Ar Pair Research Articles

Thermodynamic Analysis of ArxXe1-x Solid Solutions Based on Kirkwood–Buff Theory

Kirkwood–Buff Integral (KBI) theory is an important method for the analysis of the structural and thermodynamic properties of liquid solutions. For solids, the calculation of KBIs has become possible only recently through the finite-volume generalisation of KBI theory, but it has so far only been applied to monoatomic crystals. Here, we show that KBI theory can be applied to solid mixtures and compute the KBIs of a ArxXe1−x solid solution, for 0<x<0.1 and temperature T=84−86 K, from pair distribution functions obtained by Monte Carlo simulation. From the KBIs, the isothermal compressibility, partial molar volumes, and thermodynamic factors are calculated and found to be in good agreement with alternative theoretical methods. The analysis of the KBIs and the partial molar volumes give insight into the structure of the mixture. The KBI of Ar pairs is much larger than that of Xe pairs, which indicates the tendency of Ar impurities to accumulate. The evolution of the partial molar volumes with increasing Ar molar fraction x shows a transition at x≈0.06, which reflects the formation of Ar clusters, precursors of the Ar-rich liquid phase. The calculated thermodynamic factors show that the solid(Xe) phase becomes unstable at x≈0.1, indicating the start of the solid (Xe)–liquid (Ar) equilibrium. The chemical potentials of Ar and Xe are obtained from the thermodynamic factor by integration over lnx, and by fitting the data to the Margules equations, the activity coefficients can be estimated over the whole composition range. The present findings extend the domain of applicability of the KBI solution theory from liquids to solids.

Rotational spectroscopy of the argon dimer by time-resolved Coulomb explosion imaging of rotational wave packets

We report time-domain rotational spectroscopy of the argon dimer, Ar2, by implementing time-resolved Coulomb explosion imaging of rotational wave packets. The rotational wave packets are created in Ar2 with a linearly polarized, nonresonant, ultrashort laser pulse, and their spatiotemporal evolution is fully characterized by measuring angular distribution of the fragmented Ar+ promptly ejected from Ar22+ generated by the more intense probe pulse. The pump-probe measurements have been carried out up to a delay time of 16 ns. The alignment parameters, derived from the observed images, exhibit periodic oscillation lasting for more than 15 ns. The pure rotational spectrum of Ar2 is obtained by Fourier transformation of the time traces of the alignment parameters. The frequency resolution in the spectrum is about 90 MHz, the highest ever achieved for Ar2. The rotational constant and the centrifugal distortion constant are determined with much improved precision than the previous experimental results: B0 = 1.72713 ± 0.00009 GHz and D0 = 0.0310 ± 0.0005 MHz. The present B0 value does not match within the quoted experimental uncertainty with that from the VUV spectroscopy, so far accepted as an experimental reference to assess theories. The present improved constants would stand as new references to calibrate state-of-the-art theoretical investigations and an indispensable experimental source for the construction of an accurate empirical intermolecular potential.

Ab initio study of the mobility of Gd+ ions in He and Ar gases

The mobility of ground-state Gd+ ions in He and Ar gases is addressed theoretically to bridge the surprising gap between the experimental and previous theoretical data. An accurate ab initio coupled cluster approach is used to compute scalar-relativistic interaction potentials for all the molecular states of the Gd+-He and Gd+-Ar pairs. Then a multireference configuration interaction method is used to account for the spin-orbit coupling between the states. This greatly improves the theoretical mobility values at low fields for the 156Gd+(4f75d6s 10Do5/2) ion, bringing them in close agreement with the room-temperature measurements. Several approximations to transport cross sections for the multistate open-shell collisions are assessed and discussed. Accurate description of the transport properties of the heavy open-shell ions is shown to require a demanding high-level ab initio approach. Temperature- or field-dependent mobility data are needed for a more in-depth assessment of the ab initio interaction potentials and dynamic approximations to the cross sections for open-shell ions.

Molecular Theory and the Effects of Solute Attractive Forces on Hydrophobic Interactions.

