Some experiments on γ-scattering between 122 and 145 keV at small angles

Differential cross section for scattering of 145.4 keV gamma rays by B, C, Al, Cu and Cd have been measured from 5–25°. For angles <10° it was not possible to separate Rayleigh and Compton scattering; therefore, the sum of the cross sections is given. Rayleigh cross sections have been measured for Pb at 122.1 and 136.5 keV at angles between 20° and 70°. The experimental results are compared with the form factor theory for Rayleigh scattering and the incoherent scattering factor theory for Compton scattering.     

Accuracy of recent potential energy surfaces for the He-N2 interaction. II. Molecular beam scattering and bulk gas relaxation phenomena

A new semiempirical exchange-Coulomb model potential energy surface for the N(2)-He interaction was reported recently [A. K. Dham et al., J. Chem. Phys. 127, 054302 (2007)] and, using it, the temperature dependence of bulk gas properties of N(2)-He mixtures, such as the second virial coefficient and traditional transport phenomena, most of which depend primarily on the isotropic component of the interaction potential energy surface, was determined. Values of these properties, along with values calculated using two high-quality ab initio potential energy surfaces [C.-H. Hu and A. J. Thakkar, J. Chem. Phys. 104, 2541 (1996); K. Patel et al., ibid 119, 909 (2003)] were compared critically to available experimental data. The present paper reports on the ability of the same three potential energy surfaces to predict state-to-state and total differential cross sections, total integral cross sections, and the temperature dependence of bulk gas relaxation phenomena (including magnetic field effects on transport coefficients). While all three potential energy surfaces give total differential and higher speed integral scattering results that fall within the experimental uncertainties, integral scattering results and state-to-state differential cross section measurements consistently exceed the calculated values. All three surfaces give similar agreement with the relaxation properties of N(2)-He binary mixtures, with the semiempirical exchange-Coulomb model potential energy surface giving slightly better overall agreement with experiment than the two ab initio potential energy surfaces.     

Photon correlation spectroscopy of polystyrene near the glass–rubber relaxation

Polarized Rayleigh scattering is studied near the glass-rubber relaxation in atactic polystyrene using photon correlation spectroscopy. Average relaxation times determined from the data agree well with previous results obtained using depolarized Rayleigh scattering. The light scattering results follow the same trend observed by dielectric and mechanical relaxation studies, but the times for orientational relaxation are longer by approximately two orders of magnitude. Also, an extensive discussion of the experimental techniques necessary to use photon correlation spectroscopy of polymers near the glass-rubber relaxation is presented.     

Quantum mechanical scattering cross sections and permeability coefficients for ions crossing the human red cell and resting squid axon membranes

Quantum mechanical calculations of elastic scattering cross sections for some permeant ions crossing the human red blood cell and resting axolemma squid axon membranes have been carried out using the three-dimensional spherically symmetric square potential well. Making the assumption that the permeability coefficient is inversely proportional to scattering cross section, we obtain the order of membrane selectivity for the ions as well as values for the permeability coefficients. Despite the relatively simple method used, good agreement between calculated permeability coefficients and data available in the literature is obtained. We suggest that elastic scattering cross section measurements for ions in various membranes would be valuable not only because they give a precise idea about the permeability ratios between ions but they also determine the form of the potential the ions are moving in.     

Experimental and theoretical study of proton spin-lattice relaxation in H2-Ar gas mixtures: critical examination of the XC(fit) potential energy surface

Proton nuclear magnetic resonance spin-lattice relaxation time measurements have been carried out at 500 MHz proton Larmor frequency on two hydrogen-argon gas mixtures with 1.90% and 3.93% hydrogen at four different temperatures in the range 225 K < T < 337 K and at two different number densities. The results for different hydrogen mole percentages have been extrapolated to infinite dilution to obtain the contributions to the overall relaxation times arising from the hydrogen-argon interaction. The extrapolated relaxation times fall in the reciprocal regime in which relaxation times are inversely proportional to the density. Relaxation times have also been calculated using quantum mechanical close-coupled computations based on the H2-Ar XC(fit) potential energy surface obtained by Bissonnette et al. [J. Chem. Phys. 105, 2639 (1996)]. Significant differences found between the experimental and theoretical results indicate that the short-range anisotropy of the XC(fit) potential surface is too weak. The reciprocal regime is shown to have a much higher sensitivity to changes in the anisotropic component of the intermolecular potential energy surface.     

