Evaporation of the liquid stationary phase in gas chromatography

Summary The theory of the evaporation of the liquid stationary phase is elaborated and experimentally verified. On the basis of this theory the role played by the losses in the amount of liquid phase present is quantitatively determined. General techniques are examined which minimize the losses; these techniques are based on saturating the incoming carrier gas with liquid phase vapours and raising the pressure of the carrier gas in the column, e.g. by connecting a capillary to the column outlet in order to offer resistance to the gas flow. The application of these techniques ensures stable performance of the gas chromatographic columns using a volatile liquid phase.     

Theory of nonstationary diffusion growth of gas bubble in the supersaturated solution of gas in liquid

General theory of nonstationary diffusion growth of gas bubble in the supersaturated solution of gas in liquid is constructed using the ideas of similarity and self-similar solutions. The balance between the number of gas molecules in solution and in the bubble that displaces incompressible liquid solvent with an increase in bubble size is taken into account at the material isolation of the solution and the bubble. The dependences of the rate of growth of bubble radius on the solubility of gas and the supersaturation of solution are found. The nonstationary effect of a rapid increase in the rate of bubble growth with an increase in the product of gas solubility and solution supersaturation is elucidated. The upper limit of this product at which bubble growth can be considered as isothermal process is established. The theory is constructed at the arbitrary gas solubility.     

Analytical theory for permeation of condensable gases in a nanopore

The permeation of a condensable gas mixture in a pressure gradient is examined within a dynamic density functional theory (DDFT). The non-equilibrium density and flux profiles of gas molecules trapped within a nanopore are calculated for each species as a function of feed gas density. Because of important fluid–fluid interaction close to condensation the flux and density gradients are not related by constant transport diffusivities with the thermodynamic correction of uniform density. For long narrow pores the relation of the selectivity to the equilibrium adsorption isotherms is validated. Improved separation is achieved by combining preferential wall interaction and advantageous attraction between gas molecules of different species and examples are discussed. Results from experiments and simulations of permeation in binary mixtures near condensation are still rare and the theory provides a simple basis to study qualitative trends using known parameters.     

Allowing for Intermolecular Vibrations in the Thermodynamic Functions of a Liquid Inert Gas

Russian Journal of Physical Chemistry A - Ways of calculating interatomic vibrations in a fluid are considered using the example of an inert gas. Discrete continuum theory in the lattice gas model...     

Gas adsorption in active carbons and the slit-pore model 2: Mixture adsorption prediction with DFT and IAST

We use a fast density functional theory (a "slab-DFT") and the polydisperse independent ideal slit-pore model to predict gas mixture adsorption in active carbons. The DFT is parametrized by fitting to pure gas isotherms generated by Monte Carlo simulation of adsorption in model graphitic slit-pores. Accurate gas molecular models are used in our Monte Carlo simulations with gas-surface interactions calibrated to a high surface area carbon, rather than a low surface area carbon as in all previous work of this type, as described in part 1 of this work. We predict the adsorption of binary mixtures of carbon dioxide, methane, and nitrogen on two active carbons up to about 30 bar at near-ambient temperatures. We compare two sets of results; one set obtained using only the pure carbon dioxide adsorption isotherm as input to our pore characterization process, and the other obtained using both pure gas isotherms as input. We also compare these results with ideal adsorbed solution theory (IAST). We find that our methods are at least as accurate as IAST for these relatively simple gas mixtures and have the advantage of much greater versatility. We expect similar results for other active carbons and further performance gains for less ideal mixtures.     

Significant increase in the stability of rare gas hydrides on insertion of beryllium atom

Chemical binding between a rare gas atom with other elements leading to the formation of stable chemical compounds has received considerable attention in recent years. With an intention to predict highly stable novel rare gas compounds, the process of insertion of beryllium atom into rare gas hydrides (HRgF with Rg=Ar, Kr, and Xe) has been investigated, which leads to the prediction of HBeRgF species. The structures, energetic, and charge distributions have been obtained using MP2, density functional theory, and CCSD(T) methods. Analogous to the well-known rare gas hydrides, HBeRgF species are found to be metastable in nature; however, the stabilization energy of the newly predicted species has been calculated to be significantly higher than that of HRgF species. Particularly, for HBeArF molecule, it has been found to be an order of magnitude higher. Strong chemical binding between beryllium and rare gas atom has also been found in the HBeArF, HBeKrF, and HBXeF molecules. In fact, the basis set superposition error and zero-point energy corrected Be-Ar bond energy calculated using CCSD(T) method has been found to be 112 kJ/mol, which is the highest bond energy ever achieved for a bond involving an argon atom in any chemically bound neutral species. Vibrational analysis reveals a large blueshift (approximately 200 cm(-1)) of the H-Be stretching frequency in HBeRgF with respect to that in BeH and HBeF species. This feature may be used to characterize these species after their preparation by the laser ablation of Be metal along with the photolysis of HF precursor in a suitable rare gas matrix. An analysis of the nature of interactions involved in the present systems has been performed using theory of atoms in molecules (AIM). Geometric as well as energetic considerations along with the AIM results suggest a substantial covalent nature of Be-Rg bond in these systems. Thus, insertion of a suitable metal atom into rare gas hydrides is a promising way to energetically stabilize the HRgX species, which eventually leads to the formation of a new class of insertion compounds, viz., rare gas metallohydrides.     

