Structure and equilibrium optical properties of liquid CS2

Equilibrium optical properties of liquid carbon disulphide (CS₂), i.e. its refractive index, Kerr constant and depolarized light scattering intensity are calculated using two models of optical response of the fluid. The first one, the point polarizability approximation (PPA) assumes that a point dipole, proportional to the total polarizability, is induced in each molecule. The second one, the point atomic polarizability approximation (PAPA) assumes that point dipoles are induced in individual atomic sites.

The symmetry components of the intermolecular pair distribution function needed to calculate optical properties of the fluid are obtained by Monte Carlo computer simulation on a hard triatomic model of CS₂ as well as by two approximate approaches using this same model. The approximations are both based on the use of the site superposition approximation (SSA) for the intermolecular pair distribution function. In the first approach, the SSA pair distribution is obtained using the Monte Carlo site-site functions and in the second using the site-site functions calculated using the reference interaction site model (RISM) equations.

Extensive comparisons are carried out between the Monte Carlo results and the two approximations in order to examine the influence of the SSA and the RISM equations on optical properties of CS₂. We conclude that, while these approximations, especially the SSA, have a substantial effect on individual symmetry components of the pair distribution, they predict measurable optical properties with satisfactory accuracy.     

Structural studies of liquid alcohols by neutron diffraction

Neutron diffraction measurements have been made on methyl alcohol at room temperature for an incident wavelength of 0·94 Å. Cross sections have been obtained for CD₃OD, CD₃OH, and mixtures of these compounds. These data are subtracted to obtain the separated structure factors for intermolecular H₀H₀, the hydroxyl components, and the non-hydroxyl components. The Fourier transformations of the structure factors show components of both intra- and intermolecular distribution functions. Width parameters obtained from model fits are too large for thermal vibrations and are interpreted as geometrical broadening due to the stretching of bonds, variations in bond angles, and rotation of the methyl group. Differences in the real space distribution function between hydrogen and deuterium are noted.     

Structure and elastic properties of a nematic liquid crystal: A theoretical treatment and molecular dynamics simulation

The Frank elasticity constants which describe splay (K ₁), twist (K ₂), and bend (K ₃) distortion modes are investigated for 4-n-pentyl-4'-cyanobiphenyl (5CB) in the nematic liquid crystal. The calculations rest on statistical-mechanical approaches where the absolute values of K _i (i=1,2,3) are dependent on the direct correlation function (DCF) of the corresponding nematic state. The DCF was determined using the pair correlation function by solving the Ornstein-Zernike equation. The pair correlation function, in turn, was obtained from molecular dynamics (MD) trajectory. Three different approaches for calculations of the elasticity constants were employed based on different level of approximation about the orientational order and molecular correlations. The best agreement with experimental values of elasticity constants was obtained in a model where the full orientational distribution function was used. In addition we have investigated the approximation about spherical distribution of the intermolecular vectors in the nematic phase, often used in derivation of various mean-field theories and employed here for the construction of the DCF. We found that this assumption is not strictly valid, in particular a strong deviation from the isotropic distribution is observed for short intermolecular distances. Received 22 March 2000 and Received in final form 9 June 2000     

Local structure of Ni2Si

The local structure around the Ni atom in Ni₂Si has been studied by measuring and analyzing the Extended Energy Loss Fine Structure above the Ni M_2,3 edge. The radial distribution function we found is characterized by the efficiency of the only nearest neighbours Si atoms. We also obtained the phase shift of the Ni-Si pair and the backscattering amplitude.     

