New mechanism for the influence of Xe on the concentration of CO<Subscript>2</Subscript> molecules in self-sustained CO-laser discharges

The dependence of the CO₂ concentration on the discharge conditions and the mixture composition in a CO laser is studied experimentally. The experimental data are compared with the calculated results. A scheme of the reactions that govern the concentration of CO₂ molecules under the experimental conditions in question is constructed. It is shown that, in a gas-discharge plasma, an admixture of Xe in a mixture containing CO molecules gives rise to a new mechanism for the dissociation of CO₂ molecules by metastable xenon atoms. Under conditions close to the operating conditions of sealed-off CO lasers, the dissociation of CO₂ molecules in collisions with metastable. Xe(³P₂) atoms becomes the dominant dissociation mechanism in a He: CO mixture because it proceeds at a fast rate. This explains the observed decrease in the CO₂ concentration in a xenon-containing He: CO mixture.     

Effect of a small C3O2 additive on the vibrational distribution function of CO molecules in a low-temperature plasma

The concentration of carbon suboxide (C₃O₂) in the plasmas of sealed-off discharges in mixtures of CO with noble gases is measured for the first time by mass-spectroscopic technique. It is shown that the production of C₃O₂ (and, possibly, more complex carbon oxides) in a gas-discharge plasma significantly boosts the vibrational relaxation of CO molecules and thus greatly affects their vibrational populations. Adding xenon to a He: CO mixture reduces the concentration of C₃O₂. The effect of pulsed UV radiation on the vibrational populations of CO molecules is studied experimentally. It is shown that UV irradiation of the gas mixture after long-term discharge operation increases vibrational populations in the plateau region up to the values observed at the beginning of the discharge. This effect is attributed to the decay of C₃O₂ molecules under the action of UV radiation.     

Plasma chemistry of the sealed-off slab CO laser active medium pumped by radio-frequency discharge with liquid-nitrogen-cooled electrodes

The long-term time behavior of the output power of a sealed-off cryogenic slab CO laser pumped by a repetitively pulsed RF discharge and operating on the overtone (λ = 2.6–3.5 μm) vibrational?rotational transitions of the CO molecule was studied experimentally. It is shown that adding of an anomalously large amount of oxygen (up to 50% with respect to the CO concentration) to the initial gas mixture CO : He = 1 : 10 leads to a manyfold (by several tens of times) increase in the duration of the laser operating cycle (until lasing failure due to the degradation of the active medium). In this case, the laser life-time without replacement of the active medium reaches 10⁵–10⁶ pulses. Using various diagnostics (including luminescence spectroscopy and IR and UV absorption spectroscopy), regularities in the time-behavior of the concentrations of the main component of the active medium (CO molecules) and the products of plasmachemical reactions (O₃, CO₂) generated in the discharge gap during the laser operating cycle are revealed. Time correlation between the characteristics of the active medium and the laser output power are analyzed. A phenomenological approach to describing the entirety of plasmachemical, purely chemical, gas-dynamic, and diffusion processes determining the behavior of the laser output characteristics throughout the laser operating cycle is offered.     

Kinetics of the processes,plasma parameters,and output characteristics of a UV emitter operating on XeI molecules and iodine molecules and atoms

A kinetic model of the processes occurring in the plasma of a high-power low-pressure gas-discharge lamp is presented, and the output characteristics of the lamp are described. The lamp is excited by a longitudinal glow discharge and emits the I₂(D′-A′) 342-nm and XeI(B-X) 253-nm bands and the 206.2-nm spectral line of atomic iodine. When the emitter operates in a sealed-off mode on the p(He): p(Xe): p(I₂) = 400: 120: (100–200) Pa mixture, the fractions of the UV radiation power of iodine atoms, exciplex molecules of xenon iodide, and iodine molecules comprise 55, 10, and 35%, respectively. At the optimal partial pressure, the maximum total radiation power of the lamp reaches 37 W, the energy efficiency being about 15%.     

On the significance of hydrogen bonds for the discrimination between CO and O2 by myoglobin

 Quantum chemical geometry optimisations have been performed on realistic models of the active site of myoglobin using density functional methods. The energy of the hydrogen bond between the distal histidine residue and CO or O₂ has been estimated to be 8 kJ/mol and 32 kJ/mol, respectively. This 24 kJ/mol energy difference accounts for most of the discrimination between CO and O₂ by myoglobin (about 17 kJ/mol). Thus, steric effects seem to be of minor importance for this discrimination. The Fe—C and C—O vibrational frequencies of CO-myoglobin have also been studied and the results indicate that CO forms hydrogen bonds to either the distal histidine residue or a water molecule during normal conditions. We have made several attempts to optimise structures with the deprotonated nitrogen atom of histidine directed towards CO. However, all such structures lead to unfavourable interactions between the histidine and CO, and to ν_CO frequencies higher than those observed experimentally. Received: 7 July 1998 / Accepted: 26 October 1998     

