Methane hydrate phase equilibrium in the presence of NaBr,KBr, CaBr2, K2CO3, and MgCl2 aqueous solutions: Experimental measurements and predictions of dissociation conditions

In this communication, experimental data for dissociation conditions of methane hydrates in the presence of 0.05 and 0.1 mass fractions NaBr, KBr, K₂CO₃, and MgCl₂ aqueous solutions and in the presence of 0.05 and 0.15 mass fractions CaBr₂ aqueous solutions are reported. The experimental data were generated using an isochoric pressure-search method. The new experimental dissociation data for methane hydrates in the presence of 0.1 mass fraction MgCl₂ aqueous solution are compared with some selected experimental data from the literature and the agreements are generally found acceptable. Some of new data are finally compared with the predictions of a correlation, which is generally used in the absence of experimental data, and acceptable agreements between the experimental and predicted data are observed.     

Methane hydrate phase equilibrium in the presence of salt (NaCl, KCl, or CaCl2) + ethylene glycol or salt (NaCl, KCl, or CaCl2) + methanol aqueous solution: Experimental determination of dissociation condition

In this communication, we report dissociation conditions of methane hydrates in the presence of salt (NaCl, KCl, or CaCl₂) + ethylene glycol or salt (NaCl, KCl, or CaCl₂) + methanol aqueous solutions at different temperatures. The equilibrium data were generated using an isochoric pressure-search method. These data are compared with some selected experimental data from the literature on dissociation conditions of methane hydrates in the presence of pure water to show the inhibition effects of the above mentioned aqueous solutions.     

Phase equilibria of hydrogen sulphide clathrate hydrates in the presence of single or mixed salt aqueous solution

This work reports the dissociation pressures of hydrogen sulphide clathrate hydrates in the presence of single and mixed aqueous solutions of NaCl, KCl and CaCl₂ at different temperatures and various concentrations of salt(s) in aqueous solution. The equilibrium data were generated using an isochoric pressure-search method. These data are compared with some selected experimental data from the literature on the dissociation conditions of hydrogen sulphide clathrate hydrates in the presence of pure water to show the inhibition effects of the aforementioned aqueous solutions. Comparisons between our experimental data and the corresponding literature data show some disagreements in the literature data.     

Experimental determination and calculation of the critical curves for the binary systems of CO2 containing ketone, alkane, ester and alcohol, respectively

A set of variable-volume autoclave with a quartz window was used for the experimental determination of the high-pressure phase equilibria and critical curves. The critical temperatures, pressures, densities and mole volumes in the region near the critical point of CO₂ were examined for eleven binary systems of supercritical CO₂ (SC CO₂) with different kinds of substances (ketone, alkane, ester and alcohol), respectively. The critical curves of the above binary systems were also calculated using an equation of state. The equation consists of a hard body repulsion term and an additive perturbation term, which takes care of the attractive molecular interaction. The calculated data were compared with the experimental data, and yielded good agreements. At the same time, the values of the adjustable parameters, λ, k_σ and k_? were obtained. The critical curves of the above eleven binary systems at higher temperatures and pressures all belong to type I.     

Methane hydrate dissociation above 0 °C and below 0 °C

The kinetic data of methane hydrate dissociation at various temperatures and pressures were measured in a sapphire cell apparatus by depressurizing method. When the temperature was higher than 0 °C, the experimental results showed that the hydrate dissociation rate was controlled by intrinsic dissociation reaction. When the temperature was lower than 0 °C, water generated from the hydrate dissociation would transform into ice rapidly at the surface of hydrate crystal. The released gas diffused from the hydrate and ice mixture to the bulk of gas phase. With the hydrate continuous dissociation, the boundary of ice–hydrate moved toward water/ice phase. The hydrate dissociation was controlled by gas diffusion, and the hydrate dissociation process was treated as a moving boundary problem. Corresponding kinetic models for hydrate dissociation were established and good agreements with experimental data were achieved.     

Basis-independent potential energy curves for the neutral diatomics of Li,Na and K evaluated by means of Hartree-Fock and different density functional potentials

Summary The solution of the Schrödinger equation for diatomic molecules when the finite element method is used gives the possibility to evaluate highly accurate basis-independent potential energy curves. In this work such types of numerically accurate potential energy curves on the HF level have been evaluated for Li₂, Na₂ and K₂ and could be used as benchmarks in the optimization of basis sets. A comparison between recent LCAO HF calculations in which extended basis sets are used and the accurate values determined in this work show that there is a difference in total energy of 4×10^–5 and 10^–3 a.u. for Li, Li₂, and Na, Na₂, respectively. Evaluated dissociation energies are, however, due to the cancellation of numerical errors in much better agreement. Further, it is found that different exchange correlation potentials for the heavier molecules such as those given by von Barth-Hedin and Vosko, Wilk and Nusair reproduce experimental properties such as dissociation energies, vibrational frequencies almost as well as those achieved with advanced CI methods. TheX potential gives accurate bond lengths for Na₂ and K₂, whereas the dissociation energies are too small.     