Double electron-electron resonance in electron spin echo: Conformations of spin-labeled poly-4-vinilpyridine in glassy solutions

Double electron-electron resonance in electron spin echo has been used to study the glassy solutions of poly-4-vinylpyridine doped by nitroxyl radicals frozen in liquid nitrogen. The phase relaxation of spin labels due to spin-spin interaction of unpaired electrons has been studied. The intramolecular and intermolecular contributions of the dipole-dipole interaction of spin labels into relaxation process have been separated. It has been established that both the intramolecular and intermolecular spin-spin interaction of spin labels lead to the dependence of echo signal on timeT of the exp (?aT ^q ) type. It is shown that for the intramolecular interaction the experimentalq value is 0.3, for the intermolecular one it is 2. The assumption has been made of the linear structure of polymeric molecules due to the presence of a sufficiently high density of an electric charge on polymeric molecules.     

A neutron diffraction study of liquid chloroform

Neutron diffraction studies of the isotopes of chloroform CD³⁵Cl₃ and CD³⁷Cl₃ in liquid phase were carried out at 20°C and a wavelength of 0·7 Å. The data were corrected for background, absorption, multiple scattering and inelastic effects. The coherent distinct differential cross section was separated into intramolecular and intermolecular contributions. The latter, together with intermolecular contributions from neutron scattering data on chloroform of natural isotopic chlorine composition and from X-ray data, was employed to determine four expansion coefficients of the molecular pair correlation function.     

DRDF analysis of wide-angle x-ray scattering of natural rubber

A differential radial distribution function (DRDF) of molten natural rubber (NR) was derived from its wide-angle x-ray scattering (WAXS) data. Three peaks with Bragg spacings 4.85, 2.31, and 1.23 Å corresponding, respectively, to interchain, C₁-C₃ and C₁ -C₂ distances are present in the corrected WAXS curve. The derived DRDF, which is shown to be greatly improved than the ones published so far, contains six peaks located at 1.51, 2.48, 3.98, 5.68, 10.65, and 15.25 Å. Maximum contributions of intramolecular atomic distances to the radial distribution peaks were estimated and compared with the experimental results. The comparison shows that the first three peaks are intramolecular in origin and the remaining peaks are predominantly due to intermolecular regularities. A periodicity of an approximately constant interval of 5.26 Å is found between the intermolecular peaks and is attributable to the organizations of more or less parallel chain segments in the material. This finding provided additional evidence for the presence of local lateral ordering to the extent of about 30 Å in molten-state natural rubber.     

The inter molecular potential of NH+ 4-Ar II. Calculations and experimental measurements for the rotational structure of the v 3 band

The mid-infrared spectrum of the v ₃,(t ₂) transition of the NH⁺ ₄-Ar complex has been recorded at rotational resolution using photofragmentation spectroscopy. The spectrum is divided into perpendicular and parallel subbands corresponding to transitions between different hindered internal rotor states. The P and R branches of the strongest perpendicular subbands are rotationally resolved providing rotational and centrifugal distortion constants. The widths of individual rotational lines are limited by the laser bandwidth of 0.02 cm^?1, giving a lower limit of 250 ps for the lifetime of the excited states. Effective intermolecular separations for each internal rotor state are determined from its rotational constant, after correction for the contribution due to Coriolis coupling between the internal and total rotational angular momenta. The absolute energies, rotational and distortion constants for the first few intermolecular bending and stretching levels of the ground intramolecular vibrational state are determined in a numerical solution to the rotation-intermolecular vibration Hamiltonian, employing a three-dimensional ab initio intermolecular potential. The results are compared with the experimental constants in order to assess the accuracy of the calculated potential. The relative energy levels from this calculation are also compared with those from a two-dimensional representation of the potential energy surface (‘fixed-R’ model) in order to judge directly the influence of the radial dependence of the potential.     

