Inelastic neutron scattering study of water in hydrated LTA-type zeolites

The vibrational dynamics of water molecules encapsulated in synthetic Na-A and Mg-exchanged A zeolites were studied versus temperature by inelastic neutron scattering (INS) measurements (30-1200 cm(-1)) as a function of the induced ion-exchange percentage by using the indirect geometry tof spectrometer TOSCA at the ISIS pulse neutron facility (RAL, UK). The experimental INS spectra were compared with those of ice Ih to characterize the structural changes induced by confinement on the H2O hydrogen-bonded network. We observed, after increasing the Mg2+ content, a tendency of water molecules to restore the bulklike arrangements together with more hindered dynamics. These results are confirmed by the analysis of the evaluated one-phonon amplitude-weighted proton vibrational density of states aimed, in particular, to follow the evolution of the water molecules librational mode region.     

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     

Inelastic neutron scattering study of methyl groups rotation in some methylxanthines

The three isomeric dimethylxanthines and trimethylxanthine are studied by neutron spectroscopy up to energy transfers of 100 meV at energy resolutions ranging from 0.7 microeV to some meV. The loss of elastic intensity with increasing temperature can be modeled by quasielastic methyl rotation. The number of inequivalent methyl groups is in agreement with those of the room temperature crystal structures. Activation energies are obtained. In the case of theophylline, a doublet tunneling band is observed at 15.1 and 17.5 microeV. In theobromine, a single tunneling band at 0.3 microeV is found. Orientational disorder in caffeine leads to a 2.7 microeV broad distribution of tunneling bands around the elastic line. At the same time, broad low energy phonon spectra characterize an orientational glassy state with weak methyl rotational potentials. Librational energies of the dimethylxanthines are clearly seen in the phonon densities of states. Rotational potentials can be derived which explain consistently all observables. While their symmetry in general is threefold, theophylline shows a close to sixfold potential reflecting a mirror symmetry.     

Inelastic neutron scattering from matrix isolated species

Matrix isolation is an experimental method in chemistry that is widely used for the preparation of samples for spectroscopic studies. It makes it possible to stabilize species which are unstable at room temperature, to isolate molecules in solids from each other, and to carry out molecular spectroscopy at low temperatures. Matrix isolated molecules are studied by a variety of techniques. In this paper the application of inelastic neutron scattering (INS) to matrix isolation is reviewed. Molecules that contain hydrogen atoms are diluted in inert gases such as argon, krypton and nitrogen, and then condensed inside a liquid helium cryostat of the type that is in use at many neutron spectrometers. For this work we developed a technique for the vapor deposition of inert gases with dopants that have low vapor pressure. INS focuses on some aspects of matrix isolation which are not appropriately covered by other spectroscopies, mainly solid state aspects of matrix isolated molecular aggregates and of the matrix itself. Neutron scattering is used to observe optically forbidden excitations such as methyl librations, tunnelling transitions, phonons, and rotational transitions involving a nuclear spin flip. Moreover, a direct correlation of spectroscopic data with the powder diffraction pattern of the matrix is possible with this technique. Localized modes and phonon densities of states can be observed in the same sample and may then be related to the respective diffraction pattern. The vapor deposited samples can be characterized in this way, and possible structural faults in rare-gas lattices revealed which are not usually recognized by other techniques. The structure of molecular aggregates has been elucidated by neutron spectroscopy of their low frequency internal intermolecular modes. HCN forms long linear chains, CH₃CN antiparallel dimers. Both species may, be understood as intermediates for the formation of the respective crystals. The structure of the matrix cages, in which single molecules are embedded, is explored by recording rotational and translational localized modes of these molecules. The single particle rotations of HCl, H₂O, NH₃, and CH₄ were studied directly. Tunnel splittings and librational spectra were recorded from molecules with methyl groups [CH₃CN, CH₃I, CH₃COCH₃, C₃H₈, C₄H₁₀, Sn(CH₃)₄, and others]. The mutual influence of translational modes of the guest and phonons of the host can be studied, since the neutron spectra are directly connected with the phonon density of states of the system. The librational spectrum of N₂ in Ar and a local mode of H₂ in solid D₂ are presented and compared with theoretical calculations.     

