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In a previous paper [J. L. McAfee and B. Poirier, J. Chem. Phys. 130, 064701 (2009)], using spin-polarized density functional theory (DFT), the authors reported a binding energy of 0.755 eV, for a single hydrogen atom adsorbed on a pristine (unrelaxed) (5,5) single-walled carbon nanotube (SWNT) substrate. A full three-dimensional (3D) potential energy surface (PES) for the SWNT-H system was also developed, and used in a quantum dynamics calculation to compute all rovibrational bound states, and associated equatorial and longitudinal adsorbate migration rates. A highly pronounced preference for the latter migration pathway at ambient temperatures was observed. In this work, we extend the aforementioned study to include multiple H-atom adsorbates. Extensive DFT calculations are performed, in order to ascertain the most relevant dynamical pathways. For two adsorbates, the SWNT-H-H system is found to exhibit highly site-specific binding, as well as long-range correlation and pronounced binding energy enhancement. The latter effect is even more pronounced in the full-hydrogenation limit, increasing the per-adsorbate binding energy to 2.6 eV. To study migration dynamics, a single-hole model is developed, for which the binding energy drops to 2.11 eV. A global 3D PES is developed for the hole migration model, using 40 radial × 18 cylindrical ab initio geometries, fit to a Fourier basis with radially dependent expansion coefficients (rms error 4.9 meV). As compared with the single-adsorbate case, the hole migration PES does not exhibit separate chemisorption and physisorption wells. The barrier to longitudinal migration is also found to be much lower. Quantum dynamics calculations for all rovibrational states are then performed (using a mixed spectral basis/phase-space optimized discrete variable representation), and used to compute longitudinal migration rates. Ramifications for the use of SWNTs as potential hydrogen storage materials are discussed. [ABSTRACT FROM AUTHOR]

Simplified fluid-substrate interaction models derived from the Lennard-Jones potential are widely used in the simulation of gas physisorption phenomena. In this paper, we reinterpret the well known Steele 10-4-3 potential for a gas molecule interacting with a planar surface, and use the resultant scheme to derive new potentials for cylindrical and spherical pore geometries. These new potentials correctly recover the Steele result in the limit of infinite pore radius, a useful improvement over existing models. We demonstrate the new cylindrical Steele 10-4-3 potential in calculations of argon adsorption via fluid density functional theory. This potential yields markedly different adsorption behavior than existing cylindrical potentials, which follow from small but significant differences in both the strength and the shape of the fluid-surface interaction. These differences cannot be fully reconciled simply by reparameterizing (scaling) the existing models; the new potential is more realistic in design, and is especially to be preferred in studies where comparison with planar substrates is made. Finally, we discuss extensions of this approach to more complicated pore geometries, yielding a family of Steele-like potentials that all satisfy the correct planar limit. [ABSTRACT FROM AUTHOR]

The hydrogen storage system LiH + NH3 ↔ LiNH2 + H2 is one of the most promising hydrogen storage systems, where the reaction yield can be increased by replacing Li in LiH with other alkali metals (Na or K) in order of Li < Na < K. In this paper, we have studied the alkali metal M (M = Li, Na, K) dependence of the reactivity of MH with NH3 by calculating the potential barrier of the H2 desorption process from the reaction of an M2H2 cluster with an NH3 molecule based on the ab initio structure optimization method. We have shown that the height of the potential barrier becomes lower in order of Li, Na, and K, where the difference of the potential barrier in Li and Na is relatively smaller than that in Na and K, and this tendency is consistent with the recent experimental results. We have also shown that the H-H distance of the H2 dimer at the transition state takes larger distance and the change of the potential energy around the transition state becomes softer in order of Li, Na, and K. There are almost no M dependence in the charge of the H atom in NH3 before the reaction, while that of the H atom in M2H2 takes larger negative value in order of Li, Na, and K. We have also performed molecular dynamics simulations on the M2H2-NH3 system and succeeded to reproduce the H2 desorption from the reaction of Na2H2 with NH3. [ABSTRACT FROM AUTHOR]

This paper reports on the absorption of molecular oxygen in the region of the A-band near 760 nm under atmospheric conditions relevant for satellite retrieval studies. We use pulsed laser cavity ring-down spectroscopy with a narrow bandwidth laser and use pressure scans to increase the accuracy of the measured oxygen extinction coefficients. Absolute binary absorption coefficients in minima between absorption lines of the A-band spectrum have been measured and tabulated. We use the so-called adjustable branch coupling model including line mixing to calculate the magnetic dipole absorption in order to determine the contribution of collision induced absorption. The line mixing model has been optimized such that the collision induced absorption spectrum is smooth. [ABSTRACT FROM AUTHOR]

Transition state theory (TST) has long been applied to gas–surface reactions for the purpose of characterizing and predicting desorption rate coefficients, but current perceptions of its use exclude TST from meaningful description of the sticking coefficient for partially covered surfaces. This paper re-examines the application of TST to partially covered surfaces, and in particular to Langmuir systems with barrierless adsorption. An analogy is drawn in the present work between barrierless adsorption processes and gas-phase radical–radical recombination reactions that reveals a simple method by which TST can describe kinetics in Langmuir systems. Construction of a model ‘‘hard-wall’’ Hamiltonian that reflects the exclusion of adsorbates from occupied sites enables the kinetics in Langmuir systems to be described by constructing a transition state surface that does not permit adsorption at occupied sites. This method gives analytic expressions for the TST sticking coefficient (and desorption rate coefficient) in two important limiting cases for the gas–surface dynamics, and resolves previous difficulties in the TST description of Langmuir systems. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Soft porous coordination polymers (SPCPs) are materials with exceptional potential because of their ability to incorporate the properties of nominally rigid porous materials like metal–organic frameworks (MOFs) and those of soft matter, such as polymers of intrinsic microporosity (PIMs). This combination could offer the gas adsorption properties of MOFs together with the mechanical stability and processability of PIMs, opening up a space of flexible, highly responsive adsorbing materials. In order to understand their structure and behavior, we present a process for the construction of amorphous SPCPs from secondary building blocks. We then use classical molecular dynamics simulations to characterize the resulting structures based on branch functionalities (f), pore size distributions (PSDs), and radial distribution functions and compare them to experimentally synthesized analogs. In the course of this comparison, we demonstrate that the pore structure of SPCPs is due to both pores intrinsic to the secondary building blocks, and intercolloid spacing between colloid particles. We also illustrate the differences in nanoscale structure based on linker length and flexibility, particularly in the PSDs, finding that stiff linkers tend to produce SPCPs with larger maximum pore sizes. [ABSTRACT FROM AUTHOR]