The fluctuation hypothesis of hydrogen bonding

To describe the distortion perturbations of the water molecules in solutions the quantum-mechanical method of partial oscillators has been developed. This method allows one to separate in a simple manner two types of influences on vibrations of OH oscillators, viz. the intermolecular perturbations resulting in the different strengths of hydrogen bonds and intramolecular coupling between stretching vibrations. Here the coupling is treated quite strictly, whereas the intermolecular perturbations are introduced phenomenologically. The calculation of the distribution of distortions among molecules in liquid H₂O and D₂O induced by non-equivalency of the two hydrogen bonds of the water molecule has been made on the basis of the method developed, and the parameters of the mean statistical molecule have been found from the experimental spectrum of HOD. The depolarization ratios of vibrations in Raman spectra of the mean statistical molecules H₂O and D₂O have been computed as an illustration of the possibilities of the proposed calculation method. All the estimates show that the stretching oscillators of H₂O and D₂O molecules are significantly coupled in spite of the great distortion of the symmetry of water molecules in the liquid state.     

Vibrational spectra of crystalline H2SeO3 in the O-H stretching region

IR transmission, near-normal IR reflection and polarized Raman spectra of single crystals of selenious acid (H₂SeO₃) have been obtained at temperatures between 77°K and 335°K. The observed bands in the O-H stretching region have been assigned to two types of O-H….O bonds in the crystal structure. The spectral results appear to rule out correlation field splittings, intramolecular coupling, or Fermi resonance as explanations for the observed splitting. A proposed model based on intermolecular coupling of O-H….O vibrations to explain the splitting of the O-H stretching region is given.     

Neutron diffraction studies of liquid carbon suboxide

The structure factor S _m(Q) for liquid carbon suboxide has been determined for a Q-value range of 0·4 to 60 Å^-1 by neutron diffraction measurements using a steady-state (reactor) and a pulsed (linac) neutron source. The bond lengths of the molecule have been determined from the data and give good agreement with the results of electron diffraction measurements on the vapour phase after application of a molecular recoil correction term. The quasi-linear nature of the molecule is confirmed but the shape of the form factor indicates that large amplitude bending motion probably occurs in the liquid phase.

Oscillations in the intermolecular pair correlation function are observed to have a regular periodicity extending to 12 Å but details of orientational effects cannot be established from a single diffraction measurement.     

非晶态Mg70Zn30合金结构因子的预峰

利用X射线衍射仪对非晶态Mg₇₀Zn₃₀合金的结构进行了研究,获得了强度曲线、结构因子、双体分布函数和原子间最近邻距离.结果表明,Mg₇₀Zn₃₀合金在小Q区间存在强烈的预峰现象.根据预峰的特性,提出了Mg₇₀Zn₃₀熔体的结构模型,即Mg原子位于中心,8个Zn原子位于顶角所形成的简单立方结构模型.该模型以共享顶点的方式相连接,能够满足预峰对Mg—Mg原子间距离的要求.Mg< 关键词： 非晶态 70Zn₃₀合金')" href="#">Mg₇₀Zn₃₀合金 结构因子 预峰     

Prediction of transport properties of CO2-N2 binary mixtures via the inversion of reduced-viscosity collision integrals

In the present study, the main purpose is to extract information about the effective intermolecular potential energy function for binary mixture of nitrogen and carbon dioxide by the usage of a direct inversion of the experimentally reduced viscosity and second virial coefficient data and then to reproduce the dilute gas transport properties from the inverted potential energy. The Lennard-Jones (12, 6) potential energy function has been employed as the initial potential model required by the inversion method. The MSV potential obtained in this way is in reasonable agreement with the independently known Co₂-N₂ potential energy function. Using the inverted pair potential energies, the Chapman-Enskog scheme is employed to calculate transport properties of CO₂-N₂ in a wide composition and temperature range. The close agreement between the predicted values and the literature results of transport properties demonstrate the predictive power of the inversion scheme.     

The structure of concentrated NiCl2 aqueous solutions: what is molecular simulation revealing about the neutron scattering methodologies?