Comparison of CO<Subscript>2</Subscript> and H<Subscript>2</Subscript>O fluxes over grassland vegetations measured by the eddy-covariance technique and by open system chamber

Measurements of CO₂ and H₂O fluxes were carried out using two different techniques—eddy-covariance (EC) and open system gas exchange chamber (OC)—during two-years’ period (2003–2004) at three different grassland sites. OC measurements were made during fourteen measurement campaigns. We found good agreement between the OC and EC CO₂ flux values (n = 63, r ² = 0.5323, OC F_CO2 = −0.6408+0.9508 EC F_CO2). The OC F_H2O values were consistently lower than those measured by the EC technique, probably caused by the air stream difference inside and outside the chamber. Adjusting flow rate within the chamber to the natural conditions would be necessary in future OC measurements. In comparison with EC, the OC proved to be a good tool for gas exchange measurements in grassland ecosystems.     

Vibrational spectra and structure of M(CO2) and M2(CO2) molecules

Atomic Na, K and Cs were codeposited with CO₂ in excess of matrix gas at the temperature of 12 K. The IR spectra revealed the presence of ionic aggregates corresponding to the molecules M(CO)₂ and M₂(CO₂) (M=Na, K, Cs). Both molecular species have C_2v symmetry; M(CO₂) species have a planar ring structure while M₂(CO₂) have a W-shape structure. M₂(CO₂) molecules with C_s symmetry were also identified. The geometrical parameters of all the molecules were determined by ¹²C/¹³C and ¹⁶O/¹⁸O isotopic shifts. Raman spectra were also recorded and the results are reported in this study. The effect of photolysis on the structure of these molecules was examined. It was determined that photolysis promotes the formation of Na(CO₂) and transforms the M₂(CO₂) molecules with C_2v symmetry into C_s symmetry isomers.     

Catalytic mechanisms of Au<Subscript>11</Subscript> and Au<Subscript>11-n</Subscript>Pt<Subscript>n</Subscript> (<Emphasis Type="Italic">n</Emphasis>=1–2) clusters: a DFT investigation on the oxidation of CO by O<Subscript>2</Subscript>

The oxidation of CO catalyzed by clusters of Au₁₁, Au₁₀Pt and Au₉Pt₂ was investigated using the M06 functional suite of the density functional theory. Au and Pt atoms were described with the double-ζ valence basis set Los Alamos National Laboratory 2-double-z (LanL2DZ), whereas the standard 6-311++G(d,p) basis set was employed for the C and O atoms. Our theoretical model showed that (1) after coordination to Au and Au-Pt cluster, O₂ and CO are apparently activated, and Mulliken charges show that the gold atoms in the active sites of Au11 are negatively charged; (2) Au-Pt clusters with 11 atoms can effectively catalyze the oxidation of CO by O₂; (3) Au₁₁ exhibits good catalytic performance for the oxidation of CO; (4) oxidation of CO occurs preferably on the Au–Pt active sites in Pt-doped clusters, and the single-center mechanisms are more favorable energetically than the two-center mechanisms; (5) after adsorption, an O₂ molecule oxidates two CO molecules via stepwise mechanisms; and (6) the catalytic processes are highly exothermic.     

Kinetics of Atomic Recombination on Silicon Samples in Chlorine Plasma

The recombination kinetics of chlorine atoms on the wall of a plasmachemical reactor and on silicon samples in the positive column of a glow discharge in Cl₂ has been studied experimentally. The rate constants and probabilities of the heterogeneous recombination of chlorine atoms on the plasma limiting surfaces, as well as of the chemical interaction of chlorine atoms with silicon, are calculated. The temperature and time dependences of the probabilities of the chemical interaction of chlorine atoms with silicon are analyzed, and optimal conditions for conducting pulse relaxation experiments are determined.     

Why are dimethyl sulfoxide and dimethyl sulfone such good solvents?