The role of solute attractive forces on hydrophobic interactions is studied by coordinated development of theory and simulation results for Ar atoms in water. We present a concise derivation of the local molecular field (LMF) theory for the effects of solute attractive forces on hydrophobic interactions, a derivation that clarifies the close relation of LMF theory to the EXP approximation applied to this problem long ago. The simulation results show that change from purely repulsive atomic solute interactions to include realistic attractive interactions diminishes the strength of hydrophobic bonds. For the Ar-Ar rdfs considered pointwise, the numerical results for the effects of solute attractive forces on hydrophobic interactions are opposite in sign and larger in magnitude than predicted by LMF theory. That comparison is discussed from the point of view of quasichemical theory, and it is suggested that the first reason for this difference is the incomplete evaluation within LMF theory of the hydration energy of the Ar pair. With a recent suggestion for the system-size extrapolation of the required correlation function integrals, the Ar-Ar rdfs permit evaluation of osmotic second virial coefficients B2. Those B2's also show that incorporation of attractive interactions leads to more positive (repulsive) values. With attractive interactions in play, B2 can change from positive to negative values with increasing temperatures. This is consistent with the puzzling suggestions of decades ago that B2 ≈ 0 for intermediate cases of temperature or solute size. In all cases here, B2 becomes more attractive with increasing temperature.

Differential study on molecular suppressed ionization in intense linearly and circularly polarized laser fields

We present a differential study on above-threshold ionization of the O{sub 2} (N{sub 2}) molecule as well as the companion atom Xe (Ar) (with close ionization potential) produced by linearly and circularly polarized laser fields (25 fs, 795 nm). The photoelectron angular distributions of the companion target are similar at the same laser condition. In both linearly and circularly polarized fields, we observe that the photoelectron yields of O{sub 2} are suppressed in the entire energy spectral range as compared with Xe with fully differential measurements, but not for the N{sub 2}-Ar pair. This is different from the prediction of photoelectron energy spectra by the model including the interference terms [Phys. Rev. Lett. 85, 2280 (2000)], from which the low-energy photoelectrons of O{sub 2} were expected to be strongly suppressed in both linearly and circularly polarized laser fields. Resorting to the basic strong-field ionization picture, we believe that the lower orbital-dependent multiphoton excitation or tunneling possibility of O{sub 2} as compared with Xe is responsible for this effect. High-resolution fully differential data pose a stringent test on the current strong-field calculations on molecules.

Apparent conflicting indications on the conformation of dimethylether–argon from the rotational spectra of the d6 and 13C species

The rotational spectra of dimethylether– d 6–Ar and dimethylether– 13C–Ar have been investigated by molecular beam Fourier transform microwave spectroscopy. The obtained rotational constants, when compared to those of the normal species, give contrasting indications on the conformation of the complex. We believe the indications given by the dimethylether–Ar/dimethylether– 13C–Ar pair to be the correct ones. They suggest the Ar atom to lie in the σ ν symmetry plane of dimethylether perpendicular to the C–O–C plane, shifted towards the oxygen atom. This conformation is in agreement with several kinds of theoretical calculations. Probably isotopic substitution effects make the data from dimethylether– d 6–Ar not directly compatible with those of the other species.

Modelling the polarizability of the dihydrogen–argon pair

The collision-induced (CI) pair polarizability of dihydrogen–argon ( H 2 – Ar ) pair has been studied using quantum-chemical and analytical methods. We have proposed a procedure to determine the rotationally adapted (symmetry adapted) components of the CI polarizability tensor. The corresponding irreducible spherical symmetry adapted components are computed for the H 2 – Ar pair. We have analyzed the dipole-induced dipole (DID) model pair polarizability tensor for H 2 – Ar . Good agreement between the DID allowed rotationally adapted CI H 2 – Ar polarizability components and those computed by quantum-chemical methods is found.

Collision-induced hyper-Rayleigh spectrum of H2–Ar gas mixture

The collision-induced hyper-Rayleigh (CIHR) spectra of the gaseous H(2)-Ar mixture are discussed in the binary regime on the basis of our ab initio computed H(2)-Ar collision-induced (CI) first dipole hyperpolarizability tensor Deltabeta(R). A method for the computation of the spherical, rotationally adapted components Deltabeta(lambdaL) ((s,K))(R) of Deltabeta(R) needed for spectroscopic line shape analysis is proposed. Both the vector and the septor parts of the H(2)-Ar CIHR spectrum are evaluated at room (T=295 K) temperature. The spectra are calculated assuming the full quantum computations based on the Schrodinger equation of the relative translational motion of H(2)-Ar as well as semiclassical methods (classical trajectory approach and Birnbaum-Cohen model translational profiles). The H(2)-Ar pair CIHR septor spectrum has been found stronger than the vector one.

