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Recent work on understanding why protons migrate with increasing temperature in short, strong hydrogen bonds is extended here to three more organic, crystalline systems. Inelastic neutron scattering and density functional theory based simulations are used to investigate structure, vibrations, and dynamics of these systems as functions of temperature. The mechanism determined in a previous work on urea phosphoric acid of low frequency vibrations stabilizing average crystal structures, in which the potential energy well of the hydrogen bond has its minimum shifted towards the center of the bond, is found to be valid here. The new feature of the N–H...O hydrogen bonds studied in this work is that the proton is transferred from the donor atom to the acceptor atom. Molecular dynamics simulations show that in an intermediate temperature regime, in which the proton is not completely transferred, the proton is bistable, jumping from one side of the hydrogen bond to the other. In the case of 3,5-pyridine dicarboxylic acid, which has been studied in most detail, specific phonons are identified, which influence the potential energy surface of the proton in the short, strong hydrogen bond. [ABSTRACT FROM AUTHOR]

The lattice dynamics and molecular vibrations of benzene and deuterated benzene crystals are calculated from force constants derived from density-functional theory (DFT) calculations and compared with measured inelastic neutron-scattering spectra. A very small change (0.5%) in lattice parameter is required to obtain real lattice-mode frequencies across the Brillouin zone. There is a strong coupling between wagging and breathing modes away from the zone center. This coupling and sensitivity to cell size arises from two basic interactions. Firstly, comparatively strong interactions that hold the benzene molecules together in layers. These include an intermolecular interaction in which H atoms of one molecule link to the center of the aromatic ring of a neighboring molecule. The layers are held to each other by weaker interactions, which also have components that hold molecules together within a layer. Small changes in the lattice parameters change this second type of interaction and account for the changes to the lattice dynamics. The calculations also reveal a small auxetic effect in that elongation of the crystal along the b axis leads to an increase in internal pressure in the ac plane, that is, elongation in the b direction induces expansion in the a and c directions. [ABSTRACT FROM AUTHOR]

We study the inclusion of the metallocene ferrocene Fe(C[sub 5]H[sub 5])[sub 2] molecules in the supercages of NaY zeolite. To find the exact location of the ferrocene molecules within the supercages we perform neutron and powder x-ray diffraction on bare NaY zeolite, and on NaY zeolite loaded with one or two ferrocene molecules per supercage. Using the complementary properties of both techniques we show that the ferrocene molecules are located just above a line joining two neighboring sodium ions at the SII positions in the zeolite supercage. The C[sub 5]H[sub 5] rings are oriented towards the sodium ions in an ordered manner. This structure is confirmed by quantum chemistry calculations. The geometry of the ferrocene molecules barely changes, indicating that the increased reactivity of ferrocene upon adsorption in a zeolite is thus a result of the position of the molecules. The main interactions responsible for this position are Coulombic attraction and hydrogen bonding. The inclusion of ferrocene in a Y-type zeolite provides a homogeneous distribution of iron throughout the zeolite at well-defined locations. [ABSTRACT FROM AUTHOR]

The inelastic neutron scattering (INS) spectra of some ammonium halides and ammonium nitrate are analyzed in terms of the internal vibrations of the NH+4 ion convoluted with multiphonon lattice modes up to the eighth term. Due to the low mass of this ion, most of the intensity of the internal-mode region of the INS spectra is in the ‘‘phonon wings,’’ differences between the salts in this spectral region being mainly due to differences in their lattice-mode spectra. Refinement of the lattice Debye–Waller factor is included in the profile refinement of interatomic force constants. [ABSTRACT FROM AUTHOR]

Inelastic neutron scattering (INS) measurements are presented for solid imidazole at 20 K. Both powder and oriented platelets have been measured. Assignments of bands are consistent with earlier infrared and Raman studies except for a band at 961 cm-1 which is definitely assigned as A″. A fit with the proton-weighted vibrational density of states is presented and a full harmonic force field has been established. The mean deviation between measured eigenvalues and those predicted from the force field is ±3 cm-1. The potential energy distribution is compared to quantum chemistry calculations and is found to be in best agreement with an unscaled calculation [J. Sadlej, A. Jaworski, and K. Miaskiewicz, J. Mol. Struct. 274, 247 (1992)]. The INS spectra from a scaled ab initio force field [K. Fan, Y. Xie, and J. Boggs, Theochem, 29, 339 (1986)] are calculated both before and after refinement against only infrared frequencies and compared to experiment. It is demonstrated that comparison with the INS spectrum is a more stringent test of the force field than with just the infrared frequencies alone. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]