Separation of mixtures of solvents with wide bore capillary columns connected in series

The gas chromatographic separation of the components of a complex mixture of industrial solvents, not possible on a single stationary phase owing to coelution of some of the compounds, has been achieved on two serially connected wide bore capillary columns of different polarity. The analysis of a mixture of twenty five compounds was optimized using the serial gas chromatography theory of Purnell and Williams. The capacity factors of sample components measured with the two columns coupled in series were found to agree with those predicted by theory, regardless of the order in which they were connected.     

On the applicability of the theory of ideal adsorption solutions to equilibria in the N2-O2-zeolite NaX system

Experimental data on the joint adsorption of a nitrogen-oxygen gas mixture on zeolite NaX were analyzed using the theory of ideal adsorption solutions. Acceptable accuracy was obtained in calculations of binary isotherms from the adsorption isotherms of the pure gas mixture components. The calculated binary isotherms can be used to determine the parameters of the mathematical models of binary adsorption, for instance, competitive adsorption models.     

Gas flow headspace liquid phase microextraction

There is a trend towards the use of enrichment techniques such as microextraction in the analysis of trace chemicals. Based on the theory of ideal gases, theory of gas chromatography and the original headspace liquid phase microextraction (HS-LPME) technique, a simple gas flow headspace liquid phase microextraction (GF-HS-LPME) technique has been developed, where the extracting gas phase volume is increased using a gas flow. The system is an open system, where an inert gas containing the target compounds flows continuously through a special gas outlet channel (D = 1.8 mm), and the target compounds are trapped on a solvent microdrop (2.4 μL) hanging on the microsyringe tip, as a result, a high enrichment factor is obtained. The parameters affecting the enrichment factor, such as the gas flow rate, the position of the microdrop, the diameter of the gas outlet channel, the temperatures of the extracting solvent and of the sample, and the extraction time, were systematically optimized for four types of polycyclic aromatic hydrocarbons. The results were compared with results obtained from HS-LPME. Under the optimized conditions (where the extraction time and the volume of the extracting sample vial were fixed at 20 min and 10 mL, respectively), detection limits (S/N = 3) were approximately a factor of 4 lower than those for the original HS-LPME technique. The method was validated by comparison of the GF-HS-LPME and HS-LPME techniques using data for PAHs from environmental sediment samples.     

Practical fast gas chromatography for contract laboratory program pesticide analyses

An approach to shortening the analysis time for practical fast gas chromatography (GC) by using Method Translator software, which can be downloaded free from the Internet, is presented. This software simplifies the process of optimizing temperature programming while changing column dimensions, carrier gas type, and flow. Basic chromatographic theory is employed in a practical manner for adjusting column dimensions for optimal performance. In addition, electronic pneumatic control and high oven ramp rates make it easier to achieve fast analysis times without reproducibility problems. This practical approach is demonstrated using Contract Laboratory Program pesticide analytes. The factors found to be most important in decreasing the analysis time without a loss of performance are utilization of GC columns having smaller diameters and substitution of hydrogen for helium as the carrier gas.     

Dichlorocarbene addition to cyclopropenes: a computational study

Reaction paths for addition of dichlorocarbene to 1,2-disubstituted cyclopropenes were calculated using hybrid density functional theory (B3LYP/6-31G) in the gas phase and in the presence of a continuum solvation model corresponding to acetonitrile. In both the gas phase and acetonitrile, :CCl2-cyclopropene addition follows an asymmetric, non-least-motion approach. Barriers to addition range from 0 to 2 kcal/mol. The reactions proceed in concerted fashion in both the gas phase and solution to yield 1,3-dienes or bicyclobutanes. The reaction pathway on this complex potential energy surface of this reaction appears to bifurcate, and the product distribution is believed to be controlled by reaction dynamics. At the present level of theory, there appears to be no minimum on the potential energy surface corresponding to a dipolar intermediate.     