Low-temperature rotational relaxation of CO in self-collisions and in collisions with Ne and He

The low-temperature rotational relaxation of CO in self-collisions and in collisions with the rare-gas atoms Ne and He has been investigated in supersonic expansions with a combination of resonance-enhanced multiphoton ionization (REMPI) spectroscopy and time-of-flight techniques. For the REMPI detection of CO, a novel 2 + 1' scheme has been employed through the A(1)Pi state of CO. From the measured data, average cross sections for rotational relaxation have been derived as a function of temperature in the range 5-100 K. For CO-Ne and CO-He, the relaxation cross sections grow, respectively, from values of approximately 20 and 7 A(2) at 100 K to values of approximately 65-70 and approximately 20 A(2) in the 5-20 K temperature range. The cross section for the relaxation of CO-CO grows from a value close to 40 A(2) at 100 K to a maximum of 60 A(2) at 20 K and then decreases again to 40 A(2) at 5 K. These results are qualitatively similar to those obtained previously with the same technique for N(2)-N(2), N(2)-Ne, and N(2)-He collisions, although in the low-temperature range (T < 20 K) the CO relaxation cross sections are significantly larger than those for N(2). Some discrepancies have been found between the present relaxation cross sections for CO-CO and CO-He and the values derived from electron-induced fluorescence experiments.     

Experimental determination of molecular polarizability anisotropy of nematogens by depolarized Rayleigh light scattering

The anisotropy of molecular polarizability Δα of several nematogens has been determined by depolarized Rayleigh light scattering. The experimentally determined values were found to be noticeably smaller than those obtained by MOPAC calculation. Δα values determined from order parameter S and refractive indices show reasonably good agreement with those determined in the present experiment. This result warns us that Δα calculated by MOPAC, particularly at a finite wavelength, may be larger than the true value.     

Molecular dynamics simulations for CO2 spectra. III. Permanent and collision-induced tensors contributions to light absorption and scattering

Classical molecular dynamics simulations have been performed for gaseous CO(2) starting from an accurate anisotropic intermolecular potential. Through calculations of the evolutions of the positions and orientations of a large number of molecules, the time evolutions of the permanent and collision-induced electric dipole vector and polarizability tensor are obtained. These are computed from knowledge of static molecular parameters taking only the leading induction terms into account. The Laplace transforms of the auto-correlation functions of these tensors then directly yield the light absorption and scattering spectra. These predictions are, to our knowledge, the first in which the contributions of permanent and collision-induced tensors are simultaneously taken into account for gaseous CO(2), without any adjusted parameter. Comparisons between computations and measurements are made for absorption in the region of the ν(3) infrared band and for depolarized Rayleigh scattering in the roto-translational band. They demonstrate the quality of the model over spectral ranges from the band center to the far wings where the spectrum varies by several orders of magnitude. The contributions of the permanent and interaction-induced (dipole and polarizability) tensors are analyzed for the first time, through the purely permanent (allowed), purely induced, and cross permanent∕induced components of the spectra. It is shown that, while the purely induced contribution is negligible when compared to the collision-broadened allowed component, the cross term due to interferences between permanent and induced tensors significantly participates to the wings of the bands. This successfully clarifies the long lasting, confusing situation for the mechanisms governing the wings of the CO(2) spectra considered in this work.     

Microwave spectra, structure, and dynamics of the weakly bound complex, N2 CO2

The Fourier transform microwave spectra of the various isotopologs of the weakly bound complex of carbon dioxide with the most abundant molecule in the atmosphere, nitrogen, have been measured. The structure of the complex has been determined and evidence for the inversion of the N(2) is presented. The molecule is T-shaped, with the OCO forming the cross of the T, a structure consistent with that deduced from a previous rotationally resolved infrared experiment. A significant wide-amplitude bending motion of the N(2) is deduced from the values of the (nearly identical) nuclear quadrupole coupling constants of the nitrogen nuclei. The spectroscopic results are compared with high-quality ab initio calculations. We examine the consequences of the N(2) CO(2) complex formation in the atmosphere upon the greenhouse warming potential of carbon dioxide.     

Calculation of the transport and relaxation properties of methane. I. Shear viscosity, viscomagnetic effects, and self-diffusion

Transport properties of pure methane gas have been calculated in the rigid-rotor approximation using the recently proposed intermolecular potential energy hypersurface [R. Hellmann et al., J. Chem. Phys. 128, 214303 (2008)] and the classical-trajectory method. Results are reported in the dilute-gas limit for shear viscosity, viscomagnetic coefficients, and self-diffusion in the temperature range of 80-1500 K. Compared with the best measurements, the calculated viscosity values are about 0.5% too high at room temperature, although the temperature dependence of the calculated values is in very good agreement with experiment between 210 and 390 K. For the shear viscosity, the calculations indicate that the corrections in the second-order approximation and those due to the angular-momentum polarization are small, less than 0.7%, in the temperature range considered. The very good agreement of the calculated values with the experimental viscosity data suggests that the rigid-rotor approximation should be very reasonable for the three properties considered. In general, the agreement for the other measured properties is within the experimental error.     