We have carried out B3PW91 and MP2-FC computational studies of dimethyl sulfoxide, (CH₃)₂SO, and dimethyl sulfone, (CH₃)₂SO₂. The objective was to establish quantitatively the basis for their high polarities and boiling points, and their strong solvent powers for a variety of solutes. Natural bond order analyses show that the sulfur–oxygen linkages are not double bonds, as widely believed, but rather are coordinate covalent single S⁺→O⁻ bonds. The calculated electrostatic potentials on the molecular surfaces reveal several strongly positive and negative sites (the former including σ-holes on the sulfurs) through which a variety of simultaneous intermolecular electrostatic interactions can occur. A series of examples is given. In terms of these features the striking properties of dimethyl sulfoxide and dimethyl sulfone, their large dipole moments and dielectric constants, their high boiling points and why they are such good solvents, can readily be understood. Figure Dimers of dimethyl sulfoxide (DMSO; left) and dimethyl sulfone (DMSO2; right) showing O S—O -hole bonding and C H—O hydrogen bonding. Sulfur atoms are yellow, oxygens are red, carbons are gray and hydrogens are white     

New method of respiratory gas analysis: light spectrometer

A multigas concentration analyzer particularly suited for respiratory gas analysis has been developed using a new principle based on the measurement of the intensity of light emitted by excited atoms or ions in a direct current glow discharge. This glow discharge spectral emission gas analyzer (GDSEA), or light spectrometer, simultaneously measures O2, N2, CO2, He, and N2O gas concentrations with a 0-90% response time of 100 ms and a sample rate of less than 20 ml/min in a short gas sample line configuration. Mole accuracy and resolution of the GDSEA using a short sample line were determined in the laboratory to be +/- 0.15 to +/- 0.7% and 0.02-0.05%, respectively. In the clinical setting a comparative evaluation was made with a mass spectrometer in a long sample line, computerized, multibed, respiratory monitoring system. Results indicate a close agreement between the two instruments with differences in mixed inspiratory or expiratory O2 and CO2 concentrations of less than 2% and of derived variables, such as O2 consumption, CO2 production, and respiratory exchange ratio, of less than 5%.     

Myoglobin models and steric origins of the discrimination between O2 and CO

 Synthetic models of the myoglobin active site have provided much insight into factors that affect CO and O₂ binding in the proteins. "Capped" and "pocket" metal porphyrin systems have been developed to probe how steric factors affect ligand binding and ultimately to elucidate important aspects of the mechanism of CO discrimination in the proteins. These model porphyrins are among the most thoroughly characterized systems to date. From the twenty-one known crystal structures, analysis of the types of distortion that occur upon ligand binding under the cap, including porphyrin doming and ruffling, lateral and horizontal movement of the cap, and bending and tilting of the Fe–C–O bond, provides an indication of how steric interactions will affect structure in Hb and Mb. The model porphyrin systems discussed range from those that discriminate against O₂ binding compared to biological systems to those with similar CO and O₂ binding strength to myoglobin, and also to those that bind both O₂ and CO very weakly or not at all. The primary type of distortion observed upon CO binding is vertical or lateral movement of the cap and some ruffling of the porphyrin plane. Minimal bending or tilting of the M–C–O bond is observed, suggesting that the Fe–C–O bending that has been found from crystal structures of the hemoproteins is unlikely. Received, accepted: 23 May 1997     

Rates of fixation by lightning of carbon and nitrogen in possible primitive atmospheres

A thermochemical-hydrodynamic model of the production of trace species by electrical discharges has been used to estimate the rates of fixation of C and N by lightning in the primitive atmosphere. Calculations for various possible mixtures of CH4, CO2, CO, N2, H2, and H2O reveal that the prime species produced were probably HCN and NO and that the key parameter determining the rates of fixation was the ratio of C atoms to O atoms in the atmosphere. Atmospheres with C more abundant than O have large HCN fixation rates, in excess of 10(17) molecules J-1, but small NO yields. However, when O is more abundant than C, the NO fixation rate approaches 10(17) molecules J-1 while the HCN yield is small. The implications for the evolution of life are discussed.     

Carbon and energy yields in prebiotic syntheses using atmospheres containing CH4, CO and CO2

Yields based on carbon are usually reported in prebiotic experiments, while energy yields (moles cal^–1) are more useful in estimating the yields of products that would have been obtained from the primitive atmosphere of the earth. Energy yields for the synthesis of HCN and H₂CO from a spark discharge were determined for various mixtures of CH₄, CO, CO₂, H₂, H₂O, N₂ and NH₃. The maximum yields of HCN and H₂CO from CH₄, CO, and CO₂ as carbon sources are about 4×10^–8 moles cal^–1.     