Inelastic neutron scattering studies of polyethylene and N-alkanes

High quality inelastic neutron scattering (INS) spectra of randomly oriented polycrystalline polyethylene, perdeuteropolyethylene and highly oriented polyethylene are presented. The instrumental resolution was significantly better than previous work and has revealed increased detail in the 0 − 600 cm⁻¹ region. For the polycrystalline sample, comparison with the best available dispersion curves shows that these qualitatively reproduce the INS spectrum, apart from the energy of the maximum in the in-plane C-C bending mode v5. For the oriented samples, comparison with calculated INS spectra show fair, but not exact, agreement with the experimental spectra.     

Inelastic electron scattering in the adsorbed system

The study revealed additional channels of inelastic electron scattering, which accompany the threshold excitation of the substrate Pt4d level — ionization of the valent states of adsorbed particles chemically bonded to the excited atom, and excitation of the surface plasmon vibrations. The conjugate excitation of this type shows up as a series of typical satellites in the spectra of disappearance potentials, which reflects the structure of valent states of adsorbed particles. Analysis of the satellite structure revealed the intermediate formation of NH _x,ads particles in the reaction NO_gas + H_ads on the surface of Pt(100) single crystal and, taking into account the earlier data, made it possible to formulate a general mechanism of selfoscillations in the NO + H₂ reaction on platinum metals. Mathematical modeling of reaction kinetics on the Pt(100) surface within the suggested mechanism demonstrated the presence of regular self-oscillations of the reaction rate at invariable values of the step constants.     

Inelastic neutron scattering studies on dichloro-1,4-benzoquinones

Inelastic neutron scattering (INS) spectra of 2,6-dichloro- and 2,5-dichloro-1,4-benzoquinone were compared with Raman and infra-red (IR) spectra and analysed in detail below 1800 cm(-1). The analysis was based on calculations tending towards simulation of spectra by using GAUSSIAN (HF, DFT/B3LYP and BLYP/6-31G(d,p)), and auntieCLIMAX programs. The correlations between calculated and experimental (either INS or Raman and IR) frequencies enabled to analyse the problem of scaling factors (SFs). The advantages of INS technique was shown in studies of low frequency vibrations with participation of H-atoms. The macroscopic lattice effect at low temperatures on INS spectra is discussed.     

Inelastic neutron scattering on three mixed-valence dodecanuclear polyoxovanadate clusters

The magnetic exchange interactions in the mixed-valence dodecanuclear polyoxovanadate compounds Na(4)[V(IV)(8)V(V)(4)As(III)(8)O(40)(H(2)O)].23H(2)O, Na(4)[V(IV)(8)V(V)(4)As(III)(8)O(40)(D(2)O)].16.5D(2)O, and (NHEt(3))(4)[V(IV)(8)V(V)(4)As(III)(8)O(40)(H(2)O)].H(2)O were investigated by an inelastic neutron scattering (INS) study using cold neutrons. In addition, the synthesis procedures and the single-crystal X-ray structures of these compounds have been investigated together with the temperature dependence of their magnetic susceptibilities. The magnetic properties below 100 K can be described by simply taking into account an antiferromagnetically exchange coupled tetramer, consisting of four vanadium(IV) ions. Up to four magnetic transitions between the cluster S = 0 ground state and excited states could be observed by INS. The transition energies and the relative INS intensities could be modeled on the basis of the following exchange Hamiltonian: H(ex) = -2J(12)(xy)[S(1x)S(2x)+ S(3x)S(4x)+ S(1y)S(2y)+ S(3y)S(4y)] - 2J(12)(z)[(S(1z)S(2z)+ S(3z)S(4z)] - 2J(23)(xy)[(S(2x)S(3x)+ S(1x)S(4x)+ S(2y)S(3y)+ S(1y)S(4y)] - 2J(23)(z)[(S(2z)S(3z)+ S(1z)S(4z)]. The following sets of parameters were derived: for Na(4)[V(12)As(8)O(40)(H(2)O)].23H(2)O, J(12)(xy)() = J(12)(z)= -0.80 meV, J(23)(xy) = J(23)(z) = -0.72 meV; for Na(4)[V(12)As(8)O(40)(D(2)O)].16.5D(2)O, J(12)(xy) = J(12)(z) = J(23)(xy) = J(23)(z = -0.78 meV; for (NHEt(3))(4)[V(12)As(8)O(40)(H(2)O)].H(2)O, J(12)(xy) = -0.80 meV, J(12)(z) = -0.82 meV, J(23)(xy)() = -0.67 meV, J(23)(z) = -0.69 meV. This study of the same [V(12)As(8)]-type cluster in three different crystal environments allows us to draw some conclusions concerning the applicability on INS in the area of nondeuterated molecular spin clusters. In addition, the effects of using nondeuterated samples and different sample container shapes for INS were evaluated.     