An analysis is made of adequacy and limitations of the neutron weighted ion distribution function for the determination of the hydration ion structure in hydrothermal solutions. Our analysis indicates that the coordination number based on the O—Ni²⁺ interactions is unambiguously defined by the first peak of the neutron weighted cation distribution function G _Ni(r), but that the corresponding H—Ni⁺² and H—Cl^? coordination numbers may be ill-defined due to the occurrence of Ni⁺²-Cl^? ion pairing. For the system considered in this work, this effect contributes about 1.5 units to the H—Ni^?2 and 0.85 units to the H—Cl^? coordination numbers, respectively, for a 3.9 M NiCl₂ aqueous solution under ambient conditions. A comparison under ambient conditions between the most reliable NDIS data on Ni²⁺ hydration and our simulation results suggests that the present intermolecular potential models underestimate the O-Ni⁺² coordination numbers by about 1.5 units, and it might indicate the need for a reparametrization of the current ion-water intermolecular potentials. The hydration structure exhibits practically no temperature dependence for the isochore studied (1.356 g cm^?3) and composition. The Ni⁺²-Cl^? ion pair formation appears to affect the location of the shoulder in the neutron weighted distribution functions G _Ni(r) and G _Cl(r), although it does not affect the magnitude of the ion—water coordination.     

IR spectra of stigmastane series brassinosteroids differing in configuration and number of diol groups

IR spectra of steroid phytohormones of the stigmastane series (22R, 23R)-28-homocastasterone and (22R,23R)-28-homosecasterol and their isomers (22S,23S)-28-homocastasterone and (22S,23S)-28-homosecasterol have been analyzed. The 28-homocastasterone molecule contains diol groups in ring A and in the side chain whereas that of 28-homosecasterol has one diol group in the side chain. The lack of two OH groups in ring A of homosecasterol compared to homocastasterone results in the appearance of stretching vibrational bands of H–C= (ν_max = 3025 cm^–1) and –C=C (ν_max = 1656 cm^–1) groups of ring A. Substantial changes are observed in the area of OH stretching vibrations. Homocastasterones pressed in KBr possess twice as many OH groups as homosecasterols such that absorption band total intensities in IR spectra of both isomers caused by H-bonds of the diol groups in the side chain amount to 65% whereas the share of the 2α,3α group is only 35% of the total intensity. Hence the contribution from the side-chain OH groups of the studied brassinosteroids to the integral optical density of the bands exceeds that from the ring-A OH groups. In dilute CHCl3 solutions of the brassinosteroids, the conformations of the brassinosteroid side chains are not the same. As a result, intramolecular H-bonds of different energy are created. The optical density D_max in band maxima of free OH groups for homocastasterones is three times higher than that for the corresponding band maxima of homosecasterol. This implies that D_max for bands of free OH groups of the homocastasterone ring-A diol group is greater, in contrast with the relatively greater D_max for bands of homosecasterol side-chain OH groups bound by an intermolecular H-bond. The homocastasterone diol groups also form intramolecular Hbonds more actively. The lack of the diol group in ring A of the homosecasterols does not affect the frequencies of the C=O stretching vibrations. This leads to the conclusion that the C=O group forms intermolecular H-bonds only with the side-chain OH groups of brassinosteroids pressed in KBr.     

DFT studies on catalytic dehydrogenation of ammonia borane by Ni(NHC)2

The catalytic dehydrogenation mechanism of ammonia borane (NH₃BH₃, AB) by Ni N‐heterocyclic carbene (NHC) complexes has been investigated by density functional theory. Two possible mechanisms of the dehydrogenation of NH₃BH₃ have been theoretically studied: intramolecular reaction at Ni dicarbene and intermolecular reaction at Ni monocarbene and dicarbene. The facile occurrence of the dehydrogenation of AB was demonstrated by the low activation barriers of the rate‐determining step. It was found that the intramolecular pathway is mostly kinetically favorable with lower activation barrier of 15.51 kcal/mol than the intermolecular pathway. Moreover, for intermolecular dehydrogenation of AB, the activation prefers to take place at monocarbene Ni(NHC) catalyst. Copyright © 2014 John Wiley & Sons, Ltd.     

