and 529.w19xF. Avbelj, B. Orel, M. Klanjsek, D. Hadzi, Spectrochim.Acta 41A Ž 1985.75.