An elastic neutron scattering on dynamical transition in hydrogen-bonded systems

Elastic incoherent neutron scattering measurements have been performed on trehalose and sucrose/D₂O mixtures as a function of temperature. The study provides an effective way for characterizing the dynamical behaviour, furnishing a set of parameters characterizing the ‘flexibility’ and the ‘rigidity’ that justifies the better cryptobiotic effect of trehalose with respect to sucrose. Elastic scans make evident a non-Gaussian behaviour of the intensity profiles, which is more marked for sucrose, with a dynamical transition at T253 and 250 K for trehalose/D₂O and sucrose/D₂O mixtures, respectively.     

The high resolution inelastic neutron scattering spectrum of ammonium fluoride

The measured high resolution (deltaE/E approximately 2-3%) incoherent inelastic neutron scattering spectrum of ammonium fluoride is presented and discussed with reference to the available optical spectra. In addition, a full set of dispersion curves have been obtained from a new ab initio lattice dynamics calculation and these have been used to produce a rigorous interpretation of the spectrum. The librational modes of the ammonium ion occur at 560 cm(-1) and the anharmonicity in these modes is estimated to be 4%, about half that observed in the other ammonium halides. The reduction in anharmonicity is attributed to stronger hydrogen bonding and a deeper potential well. The calculations agree well with the observed spectrum apart from the librational modes which are shifted upwards by around 40 cm(-1) from the measured values. Dispersion and LO-TO splitting are important in this system with modes changing frequency by up to 135 cm(-1). The nature of the calculated LO-TO splitting in the optic mode region is indicative of a pseudo-cubic system confirming that the site symmetry of the ammonium ion is very close to T(d). Because of LO-TO splitting the ammonium ion asymmetric stretch, nu3, has components calculated to be at higher frequencies than those of the symmetric stretch, nu1, which contradicts the assignment scheme produced from optical data.     

Series expansion calculation of the elastic neutron diffuse scattering: Ice Ih

A theoretical systematic method for the evaluation of the neutron elastic, coherent, diffuse scattering intensity of ice Ih is presented. For each Q vector, the latter is expressed directly as a series expansion of graphs. This technique is analogous to the Ising susceptibility calculation and avoids the explicit use of individual correlation functions. Therefore the series can be easily and exactly expanded to rather high orders. For the computation of the scattering intensity, the correlation model used is based on the ice rules and the normalized probability is written according to Nagle's formulation. The resulting series is convergent and theoretical isodiffusion patterns have been drawn by extending the series up to the 8th order; the accordance with experiment is good.     

Study of the conformation of polyelectrolytes in dilute solutions by elastic neutron scattering

Experiments using small angle neutron scattering have been performed on dilute solutions of polyelectrolytes without added salt; they allow to show that this method is very suitable for measuring the radius of gyration and determining the conformation of polyions in solution.     

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.     

High resolution and the accurate measurement of intensities

Absolute intensities in several vibration rotation bands of CO₂ and in the pure rotation spectrum of CO have been measured. The results have been used to assess the precision and accuracy which such measurements can attain when made with a high resolution Fourier transform spectrometer.     

Assessing sediment pollution from the Julian Adame-Alatorre dam by instrumental neutron activation analysis

The rapid industrial development in regions of Mexico during recent years has had the side effect of introducing toxic metals, fertilizers, and pesticides into the ecosystem. Sediment cores were collected from eight locations around the Julian Adame-Alatorre dam located in the Municipality of Villanueva in the State of Zacatecas, México. The cores were analyzed for 32 major, trace, and rare earth elements (As, La, Lu, Nd, Sm, U, Yb, Ce, Co, Cr, Cs, Eu, Fe, Hf, Rb, Sb, Sc, Sr, Ta, Tb, Th, Zn, Zr, Al, Ba, Ca, Dy, K, Mn, Na, Ti, and V) in order to estimate the health risk. The samples were analyzed by instrument neutron activation analysis (INAA) using thermal neutron fluxes of 8 × 10¹³ and 5 × 10¹³ n cm⁻² s⁻¹ for short and long irradiations, respectively. The results of the contamination levels for elements such as As, Ba, Cr, Fe, Mn, Ta, V, and Zn were compared with the Mexican regulations and the guidelines of USEPA. Enrichment factors for quantified elements identified high As, Sb, Hf, and Cs contents using Fe as a crustal reference.     