A thermodynamic theory of mechanical relaxation due to energy transfer between strain-sensitive and strain-insensitive modes in polymers

A thermodynamic theory has been developed on the relaxation strength of mechanical relaxation due to energy-transfer between strain-sensitive (intermolecular) modes and strain-insensitive (intramolecular) modes. Assuming that the strain-insensitive modes exhibit an overwhelmingly large heat capacity and function as a heat reservoir of constant temperature during the relaxation process, the relaxation strength ΔG, the difference between instantaneous and equilibrium moduli, is given as ΔG = TG² _Tα² ₁/C_z1, where T is the absolute temperature, G_T is the isothermal elastic modulus, and ₁ and C_z1 are the thermal expansion coefficient and the specific heat at constant strain of the strainsensitive modes, respectively. This assumption is reasonable for a polymeric solid in which backbone chains have a lot of vibrational degrees of freedom whose energy is rather insensitive to intermolecular distance and, on the contrary, the potential for localized motion such as motion of side chains is highly sensitive to intermolecular distance.

In a special case where the strain-sensitive modes are a set of vibrators with an angular frequency ω, ΔG = γ²C_z1T, where γ is the Grüneisen constant, defined by the strain-derivative of ω, = ?? In ω?z. The case where the strainsensitive modes are a set of rotators in an n-fold symmetrical potential can be treated as an extension of the above case. In the case where the strain-sensitive modes involve the transition between two states, ΔG in the present theory is reduced to that of the well-known two-state transition theory. Agreement is obtained between theory and experiment for a-methyl relaxation of poly(methy1 methacrylate).     

CIDNP investigation of electron excitation energy transfer in the photolysis of isotope-mixed molecules of benzophenone in cyclohexane-D12

Isotopic effects arising in the photolysis of benzophenones-H₁₀/D₁₀ mixtures in cyclohexane-D₁₂ at room temperature were studied by CIDNP method. Nonlinear dependences of these effects on a relative concentration of benzophenone-D and benzophenone-H molecules have been obtained. To interpret the results, a theoretical model involving intermolecular and intramolecular energy transfers and radical-pair mechanism of CIDNP formation has been proposed. Nonlinearity of isotopic effects is determined by the difference in electron excitation energy between benzophenone-D and benzophenone-H molecules.     

Triply hydrogen bonded van der Waals complexes: CH2F2?CF2CH2 and CH2F2?CF2CHF

The results of molecular beam Fourier transform microwave (FTMW) investigations of the van der Waals complexes of difluoromethane with 1,1-difluoroethene (DFE) and trifluoroethene (TFE) are reported. The rotational parameters of the complexes have been interpreted in terms of a C_s geometry with the two H or F atoms of CH₂F₂ lying out of the σ_v symmetry plane of the complexes. The complexes are bound by three hydrogen bonds, of which one is the stronger C-F?H-C type, and two are the weaker C-H?F?H-C or C-F?H?F-C types for DFM-DFE or DFM-TFE, respectively. The most notable difference in the two complexes is that the out of plane atoms are two hydrogens for DFM-DFE, but are two fluorines for DFM-TFE. Additional information on the structure and hydrogen bonding has been obtained from ab initio calculations.     

Electron density distribution in Si and Ge using multipole, maximum entropy method and pair distribution function analysis

The local, average and electronic structure of the semiconducting materials Si and Ge has been studied using multipole, maximum entropy method (MEM) and pair distribution function (PDF) analyses, using X-ray powder data. The covalent nature of bonding and the interaction between the atoms are clearly revealed by the two-dimensional MEM maps plotted on (100) and (110) planes and one-dimensional density along [100], [110] and [111] directions. The mid-bond electron densities between the atoms are 0.554 e/?³ and 0.187 e/?³ for Si and Ge respectively. In this work, the local structural information has also been obtained by analyzing the atomic pair distribution function. An attempt has been made in the present work to utilize the X-ray powder data sets to refine the structure and electron density distribution using the currently available versatile methods, MEM, multipole analysis and determination of pair distribution function for these two systems.