Neutron inelastic scattering spectra and phase transition of 1,2-dichloroethane crystal

Neutron inelastic scattering spectra of 1,2-dichloroethane crystal have been measured at three temperatures above and below 177°K where the crystal undergoes a broad phase transition. Two and three peaks have been observed in the high and low temperature phases, respectively. The frequency distribution g(v) has been calculated in the first Brillouin zone for the low temperature phase and is compared with the observed spectra. The results show that the phase transition at 177°K is associated with rotational motion of the molecule around the axis passing through two chlorine atoms.     

Incoherent inelastic neutron scattering of the strong (NHN) bond in H3Co(CN)6

The vibrations of the strong symmetric NHN hydrogen bond in H₃Co(CN)₆ have been investigated by inelastic neutron scattering spectroscopy at 80 K. The two deformation modes δ(NH) and γ(NH) have been identified. A strong protonic mode is observed in the low energy transfer region and is assigned to the antisymmetric stretching vibration of the hydrogen bond. Relative intensities of the ν(NH) fundamental, its first overtone and bending fundamentals are interpreted as suggesting a strongly anisotropic Debye-waller factor. At 5 K the ν(NH) fundamental shows a complex structure which is interpreted in terms of coupling with low frequency lattice modes.     

Erratum

A semi-empirical method for infrared, Raman and neutron inelastic scattering AH stretching band shape analysis in strong hydrogen bonded systems is proposed. The fast (n) and the slow (N) stretching modes are supposed to be adiabatically separable. In the fundamental state of the fast mode the adiabatic potential function for the slow mode is assumed to be harmonic and wavefunctions are known exactly. Profiles are assumed to be dominated by the short-time effects on the time dependent formulation for spectral intensities which are then determined by the overlap between the adiabatic wavefunction in the fundamental state and an Airy function depending on the slope of the upper potential function at the classical turning point, multiplied by the transition moment. Infrared, Raman and neutron inelastic scattering transition probabilities are derived including high order terms of the n,N electrical coupling. This method does not contradict earlier theories but it is believed to provide a more realistic band profile determination. Examples of computed spectra at low temperature are given which show that short-time effects and electrical coupling may account for typical band shapes of strong hydrogen bonded systems (ABC bands for instance) without supposing Fermi resonances. Evans-type interactions or a double minimum potential function for the proton motions.     

On some controversy regarding νOH assignments in CsH2PO4

Spectroscopic methods such as Raman scattering and IR absorption, as well as structural methods such as x-ray and n-diffraction, offer rather ambiguous interpretation of the high-frequency phonon spectrum of CsH₂PO₄ (CDP), especially regarding the proper assignment of proton vibrational modes. Empirical lattice dynamics (LD) simulations of the proton vibrational density of states ((H-VDOS)) in CDP also reveal a discrepancy in the frequency assignments of the OH stretching modes of its two non-equivalent hydrogen bonds. This may be resolved by accounting for the LD simulated directional (H-VDOS) along the three Cartesian axes of CDP, from which the corresponding anisotropy of the proton kinetic energy tensor can be deduced. The results may then be tested by a simple deep inelastic neutron scattering (DINS) measurement on a single crystal of para-electric CDP at room temperature.     

An inelastic neutron scattering study of hydrogen bonding in potassium hydrogen dichloromaleate

The inelastic neutron scattering (INS) spectrum (350–2000 cm^?1) of potassium hydrogen dichloromaleate (solid slate) has been obtained. Two of the normal modes of vibration of the hydrogen bond [γ(OHO) and δ(OHO)] were observed and assigned. No INS band v_as(OHO) was observed in the region 500–1300 cm^?1. This conflicts with expectations from infrared data.     

Inelastic neutron scattering spectrum of Cs2[B12H12]: reproduction of its solid-state vibrational spectrum by periodic DFT

The inelastic neutron scattering (INS) spectrum of polycrystalline Cs2[B12H12] is assigned through 1200 cm(-1) on the basis of aqueous and solid-state Raman/IR measurements and normal mode analyses from solid-state density functional theory. The Cs+ cations are responsible for frequency shifts of the internal cage vibrational modes and I(h) cage mode splittings due to the crystal T(h) site symmetry. These changes to the [B12H12]2- molecular modes make isolated-molecule calculations inadequate for use in complete assignments. Solid-state calculations reveal that 30/40 cm(-1) shifts of Tg/Hg molecular modes are responsible for structure in the INS spectrum unobserved by optical methods or in aqueous solutions.     