Conformational studies of methyl 3-O-methyl-alpha-D-arabinofuranoside: an approach for studying the conformation of furanose rings

A computational method for probing furanose conformation has been developed using a methylated monosaccharide derivative 1. First, a large library of conformers was generated by a systematic pseudo Monte Carlo search followed by optimization with the AMBER molecular mechanics force field. A subset of these conformers was then subjected to ab initio and density functional theory calculations in both the gas and aqueous phases. These calculations indicate that entropic contributions to the Gibbs free energy are important determinants of the Boltzmann distribution for the conformational preferences of 1 in the gas phase. The results obtained at each level of theory are discussed and compared with the experimentally determined conformer distribution from NMR studies in aqueous solution. In addition, the ability of each level of theory to reproduce the experimentally measured 1H-1H coupling constants in 1 is discussed. Empirical Karplus equations and density functional theory methods were used to determine average 3J(H1,H2), 3J(H2,H3), and 3J(H3,H4) for each level of theory. On the basis of this comparison, consideration of solvation with the MN-GSM model provided good agreement with the experimental data.     

Investigating the nature of intermolecular and intramolecular bonds in noble gas containing molecules

This article presents a theoretical study on a number of selected noble gas containing systems of the general formula FNgR and NgR (Ng = He, Ne, Ar, Kr, Xe and R = CH₃, CN, CCH, BO, BNH, H, BeO, and AuF). The principal structures, bond energies, spectroscopic, and electronic properties of 28 noble gas containing molecules were investigated using density functional theory at the BMK level. Quantum theory of atoms in molecules, natural bond orbital, and several other analysis methods have been used to provide more insight into the nature of noble gas bonds. Although both F? Ng and Ng? R bonds in the investigated molecules are assigned to have partially covalent and partially electrostatic nature, the covalent character is dominant in Ng? R bonds. In the second part, the intermolecular interactions between FNgR molecules and hydrogen fluoride are overviewed with emphasis on the hydrogen bonding through the fluorine side of noble gas molecule with hydrogen of HF. The calculated interaction energies were found to decrease in magnitude going down the noble gas series. For all noble gases, the strongest hydrogen bond has been observed in the case R=CH₃. On the contrary, using R=CN in the FNgR moiety weakens the interaction strength. © 2014 Wiley Periodicals, Inc.     

在确定的热力学状态下测量气体导热系数与黏度

气体的导热系数和黏度是重要的热物性参数,其数值大小取决于所处的热力学状态。在目前的导热系数和黏度主要测量方法中,待测工质在测量时需经历非定常的过程或处于具有物性梯度的非平衡态之下,使得待测工质的物性在时间或者空间上不处于一个确定的热力学状态。本文利用圆柱定程干涉法,通过分析气体导热系数和黏度导致的声波能量耗散,结合气体输运理论中对稀疏气体的描述,探索了在确定的热力学状态下同时测量气体导热系数和黏度的方法,并以氩(Ar)为例进行了实验验证。测量结果与已有文献一致性较好,初步证实了方法的可行性。     

SurfKin: An ab initio kinetic code for modeling surface reactions

In this article, we describe a C/C++ program called SurfKin (Surface Kinetics) to construct microkinetic mechanisms for modeling gas–surface reactions. Thermodynamic properties of reaction species are estimated based on density functional theory calculations and statistical mechanics. Rate constants for elementary steps (including adsorption, desorption, and chemical reactions on surfaces) are calculated using the classical collision theory and transition state theory. Methane decomposition and water–gas shift reaction on Ni(111) surface were chosen as test cases to validate the code implementations. The good agreement with literature data suggests this is a powerful tool to facilitate the analysis of complex reactions on surfaces, and thus it helps to effectively construct detailed microkinetic mechanisms for such surface reactions. SurfKin also opens a possibility for designing nanoscale model catalysts. © 2014 Wiley Periodicals, Inc.     

H2O3 as a reactive oxygen species: formation of 8-oxoguanine from its reaction with guanine

Reaction of guanine with H2O3 in the absence and presence of a water molecule leading to the formation of 8-oxoguanine (8-oxoG) was investigated. Initial calculations were performed using imidazole (Im) as a model for the five-membered ring of guanine. The reactant, intermediate, and product complexes as well as transition states were obtained in gas phase at the B3LYP/6-31+G* and B3LYP/AUG-cc-pVDZ levels of theory. In all the cases, except for the reactions involving imidazole, single-point energy calculations were performed in gas phase at the MP2/AUG-cc-pVDZ level of theory. Solvation calculations in aqueous media were carried out using the polarizable continuum model (PCM) of the self-consistent reaction field (SCRF) theory. Vibrational frequency analysis and intrinsic reaction coordinate (IRC) calculations were performed to ensure that the transition states connected the reactant and product complexes properly. Zero-point energy (ZPE)-corrected total energies and Gibbs free energies at 298.15 K in gas phase and aqueous media were obtained. When a reaction of H2O3 in place of H2O2 with guanine is considered, the major barrier energy which is encountered at the first step is almost halved showing that H2O3 would be much more reactive than H2O2. Considering the reaction schemes investigated here and the observed fact that H2O3 is dissociated easily under ambient conditions, it appears that H2O3 would serve as an effective reactive oxygen species.     