A machine learning (ML) methodology that uses a histogram of interaction energies has been applied to predict gas adsorption in metal–organic frameworks (MOFs) using results from atomistic grand canonical Monte Carlo (GCMC) simulations as training and test data. In this work, the method is first extended to binary mixtures of spherical species, in particular, Xe and Kr. In addition, it is shown that single-component adsorption of ethane and propane can be predicted in good agreement with GCMC simulation using a histogram of the adsorption energies felt by a methyl probe in conjunction with the random forest ML method. The results for propane can be improved by including a small number of MOF textural properties as descriptors. We also discuss the most significant features, which provides physical insight into the most beneficial adsorption energy sites for a given application. [ABSTRACT FROM AUTHOR]

In the hybrid sulfur (HyS) cycle, the reaction between SO2 and H2O is manipulated to produce hydrogen with water and sulfuric acid as by-products. However, sulfur poisoning of the catalyst has been widely reported to occur in this cycle, which is due to strong chemisorption of sulfur on the metal surface. The catalysts may deactivate as a result of these impurities present in the reactants or incorporated in the catalyst during its preparation and operation of the HyS cycle. Here, we report a density functional theory investigation of the interaction between S, SO, and SO3 with the Pt (001), (011), and (111) surfaces. First, we have investigated the adsorption of single gas phase molecules on the three Pt surfaces. During adsorption, the 4F hollow sites on the (001) and (011) surfaces and the fcc hollow site on the (111) surface were preferred. S adsorption followed the trend of (001)4F > (011)4F > (111)fcc, while SO adsorption showed (001)4F > (011)bridge/4F > (111)fcc and SO3 adsorption was most stable in a S,O,O bound configuration on the (001)4F > (011)4F > (111)fcc sites. The surface coverage was increased on all the surfaces until a monolayer was obtained. The highest surface coverage for S shows the trend (001)S = (111)S > (011)S, and for SO it is (001)SO > (011)SO > (111)SO, similar to SO3 where we found (001)SO3 > (011)SO3 > (111)SO3. These trends indicate that the (001) surface is more susceptible to S species poisoning. It is also evident that both the (001) and (111) surfaces were reactive toward S, leading to the formation of S2. The high coverage of SO3 showed the formation of SO2 and SO4, especially on the (011) surface. The thermodynamics indicated that an increased temperature of up to 2000 K resulted in Pt surfaces fully covered with elemental S. The SO coverage showed θ ≥ 1.00 on both the (001) and (011) surfaces and θ = 0.78 for the (111) surface in the experimental region where the HyS cycle is operated. Lower coverages of SO3 were observed due to the size of the molecule. [ABSTRACT FROM AUTHOR]

The porous glass MCM-41 is an important adsorbent to study the process of adsorption of gases onto a cylindrical surface. In this work, we study the adsorption of oxygen, nitrogen, deuterium, and deuteriated methane gases into MCM-41 using a combination of neutron diffraction analysis and atomistic computer modeling to interpret the measured data. Adsorption is achieved by immersing a sample of MCM-41 in a bath of the relevant gas, keeping the gas pressure constant (0.1 MPa), and lowering the temperature in steps toward the corresponding bulk liquid boiling point. All four gases have closely analogous behaviors, with an initial layering of liquid on the inside surface of the pores, followed by a relatively sharp capillary condensation (CC) when the pore becomes filled with dense fluid, signaled by a sharp decrease in the intensity of (100) Bragg diffraction reflection. At the temperature of CC, there is a marked distortion of the hexagonal lattice of pores, as others have seen, which relaxes close to the original structure after CC, and this appears to be accompanied by notable excess heterogeneity along the pore compared to when CC is complete. In none of the four gases studied does the final density of fluid in the pore fully attain the value of the bulk liquid at its boiling point at this pressure, although it does approach that limit closely near the center of the pore, and in all cases, the pronounced layering near the silica interface seen in previous studies is observed here as well. [ABSTRACT FROM AUTHOR]

The monolayer CO adsorbed on NaCl(001) was studied with helium atom diffraction. At Tsample = 45 K the adlayer is commensurate to the substrate and of (1×1) symmetry. At Tsample ≤ 30 K a commensurate (2×1) superstructure is formed. The unit cell of this low-temperature phase contains two molecules and a glide plane. [ABSTRACT FROM AUTHOR]

Gas adsorption is a standard method for measuring pore-size distributions of nanoporous materials. This method is often based on assuming the pores as separate entities of a certain simple shape: slit-like, cylindrical, or spherical. Here, we study the effect of interconnections on gas adsorption in materials with spherical pores, such as three-dimensionally ordered mesoporous (3DOm) carbons. We consider interconnected systems with two, four, and six windows of various sizes. We propose a simple method based on the integration of solid–fluid interactions to take into account these windows. We used Monte Carlo simulations to model argon adsorption at the normal boiling point and obtained adsorption isotherms for the range of systems. For a system with two windows, we obtained a remarkably smooth transition from the spherical to cylindrical isotherm. Depending on the size and number of windows, our system resembles both spherical and cylindrical pores. These windows can drastically shift the point of capillary condensation and result in pore-size distributions that are very different from the ones based on a spherical pore model. Our results can be further used for modeling fluids in a system of interconnected pores using Monte Carlo and density functional theory methods. [ABSTRACT FROM AUTHOR]

Interactions of trapped reservoir gases within organic-rich and brine-bearing sedimentary rocks have direct relevance to many geoenergy applications. Extracting generalizable information from experimental campaigns is hindered by the fact that geological systems are extremely complex. However, modern computational tools offer the opportunity of studying systems with controlled complexity, in an effort to better understand the mechanisms at play. Employing molecular dynamics, we examine here adsorption and transport of gases containing CH4 and either CO2 or H2S within amorphous silica nanopores filled with benzene. We explicitly quantify the effect of small amounts of water/brines at geological temperature and pressure conditions. Because of wetting, the presence of brines lessens the adsorption capacity of the aromatic-filled pore. The simulation results show salt-specific effects on the transport properties of the gases when either KCl or CaCl2 brines are considered, although adsorption was not affected. The acid gases considered either facilitate or hinder CH4 transport depending on whether they are more or less preferentially adsorbed within the pore as compared to benzene, and this effect is mediated by the presence of water/brines. Our simulation results could be used to extract thermodynamic quantities that in the future will help to optimize transport of various gases through organic-rich and brine-bearing sedimentary rocks, which is likely to have a positive impact on both hydrocarbon production and carbon sequestration applications. As a first step, a phenomenological model is presented here, which allows one to predict permeability based on interatomic energies. [ABSTRACT FROM AUTHOR]

