Inelastic neutron scattering , hydrogen bonds

The usefulness of potential energy hypersurfaces in describing reaction dynamics and chemical reactivity is well illustrated by Levine and Bernstein [84] and Shaik et al. [85] books. See also the fundamental paper of Hase [86]. This success does not assure that the coordinate representation of quantum system is necessarily truthful. It goes without saying, the coordinate representation is an extremely useful mathematical model. However, from recent inelastic neutron scattering experiments on hydrogen bonded system, the idea that the BO approximation may be inadequate has been advanced by Kearley and coworkers[87]. 

Adsorbed hydrogen on metal surfaces is of particular interest from both theoretical and experimental points of view. Vibrational spectroscopy data on hydrogen adsorbed from the gas phase have been obtained from IR reflection-absorption experiments as well as from electron energy loss spectroscopy and inelastic neutron scattering techniques [39-41]. In UHV, absorption bands for the M-H bond have been reported in the mid- and far-infrared regions [41, 42],... 

Inelastic neutron scattering studies have shown [7-11] that many hydrogen-bonded crystals [potassium carbonate (i.e., KHCO3), various polyanilines, Ca(OH)2, and others] are characterized by the proton dynamics that is very decoupled from the backbone lattice. 

The normal-mode calculation produced 597 frequencies and eigenvectors. We represent the former in Fig. 33 by giving a histogram of the calculated frequencies below 1700 cm", plotting the number of normal modes in each 5-cm interval. Such a density of vibrational states, which can be probed by inelastic neutron scattering (Jacrot et al., 1982), is not observed directly by IR or Raman. As we noted, and will consider further below, the IR spectrum is determined by the dji/dQ in each normal mode, which remains to be calculated. Since hydrogen bonding and TDC have also not been included in this calculation, the values of the amide frequencies and the distribution of the low-frequency modes cannot be considered finalized as yet. 

J. Howard, J. Tomkinson, J. Eckert, J.A. Goldstone A.D. Taylor (1983). J. Chem. Phys., 78, 3150-3155. Inelastic neutron scattering studies of some intramolecular hydrogen bonded complexes a new correlation of gamma-(OHO) vs R(00). 

J.C. Li (1996). J. Chem. Phys., 105, 6733-6755. Inelastic neutron scattering studies of hydrogen bonding in ices. 

P. Papanek, J.E Fischer N.S. Murthy (1996). Macromol., 29, 2253-2259. Molecular vibrations in nylon 6 studied by inelastic neutron scattering. P.Papanek, J.E Fischer N.S. Murthy (2002). Macromol., 35, 4175-4182. Low-frequency amide modes in different hydrogen-bonded forms of nylon-6 studied by inelastic neutron-scattering and density-functional calculations. 

F. Fillaux J. Tomkinson (1992). J. Mol Struct., 270, 339-349. Proton transfer dynamics in the hydrogen bond. Inelastic neutron scattering spectra of Na, Rb and Cs hydrogen carbonates at low temperature. 

C.N. Tam, P. Bour, J. Eckert F.R. Trouw (1997). J. Phys. Chem. A, 101, 5877-5884. Inelastic neutron scattering study of hydrogen-bonded solid formamide. 

J. Tomkinson, I.J. Braid, J. Howard T.C. Waddington (1982). Chem. Phys., 64, 151-157. Hydrogen-bonding in potassium hydrogen maleate and some simple derivatives studied by inelastic neutron-scattering spectroscopy. 

F. Fillaux, B. Marchon, A. Novak J. Tomkinson (1989). Chem. Phys., 130, 257-270. Proton dynamics in the hydrogen bond, inelastic neutron scattering by single crystals of CSH2PO4 at 20 K. 

In this chapter we will consider molecular crystals with normal hydrogen bonds in which the donor A H interacts with an acceptor B. The so-called bifurcated and trifurcated H-bonds [1] as well as the new multiform unconventional H-bonds [2] are beyond the scope of the present chapter. We will focus on the proton dynamics in molecular crystals with strong and moderate H-bonds [3] in the ground electronic state. Attention will be focused on the interpretation of the structural and spectroscopic manifestations of the dynamics of the bridging proton as established in X-ray, neutron diffraction, infrared, and inelastic neutron scattering (INS) studies of H-bonded crystals. 

---