Comparative theoretical study of the dissociation process of the isoelectronic molecules BH3CO,CH2CO,HNCO, CO2 and BH3N2, CH2N2, HN3, N2O

Anab initio study of the electronic structure of several 22-electrons molecules is presented. The equilibrium geometries of their ground state are calculated at the SCF level using the 6–31G basis set and are found to be in good agreement with the experimental geometries. The dissociation process of these molecules leading to the isoelectronic products CO or N₂ on the one hand and BH₃, CH₂, NH and O on the other hand is studied. The least-energy dissociation paths of the ground states determined at the SCF level are compared on the basis of electron density interactions. The dissociation energies corresponding to the two lowest dissociation channels are calculated. In these calculations, the correlation energy is taken into account using a non-variational method developed previously. The calculated values of dissociation energies are in good agreement with the existing experimental values. The results permit to predict values for HNCO, BH₃CO and CH₂N₂ and to confirm the instability of BH₃N₂.Aspirant du Fonds National Belge de la Recherche Scientifique.     

Mechanism of etching and of surface modification of polyimide in RF and LF SF6-O2 discharges

Etch rates of Kapton H polyimide film in SF₆-O₂ plasmas (0.25 torr) were studied as a function of the input gas mixture, the excitation frequency (25–450 kHz; 13.56 MHz), and the biasing mode. The treated surface was examined by X ray photoelectron spectroscopy (ESCA), scanning electron microscopy (SEM), and contact angle measurement. The ion and neutral species of the plasma were sampled and analyzed by mass spectrometry. Etch rates are found to depend on the positive ion flux and the degree of dissociation of neutral molecules. Plasma-treated surfaces are always covered with a deposited material (C_nH_mO_xF_y) which partially obstructs the etching reaction by a masking effect and causes surface roughness. A proposed kinetic analysis of the etching mechanism is in good agreement with the experimental data.     

Polyatomic ions in inductively coupled plasma–mass spectrometry: Part II: Origins of N2H and HxCO ions using experimental measurements combined with calculated energies and structures

Several polyatomic ions in inductively coupled plasma–mass spectrometry are studied experimentally and by computational methods. Novel calculations based on spin-restricted open shell second order perturbation theory (ZAPT2) and coupled cluster (CCSD(T)) theory are performed to determine the energies, structures and partition functions of the ions. These values are combined with experimental data to evaluate a dissociation constant and gas kinetic temperature (T_gas) value. In our opinion, the resulting T_gas value can sometimes be interpreted to deduce the location where the polyatomic ion of interest is generated. The dissociation of N₂H⁺ to N₂⁺ leads to a calculated T_gas of 4550 to 4900 K, depending on the computational data used. The COH⁺ to CO⁺ system yields a similar temperature, which is not surprising considering the similar energies and structures of COH⁺ and N₂H⁺. The dissociation of H₂CO⁺ to HCO⁺ leads to a much lower T_gas (< 1000 to 2000 K). Finally, the dissociation of H₂COH⁺ to HCOH⁺ generates a T_gas value between those from the other H_xCO⁺ ions studied here. All of these measured T_gas values correspond to formation of extra polyatomic ion in the interface or extraction region. The computations reveal the existence of isomers such as HCO⁺ and COH⁺, and H₂CO⁺ and HCOH⁺, which have virtually the same m/z values and need to be considered in the interpretation of results.     

Ab initio SCF MO calculations on the CH3Br molecule

Self-consistent fieldab initio calculations have been performed for CH₃Br. The calculated equilibrium conformation is in good agreement with experiment. Valence and core level ionization potentials, Mulliken population analysis and electronic properties are presented. The ionization potentials are in good agreement with the experimental values, except for one case in which the experimental value may be wrongly assigned. The calculated dipole moment, 2.43 Debye, is 34% or 0.6 Debye larger than the experimental value.Two of us (C.W.B. and G. del C.) wish to acknowledge the hospitality of the IBM San José Research Laboratory and joint study agreements between the IBM and the Lawrence Berkeley Laboratory and the National University of Mexico respectively.     