Hydrogen bond dynamics and water structure in glucose-water solutions by depolarized Rayleigh scattering and low-frequency Raman spectroscopy

The effect of glucose on the relaxation process of water at picosecond time scales has been investigated by depolarized Rayleigh scattering (DRS) experiments. The process is assigned to the fast hydrogen bonding dynamics of the water network. In DRS spectra this contribution can be safely separated from the slower relaxation process due to the sugar. The detected relaxation time is studied at different glucose concentrations and modeled considering bulk and hydrating water contributions. As a result, it is found that in diluted conditions the hydrogen bond lifetime of proximal water molecules becomes about three times slower than that of the bulk. The effect of the sugar on the hydrogen bond water structure is investigated by analyzing the low-frequency Raman (LFR) spectrum sensitive to intermolecular modes. The addition of glucose strongly reduces the intensity of the band at 170 cm(-1) assigned to a collective stretching mode of water molecules arranged in cooperative tetrahedral domains. These findings indicate that proximal water molecules partially lose the tetrahedral ordering typical of the bulk leading to the formation of high density environments around the sugar. Thus the glucose imposes a new local order among water molecules localized in its hydration shell in which the hydrogen bond breaking dynamics is sensitively retarded. This work provides new experimental evidences that support recent molecular dynamics simulation and thermodynamics results.     

Vibrational spectrum of the spin crossover complex [Fe(phen)(2)(NCS)(2)] studied by IR and Raman spectroscopy, nuclear inelastic scattering and DFT calculations

The vibrational modes of the low-spin and high-spin isomers of the spin crossover complex [Fe(phen)(2)(NCS)(2)] (phen = 1,10-phenanthroline) have been measured by IR and Raman spectroscopy and by nuclear inelastic scattering. The vibrational frequencies and normal modes and the IR and Raman intensities have been calculated by density functional methods. The vibrational entropy difference between the two isomers, DeltaS(vib), which is--together with the electronic entropy difference DeltaS(el)--the driving force for the spin-transition, has been determined from the measured and from the calculated frequencies. The calculated difference (DeltaS(vib) = 57-70 J mol(-1) K(-1), depending on the method) is in qualitative agreement with experimental values (20-36 J mol(-1) K(-1)). Only the low energy vibrational modes (20% of the 147 modes of the free molecule) contribute to the entropy difference and about three quarters of the vibrational entropy difference are due to the 15 modes of the central FeN(6) octahedron.     

Solute-solvent contact by intermolecular cross relaxation. I. The nature of the water-hydrophobic interface

Intermolecular cross-relaxation rates between solute and solvent were measured by {1H} 19F nuclear magnetic resonance experiments in aqueous molecular solutions of ammonium perfluoro-octanoate and sodium trifluoroacetate. The experiments performed at three different magnetic fields provide frequency-dependent cross-relaxation rates which demonstrate clearly the lack of extreme narrowing for nuclear spin relaxation by diffusionally modulated intermolecular interactions. Supplemented by suitable intramolecular cross-relaxation, longitudinal relaxation, and self-diffusion data, the obtained cross-relaxation rates are evaluated within the framework of recent relaxation models and provide information about the hydrophobic hydration. In particular, water dynamics around the trifluoromethyl group in ammonium perfluoro-octanoate are more retarded than that in the smaller trifluoroacetate.     

From fractal to nanorod porphyrin J-aggregates. Concentration-induced tuning of the aggregate size

A structural change from fractal to nanorod J-aggregates of tetrakis(4-sulfonatophenyl)porphyrin has been obtained by acting on the intermolecular interaction potential. The size and shape of these self-assembled porphyrin clusters have been monitored under different experimental conditions, by means of polarized and depolarized dynamic light scattering and small and wide angle elastic light scattering. At sufficiently low porphyrin concentration and high ionic strength, the shielded repulsive potential seems to be responsible for the fractal structure of the aggregates. On the contrary, at low ionic strength (nonshielded potential) and high porphyrin concentration, these species self-assemble in a rodlike arrangement. The length of the so-formed rod-shaped aggregates decreases on increasing porphyrin concentration. Moreover, both fractals and rods display a structure-dependent optical activity induced by a chiral template.     