An analytical method for the calculation of the relative intensities of bending and stretching modes in inelastic neutron scattering spectra

An approximate analytical method is presented for calculating the relative intensities of the bands due to the stretching (ν) and doubly degenerate b     

Multivariate curve resolution methods in imaging spectroscopy: influence of extraction methods and instrumental perturbations

Imaging spectroscopy is becoming a key field of analytical chemistry. In the face of more and more complex samples, we actually need accurate microscopic insight. Nowadays, the methods used to produce concentration maps of the pure compounds from spectral data sets are based on the classical univariate approach although multivariate approaches are sometimes investigated. But in any case, the analytical quality of the chemical images thus provided cannot be discussed since no reference methods are at our disposal. Thus the proposed research focuses on the application of multivariate methods such as Orthogonal Projection Approach (OPA), SIMPLE-to-use Self-modeling Mixture Analysis (SIMPLISMA), Multivariate Curve Resolution - Alterning Least Squares (MCR-ALS), and Positive Matrix Factorization (PMF) for imaging spectroscopy. A systematic and quantitative characterization of the accuracy of spectra and images extraction is investigated on mid-infrared spectral data sets. Of special interest is the influence of instrumental perturbations such as noise and spectral shift on the extraction ability to access the algorithm's robustness.     

Effect of the components of the raman scattering tensor on band intensities

The Raman scattering (RS) tensor as a function of excitation radiation frequency is studied taking into account, the vibration quantum. It is shown that the corresponding asymmetric components of the tensor are described by fast converging series. Expressions are obtained for the antisymmetric and symmetric components of the RS tensor. A convenient formula is proposed for the calculation of the RS tensor and its components, where the tensor is represented as a sum of the derivative of static polarizability and the terms containing fast converging series. The formula makes it possible to avoid infinite summation. Computer experiments simulating the dependence of the intensity of RS bands and their components on the excitation frequency are carried out. Moscow State University of Building Engineering. V. I. Vernadskii Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 6, pp. 1006–1015, November–December, 1996. Translated by I. Izvekova     

Theoretical aspects of small-angle neutron scattering from multiphase polymers

A molecular theory for small-angle neutron scattering from polymer mixtures is reviewed and extended to consider multiphase polymer systems such as block copolymers and their blends with homopolymers. Methods are developed for the isolation of scattering functions for individual components in these blends. These methods rely on two contrast-matching techniques: the concept of “composition matching,” where a mixture of deuterium-labeled and protonated species is used to match the contrast of a third component; and the synthesis of “phase-matched” block copolymers, where the contrast of the block copolymer sequences are matched. Methods are discussed specifically for the isolation of single chain and single sequence scattering functions for diblock and triblock copolymers, their blends with homopolymers, and star copolymers.     

Elemental analysis of soils from Toshki by using instrumental neutron activation analysis techniques

Instrumental neutron activation analysis has allowed performing a complete characterization of the chemical composition of thirty soil samples soil samples from ten locations collected from the Toshki area in Upper Egypt. The samples were irradiated with thermal neutrons at the Mainz TRIGA research reactor. Gamma-ray spectra were recorded using a high efficiency HPGe detector to determine the contents of major, minor and trace elements in these samples. As a result of the analysis, altogether 32 elements were identified qualitatively and quantitatively. These elements are: Al, As, Ba, Ca, Ce, Co, Cr, Cs, Eu, Fe, Ga, Hf, K, La, Lu, Mg, Mn, Na, Nb, Nd, Rb, Sc, Sm, Sn, Ta, Ti, Th, U, V, Yb, Zn and Zr. In several cases, X-ray fluorescence analysis was used for comparison. The results obtained for U and Th were compared with data reported in literature.     

Absolute intensities of Raman trace scattering from bicyclo-[1.1.1]-pentane

Our previous theoretical studies have identified the Raman intensity parameter for the bridgehead C-H stretch in bicyclo-[1.1.1]-pentane as the largest for any saturated hydrocarbon yet considered, while the methylene C-H parameter is predicted to be ordinary. Theoretical methods including self-consistent field, static and time dependent density functional theory, and coupled cluster, all predict a large bridgehead intensity parameter, but differ widely in the actual value. We have synthesized bicyclo-[1.1.1]-pentane and recorded the absolute intensity Raman trace scattering spectra. The recorded intensity of a resonance polyad in the C-H stretching region has been resolved and distributed onto the fundamental modes through an anharmonic resonance analysis from a computed quartic force field. The experimental internal coordinate intensity parameters have been obtained and compared with those computed. Although the static and dynamic density functional values overestimate the parameter by 10%-18%, the values predicted at the coupled-cluster level are found to be correct to within experimental error.     

