Search Results - (Author, Cooperation:A. Jameson)

2016-02-26

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2018-02-22

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Low-Energy, Multiple-Particle Dynamics

2013-07-06

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1089-7690

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The nitrogen shielding surface in ammonia is calculated using the localized orbital-local origin (LORG) method of Hansen and Bouman, in terms of the symmetry coordinates for the molecule. With respect to the inversion coordinate, the N shielding surface has a shape similar to the potential surface. Rovibrational averaging of the N shielding in NH3 and ND3 molecules is carried out using numerical wave functions which are solutions to the inversion potential which best fits the spectra of all isotopomers. The other coordinates are vibrationally averaged in the usual way, assuming small amplitude motions. The calculated temperature dependence of the N shielding due to inversion is in the opposite sense to that observed for a large number of molecules, and is nearly canceling the contributions from all the other coordinates. The temperature dependence of the nitrogen shielding in ammonia has been measured in the range 300–400 K in samples with densities in a hundredfold range (0.37–33 amagat). When the temperature-dependent intermolecular effects are separated out, the remaining temperature dependence is small and is consistent with the calculations. The inversion contribution to the deuterium-induced isotope shift is of opposite sign to the contributions from all other coordinates. The agreement with the experimental isotope shift in the liquid phase is satisfactory.

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1089-7690

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The average 129Xe nuclear magnetic resonance (NMR) chemical shift for xenon atoms in alpha cages of zeolite CaA is observed in a single peak dependent on xenon loading (〈n〉=0.5–8.9 Xe atoms/alpha cage) and temperature (240–360 K). The general increase of the shift with increasing average number of xenon atoms per alpha cage is shown to be due largely to the changing distribution of occupancies with increasing 〈n〉, coupled with increasing increments in the chemical shifts of Xen with increasing n. Except at the highest loadings, the results obtained for xenon in CaA are predicted nicely on the basis of δav(T)=(1/〈n〉)Σnnδn(T)Pn (〈n〉,T), where the fractions Pn of alpha cages containing n Xe atoms are imported from the Pn measured in xenon in zeolite NaA. The high loading data in CaA are interpreted in terms of contributions to the average 129Xe chemical shifts associated with xenon atoms in the window positions.

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1089-7690

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The values of the collisional efficiency for rotational angular momentum transfer bij are compared for various probe molecules (i=NNO, CO2, CO, N2, CF4, CH4, and SF6 ) with different collision partners ( j=same set as i plus Ar, Kr, Xe, and HCl). The trends in bij are consistent with an underlying kinematic factor which is the same as the collisional efficiency for angular momentum transfer in the perfectly rough hard sphere model of Chandler, modified by electronic factors which depend primarily on the anisotropy of the molecule and secondly on the polarizability and anisotropy of the collision partner.

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Nuclear spin–lattice relaxation times (T1) have been measured for 19F in the CF4 molecule in pure CF4 gas and in Ar, Kr, Xe, N2, CO, HCl, CO2, CH4, SF6, and C2H6, from which the following relaxation cross sections for the rotational angular momentum vector in CF4 have been obtained: σJ/A(ring)2=19.2±0.7 (CF4–Ar), 29.2±0.8 (CF4–Kr), 34.3±0.8 (CF4–Xe), 12.7±0.2 (CF4–N2), 12.8±0.6 (CF4–CO), 22.0±0.1 (CF4–HCl), 29.7±0.7 (CF4–CO2), 12.2±0.2 (CF4–CH4), 39.4±0.8 (CF4–CF4), 58.0±1.2 (CF4–SF6), and 20.8±0.7 (CF4–C2H6). The temperature dependence of these cross sections in the range 210–400 K is essentially T−1 except for (CF4–Ar) and (CF4–CO, N2) for which it is T−0.5 and T−0.7, respectively.

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We have observed the equilibrium distribution of Xe atoms trapped in the alpha cages of zeolite NaA at 300 and at 360 K for low to high xenon loadings. The experimental distributions obtained by nuclear magnetic resonance (NMR) spectroscopy differ from two previously proposed statistical distributions. The experimental deviations from these statistical models can be explained by the attractive Xe–Xe interactions which favor clustering at low to medium loading, and the higher energies associated with the overcrowded cage disfavoring clusters of eight Xe atoms at high loadings. The temperature dependence of the 129Xe NMR chemical shift of each cluster has been measured in the range 188–421 K, except that for Xe8, which was determined only up to 300 K. The observed shifts and their temperature dependence are interpreted by using the results of ab initio calculations of the intermolecular shielding function in the 39Ar system as a model for the 129Xe system.

