Neutron diffraction spectra

Figure 16.2 (a) Neutron diffraction spectra of the bare single-walled nanotube bundle... 

Figure 2. Neutron diffraction spectra of acid Nafion membrane for wet and dr samples. Instrument is D 1B. Incident wave-length is Ao = 2.56 A. The horizontal axis is expressed either in scattering angles (20) units, or in neutron momentum transfer (Q = tt/Xo stn0) units. Note the different vertical scales for the two samples. The separation between the amorphous and crystalline contributions to the large angle peak is shown for the dry sample. Temperature 25°C.

This chapter discusses the form and parameterization of the potential energy terms that are used for the atomistic simulation of polymers. The sum of potential terms constitutes a molecular force field that can be used in molecular mechanics, molecular dynamics, and Monte Carlo simulations of polymeric systems. Molecular simulation methods can be used to determine such properties as PVT data, selfdiffusion coefficients, modulus, phase equilibrium, x-ray and neutron diffraction spectra, small molecule solubility, and glass transition temperatures with considerable accuracy and reliability using current force fields. Included in the coverage of Chapter 4 is a review of the fundamentals of molecular mechanics and a survey of the most widely used force fields for the simulation of polymer systems. In addition, references to the use of specific force fields in the study of important polymer groups are given. 

Hg(CN)2 in the solid state has a structure (I42d neutron diffraction), completely different from that of Cd(CN)2 Almost-linear molecules (r(Hg—C) 201.9, r(C—N) 116.0pm (corrected for thermal motion) a(C—Hg—C) 175.0°) are arranged such that four secondary bonds N" Hg (274.2 pm) yield the often-occurring 2 + 4 coordination around Hg.103 Analysis of the 199Hg MAS NMR spectrum of Hg(CN)2 has yielded the chemical shift and shielding tensor parameters.104... 

Other studies, such as infrared and Raman spectra of gaseous benzene, neutron diffraction studies of crystalline benzene, and electron diffraction and microwave spectral studies, are equally incapable, according to critical analysis [87AG(E)782], of resolving unanimously the Dih—Deh structural dilemma of the benzene molecule. Furthermore, no decisive conclusion could be drawn from photoelectron spectra or H—NMR spectrum measurements of benzene molecules in a liquid crystal environment. The latter experiments merely indicate that the average lifetime of a Dih structure (if it appears on the PES) is less than 10 4 sec corresponding to the energy barrier of the Dih- >D6h-+D h interconversion of approximately 12 kcal/mol. 

Direct Synthesis reaction of, 6 395 fluoride, 21 235, 237, 239, 249 homopolyatomic cations, 17 82 ion, stereochemistry, 2 40-41, 44-45 isocyanates, preparation, 9 158 properties, 9 157 isothiocyanates, properties, 9 177 mixed valence compounds of, 10 375-381 crystal structure of, 10 376 diffuse reflectance spectrum of, 10 380 structure of Pb," ion, 10 381 nuclear magnetic shielding, 22 224 organometallic compounds, 2 82, 88, 89 oxide, neutron diffraction studies on, 8 231-233... 

Another new approach to the water structure has been provided by Brookhouse s [9] study of inelastic neutron diffraction from water, Here he is measuring not, as in elastic neutron scattering, the relative positions of the proton in the water, but the velocity spectrum of the water molecules, He finds here that the molecules appear to behave very largely like those for gas, that is, they appear to be moving in relatively free space. 

The observation of a bent Cr-H-Cr bond in the tetraethylammonium salt without an accompanying substantial deformation of the linear architecture of the nonhydrogen atoms in the [Cr2(CO)io(M2-H)] monoanion reflects the inherent flexibility of the bond. The deformability of the[M2(CO)io(M2-H)] monoanion species to adopt an appreciably bent, staggered carbonyl structure was first reported by Bau and co-workers (23) from neutron diffraction studies of two crystalline modifications of the electronically equivalent, neutral W2(CO)9(NO)(m2-H) molecule. Subsequent x-ray diffraction studies (15) of the analogous [W2(CO)io(m2-H)] monoanion found that the nonhydrogen backbone can have either an appreciably bent structure for the bis(triphenylphosphine)-iminium salt or a linear structure for the tetraethylammonium salt, with the W-W separation 0.11 A less in the bent form. Crystal packing forces probably were responsible (15) for the different molecular configurations of the monoanion in the two lattices. In solution, however, all known salts of the [W2(CO)io(m2-H)] monoanion exhibit the same three-band carbonyl ir absorption spectrum char-... 

Evidence that the proton lies midway between the fluorine atoms in the crystal KHF has been provided by entropy measurements,28 study of the polarized infrared spectrum,29 neutron diffraction,80 and nuclear spin magnetic resonance.81 The uncertainty in the location of the proton at the midpoint between the fluorine atoms is reported to be 0.10 A for the neutron diffraction study and 0.06 A for the nuclear magnetic resonance study. 

In the case of the ethane (45) and butane (46) monolayers adsorbed on graphite, it has been possible to analyze the neutron diffraction patterns using all three Euler angles of the molecule as orientational parameters. Here we limit discussion to the butane monolayer which we have taken as a model system and whose vibrational spectrum was discussed in Sec. II. 

The NMR experiment was performed on nB in C11B2O4. The spectrum in the commensurate phase is discussed. The magnetic moment at the Cu(A) site is estimated to be 0.45 /tg, which is almost 50% of the moment derived from the neutron diffraction experiment. The magnetic moment at the Cu(B) sites is absent in the commensurate phase. The asymmetric nature of the spectral pattern is not understood so far. A phenomenological understanding of the commensurate to incommensurate transition is discussed on the basis of the molecular-field approximation. More precise discussion of the commensurate phase and also of the incommensurate phase will be presented in the near future. 

Coexistence of a SC and the IC AF phases at low temperatures was confirmed recently by the neutron diffraction experiments [31] for La2 xSrxCu04 (x = 0.10) in the vortex state. (The coexistence of SC and AF formations was found also from the //S11 spectra [32]). The way of the "coexistence of SC and the stripe order in the same sample remains unresolved one view treats the new stripe symmetry as a superstructure superimposed on the Fermi surface that changes the energy spectrum like any SDW/CDW can do it (e.g. [33]). Another plausible alternative would be a spatially inhomogeneous coexistence of the nonsuperconducting IC AF phase and a "metallic phase with strong fluctuations. 

The 13C-CP-MAS spectrum has also been used to complement the diffraction data relating to the crystal structure of cellulose, but hitherto the results have been inconclusive [242]. A significant amount of H single crystal spectroscopy has been reported and has been used as a basis for correlation with neutron diffraction data and with theoretical ab-initio molecular orbital calculations [243, 244]. 

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