Ionic liquid (IL)-based solid polymer electrolytes (SPE) with stable thermal properties and low electrical resistivity have been evaluated. Two candidates for the polymer component of the SPE, poly(ethylene glycol) diacrylate (PEGDA) and Nafion, were considered. Differential scanning calorimetry analysis and electrical resistivity tests revealed that PEGDA, in comparison to Nafion, enables the formation of uniform SPEs with lower electrical resistivity and better thermal stability within a range of 25 °C–170 °C. Therefore, PEDGA was selected for further evaluation of the IL component effect on the resulting SPE. Six IL candidates, including 1-butyl-3-methylimidazolium methanesulfonate ± methanesulfonic acid (BMIM.MS ± MSA), diethylmethylammonium triflate ±bis(trifluoromethanesulfonyl)imine (Dema.OTF±HTFSI), and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ± bis(trifluoromethanesulfonyl)imine (BMIM.TFSI ± HTFSI), were selected to test the effect of hydrophobicity/hydrophilicity of the IL on the resulting SPE. Fourier transformation infrared spectrometer analysis revealed that the BMIM.MSA-based electrolytes have the highest tendency to absorb from the environment and keep the moisture, while Dema.OTF has the fastest curing time. The SPE candidates were further evaluated for absorption characteristics of different gasses and vapors, such as N2, O2, ethanol vapor, and diluted CO/N2, that were tested with the in situ quartz crystal microbalance (QCM) technique. Among all six candidates, BMIM.MS showed the largest N2 and O2 absorption capacity from the environment. Dema.OTF + HTFSI, meanwhile, demonstrated a higher level of interactions with the ethanol vapor. In the case of CO/N2, QCM analysis revealed that BMIM.MS+MSA has the largest, ∼13 µg/cm2, absorption capacity that is reached within 400 s of being exposed to the gas mixture. [ABSTRACT FROM AUTHOR]

A molecular-level understanding of CO adsorption behavior would be greatly beneficial to resolving the problem of CO poisoning in fuel cells and medical science. Herein, an efficient borrowing strategy based on surface enhanced Raman scattering (SERS) has been developed to investigate the adsorption behavior of CO at the gas–solid interface. A composite SERS substrate with high uniformity was fabricated by electrochemical deposition of optimal Pt over-layers onto an Au nanoparticle film. The results indicated that the linearly bonded mode follows the Langmuir adsorption curve (type I), while the multiply bonded did not. It took a longer time for the C–OM vibration to reach the adsorption equilibrium than that of C–OL. The variation tendency toward the Pt–COL frequency was in opposition to that of C–OL, caused by the chemical and dipole–dipole coupling effects. The increase in dynamic coupling effects of the CO molecules caused a blue shift in νCO and a red shift of the Pt–CO band, while its shielding effect on SERS intensity cannot be ignored. Additionally, higher pressure is more conducive for linear adsorption to achieve saturation. Density functional theory calculations were employed to explore the adsorption mechanisms. It should also be noted that the substrate with good recycling performance greatly expands its practical application value. The present study suggested that the SERS-based borrowing strategy shows sufficient even valuable capacity to investigate gas adsorption kinetics behavior. [ABSTRACT FROM AUTHOR]

We have implemented a scheme for classical molecular dynamics simulations of collision-induced absorption. The program has been applied to a gas mixture of argon (Ar) and krypton (Kr). The simulations are compared with accurate quantum dynamical calculations. The comparisons of the absorption coefficients show that classical molecular dynamics is correct within 10% for photon wave numbers up to 220 cm−1 at a temperature of 200 K for this system. At higher temperatures, the agreement is even better. Molecular dynamics accounts for many-body interactions, which, for example, give rise to continuous dimer formation and destruction in the gas. In this way, the method has an advantage compared with bimolecular classical (trajectory) treatments. The calculations are carried out with a new empirical Ar–Kr pair potential. This has been obtained through extensive analysis of experimental thermophysical and transport properties. We also present a new high level ab initio Ar–Kr potential curve for comparison, as well as ab initio interaction-induced dipole curves computed with different methods. In addition, the Ar–Kr polarizability and hyperpolarizability are reported. A comparison of the computed absorption spectra with an experiment taken at 300 K shows satisfactory agreement although a difference in absolute magnitude of 10%–15% persists. This discrepancy we attribute mainly to experimental uncertainty. [ABSTRACT FROM AUTHOR]

Adsorption of flue gases by single-wall carbon nanotubes (SWCNT) has been studied by means of Monte Carlo simulations. The flue gas is modeled as a ternary mixture of N2, CO2, and O2, emulating realistic compositions of the emissions from power plants. The adsorbed flue gas is in equilibrium with a bulk gas characterized by temperature T, pressure p, and mixture composition. We have considered different SWCNTs with different chiralities and diameters in a range between 7 and 20 Å. Our results show that the CO2 adsorption properties depend mainly on the bulk flue gas thermodynamic conditions and the SWCNT diameter. Narrow SWCNTs with diameter around 7 Å show high CO2 adsorption capacity and selectivity, but they decrease abruptly as the SWCNT diameter is increased. For wide SWCNT, CO2 adsorption capacity and selectivity, much smaller in value than for the narrow case, decrease mildly with the SWCNT diameter. In the intermediate range of SWCNT diameters, the CO2 adsorption properties may show a peculiar behavior, which depend strongly on the bulk flue gas conditions. Thus, for high bulk CO2 concentrations and low temperatures, the CO2 adsorption capacity remains high in a wide range of SWCNT diameters, although the corresponding selectivity is moderate. We correlate these findings with the microscopic structure of the adsorbed gas inside the SWCNTs. [ABSTRACT FROM AUTHOR]

The surface composition of bimetallics can be strongly altered by adsorbing molecules where the metal with the strongest interaction with the adsorbate segregates into the surface. To investigate the effect of reactive gas on the surface composition of Au-Cu alloy, we examined by means of density functional theory to study the segregation behavior of copper in gold matrices. The adsorption mechanisms of CO, NO, and O2 gas molecules on gold, copper, and gold-copper low index (111), (100), and (110) surfaces were analyzed from energetic and electronic points of view. Our results show a strong segregation of Cu toward the (110) surface in the presence of all adsorbed molecules. Interestingly, the Cu segregation toward the (111) and (100) surface could occur only in the presence of CO and at a lower extent in the presence of NO. The analysis of the electronic structure highlights the different binding characters of adsorbates inducing the Cu segregation. [ABSTRACT FROM AUTHOR]