Inelastic neutron scattering in the glassy and metastable phases of methoxy-benzylidene-butyl-aniline

The inelastic neutron scattering (INS) spectra were measured in the glassy and in the metastable phases of methoxy-benzylidene-butyl-aniline. For comparison, the spectrum of benzylidene-aniline was also recorded. Complementary information was provided by Raman scattering measurements which analyzed more accurately the vibrational motions. Strong differences between the INS and Raman spectra were evidenced. The assignment of some lines is discussed to elucidate the respective role of the core and the tail of the molecule in the appearance of the successive metastable phases.     

Inelastic neutron scattering studies on low frequency vibrations of pentachlorophenol

Inelastic neutron scattering (INS) and DFT theoretical studies on pentachlorophenol (PCP) and d-PCP were performed. IR and Raman spectra were also measured for comparison. A special attention was focused on low frequency modes in INS spectra, which provide information about modes into which the co-ordinates of the hydrogen and chlorine atoms are involved. The intensity of respective INS bands is discussed based on the cross-sections of nuclei and calculated relative amplitudes of vibrations. The appearance of overtones and summation frequencies in INS spectra was evidenced.     

Inelastic neutron scattering and infrared spectroscopic study of furan adsorption on alkali-metal cation-exchanged faujasites.

Inelastic neutron scattering (INS) as well as infrared (IR) transmission and diffuse reflection infrared Fourier transform (DRIFT) spectra of furan adsorbed on Li-LSX, NaY, NaX, K-LSX, and CsNaX zeolites have been measured in the range 2000-200 and 4000-1300 cm(-1), respectively. On the basis of an assignment of normal modes of furan taken from the literature and our own quantum chemical calculations of vibrational frequencies, the observed frequency shifts between bulk furan and furan adsorbed on the zeolites mentioned above have been interpreted in view of the interactions between furan and zeolite. For an explanation of frequency shifts of CH out-of-plane bendings, CH stretchings and some ring vibrations, it has to be assumed that in addition to the interaction between furan and the corresponding cation of the zeolite, a further interaction between the CH bonds and lattice oxygen atoms exists.     

Inelastic electron tunnelling spectra of some derivatives of naphthalene

Inelastic electron tunnelling spectra of 1- and 2-naphthylacetic acid, 1- and 2-naphthoic acid, 1- and 2-naphthylacetate and 1-naphthylacetamide were measured at 4.2 K in the range 100–4000 cm⁻¹. The molecules were transferred onto the alumina surface by the vacuum deposition method. The spectra showed intensive and sharp peaks originating from aromatic rings. However, the spectra from naphthylacetates were of poorer quality than those from the other compounds and hence their adsorption orientation was concluded to be more parallel with respect to the surface of alumina. The adsorption of the naphthylacetates was possibly followed by dissociation of the molecules. The I, U characteristics were also measured, and it was observed that the naphthoic acids yielded the most nonlinear curves.     

Inelastic scattering of fast electrons and the electron density of the scattering molecules

Theoretical and Experimental Chemistry -     

Inelastic incoherent neutron scattering studies of water interacting with biological macromolecules.

The interaction between water and biological macromolecules in living organisms is of fundamental importance in a range of processes. We have studied water-DNA and water-proteolipid membrane systems over a range of hydration states using inelastic incoherent neutron scattering. We find a relatively sharp transition for both systems at a water concentration above which bulk solvent can be detected. Below this concentration, bulk water is essentially absent, i.e., all the water in the system is interacting with the biological macromolecules. This water is strongly perturbed as judged by its energy transfer spectrum, with a broader and lower energy transition than bulk water in the 50-75 meV (approximately 400-600 cm(-1)) range. Taking into account the differing geometry of (cylindrical) DNA and (planar) membranes, the number of water shells perturbed by each system was estimated. A conclusion is that in living organisms a large proportion of the cellular water will be in a state quite distinct from bulk water. The data add to the growing evidence that water structure in the vicinity of biological macromolecules is unusual and that the proximal water behaves differently compared to the bulk solvent.     