Experimental determination of dissociation constant,Henry's constant,heat of reactions,SO2 absorbed and gas bubble–liquid interfacial area for dilute sulphur dioxide absorption into water

The experimental results for dissociation constant, Henry's constant, heat of reactions and gas bubble–liquid interfacial area for absorption of dilute SO₂ (SO₂ partial pressure upto 0.963 kPa) into water in bubble column are presented. The relations between Henry's constant versus temperature and dissociation constant versus temperature have been proposed based on the present experimental investigation. The temperature dependence on gas bubble–liquid interfacial area per unit volume of liquid has examined. Result shows a very small decrease in interfacial area with increase in temperature. The experimental results are used to correlate total SO₂ absorbed at saturation as function of temperature and SO₂ partial pressure. Comparisons between experimental results and literature data have also been made at different temperatures. The reasonable fittings between the data obtained from correlation and literature data shows that the proposed correlation can be successfully used for predicting the absorption equilibria of dilute SO₂ in distilled water at different temperatures.     

Analytical solution of vibrational–rotational energy transfer in the dissociation and relaxation of N2, Br2, and CO at high temperature

An approximate analytical solution of relaxation in a low-pressure system with exponential transition probabilities is given for vibrational–rotational energy transfer in the dissociation of diatomics. The main assumption is that the rotational degrees of freedom are in thermal equilibrium at all times, and that the barrier to dissociation in the vibrational–rotational plane is linear and asymmetric. The theory is applied to high-temperature dissociations of N₂, Br₂, and CO in excess argon, with satisfactory agreement with available experimental data.     

Shape changes induced by N-terminal platination of ubiquitin by cisplatin

The three-dimensional conformation of a protein is an important property and plays a key role in its biological activity. We show here that ion mobility-mass spectrometry (IM-MS) can be used to detect conformational changes in the protein ubiquitin in the gas phase induced by reaction with the anticancer drug cisplatin. The primary adduct was ubiquitin-{Pt(NH₃)₂} under denaturing conditions. Up to three different conformations appear to be generated upon platination depending on the charge state. The collision cross-sections (Ω) for each conformation indicate that the conformations of the platinated protein are contracted in size compared with unmodified ubiquitin with generally smaller Ω values. Ion mobility-tandem MS allowed determination of the platinum binding site without a requirement for prior Chromatographic separation. A rapid 30-min digestion of cisplatin-modified ubiquitin with trypsin allowed the platination site to be identified as the N-terminal methionine following low-energy collision-induced dissociation (CID) studies of the modified peptide. The data were generated using a Traveling-Wave based ion mobility-MS approach. Such cisplatin-induced shape changes may have a significant effect on its function in vivo. This work highlights the usefulness of the ion-mobility mass spectrometry technique for shedding new light on such protein interactions.     

Phase equilibria of clathrate hydrates of methane + carbon dioxide: New experimental data and predictions

In this communication, we report experimental dissociation conditions for region clathrate hydrates of methane + carbon dioxide in gas–liquid water–hydrate (G–Lw–H) equilibrium. The temperature and pressure conditions are in the range of (279.1–289.9) K and (2.96–13.06) MPa, respectively. Concentrations of carbon dioxide in the feed gas are also varied. An isochoric pressure-search method was used to perform the measurements. The dissociation data generated in this work along with the literature data are compared with the predictions of a thermodynamic model and a previously reported empirical equation. A discussion is made on the deviations between the experimental and predicted data.     

The importance of valencep functions in the bonding of Na2, K2, and Cu2 and their positive and negative ions

Summary The relative importance of the valencep functions for describing the bonding in the valence isoelectronic Na₂, K₂, and Cu₂ molecules and their positive and negative ions is investigated. In absolute magnitude the contribution of thep functions to the dissociation energy follows the trend Cu>Na>K while by percentage of the dissociation energy the importance of thep functions follows the polarizabilities, i.e. K>Na>Cu. The bonding in K₂, K ₂ ⁺ , and K ₂ ^– is analyzed to explain the observed trends.Dedicated to Prof. Klaus Ruedenberg     

A model of the chemical processes occurring in CF4/O2 discharges used in plasma etching

A model has been developed in an attempt to explain the chemistry which occurs in plasmas produced in mixtures of CF₄ and O₂. Emphasis is placed on gas-phase free radical reactions, and the predictions of the model are compared with experimental results. Dissociation rates following electron impact are deduced mainly from experimental observations although relative dissociation rates have been calculated. An important assumption of the model is that CF₂ can be produced as a primary dissociation product following electron impact. Furthermore, this process is favored over that producing CF₃ by more than a factor of 2. Experimental evidence is presented to support this assumption. Although the model agrees well with experiment on the total amount of fluorine produced, some discrepancy exists between the predicted and measured values of [F₂]. It is suggested that the higher concentrations detected in the experiments resulted from recombination of F atoms in the sampling region. The agreement for concentrations of CO₂, CO, and COF₂ is generally better than a factor of 2 over a wide range of experimental conditions.     