The adsorption of noble gases on metallic surfaces represents a paradigmatic case of van-der-Waals (vdW) interaction due to the role of screening effects on the corrugation of the interaction potential [J. L. F. Da Silva et al., Phys. Rev. Lett. 90, 066104 (2003)]. The extremely small adsorption energy of He atoms on the Mg(0001) surface (below 3 meV) and the delocalized nature and mobility of the surface electrons make the He/Mg(0001) system particularly challenging, even for state-of-the-art vdW-corrected density functional-based (vdW-DFT) approaches [M. P. de Lara-Castells et al., J. Chem. Phys. 143, 194701 (2015)]. In this work, we meet this challenge by applying two different procedures. First, the dispersion-corrected second-order Möller-Plesset perturbation theory (MP2C) approach is adopted, using bare metal clusters of increasing size. Second, the method of increments [H. Stoll, J. Chem. Phys. 97, 8449 (1992)] is applied at coupled cluster singles and doubles and perturbative triples level, using embedded cluster models of the metal surface. Both approaches provide clear evidences of the anti-corrugation of the interaction potential: the He atom prefers on-top sites, instead of the expected hollow sites. This is interpreted as a signature of the screening of the He atom by the metal for the on-top configuration. The strong screening in the metal is clearly reflected in the relative contribution of successively deeper surface layers to the main dispersion contribution. Aimed to assist future dynamical simulations, a pairwise potential model for the He/surface interaction as a sum of effective He-Mg pair potentials is also presented, as an improvement of the approximation using isolated He-Mg pairs. [ABSTRACT FROM AUTHOR]

In spite of its importance for the evaluation of the Earth radiative budget, thus for climate change, very few measurements of the water vapor continuum are available in the near infrared atmospheric windows especially at temperature conditions relevant for our atmosphere. In addition, as a result of the difficulty to measure weak broadband absorption signals, the few available measurements show large disagreements. We report here accurate measurements of the water vapor self-continuum absorption in the 2.1 μm window by Optical Feedback Cavity Enhanced Absorption Spectroscopy (OF-CEAS) for two spectral points located at the low energy edge and at the center of the 2.1 μm transparency window, at 4302 and 4723 cm-1, respectively. Self-continuum cross sections, CS, were retrieved with a few % relative uncertainty, from the quadratic dependence of the spectrum base line level measured as a function of water vapor pressure, between 0 and 16 Torr. At 296 K, the CS value at 4302 cm-1 is found 40% higher than predicted by the MT_CKD V2.5 model, while at 4723 cm-1, our value is 5 times larger than the MT_CKD value. On the other hand, these OF-CEAS CS values are significantly smaller than recent measurements by Fourier transform spectroscopy at room temperature. The temperature dependence of the self-continuum cross sections was also investigated for temperatures between 296 K and 323 K (23-50 °C). The derived temperature variation is found to be similar to that derived from previous Fourier transform spectrometer (FTS) measurements performed at higher temperatures, between 350 K and 472 K. The whole set of measurements spanning the 296-472 K temperature range follows a simple exponential law in 1/T with a slope close to the dissociation energy of the water dimer, D0 ≈ 1100 cm-1. [ABSTRACT FROM AUTHOR]

Up until now, gas permeation through polymers under high pressure has not been able to be measured continuously. The combination of a special high pressure cell and a commercially available fluorescence-based oxygen measurement system allows in-situ monitoring of oxygen permeation through a polymer sample under pressure in an aqueous environment. The principle of the oxygen sensor is based on dynamic fluorescence quenching and measurement of the fluorescence decay time. It was observed that the decay time increases non-linearly with the applied pressure, and hence, the displayed oxygen concentration has to be corrected. This deviation between the measured and the real concentration depends not only on the pressure but also on the absolute oxygen concentration in the water. To obtain a calibration curve, tests were performed in the pressure range between 1 and 2000 bars and initial oxygen concentrations in the range between 40 and 280 μmol/l. The polynomial calibration curve was of the fourth order, describing the raw data with a coefficient of determination R² > 0.99. The effective oxygen permeation through polymeric samples can be calculated with this function. A pressure hysteresis test was undertaken but no hysteresis was found. No temperature dependence of the oxygen sensor signal was observed in the range between 20 °C and 30 °C. This study presents for the first time data showing the oxygen permeation rates through a polyethylene film in the pressure range between 1 and 2000 bars at 23 °C. [ABSTRACT FROM AUTHOR]

Previous measurements of the X-ray absorption spectra of PbCl2 at the chlorine K-edge have shown significant variation between different studies. Herein, using first principles simulations of X-ray absorption spectroscopy, we show that the observed spectral variations are due to the generation of Cl2 gas and depletion of chlorine from PbCl2, consistent with what is observed during ultraviolet absorption for the same compound. We note that Cl2 gas generation can also be initiated using higher resonant X-ray energies, including Pb X-ray absorption edges. While this casts doubt on previous interpretations of certain measurements, it does indicate a means of generating chlorine gas during in situ experiments by passing high energy x-rays through a hard x-ray transparent medium and onto PbCl2. [ABSTRACT FROM AUTHOR]

Density functional theory was used to study CO adsorption on thin Pt metal films supported on SrO- and TiO2-terminated SrTiO3(100) surfaces. Regardless of substrate-termination, significant enhancement in CO binding occurred on the Pt monolayer compared to the bulk Pt(100) surface. We also observed CO-coverage dependent shifting of Pt atoms, influenced by the nature of underlying oxide atoms. These oxide-induced effects become negligible after depositing more than 2 monolayers of Pt. Evaluating the electronic structures of oxide-supported Pt showed that the interaction of filled Pt dxz+yz and empty Pt dz² states with CO molecular orbitals can be directly related to CO adsorption on the Pt/SrTiO3(100) surface. A hybrid d-band model is able to capture the CO adsorption trends for systems that do not show large lateral distortion except for the case of Pt adsorbed above the Sr atom on the SrO-termination. For this case, charge transfer from adjacent Pt atoms leads to a large filled dz2 peak below the Fermi level that weakens the Pt-CO s bonding due to Pauli repulsion. [ABSTRACT FROM AUTHOR]