Incoherent neutron quasi-elastic scattering studies of the anisotropic self-diffusion in nematic and smectic a phases of ethyl 4-(4′acetoxy benzylidene) aminocinnamate (EABAC)

Incoherent quasi-elastic neutron scattering spectra have been measured down to low scattering vectors (Q >) 0.14 Å^?1) on magnetically aligned specimens of the nematic and smectic A phases of EABAC. Data were obtained for Q || n and Q⊥n where n denotes the direction of the unique axis) giving the diffusion coefficients D_|| and D_⊥ for the two phases as follows (in units of 10^?7 cm² s^?1); nematic: D_|| = 15.8 ± 2.0, D_⊥ = 12.3 ± 0.8; smectic A: D_|| = 4.3 ± 0.4, D_⊥ = 4.9 ± 0.8. The anisotropy is reversed between the phases and the molecules are more mobile in the nematic phase as expected. In the smectic A experiments with Q || n an apparent inconsistency between low and high Q results is interpreted as evidence for the existence of some slow motion other than translational diffusion which requires further investigation. Preliminary measurements were made to explore the application of the “fixed window” method for determining the temperature dependence of the diffusion coefficients.     

Cross section measurements for 141Pr(n,γ)142Pr reaction at neutron energies from 13.5 to 14.8?MeV

Activation cross-sections were measured for the ¹⁴¹Pr(n,??)¹⁴²Pr reaction at three different neutron energies from 13.5 to 14.8?MeV. The fast neutrons were produced via the ³H(d,n)⁴He reaction on Pd-300 neutron generator. The natural high-purity Pr₂O₃ powder was used as target material. Induced gamma activities were measured by a high-resolution gamma-ray spectrometer with high-purity germanium detector. Measurements were corrected for gamma-ray attenuations, random coincidence (pile-up), dead time and fluctuation of neutron flux. The neutron fluences were determined by the cross section of ²⁷Al(n,??)²⁴Na reaction. The neutron energy in the measurement were by the cross section ratios of ⁹⁰Zr(n,2n)^89m+gZr and ⁹³Nb(n,2n)^92mNb reactions. The data for ¹⁴¹Pr(n,??)¹⁴²Pr reaction cross sections are reported to be 3.3?±?0.2, 2.7?±?0.2 and 2.2?±?0.2 mb at 13.5?±?0.2, 14.1?±?0.2, and 14.8?±?0.2?MeV incident neutron energies, respectively. Results were discussed and compared with some corresponding values found in the literature.     

Determination of elastic shear constants of polyethylene at room temperature by inelastic neutron scattering

The elastic shear constants of both the amorphous and crystalline regions of polyethylene have been measured at room temperature. A newly developed method is used which allows the determination of elastic constants from the coherent inelastic neutron scattering of polycrystals. A deuterated and partially oriented sample is investigated on a triple-axis spectrometer and a time-of-flight instrument. The elastic constants of the crystalline regions of polyethylene are c₄₄ = 2.1 ± 0.3, c₅₅ = 2.2 ± 0.3, c₆₆ = 1.8 ± 0.2, and c′ = 1/4(c₁₁ + c₂₂ ? 2c₁₂) = 0.92 GPa. The shear modulus of the amorphous regions is obtained as G = 0.55 ± 0.03 GPa. In connection with other experimental results the elastic constant matrix is given and compared with theoretical estimates. With simple models, macroscopic moduli are calculated which are in good agreement with published experimental data.     

Elastic and inelastic neutron studies of hydrogen molybdenum bronzes

Hydrogen molybdenum bronzes H_xMoO₃ (～0.3 < x < 2.0) have been investigated with elastic and inelastic neutron scattering. Neutron diffraction studies of orthorhombic D_0.36MoO₃ show that deuterium is incorporated as -OD without any major structural change to the MoO₃ layer lattice. The inelastic neutron scattering spectra of H_xMoO₃ phases confirm that, for H_0.34MoO₃, H is present as -OH, but, for the monoclinic phases H_0.93MoO₃, H_1.68MoO₃ and H_2.0MoO₃, only peaks associated with -OH₂ groups are found.     

Ultrafast vibrational energy transfer of polyethylene investigated with picosecond laser pulses

Picosecond Raman scattering after infrared excitation is studied for polyethylene films at room temperature. Ultrafast vibrational energy redistribution (⩽ 5 ps) and a comparatively long population lifetime (T₁ = 260 ± 100 ps) are observed for the CH-stretching modes.     

Inelastic neutron scattering spectra of alkali metal (Na,K) bifluorides: The harmonic overtone of v3

Inelastic neutron scattering spectra of MFHF (M  Na and K) have been measured up to energy transfers of ca. 4000 cm^?1 Both 0 → 1 and 0 → 2 transitions of the bending (v₂), and antisymmetric stretching (v₃) modes were observed. A normal harmonic (i.e. no quartic contribution) model for the dynamics of the bifluoride ion is entirely consistent with our observations. Evidence of phonon dispersion was observed in the band shape of v₃, but no structure attributable to the LO mode could be found. The similarity of the band shapes of v₃ for both NaFHF and KFHF is interpreted in terms of a very short range coupling mechanism.     