Conformational Control of Inorganic Adhesion in a Designer Protein Engineered for Cuprous Oxide Binding

Combinatorial selection of peptides that bind technological materials has emerged as a valuable tool for room-temperature nucleation and assembly of complex nanostructured materials. At present, the parameters that control peptide-solid binding are poorly understood, but such knowledge is needed to build the next generation of hybrid materials. Here, we use a derivative of the DNA binding protein TraI engineered with a disulfide-bonded cuprous oxide binding sequence called CN225 to probe the influence of sequence composition and conformation on Cu2O binding affinity. We previously reported a statistically significant enrichment in paired arginines (RR) among a family of cuprous oxide binding peptides and hypothesized that this is a key motif for binding. However, systematic alanine (A) substitutions in the CN225 RR motif (creating RA, AR, and AA pairs) do not support the hypothesis that RR is critical for Cu2O binding by CN225. Instead, we find that the presentation of the peptide in a disulfide-constrained loop (i.e., the conformation present during combinatorial selection) is crucial for binding to the metal oxide. Our results suggest that caution should be exerted when extrapolating from statistical data and that, in some cases, conformation is more important than composition in determining peptide-inorganic adhesion.

Collision-induced hyperpolarizability and hyper-Rayleigh spectra in the He–Ar heterodiatom

The ab initio interatomic distance dependent collision induced dipole moment, polarizability and the vector b 1( R) and an irreducible 3rd rank tensor b 3( R) components of the first hyperpolarizability tensor b of He–Ar pair are obtained. The hyperpolarizability data are applied in order to calculate the collision induced He–Ar hyper-Rayleigh spectra both quantum-mechanically and semiclassically for the frequency shifts up to 1200 cm −1. Computations were carried out for two temperature values – T = 295 K and T = 95 K. Spectral features of both vector and the irreducible 3rd rank tensor parts of CIHR spectra are analyzed. The frequency dependent behavior of the HR depolarization ratio is discussed.

First-principle calculation for the high-temperature diffusion coefficients of small pairs: the H–Ar Case

Argon has been widely used as a diluent for high-temperature reacting flow experiments. Numerical simulation of such process requires accurate diffusion coefficients for the H–Ar pair. All available potential energy functions for the H–Ar system have been empirically extrapolated in the repulsive region and are probably not reliable for prediction of H–Ar binary diffusion coefficient at high temperatures. We perform calculations using the restricted coupled cluster theory with single and double excitation (plus triple corrections) [RCCSD(T)] and suitable basis sets to obtain accurate potential energies. The calculated potential energy function is corrected for basis set superposition error and is validated against molecular beam scattering data. Comparisons with previous literature potential functions are also made. Using the Chapman–Enskog theory we carried out first-principle calculation of high-temperature diffusion coefficients by direct numerical integration of the collision integrals using the RCCSD(T) potential function. The computed diffusion coefficients are validated against available experimental data. Comparisons are also made with results obtained from transport compilations and packages commonly used in combustion simulation.

Statistical physics partitioning and classical trajectory analysis of the phase space in CO 2–Ar weakly interacting pairs

This paper aims at elaborating rigorous criteria which make it possible to discriminate between true bound, quasibound, and free pair states for a system possessing a pronounced anisotropy of intermolecular interaction. The speculative statistical physics definition for various domains in the phase space of CO 2–Ar pairs is verified using extensive 3D classical trajectory calculations with selected reliable potential energy surfaces. The results of numerical trajectory simulation is shown to confirm quite nicely the validity of our ad hoc statistical physics approach.