The structure of the first layer CO adsorbed on NaCl(100) is investigated experimentally by means of quantitative low-energy electron diffraction at 25 K, and theoretically by means of density functional theory. Consistent with earlier helium atom diffraction results, the monolayer structure has p(2x 1) symmetry with a glide-plane along the longer axis of the unit cell. The structure analysis confirms the binding of CO via the carbon end to the NaCl(100) surface. The vertical distance of carbon above Na+ is 2.58 ± 0.08 Â, in good agreement with geometry optimizations based on dispersion-corrected density functional theory, and 0.15 Â lower than predicted in calculations based on the nonlocal van der Waals density functional. [ABSTRACT FROM AUTHOR]

We compare calculations of the translational collision-induced spectra and their integrated intensities of both He-Ar and Ne-Ar collisional complexes, using the quantum mechanical and a semiclassical formalism. Advanced potential energy and induced dipole functions are used for the calculations. The quantum method used is as described previously [L. Frommhold, Collision-induced Absorption in Gases (Cambridge University Press, 1993 and 2006)]. The semiclassical method is based on repeated classical atom-atom scattering calculations to simulate an ensemble average; subsequent Fourier transform then renders the binary absorption coefficient as a function of frequency. The problem of classical calculations is the violation of the principle of detailed balance, which may be introduced only artificially in classical calculations. Nevertheless, it is shown that the use of classical trajectories permits a fairly accurate reproduction of the experimental spectra, comparable to the quantum mechanical results at not too low temperatures and for collisional pairs of not too small reduced mass. Inexpensive classical calculations may thus be promising to compute spectra also of molecular pairs, or even of polyatomic collisional pairs with anisotropic intermolecular interactions, for which the quantum approach is still inefficient or impractical. [ABSTRACT FROM AUTHOR]

Freezing of gases adsorbed on open surfaces (e.g., graphite) and in narrow pores is a widespread phenomenon which is a subject of a large number of publications. Modeling of the gas/liquid-solid transition is usually accomplished with a molecular simulation technique. However, quantitative analysis of the gas/liquid-solid coexistence and thermodynamic properties of the solid layer still encounters serious difficulties. This is mainly due to the effect of simulation box size on the lattice constant. Since the lattice constant is a function of loading and temperature, once the ordering transition has occurred, the simulation box size must be corrected in the course of simulation according to the Gibbs-Duhem equation. A significant problem is also associated with accurate prediction of the two-dimensional liquid-solid coexistence because of a small difference in densities of coexisting phases. The aim of this study is thermodynamic analysis of the two-dimensional phase coexistence in systems involving crystal-like free of defects layers in narrow slit pores. A special attention was paid to the determination of triple point temperatures. It is shown that intrinsic properties of argon monolayer adsorbed on the graphite surface are similar to those of isolated monolayer accommodated in the slit pore having width of two argon collision diameters. Analysis of the latter system is shown to be clearer and less time-consuming than the former one, which has allowed for explanation of the experimentally observed two-stage melting transition of argon monolayer on graphite without invoking the periodic surface potential modulation and orientational transition. [ABSTRACT FROM AUTHOR]

We have studied the co-adsorption of Br2 and H2 on Si(001), and obtained co-adsorption energies and the surface phase diagram as a function of the chemical potential and pressure of the two gases. To do this, we have used density functional theory calculations in combination with ab initio atomistic thermodynamics. Over large ranges of bromine and hydrogen chemical potentials, the favored configuration is found to be either one with only Br atoms adsorbed on the surface, at full coverage, in a (3 × 2) pattern, or a fully H-covered surface in a (2 × 1) structure. However, we also find regions of the phase diagram where there are configurations with either only Br atoms, or Br and H atoms, arranged in a two-atom-wide checkerboard pattern with a (4 × 2) surface unit cell. Most interestingly, we find that by co-adsorbing with H2, we bring this pattern into a region of the phase diagram corresponding to pressures that are significantly higher than those where it is observed with Br2 alone. We also find small regions of the phase diagram with several other interesting patterns. [ABSTRACT FROM AUTHOR]

Using the results of extensive Monte Carlo simulations in the canonical and grand canonical ensembles, we discuss the phase behavior of mixed submonolayer films of krypton and xenon adsorbed on the graphite basal plane. The calculations have been performed using two- and three-dimensional models of the systems studied. It has been demonstrated that out-of-plane motion does not affect the properties of the films as long as the total density is well below the monolayer completion and at moderate temperatures. For the total densities close to the monolayer completion, the promotion of particles to the second layer considerably affects the film properties. Our results are in a reasonable agreement with the available experimental data. The melting point of submonolayer films has been shown to exhibit non-monotonous changes with the film composition, and reaches minimum for the xenon concentration of about 50%. At the temperatures below the melting point, the structure of solid phases depends upon the film composition and the temperature; one can also distinguish commensurate and incommensurate phases. Two-dimensional calculations have demonstrated that for the xenon concentration between about 15% and 65% the adsorbed film exhibits the formation of a superstructure, in which each Xe atom is surrounded by six Kr atoms. This superstructure is stable only at very low temperatures and transforms into the mixed commensurate [formula]° phase upon the increase of temperature. Such a superstructure does not appear when a three-dimensional model is used. Grand canonical ensemble calculations allowed us to show that for the xenon concentration of about 3% the phase diagram topology of monolayer films changes from the krypton-like (with incipient triple point) to the xenon-like (with ordinary triple point). [ABSTRACT FROM AUTHOR]

We have performed CCSD(T), MP2, and DF-LMP2 calculations of the interaction energy of CO on the MgF2(110) surface by applying the method of increments and an embedded cluster model. In addition, we performed periodic HF, B3LYP, and DF-LMP2 calculations and compare them to the cluster results. The incremental CCSD(T) calculations predict an interaction energy of Eint = -0.37 eV with a C-down orientation of CO above a Mg2+ ion at the surface with a basis set of VTZ quality. We find that electron correlation constitutes about 50% of the binding energy and a detailed evaluation of the increments shows that the largest contribution to the correlation energy originates from the CO interaction with the closest F ions on the second layer. [ABSTRACT FROM AUTHOR]

