Kinetics following addition of sulfur fluorides to a weakly ionized plasma from 300 to 500 K: rate constants and product determinations for ion-ion mutual neutralization and thermal electron attachment to SF5, SF3, and SF2

Rate constants for several processes including electron attachment to SF(2), SF(3), and SF(5) and individual product channels of ion-ion mutual neutralization between SF(6)(-), SF(5)(-), and SF(4)(-) with Ar(+) were determined by variable electron and neutral density attachment mass spectrometry. The experiments were conducted with a series of related neutral precursors (SF(6), SF(4), SF(5)Cl, SF(5)C(6)H(5), and SF(3)C(6)F(5)) over a temperature range of 300-500 K. Mutual neutralization rate constants for SF(6)(-), SF(5)(-), and SF(4)(-) with Ar(+) are reported with uncertainties of 10-25% and show temperature dependencies in agreement with the theoretical value of T(-0.5). Product branching in the mutual neutralizations is temperature independent and dependent on the electron binding energy of the anion. A larger fraction of product neutrals from the SF(6)(-) mutual neutralization (0.9 ± 0.1) are dissociated than in the SF(5)(-) mutual neutralization (0.65 ± 0.2), with the SF(4)(-) (0.7 ± 0.3) likely lying in between. Electron attachment to SF(5) (k = 2.0 × 10(-8) ±(1)(2) cm(3) s(-1) at 300 K) and SF(3) (4 ± 3 × 10(-9) cm(3) s(-1) at 300 K) show little temperature dependence. Rate constants of electron attachment to closed-shell SF(n) species decrease as the complexity of the neutral decreases.     

Enhancement of rate of mutual neutralization by an ambient gas

Computer simulated experiments are reported which demonstrate that the rate of mutual neutralization in an ambient gas may be greatly enhanced by the bound ion pairs formed. The contributing energy range (expressed in units of kθ where θ is the temperature) is remarkably wide.     

Observation of dihalide elimination upon electron attachment to oxalyl chloride and oxalyl bromide, 300-550 K

Rate coefficients have been measured for electron attachment to oxalyl chloride [ClC(O)C(O)Cl] and oxalyl bromide [BrC(O)C(O)Br] in He gas at 133 Pa pressure over the temperature range of 300-550 K. With oxalyl chloride, the major ion product of attachment is Cl2(-) at all temperatures (66% at 300 K); its importance increases slightly as temperature increases. Two other product ions formed are Cl- (18% at 300 K) and the phosgene anion CCl2O- (16% at 300 K) and appear to arise from a common mechanism. With oxalyl bromide, the Br2(-) channel represents almost half of the ion product of attachment, independent of temperature. Br- accounts for the remainder. For oxalyl chloride, the attachment rate coefficient is small [(1.8 +/- 0.5) x 10(-8) cm3 s(-1) at 300 K], and increases with temperature. The attachment rate coefficient for oxalyl bromide [(1.3 +/- 0.4) x 10(-7) cm3 s(-1) at 300 K] is nearly collisional and increases only slightly with temperature. Stable parent anions C2Cl2O2(-) and C2Br2O2(-) and adduct anions Cl- (C2Cl2O2) and Br- (C2Br3O2) were observed but are not primary attachment products. G2 and G3 theories were applied to determine geometries of products and energetics of the electron attachment and ion-molecule reactions studied. Electron attachment to both oxalyl halide molecules leads to a shorter C-C bond and longer C-Cl bond in the anions formed. Trans and gauche conformers of the neutral and anionic oxalyl halide species have similar energies and are more stable than the cis conformer, which lies 100-200 meV higher in energy. For C2Cl2O2, C2Cl2O2(-), and C2Br2O2(-), the trans conformer is the most stable conformation. The calculations are ambiguous as to the oxalyl bromide geometry (trans or gauche), the result depending on the theoretical method and basis set. The cis conformers for C2Cl2O2 and C2Br2O2 are transition states. In contrast, the cis conformers of the anionic oxalyl halide molecules are stable, lying 131 meV above trans-C2Cl2O2(-) and 179 meV above trans-C2Br2O2(-). Chien et al. [J. Phys. Chem. A 103, 7918 (1999)] and Kim et al. [J. Chem. Phys. 122, 234313 (2005)] found that the potential energy surface for rotation about the C-C bond in C2Cl2O2 is "extremely flat." Our computational data indicate that the analogous torsional surfaces for C2Br2O2, C2Cl2O2(-), and C2Br2O2(-) are similarly flat. The electron affinity of oxalyl chloride, oxalyl bromide, and phosgene were calculated to be 1.91 eV (G3), and 2.00 eV (G2), and 1.17 eV (G3), respectively.     

Kinetics of electron attachment to OH and HNO3 and mutual neutralization of Ar+ with NO2(-) and NO3(-) at 300 and 500 K

The electron attachment rate constant to nitric acid (HNO(3)) has been measured in a flowing afterglow-Langmuir probe (FALP) apparatus at 300 and 500 K using three independent methods: the traditional FALP technique of monitoring electron depletion, "one-gas" VENDAMS (variable electron and neutral density attachment mass spectrometry), and "two-gas" VENDAMS. The three measurements are in agreement with a 300 K weighted average of 1.4 ± 0.3 × 10(-7) cm(3) s(-1), 2 to 10 times higher than previously reported values. Attachment is primarily dissociative yielding NO(2)(-) as previously reported, but for the first time a small endothermic channel to produce OH(-) was also observed at 500 K. From the one-gas VENDAMS data, associative attachment to the OH produced in the primary attachment was found to occur with an effective two body rate constant of 1.2±(0.7) (3)×10(-11) cm(3) s(-1) at 300 K, the first reported rate constant for this radical species. Finally, ion-ion neutralization rate constants of NO(2)(-) and NO(3)(-) with Ar(+) were determined to be 5.2±(2.5) (1.5) × 10(-8) and 4.5 ± 2.5 × 10(-8) cm(3) s(-1) at 300 K, respectively.     