Structure determination of proteins in <Superscript>2</Superscript>H<Subscript>2</Subscript>O solution aided by a deuterium-decoupled 3D HCA(N)CO experiment

We developed an NMR pulse sequence, 3D HCA(N)CO, to correlate the chemical shifts of protein backbone ¹Hα and ¹³Cα to those of ¹³C′ in the preceding residue. By applying ²H decoupling, the experiment was accomplished with high sensitivity comparable to that of HCA(CO)N. When combined with HCACO, HCAN and HCA(CO)N, the HCA(N)CO sequence allows the sequential assignment using backbone ¹³C′ and amide ¹⁵N chemical shifts without resort to backbone amide protons. This assignment strategy was demonstrated for ¹³C/¹⁵N-labeled GB1 dissolved in ²H₂O. The quality of the GB1 structure determined in ²H₂O was similar to that determined in H₂O in spite of significantly smaller number of NOE correlations. Thus this strategy enables the determination of protein structures in ²H₂O or H₂O at high pH values.     

Structures and electron affinities of triatomic molecules consisting of Al, P and X (X = B, Al, Ga; C, Si, Ge; N, P, As; O, S and Se)

The structures and electronic properties of the triatomic molecules containing Al, P, X atoms (X = B, Al, Ga; C, Si, Ge; N, P, As; O, S and Se) and their anions are investigated at the B3LYP/cc-PVTZ and the B3LYP/aug-cc-PVTZ levels. The results show that the most stable structures of the anions are AlXP⁻ (X = B, C, N) and PAlX⁻ (X = S, Se), while for the neutral molecules, the most stable structures are PXAl (X = C, N and O). The order of the VDEs of the anions molecules and the AEAs of the neutral species are C < N < O < Si ≈ Ge < P ≈ As < Al = Ga < B < S ≈ Se and C < O < N < Si ≈ Ge < P ≈ As < B < Al ≈ Ga < S ≈ Se, respectively.     

Investigation of the mechanism for rapid heating of nitrogen and air in gas discharges

A rapid heating of nitrogen-oxygen mixtures excited by gas discharges is investigated numerically with allowance for the following main processes: the reactions of predissociation of highly excited electronic states of oxygen molecules (which are populated via electron impact or via the quenching of the excited states of N₂ molecules), the reactions of quenching of the excited atoms O(¹ D) by nitrogen molecules, the VT relaxation reactions, etc. The calculated results adequately describe available experimental data on the dynamics of air heating in gas-discharge plasmas. It is shown that, over a broad range of values of the reduced electric field E/N, gas heating is maintained by a fixed fraction of the discharge power that is expended on the excitation of the electronic degrees of freedom of molecules (for discharges in air, η_E?28%). The lower the oxygen content of the mixture, the smaller the quantity η_E. The question of a rapid heating of nitrogen with a small admixture of oxygen is discussed.     

Development of a nonequilibrium microwave discharge at the end of a cylindrical electrode in nitrogen at reduced pressures

Excitation of a microwave discharge at the end of a cylindrical electrode in nitrogen at a pressure of 1 Torr and incident powers of 60–140 W was investigated experimentally by using K-008 and K-011 video cameras and analyzing oscillograms of discharge emission. The times during which the discharge is established in the radial and axial directions are found to be on the order of 10⁻⁴ and 10⁻² s, respectively. The results obtained are analyzed using one-dimensional simulations of a discharge in nitrogen in a quasistatic approximation. The kinetic scheme includes 50 processes involving electrons, ions, and excited molecules and atoms. The time evolution of the concentrations of molecular nitrogen in the N₂(C ³II _u ) and N₂(B ³II _g ) states, responsible for the recorded discharge emission, is compared with the experimental data.     

Prebiotic synthesis in atmospheres containing CH4, CO,and CO2

The prebiotic synthesis of organic compounds using a spark discharge on various simulated primitive earth atmospheres at 25 degrees C has been studied. Methane mixtures contained H2 + CH4 + H2O + N2 + NH3 with H2/CH4 molar ratios from 0 to 4 and pNH3 = 0.1 torr. A similar set of experiments without added NH3 was performed. The yields of amino acids (1.2 to 4.7% based on the carbon) are approximately independent of the H2/CH4 ratio and whether NH3 was present, and a wide variety of amino acids are obtained. Mixtures of H2 + CO + H2O + N2 and H2 + CO2 + H2O + N2, with and without added NH3, all gave about 2% yields of amino acids at H2/CO and H2/CO2 ratios of 2 to 4. For a H2/CO2 ratio of 0, the yield of amino acids is extremely low (10(-3)%). Glycine is almost the only amino acid produced from CO and CO2 model atmospheres. These results show that the maximum yield is about the same for the three carbon sources at high H2/carbon ratios, but that CH4 is superior at low H2/carbon ratios. In addition, CH4 gives a much greater variety of amino acids than either CO or CO2. If it is assumed that an abundance of amino acids more complex than glycine was required for the origin of life, then these results indicate the requirement for CH4 in the primitive atmosphere.