We report investigations of adsorption of N2 and O2 molecules on silver cluster cations. We have first revisited structures of small silver clusters based on first-principles calculations within the framework of density functional theory with hybrid functional. The 2D to 3D transition for the neutral clusters occurs from n = 6 to 7 and for cations, in agreement with experiments, from n = 4 to 5. With the refined structures, adsorption energies of N2 and O2 molecules have been calculated. We have identified characteristic drops in the adsorption energies of N2 that further link our calculations and experiments, and confirm the reported 2D-3D transition for cations. We have found that perturbations caused by physisorbed molecules are small enough that the structures of most Ag clusters remain unchanged, even though physisorption stabilizes the 3D Ag7+ structure slightly more than the 2D counterpart. Results for pure O2 adsorption indicate that charge transfer from Agn+ to O2 occurs when n > 3. Below that size oxygen essentially physisorbes such as nitrogen to the cluster. We interpret the experimentally observed mutually cooperative co-adsorption of oxygen and nitrogen using results from density functional theory with generalized gradient approximations. The key to the enhancement is N2-induced increase in charge transfer from Agn+ cations to O2. [ABSTRACT FROM AUTHOR]

Adsorption of O2 molecule on Al surface has been a long standing puzzle for the first principles calculation. We have studied the adsorption of O2 molecule on the Al(111) surface using hybrid functionals. In contrast to the previous local-density approximation/gradient-corrected approximation, the present calculations with hybrid functionals successfully predict that O2 molecule can be absorbed on the Al(111) surface with a barrier around 0.2-0.4 eV, which is in good agreement with experiments. Our calculations predict that the lowest unoccupied molecular orbital of O2 molecule is higher than the Fermi level of the Al(111) surface, which is responsible for the barrier of the O2 adsorption. [ABSTRACT FROM AUTHOR]

We study the water bilayer on clean and hydrogen preadsorbed Rh(111) surfaces by means of density-functional theory with the generalized gradient approximation and the van der Waals density functional, to investigate the influence of adsorbed hydrogen on the adsorption state of water. We found that adsorbed hydrogen interacts repulsively with water through its 1b1 and 4a1 orbitals. The repulsion dominates at high hydrogen coverage, resulting in a hydrophobic Rh(111)-H surface. [ABSTRACT FROM AUTHOR]

We report a study of the adsorption of CF4 on graphite preplated with a monolayer of CF3Cl, using infrared reflection absorption spectroscopy combined with ellipsometry. The saturated vapor pressure of CF3Cl is nearly 3 orders of magnitude smaller than that of CF4 at the same temperature, so the main control variables are the temperature and the pressure (or chemical potential) of CF4, together with the initial coverage of CF3Cl. The temperature range covered is 60-105 K. We find that, if the initial monolayer of CF3Cl is liquid, CF4 continuously displaces CF3Cl by substitution in the monolayer. If the initial monolayer of CF3Cl is solid, due to either lower temperature or compression, CF4 condenses as a second layer on the top of the CF3Cl layer, with only slight mixing with the original layer. This behavior persists to multiple layers of CF4. [ABSTRACT FROM AUTHOR]

Introduction of a second metal can greatly modify the surface reactivity of a host metal. Recently Jeroro and Vohs found that Pd(111) deposited with 0.03-0.06 monolayer of Zn might possess unique activity to methanol steam reforming reaction. To investigate the distribution of the deposited Zn, we examined the adsorption of CO on two types of model systems. In the first model, Zn is in the top-layer of Pd(111) only, while in the second model Zn is placed in the subsurface exclusively. It is found that Zn atoms in the topmost layer show negligible effect on CO adsorption especially at hollow sites, whereas the second layer Zn atoms affect significantly the interaction of CO with the substrate. It is revealed that the negligible influence of the first layer Zn on CO adsorption is due to the offsetting of the ligand effect by the strain effect. On the other hand, the ligand effect dominates the CO adsorption in the second model where the strain effect is insignificant. It is demonstrated that the d-band centers correlate well with the binding energies of the second model, whereas no such good correlation exists for the first model. Our results show that the subsurface plays a more important role and the observed dramatic modification of surface reactivity of Pd(111) deposited with 0.03-0.06 ML Zn is most likely originated from the subsurface Zn atoms, if the coverage is not underestimated and the deposited Zn atoms are distributed uniformly within a layer. [ABSTRACT FROM AUTHOR]

The pure rotational spectrum of the CrS radical has been measured in its ground X 5Πr state using gas-phase millimeter/submillimeter direct absorption methods. The molecule was created by the reaction of chromium vapor, sublimed in a Broida-type oven, with hydrogen sulfide. Eleven rotational transitions were recorded for this free radical in the frequency range of 280-405 GHz; in most transitions, all five spin components were observed, and lambda-doubling was resolved in the Ω=0, 1, and 2 ladders. The data were fit with a Hund's case (a) Hamiltonian and rotational, spin-orbit, spin-spin, and lambda-doubling constants were established. Higher order spin and spin-orbit terms were essential in the analysis. The lambda-doubling constants indicate a nearby 5Σ+ state at an energy of ∼1500-2000 cm-1. A bond length of 2.0781 Å was derived for CrS from the data, which is larger than the value of 2.0682 Å found for MnS by ∼0.01 Å. In contrast, the bond distance for MnO is greater than that of CrO by 0.03 Å, an illustration of the subtle differences between 3d oxide and sulfides. CrS is the second molecule in a 5Π state that has been studied by rotational spectroscopy. [ABSTRACT FROM AUTHOR]

The adsorption properties of a single CO molecule on Scn (n=2–13) clusters are studied by means of a density functional theory with the generalized gradient approximation. Two adsorption patterns are identified. Pattern a (n=3, 4, 6, 8, 11, and 12), CO binds to hollow site while Pattern b (n=5, 7, 9, 10, and 13), CO binds to bridge site accompanied by significantly lengthening of the Sc–Sc bond. The adsorption energy exhibits clear size-dependent variation and odd-even oscillation for n<10 and reach the peak at n=5, 7, and 9, implying their high chemical reactivity. Similar variations are noted in C–O bond length, vibrational frequency, and charge transferred between CO and the clusters. This can be understood in light of the adsorption pattern, the atomic motif, and the relative stability of the bare Sc clusters. Compared with the free Sc clusters, the magnetic nature remains upon adsorption except n=2, 4, 12, and 13. Particularly, the moments of n=13 reduce significantly from 19 to 5μB, implying the adsorption plays an attenuation influence on the magnetism of the cluster. [ABSTRACT FROM AUTHOR]