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Nuclear spin–lattice relaxation times (T1) have been measured as functions of temperature for 14N in N2 gas and in N2–Ar, Kr, Xe, CO, CO2, HCl, CH4, CF4, and SF6 mixtures. The relaxation is dominated by the quadrupolar mechanism so that empirical values of the collision cross sections for molecular reorientation (σθ,2) could be obtained as functions of temperature. The values of σθ,2/A(ring)2 at 300 K are 29.6±0.9 (N2–N2), 33±1 (N2–Ar), 41±2 (N2–Kr), 44±2 (N2–Xe), 32±1 (N2–CO), 59±3 (N2–CO2), 46±1 (N2–HCl), 31±1 (N2–CH4), 59±2 (N2–CF4), and 73±2 (N2–SF6). For all 14N2 –buffer pairs, the temperature dependence of the cross section deviates from T−1, which is not very different from that of the collision cross section (σJ) for changes in the rotational angular momentum vector. This is the first molecule for which the collision cross sections σθ,2 and σJ have both been measured for a series of collision partners. The ratio (σθ,2/σJ) is found to be nearly constant, 2.1±0.2 for the N2 molecule with the ten collision partners. Based on Kouri's IOS factorization scheme, (σθ,2/σJ)〉1 may be true in general. The data for N2 are compared with the theoretical reduced correlation times based on existing mathematical models for molecular reorientation in fluids.

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1089-7690

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1089-7690

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Cross sections for the relaxation of the rotational angular momentum vector of the NNO molecule on collision with CO, Ar, CO2, NNO, Kr, CF4, and Xe, have been measured by 15N spin relaxation in 15N15NO molecules. The relaxation times of the two nuclei are in a ratio 1.86, independent of density, collision partner, or temperature. Except for Ar and CO, the cross sections are larger than the hard sphere cross sections and their temperature dependences range from T−0.8 to T−1.0.

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1089-7690

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The temperature dependence of the phosphorus shielding in phosphine has been remeasured in the range 300–400 K in samples with densities in the range 7–30 amagat. The shielding surfaces are calculated using the localized orbital–local origin (LORG) method of Hansen and Bouman in terms of the symmetry coordinates for the molecule. These surfaces are used to calculate the rovibrationally averaged 31P shielding. The calculated temperature dependence and the deuterium-induced isotope shift for phosphine are in agreement with experiment. The shapes of the 31P in PH3 and the 15N in NH3 shielding surfaces are very similar. With the exception of the inversion coordinate, the remarkable similarity of the surfaces becomes obvious when the shielding functions are scaled by the values of 〈r−3〉np for the ground states of the neutral P and N atoms.

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1089-7690

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The 19F spin relaxation in CF4 in oxygen gas and in SiF4 in oxygen gas has been measured as a function of density, temperature, and magnetic field. The spin–rotation (intramolecular) and the nuclear spin dipole–electron spin dipole interactions (intermolecular) are the dominant mechanisms. The field dependence of the dipolar relaxation rates is characterized for CF4–O2 and SiF4–O2, and compared with those for other spherical top-O2 systems. Agreement of theoretical estimates with the observed field dependence is satisfactory. The temperature dependences of the collision efficiencies for the CF4–O2 and SiF4–O2 intermolecular relaxation have been determined. The magnitudes are found to be roughly three times that for hard spheres.

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1089-7690

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The nuclear spin relaxation times (T1) of 129Xe in xenon–O2 gas mixtures have been measured as a function of temperature and density at different magnetic fields. This system is used to characterize the intermolecular dipolar relaxation of nuclear spins in the gas phase. An empirical Boltzmann-averaged collision cross section associated with the collision-induced transitions between 129Xe nuclear spin states is obtained as a function of temperature.