Measuring internal energy deposition in collisional activation using hydrated ion nanocalorimetry to obtain peptide dissociation energies and entropies

The internal energy deposited in both on- and off-resonance collisional activation in Fourier transform ion cyclotron resonance mass spectrometry is measured with ion nanocalorimetry and is used to obtain information about the dissociation energy and entropy of a protonated peptide. Activation of Na⁺(H₂O)₃₀ results in sequential loss of water molecules, and the internal energy of the activated ion can be obtained from the abundances of the product ions. Information about internal energy deposition in on-resonance collisional activation of protonated peptides is inferred from dissociation data obtained under identical conditions for hydrated ions that have similar m/z and degrees-of-freedom. From experimental internal energy deposition curves and Rice-Ramsperger-Kassel-Marcus (RRKM) theory, dissociation data as a function of collision energy for protonated leucine enkephalin, which has a comparable m/z and degrees-of-freedom as Na⁺(H₂O)₃₀, are modeled. The threshold dissociation energies and entropies are correlated for data acquired at a single time point, resulting in a relatively wide range of threshold dissociation energies (1.1 to 1.7 eV) that can fit these data. However, this range of values could be significantly reduced by fitting data acquired at different dissociation times. By measuring the internal energy of an activated ion, the number of fitting parameters necessary to obtain information about the dissociation parameters by modeling these data is reduced and could result in improved accuracy for such methods.     

Diffusion technique for the generation of gaseous halogen standards

Halogens are known to play an important role in the tropospheric ozone-depletion chemistry and are of special interest because of their influence on the atmospheric oxidation capacity. In this paper, we investigate the application of a capillary diffusion technique for the generation of gaseous halogen standards like Br₂, IBr, ICl and I₂. The influence of capillary dimension (i.e. length and inner diameter), ambient pressure and headspace volume of the diffusion vessel on the test gas output has been evaluated. The experimental output rates are determined from the mass loss of the analyte vessel on a regular schedule and compared with their respective theoretical predictions. We also demonstrate that a 1,3,5-trimethoxybenzene-coated diffusion denuder is capable of collecting gaseous ICl quantitatively, which provides an attractive alternative for the rapid determination of the output of test gas devices. The output rates of ICl measured by the denuder method are in close agreements with the data obtained by the gravimetric method.     

Synthesis,Characterization and Application of Amine-Functionalized Hierarchically Micro-Mesoporous Silicon Composites for CO2 Capture in Flue Gas

An efficient CO₂ adsorbent with a hierarchically micro-mesoporous structure and a large number of amine groups was fabricated by a two-step synthesis technique. Its structural properties, surface groups, thermal stability and CO₂ adsorption performance were fully investigated. The analysis results show that the prepared CO₂ adsorbent has a specific hierarchically micro-mesoporous structure and highly uniformly dispersed amine groups that are favorable for the adsorption of CO₂. At the same time, the CO₂ adsorption capacity of the prepared adsorbent can reach a maximum of 3.32 mmol-CO₂/g-adsorbent in the actual flue gas temperature range of 303–343 K. In addition, the kinetic analysis results indicate that both the adsorption process and the desorption process have rapid adsorption/desorption rates. Finally, the fitting of the CO₂ adsorption/desorption experimental data by Avrami’s fractional kinetic model shows that the CO₂ adsorption rate is mainly controlled by the intra-particle diffusion rate, and the temperature has little effect on the adsorption rate.     

A Study of C5H10 radical cations by collisionally activated dissociation,charge stripping,and low energy ion-molecule reactions

C₅H₁₀ radical cations generated from a variety of olefins and cycloalkanes were investigated by collisionally activated dissociation, charge stripping, and low energy ion-molecule reactions. It has been determined that all of the isomers studied can be distinguished by charge stripping, whereas collisionally activated dissociation and the ion-molecule reactions are less informative. The radical cations from cyclopentane, methylcyclobutane and substituted cyclopropanes retain their cyclic structures for at least a fraction of the population having lifetimes in the microsecond range.