We utilized direct rate measurements and temperature programmed desorption to investigate reactions that occur during the collisions of gaseous oxygen atoms with deuterium-covered Pt(100). We find that both D2O and D2 desorb promptly when an oxygen atom beam impinges upon D-covered Pt(100) held at surface temperatures ranging from 90 to 150 K, and estimate effective cross sections of 12 and 1.8 Å2, respectively, for the production of gaseous D2O and D2 at 90 K. The yields of D2O and D2 that desorb at 90 K are about 13% and 2%, respectively, of the initial D atom coverage, though most of the D2O product molecules (∼80%) thermalize to the surface rather than desorb at the surface temperatures studied. Increasing the surface temperature from 90 to 150 K causes the D2O desorption rate to decay more quickly during O atom exposures to the surface and results in lower yields of gaseous D2O. We attribute the production of D2O and D2 in these experiments to reactions involving intermediates that are not thermally accommodated to the surface, so-called hot precursors. The results are consistent with the production of hot D2O involving first the generation of hot OD groups from the reaction O*+D(a)→OD*, where the asterisk denotes a hot precursor, followed by the parallel pathways OD*+D(a)→D2O* and OD*+OD(a)→D2O*+O(a). The final reaction contributes significantly to hot D2O production only later in the reaction period when thermalized OD groups have accumulated on the surface, and it becomes less important at higher temperature due to depletion of the OD(a) concentration by thermally activated D2O production. [ABSTRACT FROM AUTHOR]

A kinetic lattice gas model is used to study the equilibrium properties and the desorption kinetics of CO on Ru(0001). The authors compute all relevant on-site binding and interaction energies of CO molecules within density functional theory and import them in two different models. The first model allows the CO molecules to adsorb upright on top and hollow sites. The authors calculate the phase diagram, coverage isobars, and temperature programed desorption spectra. Up to a coverage of 1/3 ML, very good agreement is obtained between theory and experiment when considering top sites only. For coverages beyond 1/3 ML, hollow sites are included and disagreement between theory and experiment occurs. The second model allows adsorption on top sites only but allows them to tilt and shift from their upright positions. The authors show that this model resolves many of the deficiencies of their first one. Furthermore, the authors demonstrate that this model is more consistent with experiment since it is the only model that is able to explain the results from IR-spectroscopy experiments. [ABSTRACT FROM AUTHOR]

The authors have performed a systematic computational study of the hydrogen storage capacity of model organometallic compounds consisting of Sc, Ti, and V transition metal atoms bound to CmHm rings (m=4–6). For all the complexes considered, the hydrogen storage capacity is limited by the 18-electron rule. The maximum retrievable H2 uptake predicted is 9.3 wt % using ScC4H4, slightly better than the 9.1 wt % hydrogen using TiC4H4, and much larger than the ∼7 wt % hydrogen with VC4H4, where only four H2 molecules can be adsorbed. The kinetic stability of these hydrogen-covered organometallic complexes is reviewed in terms of the energy gap between the highest occupied and lowest unoccupied molecular orbitals and the strength and nature of successive H2 bindings. [ABSTRACT FROM AUTHOR]

We report the results of extensive equilibrium molecular dynamics simulation of adsorption of ternary gas mixtures in a class of model nanoporous materials, namely, carbon molecular sieve membranes (CMSMs), which are being utilized for separation of gas mixtures. We use a novel three-dimensional molecular pore network model for CMSMs based on Voronoi tessellation of the simulation cell. The simulations with such a model allow us to investigate the effect of the morphology of the pore space, i.e., its pore size distribution and pore connectivity, on the adsorption isotherms. We also extend a statistical mechanical theory of adsorption of single gases and their binary mixtures, recently developed by us, to ternary mixtures and use the results of the molecular simulations to test its accuracy. The theory is shown to provide very accurate predictions for the simulation results over wide ranges of the temperature and porosity of the membrane. The performance of the theory is also compared with that of another theory of adsorption based on a virial expansion. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Recent measurements of the resonance frequency of a copper disk covered with carbon nanotube bundles have shown characteristic resonance shifts during exposure with various gas molecules. The shifts were interpreted as the change of the dielectric permittivity of the system forming the sensor due to the electric properties of the adsorbed molecules. Starting from a simplified sensor model formed by one single wall nanotube, we develop a self-consistent approach to describe the variation of the linear dielectric susceptibility of the tube at the atomic scale when molecules are adsorbed at its external surface. The sensitivity of this model sensor is tested as a function of the apolar or polar nature of the admolecules, their adsorption geometry, their concentration, and the characteristics of the tube (length, diameter,...). The comparison with data on dielectric constant changes vs adsorption, coming from measurements of the resonance frequency shifts, displays striking agreement for most of the molecular species considered. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present theoretical and experimental evidence for CO[sub 2] adsorption on different sites of single walled carbon nanotube (SWNT) bundles. We use local density approximation density functional theory (LDA-DFT) calculations to compute the adsorption energies and vibrational frequencies for CO[sub 2] adsorbed on SWNT bundles. The LDA-DFT calculations give a range of shifts for the asymmetric stretching mode from about -6 to -20 cm-1 for internally bound CO[sub 2], and a range from -4 to -16 cm-1 for externally bound CO[sub 2] at low densities. The magnitude of the shift is larger for CO[sub 2] adsorbed parallel to the SWNT surface; various perpendicular configurations yield much smaller theoretical shifts. The asymmetric stretching mode for CO[sub 2] adsorbed in groove sites and interstitial sites exhibits calculated shifts of -22.2 and -23.8 cm-1, respectively. The calculations show that vibrational mode softening is due to three effects: (1) dynamic image charges in the nanotube; (2) the confining effect of the adsorption potential; (3) dynamic dipole coupling with other adsorbate molecules. Infrared measurements indicate that two families of CO[sub 2] adsorption sites are present. One family, exhibiting a shift of about -20 cm-1 is assigned to internally bound CO[sub 2] molecules in a parallel configuration. This type of CO[sub 2] is readily displaced by Xe, a test for densely populated adsorbed species, which are expected to be present on the highest adsorption energy sites in the interior of the nanotubes. The second family exhibits a shift of about -7 cm-1 and the site location and configuration for these species is ambiguous, based on comparison with the theoretical shifts. The population of the internally bound CO[sub 2] may be enhanced by established etching procedures that open the entry ports for adsorption, namely, ozone oxidation followed by annealing in vacuum at 873 K. Xenon displacement experiments indicate that internally bound CO[sub 2] is preferentially displaced relative to the -7 cm-1 shifted species. The -7 cm-1 shifted species is assigned to CO[sub 2] adsorbed on the external surface based on results from etching and Xe displacement experiments. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