Conformational heterogeneity and low-frequency vibrational modes of proteins

Molecular dynamics simulation and normal mode analysis are used to calculate the vibrational density of states of dihydrofolate reductase complexed with nicotinamide adenine dinucleotide phosphate at 120 K and the results are compared with the experimental spectrum derived from inelastic neutron scattering. The simulation results indicate that the experimental spectrum arises from an average over proteins trapped in different conformations with structural differences mainly in the loop regions, and that these conformations have significantly different low-frequency (<20 cm(-1)) spectra. Thus, the experimentally measured spectrum is an average over the vibrational modes of different protein conformations and is thus inhomogeneously broadened. The implications of this broadening for future neutron scattering experiments and ligand binding calculations are discussed.     

Inelastic neutron scattering from isotactic polypropylene

We used inelastic neutron scattering to probe the low‐energy excitations in semicrystalline isotactic polypropylenes with different degrees of crystallinity. The contributions from the amorphous and crystalline regions to the total scattering intensity were extracted under the assumption of a weighted linear contribution of the two regions in a simplified two‐phase system. The resulting intensity from the amorphous region showed a peak at 1.2 meV that was in good agreement with the previously determined boson peak characteristic of atactic polypropylene. The possibility of a contribution to the boson peak region by longitudinal acoustic mode modes that are characteristic of semicrystalline polymers and appear in the same low‐frequency region is discussed. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2852–2859, 2001     

Solubilities of hydrogen and deuterium gases in water and their isotope fractionation factor

Solubilities of deuterium gas in water were measured at 5° intervals from 278 to 303°K with an overall precision of about 0.4%. Thermodynamic functions for the solution process were calculated for deuterium gas and compared with the corresponding quantities for hydrogen gas based on the reported data of Crozier and Yamamoto. Henry's law constantsk, obtained at different temperaturesT, were fitted to an equation of the form $$R ln ({1 \mathord{\left/ {\vphantom {1 k}} \right. \kern-\nulldelimiterspace} k}) = A + {B \mathord{\left/ {\vphantom {B {T + }}} \right. \kern-\nulldelimiterspace} {T + }}C ln T + DT$$ Isotope fractionation factors α for the D₂/H₂ system were obtained with careful error estimates. Compared at the same temperature, D₂ gas is more soluble in water than H₂ gas, showing a “normal” isotope effect, and the value of α decreases from 1.086 (±0.005) at 278°K to 1.065 (±0.006) at 303°K. The large isotope effect may be attributed, at least partly, to the difference in the zero-point energies between H₂ and D₂ molecules when they execute oscillatory motion in a solvent cage.     

Measuring molecular force fields: Terahertz,inelastic neutron scattering,Raman, FTIR,DFT, and BOMD molecular dynamics of solid l-serine

A vibrational analysis of polycrystalline l-serine is provided using experimental terahertz, FTIR, Raman and inelastic neutron scattering (INS) spectra, calculated INS spectra – and Born–Oppenheimer molecular dynamics (BOMD) simulations from which the power spectra for the electronegative elements are compared to the THz spectra. Corrections are made to density functional theory (DFT) calculations for van der Waals interactions. Assignments and potential energy distributions are included for all 3N = 336 normal modes of an eight molecule supercell, including those for 48 non-bonded whole molecule translating and rotating vibrations, of which three are acoustic modes, usually not considered. Calculated and observed frequencies differ by an average 3 cm⁻¹ (s = 4). The INS spectrum of these modes below 100 cm⁻¹, calculated from energy second derivatives, show a remarkable similarity to the experimental 10 K spectra. The calculated low frequency modes are insensitive to small changes in cell parameters and geometry. THz intensities are represented by power spectra and not calculated explicitly. Nevertheless, power spectra of 13 ps BOMD trajectories at classical temperatures of 20 K, 400 K, and 500 K are markedly similar to the experimental terahertz spectra at 77 K and 298 K. Calculations on a serine crystal supercell 2 × 2 × 2 molecules deep appear to include, in a crude but fortuitously accurate way, enough of the principle out of phase dispersion to yield a match with experimental frequencies and intensities.     