Ultrafast solvent response upon a change of the solute size in non-polar supercritical fluids

Non-polar solvation dynamics has been investigated using steady-state absorption and emission spectroscopy of the NO A 2Σ+(3sσ) Rydberg state in fluid Ar over a wide range of densities spanning the supercritical regime. Equilibrium molecular dynamics simulations were implemented to derive a new isotropic NO A(3sσ)–Ar pair potential which was further used to investigate the role of local density enhancements on the solvation process by non-equilibrium molecular dynamics simulations. These density inhomogeneities were found to have no influence on the solvation dynamics. Furthermore, the latter was shown to take place in a strongly non-linear regime, especially at low temperatures. This process results from the dramatic change of solute–solvent short range interaction associated with the large solute size change upon excitation to the Rydberg state.

Time correlation function approach to liquid phase vibrational energy relaxation: H2 and D2 solutes in Ar solvent

The theoretical treatment in Paper I [D. W. Miller and S. A. Adelman, J. Chem. Phys. 117, 2672, (2002), preceding paper] of the vibrational energy relaxation (VER) of low-frequency, large mass dihalogen solutes is extended to the VER of the high-frequency, small mass molecular hydrogen solutes H2 and D2 in a Lennard-Jones argon-like solvent. As in Paper I, values of the relaxation times T1 predicted by the theory are tested against molecular dynamics (MD) results and are found to be of semiquantitative accuracy. To start, it is noted that standard Lennard-Jones site–site potentials derived from macroscopic data can be very inaccurate in the steep repulsive slope region crucial for T1. Thus, the H–Ar Lennard-Jones diameter σUV is not taken from literature values but rather is chosen as σUV=1.39 Å, the value needed to make the theory reproduce the experimental H2/Ar gas phase VER rate constant. Next, by MD simulation it is shown that the vibrational coordinate fluctuating force autocorrelation function 〈F̃(t)F̃〉0 of Paper I decays roughly an order of magnitude more rapidly for the molecular hydrogen solutions than for the dihalogen solutions. This result implies a relatively slow decay for the molecular hydrogen friction kernels β(ω)=(kBT)−1∫0∞〈F̃(t)F̃〉0 cos ω tdt, yielding for the H2/Ar and D2/Ar systems at T=150 K physical millisecond values for T1=β−1(ωl) despite the high liquid phase vibrational frequencies ωl of H2 and D2. The rapid decay of 〈F̃(t)F̃〉0 is due to both the steepness of the repulsive slope of the H–Ar potential and the small masses of H and D. Thus, the small value chosen for σUV is needed to avoid unphysically long T1’s. Next, an analytical treatment of the H2/D2 isotope effect on T1, based on the theory, is found to predict that the H2/Ar and D2/Ar T1’s are close in value due to the compensating effects of lower ωl but slower decay of 〈F̃(t)F̃〉0 for D2/Ar, a result in qualitative agreement with experiments. Applying the theory to numerically study the isothermal ρ dependencies of the VER rate constant k(T,ρ)=T1−1 at 150 K reveals that for both H2/Ar and D2/Ar, as for the solutions of Paper I, k(T,ρ) can be factorized as in the isolated binary collision (IBC) model. Moreover, the molecular theory and IBC rate isotherms differ only slightly for both solutions, a result interpreted in terms of the form of the H–Ar pair correlation function. The theoretical and experimental rate isotherms at 150 K are then compared. Agreement is very good for the H2/Ar solution, but for the D2/Ar solution the theoretical rates are about four times too large. Finally, the isochoric T dependencies of k(T,ρ) in the range 200–1000 K are found for both solutions to conform to an Arrhenius rate law.

Calculation of the potential of mean force for chloride-argon and sodium-argon pairs in water

As a model for understanding hydrophilic-hydrophobic interactions in aqueous solutions, the potential of mean force for Cl −···Ar and Na +···Ar pairs in water was computed by molecular dynamics simulations. The Cl −···Ar potential is slightly modified by the solvent, with the appearance of a shallow secondary minimum associated to the stabilization of a solvent separated pair in addition to the stable contact pair. The Na +···Ar potential is considerably modified from the gas to the liquid. The depth of the attractive well is divided by a factor 3. At short distances, the repulsive part of the potential is dominated by the solvent-induced electrostatic repulsion rather than by the direct Pauli repulsion. The difference of the solvation properties of the two pairs is explained by a simple electrostatic model and is attributed to a size rather than a charge effect.