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1089-7690

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Investigation of competitive adsorption is carried out using the Xe–Ar mixture in zeolite NaA as a model system. The Xen clusters are trapped in the alpha cages of this zeolite for times sufficiently long that it is possible to observe individual peaks in the NMR spectrum for each cluster while the Ar atoms are in fast exchange between the cages and also with the gas outside. The 129Xe nuclear magnetic resonance spectra of 12 samples of varying Xe and Ar loadings have been observed and analyzed to obtain the 129Xe chemical shifts and the intensities of the peaks which are dependent on the average argon and xenon occupancies. The detailed distributions, f(XenArm), the fractions of cages containing n Xe atoms and m Ar atoms cannot be observed directly in this system, that is, individual peaks for XenArm mixed clusters are not observed in the NMR spectrum. This information is, however, convoluted into the observed 129Xe chemical shifts for the Xen peaks and the distributions Pn, the fraction of cages containing n Xe atoms, regardless of the number of Ar atoms, obtained from their relative intensities. Grand canonical Monte Carlo (GCMC) simulations of mixtures of Xe and Ar in a rigid zeolite NaA lattice provide the detailed distributions and the average cluster shifts, as well as the distributions Pn. The agreement with experiment is reasonably good for all 12 samples. The calculated absolute chemical shifts for the Xen peaks in all samples at 300 K range from 75 to 270 ppm and are in good agreement with experiment. The GCMC results are compared with a strictly statistical model of a binary mixture, derived from the hypergeometric distribution, in which the component atoms are distinguishable but equivalent in competition for eight lattice sites per cage under mutual exclusion. The latter simple model introduced here provides a limiting case for the distributions, with which both the GCMC simulations and the properties of the actual Xe–Ar system are compared. © 1996 American Institute of Physics.

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1749-6632

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Natural Sciences in General

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1089-7690

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We have observed the individual signals of the Xen clusters (n=1–5) trapped in the alpha cages of zeolite KA. The 129Xe NMR chemical shift of each cluster in zeolite KA is larger than that of the corresponding Xen cluster in zeolite NaA. The temperature dependence of the chemical shifts of the clusters vary systematically with cluster size as they do in NaA, but the change of the temperature coefficients with n is somewhat more pronounced for Xen in the cages of KA than in NaA. The Xen chemical shifts and their variation with temperature are reproduced by the grand canonical Monte Carlo (GCMC) simulations. GCMC simulations of the distribution of the Xe atoms among the alpha cages in KA provide the fractions of cages containing n Xe atoms which agree reasonably well with the observed equilibrium distributions. The characteristics of Xe distribution and chemical shifts in KA are compared with that in NaA. © 1995 American Institute of Physics.

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Xenon trapped in the alpha cages of zeolite NaA exhibits distinct NMR signals for clusters Xe1, Xe2, Xe3,..., up to Xe8. Using multisite magnetization transfer experiments, we have measured the rate constants kmn for the elementary processes that are involved in the cage-to-cage transfer of Xe atoms in the zeolite NaA, that is, for a single Xe atom leaving a cage containing Xen to appear in a neighboring cage containing Xem−1, thereby forming Xem. In a random walk simulation, these rate constants reproduce over a hundred magnetization decay/recovery curves that we have measured in four samples of Xe in zeolite NaA at room temperature, in selective inversion, and complementary experiments for all the significantly populated clusters. The simulations also lead to the correct experimental equilibrium distributions, that is, the fractions of the alpha cages containing Xe1,Xe2,...,Xe8.

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The equilibrium distribution of the Xe atoms among the alpha cages of the zeolite NaA have been measured directly by nuclear magnetic resonance (NMR) in ten samples ranging from very low xenon loading up to saturation. These distributions are simulated by a grand canonical Monte Carlo (GCMC) method which reproduces the experimental data quantitatively for all ten samples at 296 K and also at 360 K. The adsorption isotherm of the high loading samples has been determined directly from the chemical shift of the gas in equilibrium with the adsorbed xenon. The data compare favorably with the adsorption isotherms resulting from the simulations. The previously reported 129Xe chemical shifts of the individual Xen clusters and their temperature dependences in the range 188–420 K are reproduced quantitatively by the GCMC simulation which makes use of pairwise additive ab initio intermolecular shielding functions. These cluster shifts and their temperature dependence encode the distribution of configurations for a given Xen cluster in an alpha cage. Quantitative agreement with the three experimental measures of the distribution of Xe atoms in NaA (partitioning between the adsorbed phase and the gas phase, distribution of the intrazeolitic atoms among the alpha cages, and the distribution of Xe atoms within an alpha cage containing Xen) as a function of temperature has been achieved for the first time.

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The quantitative agreement between the results of a grand canonical Monte Carlo (GCMC) simulation and the various direct experimental measures of the distribution of the Xe atoms between adsorbed phase and gas phase, of intrazeolitic xenon among the alpha cages, and of the distribution of n Xe atoms in a Xen cluster within one alpha cage permit us to consider the structure of the adsorbed fluid in the GCMC simulation as a reasonable description of the actual structure. We provide here the adsorption sites for a single Xe atom in the alpha cage of zeolite NaA, the transition states between these adsorption sites, the one-body distribution functions for the individual clusters Xen inside the alpha cage, the Xe–Xe pair distribution functions for Xe2 through Xe8 at two temperatures, and some of the local minima in the configuration space of the clusters Xe2 through Xe8, i.e., some of the minimum energy configurations of the clusters.

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