C, N and H magnetic relaxation and anisotropic molecular reorientation of nitrobenzene

Magnetic relaxation rates of ¹³C and ¹⁴N have been measured for neat nitrobenzene and several 50% (v/v) solutions thereof. Using known values of the ¹⁴N quandrupole coupling constant and asymmetry parameter and the ¹³CH bond distances we have determined the three reorientational correlation times and the orientation of the ¹⁴N quadrupole coupling tensor. The correlation times are compared with those previously obtained for nitrobenzene from a combination of depolarized Rayleigh scattering ad ¹³C relaxation data. Knowledge of the principal components of the rotational diffusion tensor permits determination of the quadrupole coupling constant for the ortho deuterons in nitrobenzene-d₅ and the unambiguous assignment to molecular axes of previously determined components of the chemical shift tensor for ¹⁵N in nitrobenzene ¹⁵N. In addition, the implications of our assignment of the ¹⁴N quadrupole coupling tensor components for earlier studies of the alignment of nitrobenzene by electric fields and in a liquid crystal are discussed.     

Depolarised Rayleigh scattering from liquid carbon monoxide,nitrogen and oxygen

The spectral distributions and scattering cross sections of depolarised Rayleigh scattering have been determined for liquid carbon monoxide, nitrogen and oxygen at 77 K and atmospheric pressure. It is shown that the scattering arises predominantly from molecular orientational fluctuations. The experimental scattering cross sections at 488 nm are 7 ± 1 for CO, 16 ± 1 for N₂ and 46 ± 4 for O₂ in units of 10^?30 cm² sr^?1 molecule^?1, based on a recently determined value for the absolute depolarised scattering intensity for liquid argon. The estimated proportions of induced anisotropy scattering are 10% for CO, 2.5% for N₂ and 0.8% for O₂. It is shown that there is appreciable free rotation of N₂ and CO in the liquids at this temperature but for O₂ this motion is dissipated much more efficiently by molecular interactions.     

Scattering experiments with hydrogen atoms. I. Differential collision cross sections for H + Ar,D + Ar and H + CH4

Differential collision cross section measurements for the scattering of hydrogen and deuterium atoms from argon and methane have been carried out with a crossed beams scattering apparatus which uses an oscillating supersonic beam as scattering target and a cryogenic bolometer as beam detector. Diffraction oscillations have been clearly resolved. The data are analyzed with a best fit computing procedure in terms of simple intermolecular energy functions. Well depth parameters for both Ar and CH₄ are 60% larger as compared with those predicted by the geometric mean combination rule while the experimental minimum of the well positions are 10% smaller as given by the arithmetic mean combining rule.     

Oxygen accessibility to ribonuclease a: quantitative interpretation of nuclear spin relaxation induced by a freely diffusing paramagnet

The nuclear spin relaxation induced by a freely diffusing paramagnetic center provides a direct measure of intermolecular accessibility. A number of factors are involved in a quantitative interpretation of relaxation data including excluded volume effects, solvation differences, and the details of the electron spin relaxation in the paramagnetic center. In the case where the electron relaxation time is short compared with correlation times describing the electron-nuclear coupling, the nuclear spin relaxation rates may be related to the effective local concentration of the paramagnetic center at different locations about the solute of interest. The local concentrations may in turn be related to differences in the local free energies of interaction between the diffusing paramagnet and the cosolute. We demonstrate this approach for the case of ribonuclease A and deduce surface free energy differences for a large number of protein proton sites. We find that the oxygen accessibility is poorly represented by hard-sphere models such as computed solvent or steric accessibility. There is a distribution of local intermolecular interactions with a width of the order of RT that dominates the report of the intermolecular exploration of the protein by this simple solute.     

Vapor+liquid equilibrium of water, carbon dioxide, and the binary system, water+carbon dioxide, from molecular simulation

NVT- and NpT-Gibbs ensemble Monte Carlo (GEMC) simulations were applied to describe the vapor–liquid equilibrium of water (between 323 and 573 K), carbon dioxide (between 230 and 290 K) and their binary mixtures (between 348 and 393 K). The properties of supercritical carbon dioxide were determined between 310 and 520 K by NpT-MC simulations. Literature data for the effective pair potentials (for water: the SPC-, SPC/E-, and TIP4P potential models; for carbon dioxide: the EPM2 potential model) were used to describe the properties of the pure substances. The vapor pressures of water and carbon dioxide are calculated. For water, the SPC- and TIP4P models give superior results for the vapor pressure when compared to the SPC/E model. The vapor–liquid equilibrium of the binary mixture, carbon dioxide–water, was predicted using the SPC- as well as the TIP4P model for water and the EPM2 model for carbon dioxide. The interactions between carbon dioxide and water were estimated from the pair potentials of the pure components using common mixing rules without any adjustable binary parameter. Agreement of the predicted data for the compositions of the coexisting phases in vapor–liquid equilibrium and experimental results is observed within the statistical uncertainties of the simulation results in the investigated range of state, i.e. at pressures up to about 20 MPa.