Intense quasielastic neutron scattering of 4,4'-diethyoxyazoxybenzene and its connection with the inelastic neutron scattering spectrum

This is a continuation of a previous study of the odd-even effect for compounds of the series of 4,4'-di-n-alkyloxyazoxybenzenes. The reduction of the elastic component of the neutron scattering spectra (the elasticity depression), studied earlier at constant ΔT from the clearing point, has now been studied at constant temperature. The ethyoxy member of the series shows an exceptionally large elasticity depression; this is interpreted as evidence of a small steric hindrance for molecular motions due to the high nematic order parameter. This statement has been corroborated by the inelastic neutron scattering measurements for solid compounds of the series.     

Measurement of calcium and titanium from 47sc by instrumental neutron activation analysis

A procedure is outlined for measuring calcium and titanium from ⁴⁷Sc by instrumental neutron activation analysis. Precise calcium measurements obtained from ⁴⁷Sc are compared with measurements based on ⁴⁷Ca and ⁴⁹Ca. The method is particularly suitable for low levels (ca. 1%) of calcium.     

Estimating the Hartree—Fock limit from finite basis set calculations

It is demonstrated that the polarization-consistent basis sets, which are optimized for density functional methods, are also suitable for Hartree–Fock calculations, and can be used for estimating the Hartree–Fock basis set limit to within a few micro-hartree accuracy. Various two- and three-point extrapolation schemes are tested and exponential functions are found to be superior compared to functions depending on the inverse power of the highest angular momentum function in the basis set. Total energies can be improved by roughly an order of magnitude, but atomization energies are only marginally improved by extrapolation.     

Small-angle neutron scattering from surfactant-assisted aqueous dispersions of carbon nanotubes

The mechanism of surfactant-assisted dispersion of single-walled carbon nanotubes in water is studied by small-angle neutron scattering. The previously hypothesized formation of cylindrical micelles with the nanotubes forming the core of cylinders is inconsistent with the data presented. The scattering data favor a random structureless adsorption model for the dispersion of the nanotubes.     

Small-angle neutron scattering study on the transamidition of polyamide-4.6

The interchange or so-called transamidation process in polyamide-4.6 was investigated by means of small-angle neutron scattering (SANS). For this purpose, a 50/50 blend of partially deuterated and fully hydrogenous polyamide-4.6 was processed at 300°C for variable times (1–20 min). The obtained results are in line with the theory as described by Benoit to quantify the transesterfication process in homopolyesters. An exponential relaxation time τ of approximately 1500 s was obtained. On the basis of this relaxation time τ, it can be calculated that approximately four interchange reactions per chain occur in polyamide-4.6 during a typical processing time of 3 min at 300°C. The data interpretation is not complicated by the presence of some crystallinity in polyamide-4.6. © 1996 John Wiley & Sons, Inc.     

Density of phonon states in solid parahydrogen from inelastic neutron scattering

We have measured the inelastic neutron scattering spectrum of solid parahydrogen (at low pressure and T=13.3 K) using the thermal original spectrometer with cylindrical analyzers spectrometer at the ISIS pulsed neutron source (UK). From the experimental spectrum we have obtained the parahydrogen density of phonon states which has been compared with the estimates available in the literature. The present determination improves substantially the previous experimental scenario from the point of view of both statistics and accuracy. The comparison with the most recent estimate obtained from a quantum mechanical simulation of the molecular dynamics calls for an improvement of the computational methods..     

The polymerization of actin: structural changes from small-angle neutron scattering

We present a new analysis of small-angle neutron-scattering data from rabbit muscle actin in the course of the polymerization from G-actin to F-actin as a function of temperature. The data, from Ivkov et al. [J. Chem. Phys. 108, 5599 (1998)], were taken in D2O buffer with Ca2+ as the divalent cation on the G-actin in the presence of ATP and with KCl as the initiating salt. The new analysis of the data using modeling and the method of generalized indirect fourier transform (O. Glatter, GIFT, University of Graz, Austria, http://physchem.kfunigraz.ac.at/sm/) provide shapes and dimensions of the G-actin monomer and of the growing actin oligomer in solution as a function of temperature and salt concentration. This analysis indicates that the G-actin monomer, under the conditions given above, is a sphere 50-54 A in diameter as opposed to the oblate ellipsoid seen by x-ray crystallography. The F-actin dimensions are consistent with x-ray crystal structure determinations.