Size and isomer dependence of HF vibrational frequency shift in ArnHF van der Waals clusters with n=1–14

Z. Bačić, J. Chem. Soc., Faraday Trans., 1997, 93, 1459 DOI: 10.1039/A607739K

Vibrational wave functions and energy levels of large anharmonic clusters: A vibrational SCF study of (Ar)13

The vibrational ground state and the fundamental excited states of (Ar)13 were studied by vibrational self-consistent field (VSCF) calculations. These calculations treat the interaction between different modes through a mean potential approximation, and incorporate anharmonicity in full. The good accuracy of VSCF for such systems was demonstrated by test calculations for (Ar)3 and other clusters. The study of (Ar)13 focused on the properties of the wave functions and the excitation energies, on the role of the coupling between the modes and on the deviation from the harmonic approximation. It was found that SCF excitation energies for the fundamental transitions differ from the harmonic values by about 25% for the softest modes, and by about 10% for the stiffest modes. Coupling between the modes, treated by SCF, was found to be much more important than the intrinsic anharmonicity of the individual modes. For the ground state, the harmonic wave function compares well with VSCF, but for the fundamental excited states appreciable differences were found. The results for a potential field expanded to fourth-order polynomial in the normal mode displacements are found to be valid, almost indentical with those for a more elaborate sixth-order polynomial expansion. The fundamental excitation frequencies computed using the Aziz–Slaman Ar–Ar pair potential are very similar, with some quantitative deviations, to the values obtained with a Lennard-Jones potential. The differences are larger for certain specific modes, and very small for the others. These calculations demonstrate the computational power of VSCF as a tool for quantum-mechanical calculations for large clusters, at the level of specific wave functions.

結核性肉芽腫炎症の制御における宿主遺伝子の役割

Natural bactericidal resistance of Mycobacterium bovis BCG is under the control of a single gene, Bcg. The development of lung granulomas in susceptible (Bcgs) and resistant (Bcgr) mice was studied in two sets of Bcg-congenic systems, the BALB/c (Bcgs) -C.D2 (BALB/c.Bcgr) pair and the B10. A (Bcgs) -B10. Ar (Bcgr) pair, by using BCG as well as foregin-body granuloma-inducing agents, dextran beads. Large lung granulomas induced by the intratracheal challenge of either BCG or dextran beads developed in Bcgs mice. By contrast, minimal lesions were produced in Bcgr mice given BCG or dextran beads. Aqueous extracts prepared from pulmonary granuloma lesions induced in Bcgs mice by either BCG or dextran beads contained a large amount of interleukin 1 (IL-1) activity but not IL-2 or IL-4 activity. Very low IL-1 activity was detectable in extracts from Bcgr mice challenged with BCG and dextran beads. The activity of IL-1 was correlated with activity/size of the granulomatous inflammation in mice.These results incucate that the Bcg gene has pleiotropic effects on the development of granulomas induced by either BCG or nonspecific foreign body agents (dextran beads) and that macrophage-derived cytokines participate in granuloma formation.

Far infrared absorption of the gaseous CH4–Ar mixture

We present the first measurements of the rototranslational absorption spectrum of the CH4–Ar gaseous system in the far infrared region 40–600 cm−1. Measurements were made at 296, 243, and 195 K at densities as low as possible; the binary absorption spectrum of the CH4–Ar pair was then obtained. We performed a line-shape analysis of these spectra following the procedure previously introduced for pure methane. We find that the experimental data can be described at high frequencies by using ab initio values of the octopole and hexadecapole multipole moments and anisotropic overlap intensities very close to those used for pure methane. At low frequencies, the computed spectrum is significantly lower than the experimental data at any temperature. We thus include in our analysis a low-frequency spectral contribution associated with the isotropic overlap induction mechanism. Good fits of the experimental data are then obtain at each temperature over all the spectral range.

Cage effect for the abstraction of H from H2O in Ar matrices

A red wing in the absorption of H2O in the first continuum which extends beyond 200 nm is induced by the matrix. A threshold energy of 6.95 eV is found for the quantum efficiency of permanent dissociation of H2O into OH and H. This corresponds to an additional barrier of 1.8 eV due to the matrix cage. The barrier is attributed to the sum of repulsive H–Ar pair potentials in an interstitial site of D3 symmetry, which has to be surmounted by the H atom on the way from the initial substitutional H2O site to the final stable Oh site of the abstracted H atom.