High-resolution UV photoelectron spectrum of CO2

The highly-resolved HeI photoelectron spectrum of CO₂ is presented and its vibrational structure studied in detail. In the X? ²Π_g ionic state the v₃ antisymmetric mode is found to be excited in double quanta (v_1-v_2-v₃ = 0. 0. 2) with energy hv₃ = 181 meV. In the C? ²Σ_g⁺ state a single quantum of the same mode is found to be excited (hv₃ = 189 meV) in combination with a v₁ excitation. Vibronic interaction with vibrational levels in the B? ²Σ_u⁺ state of the ion is suggested to promote this (1, 0, 1) excitation. It is established that inelastic scattering processes contribute to the vibrational structure in the C? ²Σ_g⁺ band. The spin-orbit splitting in the X? ²Π_g is determined to be 19±1 meV and 10±2 eV in the ā²Π_u state. Vibronic structure is resolved in the X? ²Π_g band where the Renner-Teller coupling constant is determined to be ? = 0.21±0.02 and the vibrational energy of the v₂ mode as 60±7 meV. In the ā²Π_u state the v₂ energy is found to be hv₂ = 60 meV from the observed hot-band structure.     

Vibrational relaxation of hydrogen-bonded species in solution. I. Theory

We present a simple model to describe the phase relaxation of the ν_s(XH) mode of a hydrogen-bonded species XH?Y in solution in an inert solvent. The major assumptions made are that the ν_σ(XH?Y) stretching mode of the hydrogen bond can be represented by the Ornstein—Uhlenbeck stochastic process, and that the ν_s(XH) phase coherence is lost because of the coupling between the two vibrational modes. The model can be analysed in terms of Kubo's theory of a randomly modulated oscillator. An expression is derived for the transition dipole moment autocorrelation function which characterises the line shape of the mid infrared ν_s(XH) absorption band, and various limiting cases of the formula are discussed. It is further shown that vibrational relaxation may be expected to influence the far infrared ν_σ(XH?Y) bandshape and the one-phonon neutron inelastic scattering cross section, and that the parameters required to characterise all three types of spectra are closely related. Experimental tests of the theory are reported in the following paper.     

Theory and spectroscopy of an incarcerated quantum rotor: The infrared spectroscopy,inelastic neutron scattering and nuclear magnetic resonance of H2@C60 at cryogenic temperature

The supramolecular complex, H₂@C₆₀, represents a model of a quantum rotor in a nearly spherical box. In providing a real example of a quantum particle entrapped in a small space, the system cuts to the heart of many important and fundamental quantum mechanical issues. This review compares the predictions of theory of the quantum behaviour of H₂ incarcerated in C₆₀ with the results of infrared spectroscopy, inelastic neutron scattering and nuclear magnetic resonance. For H₂@C₆₀, each of these methods supports the quantization of translational motion of H₂ and the coupling of the translational motion with rotational motion and provides insights to the factors leading to breaking of the degeneracies of states expected for a purely spherical potential. Infrared spectroscopy and inelastic neutron scattering experiments at cryogenic temperatures provide direct evidence of a profound quantum mechanical feature of H₂ predicted by Heisenberg based on the Pauli principle: the existence of two nuclear spin isomers, a nuclear spin singlet (para-H₂) and a nuclear triplet (ortho-H₂). Nuclear magnetic resonance is capable of probing the local lattice environment of H₂@C₆₀ through analysis of the H₂ motional effects on the ortho-H₂ spin dynamics (para-H₂, the nuclear singlet state, is NMR silent). In this review we will show how the information obtained by three different forms of spectroscopy join together with quantum theory to create a complementary and consistent picture which strikingly shows the intrinsically quantum nature of H₂@C₆₀.     

Controlled incorporation of deuterium into bacterial cellulose

Isotopic enrichment has been widely used for investigating the structural and dynamic properties of biomacromolecules to provide information that cannot be carried out with molecules composed of natural abundance isotopes. A media formulation for controlled incorporation of deuterium in bacterial cellulose synthesized by Gluconacetobacter xylinus subsp. sucrofermentans is reported. The purified cellulose was characterized using Fourier Transform Infra-Red spectrophotometry and mass spectrometry which revealed that the level of deuterium incorporation in the perdeuterated cellulose was greater than 90 %. Small-angle neutron scattering analysis demonstrated that the overall structure of the cellulose was unaffected by the substitution of deuterium for hydrogen. In addition, by varying the amount of D-glycerol in the media it was possible to vary the scattering length density of the deuterated cellulose. A large disk model was used to fit the curves of bacterial cellulose grown using 0 and 100 % D-Glycerol yielding a lower bound to the disk radii, R _min = 1,132 ± 6 and 1,154 ± 3 Å and disk thickness, T = 128 ± 1 and 83 ± 1 Å for the protiated and deuterated forms of the bacterial cellulose, respectively. This agrees well with the scanning electron microscopy analysis which revealed stacked sheets in the cellulose pellicles. Controlled incorporation of deuterium into cellulose will enable new types of experiments using techniques such as neutron scattering to reveal information about the structure and dynamics of cellulose and its interactions with proteins and other (bio) polymers.