When a Ne:H[sub 2]:CO[sub 2]=800:10:1 mixture is codeposited at 4.3 K with neon atoms that have been excited in a microwave discharge, the infrared spectrum of the resulting solid deposit includes not only absorptions characteristic of experiments from which H[sub 2] is absent, but also numerous product absorptions that are common to similar experiments on Ne:HCOOH samples. The carriers of these latter absorptions include HCOOH itself, as well as trans-HOCO, HCOOH[sup +], and HCO[sub 2][sup -]. Detailed isotopic substitution studies confirm the infrared identifications of HCOOH[sup +] and HCO[sub 2][sup -] and permit the identification of two previously unassigned vibrational fundamentals each of HOCO[sup +] and DOCO[sup +]. Density functional calculations for the latter species support the proposed assignments. Evidence is presented for the stabilization of the H[sub 2]...CO[sub 2][sup -] anion complex, in which the H[sub 2] moiety becomes infrared-active. The processes that result in the stabilization of these products are considered. [ABSTRACT FROM AUTHOR]

In the control of environmental pollution, metal carbides are potentially useful for trapping and destroying sulfur dioxide (SO[sub 2]). In the present study, the density functional theory was employed to study the surface structures and electronic properties of the adsorbed SO[sub 2] on titanium carbides: metcar Ti[sub 8]C[sub 12], nanocrystal Ti[sub 14]C[sub 13], and a bulk TiC(001) surface. The geometries and orientations of SO[sub 2] were fully optimized on all these substrates. Our calculations show that, in spite of the high C/Ti ratio and C[sub 2] groups, metcar Ti[sub 8]C[sub 12] exhibits extremely high activity towards SO[sub 2]. The S–O bonds of SO[sub 2] spontaneously break on Ti[sub 8]C[sub 12]. The products of the decomposition reaction (S, O) interact simultaneously with Ti and C sites. The C atoms are not simple spectators, and their participation in the dissociation of SO[sub 2] is a key element for the energetics of this process. Nanocrystal Ti[sub 14]C[sub 13] also displays a strong interaction with SO[sub 2]. Although the dissociation of SO[sub 2] on Ti[sub 14]C[sub 13] cannot proceed as easily as that on Ti[sub 8]C[sub 12], it could occur by thermal activation even at very low temperature. SO[sub 2] is weakly bonded with the bulk TiC(001) surface. By thermal activation the dissociation of SO[sub 2] on a TiC(001) surface may also take place but it should be much more difficult than that on Ti[sub 14]C[sub 13]. Therefore, we suggest that the carbide nanoparticles (Ti[sub 8]C[sub 12] and Ti[sub 14]C[sub 13]) should have special chemical activity towards SO[sub 2] removal associated with their “magic” structures. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Studies the kinetics of reactions of D atoms with isopropyl iodide adsorbed on Pt(111), H covered Pt(111) and monolayer C covered Pt(111) surfaces using direct product detection. Formation of the gaseous product propane through propyl abstration from the halide; Reaction products at low temperatures.

Reports on the initial adsorption probability of oxygen on silicon as a function of surface temperature, incident kinetic energy and angle. Distinct kinetic energy regimes with opposite temperature and energy dependencies; Adsorption mechanisms; Estimation of trapping probabilities.

Formulates a general theory of the kinetics of adsorption, desorption and dissociation of gases at surfaces. Establishment of dissociation equilibrium; Derivation of kinetic equations for adsorption, desorption and dissociation; Employment of a lattice gas model for homonuclear molecules with both atoms and molecules present on the surface.

Examines the adsorption-desorption properties of para and ortho hydrogen gas molecules in sodium chloride. Adsorption of para-hydrogen molecules; Redshift changes in internal vibration frequencies for para-hydrogen molecules; Effects of pressure on infrared signals.

Examines the effect of gas adsorption on structural and electrical properties of activated carbon fibers (ACF). Magnetic susceptibility and electrical conductivity of ACF; Pressure dependence of electron spin resonance linewidth; Role of magnetic interaction in oxygen adsorption; Comparison of adsorption isotherms between oxygen and nitrogen.

Molecular dynamics calculations are reported on the sorption and diffusion of small gas molecules (He, H2, and O2) in amorphous polyisobutylene. An all-atom force field (with explicit hydrogen atoms) was used. In addition to the standard method of obtaining the diffusion coefficient from an equilibrium calculation via the mean-square displacement we used a nonequilibrium technique that applies a fictitious external field selectively to the gas molecules. To our knowledge, nonequilibrium MD is, for the first time, applied to the problem of gas diffusion in polymers. Results of both techniques are compared. Energy profiles of the jump events underlying gas diffusion through polymers are studied. We also discuss the possible presence of anomalous (non-Einstein) behavior of gas molecules diffusing through an amorphous polymer. Gas solubilities in polyisobutylene are calculated by particle-insertion techniques. [ABSTRACT FROM AUTHOR]

In some derivations of the Boltzmann equation, the equation which describes the time evolution of the distribution function in the phase space of the atoms of a low-density gas, it has been assumed that all of the trajectories in the phase space of the pairs of atoms are such that transforming back sufficiently far in time leads to separated atoms. In a recent development, it has been shown that if this is not true the ‘‘bound-state’’ trajectories lead to an addition to the collision integral which, in turn, leads to corrections to the values of the low-density transport coefficients. In the present alternate development, it is shown that the ‘‘bound-state’’ trajectories may be joined to trajectories in the ‘‘unbound region’’ through the introduction of a complex time and analytic continuation. With this generalization of the concept of trajectories, one may introduce the ‘‘molecular chaos assumption’’ in the usual manner, and the ‘‘collision integral’’ may be generalized to include the bound-state region of the phase space. This leads to ‘‘corrections’’ to the low-density limiting values of the transport coefficients. These effects of the bound pairs of atoms on the low-density transport coefficients of a gas of atoms which interact according to a Lennard-Jones potential are investigated numerically. It is found that the effects become important at temperatures, scaled in the usual manner by ε/k, below about unity. Specifically, it is found that at a scaled temperature of unity, corresponding to a temperature in the range of about 100–200 K, the effect is to lower the self-diffusion coefficient by about 3.7% and the viscosity and thermal conductivity by about 0.7%. The effects become significantly greater at lower temperatures. [ABSTRACT FROM AUTHOR]