Finite concentration inverse gas chromatography: Diffusion and partition measurements

Inverse gas chromatography (IGC) is a very fast, accurate, and reliable technique to measure diffusion coefficients. This technique however, has been limited to measurements in the infinite dilution region, i.e., in the region of negligible amount of solvent in the polymer. We have extended the scope of inverse gas chromatography to measure diffusion coefficients at finite concentrations of the solvent. This involves doping the carrier gas with a solvent of interest to achieve finite concentrations of solvent in the carrier gas and hence in the polymer. The carrier gas is passed through a saturator maintained at constant temperature to achieve this purpose. Diffusion coefficients for polyvinyl acetate–toluene, and polystyrene–toluene systems were determined at finite concentrations. The results were compared with the traditional gravimetric sorption and piezoelectric sorption measurements reported in the literature. The data are in excellent agreement with the values reported, correlate well with the Vrentas–Duda free volume theory, and can also be predicted from infinitely dilute data using the free volume theory. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1279–1290, 1997     

Locating the lowest free-energy point on conical intersection in polar solvent: reference interaction site model self-consistent field study of ethylene and CH2NH2(+)

We present a theoretical method for locating the lowest free-energy points on conical intersections (CIs) in solution using the reference interaction site model self-consistent field (RISM-SCF) theory. Based on the linear-response theory, the nonequilibrium free energy is defined as a quadratic function of solvation coordinates, the parameters in which are directly obtained by ab initio RISM-SCF calculations. This free energy is easily incorporated into an efficient CI optimization procedure in gas phase. The present method is applied to the cis-trans photoisomerizations of ethylene and methaniminium cation (CH2NH2(+)) in polar solvents. We show that the geometries and energies of CIs are largely affected by the solute-solvent electrostatic interaction. In particular, the hydrogen migration of ethylene observed at CIs in the gas phase disappears in protic solvents due to the large stabilization of the zwitterionic state.     

Prediction of a Neutral Noble Gas Compound in the Triplet State

Discovery of the HArF molecule associated with H?Ar covalent bonding [Nature, 2000 , 406, 874–876] has revolutionized the field of noble gas chemistry. In general, this class of noble gas compound involving conventional chemical bonds exists as closed‐shell species in a singlet electronic state. For the first time, in a bid to predict neutral noble gas chemical compounds in their triplet electronic state, we have carried out a systematic investigation of xenon inserted FN and FP species by using quantum chemical calculations with density functional theory and various post‐Hartree–Fock‐based correlated methods, including the multireference configuration interaction technique. The FXeP and FXeN species are predicted to be stable by all the computational methods employed in the present work, such as density functional theory (DFT), second‐order Møller–Plesset perturbation theory (MP2), coupled‐cluster theory (CCSD(T)), and multireference configuration interaction (MRCI). For the purpose of comparison we have also included the Kr‐inserted compounds of FN and FP species. Geometrical parameters, dissociation energies, transition‐state barrier heights, atomic charge distributions, vibrational frequency data, and atoms‐in‐molecules properties clearly indicate that it is possible to experimentally realize the most stable state of FXeP and FXeN molecules, which is triplet in nature, through the matrix isolation technique under cryogenic conditions.     

Density Functional Theory Study of Solvent Effects on the Structure and Vibrational Frequencies of Tetranitrotetraazabicyclooctane “bicyclo‐HMX”

Density Functional Theory (DFT) calculations have been performed on the high explosive compound tetranitrotetraazabicyclooctane (bicyclo‐HMX) in gas phase employing the self‐consistent field (SCF) theory and in different solvents, i.e. cyclohexane, dichloroethane, ethylalcohol, acetonitrile, and water, utilizing the self‐consistent reaction field (SCRF) theory at a B3LYP/6–31 1G** level of theory. Reasonable agreement has been found between the computed and the available experimental data. The effects of different solvents on the geometry, electronic structure, and vibrational frequencies are discussed. Various solvents under consideration are found to have similar effects on the above properties. But some properties change regularly with the increasing polarity of the solvents, and the stronger the solvent polarity is, the larger the change is. In addition, based on the vibrational analysis, standard thermodynamic properties of bicyclo‐HMX in gas phase are also computed by using the statistical thermodynamic principle.