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.     

Inelastic neutron scattering study of hydrogen in d(8)-THFD(2)O ice clathrate

In situ neutron inelastic scattering experiments on hydrogen adsorbed into a fully deutrated tetrahydrofuran-water ice clathrate show that the adsorbed hydrogen has three rotational excitations (transitions between J=0 and 1 states) at approximately 14 meV in both energy gain and loss. These transitions could be unequivocally assigned since there was residual orthohydrogen at low temperatures (slow conversion to the ground state) resulting in an observable J=1-->0 transition at 5 K (kT=0.48 meV). A doublet in neutron energy loss at approximately 28.5 meV is interpreted as J=1-->2 transitions. In addition to the transitions between rotational states, there are a series of peaks that arise from transitions between center-of-mass translational quantum states of the confined hydrogen molecule. A band at approximately 9 meV can be unequivocally interpreted as a transition between translational states, while broad features at 20, 25, 35, and 50-60 meV are also interpreted to as transitions between translational quantum states. A detailed comparison is made with a recent five-dimensional quantum treatment of hydrogen in the smaller dodecahedral cage in the SII ice-clathrate structure. Although there is broad agreement regarding the features such as the splitting of the J=1 degeneracy, the magnitude of the external potential is overestimated. The numerous transitions between translational states predicted by this model are in poor agreement with the experimental data. Comparisons are also made with three simple exactly solved models, namely, a particle in a box, a particle in a sphere, and a particle on the surface of a sphere. Again, there are too many predicted features by the first two models, but there is reasonable agreement with the particle on a sphere model. This is consistent with published quantum chemistry results for hydrogen in the dodecahedral 5(12) cage, where the center of the cage is found to be energetically unfavorable, resulting in a shell-like confinement for the hydrogen molecule wave function. These results demonstrate that translational quantum effects are very significant and a classical treatment of the hydrogen molecule dynamics is inappropriate under such conditions.     

Inelastic neutron scattering and DFT study of 2-amino-3-hydroxymethyl-1,3-propane diol (TRIS)

We report an inelastic neutron scattering (INS) study of 2-amino-3-hydroxymethyl-1,3-propane diol (TRIS). The assignment of the experimental vibrational spectra measured using several incident neutrons’ energies on HRMECS spectrometer has been made by means of DFT calculations. To simulate crystal environment both molecular cluster and solid state models were used. The study has been completed by an alternative approach, molecular dynamics (MD) calculations, done at the same level of the DFT theory. The INS spectra calculated with the solid state models (normal mode analysis, and MD) gave a better fit of the experiment than the cluster model. On the other hand, the peaks between 650 and 850 cm⁻¹ in the experimental INS spectra assigned to OH torsional modes were reproduced better by the cluster calculations. The nature of the stretching frequency of unusually long O–H bond (1.012 Å) was interpreted by means of MD calculations. The interpretation of the spectrum below 100 cm⁻¹ was based on Fourier transform of the velocity autocorrelation function of centre of mass of a molecule of TRIS.     

Inelastic neutron scattering studies of the interaction between water and some amino acids

The interaction between water and some of amino acids (glycine, L-glutamine, L-threonine, L-cysteine and L-serine) was studied by inelastic incoherent neutron scattering (IINS). The vibrational spectra of dry amino acids and amino acids with a water content (e.g., 1 mol water/1 mol amino acid) were recorded. Comparing the difference spectra obtained by subtracting the spectrum of dry sample from those of wet sample with the spectra of ice Ih, we obtained that the difference spectrum for serine changed greatly from normal ice spectrum; but on the other hand, the difference spectra for the other amino acids such as glycine, glutamine, threonine, and cysteine changed slightly. The results demonstrate that serine has stronger hydrophilic character than glycine, glutamine, threonine, and cysteine. This is the first time the hydrophilic or hydrophobic character of amino acids was studied by using inelastic neutron scattering techniques, which provides important information for theoretical modeling and force field refinement for the interaction between water and the amino acids studied here.