Reaction of HO with glycolaldehyde, HOCH2CHO: rate coefficients (240-362 K) and mechanism

Absolute rate coefficients for the title reaction, HO+HOCH2CHO-->products (R1), were measured over the temperature range 240-362 K using the technique of pulsed laser photolytic generation of the HO radical coupled to detection by pulsed laser induced fluorescence. Within experimental error, the rate coefficient, k1, is independent of temperature over the range covered and is given by k1(240-362 K)=(8.0+/-0.8)x10(-12) cm3 molecule-1 s-1. The effects of the hydroxy substituent and hydrogen bonding on the rate coefficient are discussed based on theoretical calculations. The present results, which extend the database on the title reaction to a range of temperatures, indicate that R1 is the dominant loss process for HOCH2CHO throughout the troposphere. As part of this work, the absorption cross-section of HOCH2CHO at 184.9 nm was determined to be (3.85+/-0.2)x10(-18) cm2 molecule-1, and the quantum yield of HO formation from the photolysis of HOCH2CHO at 248 nm was found to be (7.0+/-1.5)x10(-2).     

The pressure and composition dependence of mutual diffusion in the system helium-nitrogen at 300 K

Diffusion coefficients for the system helium-nitrogen have been measured as a function of pressure at 300 K from 1 to 12.5 atmospheres. These measurements include composition dependences at 1, 3, 5 and 7 atmospheres. The results show reasonable agreement with the Thorne-Enskog theory for moderately dense gases.     

Site-specific rate coefficients for reaction of OH with ethanol from 298 to 900 K

The rate coefficients for reactions of OH with ethanol and partially deuterated ethanols have been measured by laser flash photolysis/laser-induced fluorescence over the temperature range 298-523 K and 5-100 Torr of helium bath gas. The rate coefficient, k(1.1), for reaction of OH with C(2)H(5)OH is given by the expression k(1.1) = 1.06 × 10(-22)T(3.58)?exp(1126/T) cm(3) molecule(-1) s(-1), and the values are in good agreement with previous literature. Site-specific rate coefficients were determined from the measured kinetic isotope effects. Over the temperature region 298-523 K abstraction from the hydroxyl site is a minor channel. The reaction is dominated by abstraction of the α hydrogens (92 ± 8)% at 298 K decreasing to (76 ± 9)% with the balance being abstraction at the β position where the errors are 2σ. At higher temperatures decomposition of the CH(2)CH(2)OH product from β abstraction complicates the kinetics. From 575 to 650 K, biexponential decays were observed, allowing estimates to be made for k(1.1) and the fractional production of CH(2)CH(2)OH. Above 650 K, decomposition of the CH(2)CH(2)OH product was fast on the time scale of the measured kinetics and removal of OH corresponds to reaction at the α and OH sites. The kinetics agree (within ±20%) with previous measurements. Evidence suggests that reaction at the OH site is significant at our higher temperatures: 47-53% at 865 K.     

The pressure dependence of the mutual diffusion coefficients of binary mixtures of helium and six other gases at 300 K: tests of Thorne's equation

A Loschmidt cell has been used to measure the pressure dependence, at constant composition and temperature, for six systems consisting of binary mixtures of helium. The results are compared with the Thorne—Enskog theory for moderately dense gases of rigid spheres, and an extension to the theory suggested.     

The pressure dependence of the mutual diffusion coefficients of binary mixtures of helium with ten other gases at 300 K: tests of Thorne's equation

The pressure dependences of the binary diffusion coefficients of ten systems containing helium have been measured at constant composition at 300 K. The results are compared with the Thorne—Enskog theory for moderately dense gases of rigid spheres, and a previously suggested empirical extension to the theory tested.     

Measurement of mutual diffusion coefficients,densities, viscosities,and osmotic coefficients for the system KSCN-H2O at 25°C

Mutual diffusion coefficients measured on the volume-fixed frame of reference are reported for KSCN-H₂O at 25°C over the concentration range 0.0 to 10.26 mol-dm^–3. The diffusion coefficient at infinite dilution was obtained from limiting ionic equivalent conductances of K⁺ and SCN^–. Low concentration conductances of KSCN-H₂O at 25°C used to obtain the limiting ionic equivalent conductance of SCN^– are reported. Values of density and viscosity for this system are reported from 0.0 to 10.30 mol-dm^–3. Osmotic coefficienss of KSCN-H₂O at 25°C were measured by the isopiestic method. These are reported over the concentration range of 0.30 to 24.94 molal (saturation). Values of thermodynamic diffusion coefficients for the concentration range 0.0 to 10.26 mol-dm^–3 are tabulated. Results are compared to other potassium salts with monovalent anions at 25°C.     

Molecular dynamics simulation of self- and mutual diffusion coefficients for confined mixtures

The self- and mutual diffusion coefficients for binary mixtures of Ar-Kr both in the bulk and in the nanopores were studied by molecular dynamics simulations. The composition dependences and the relationships between the self- and the mutual diffusion coefficients both in the bulk and in the nanopores were further discussed. It was found that the simulation results (D(c.m.)) are close to the calculated ones (D(s)) for the Ar-Kr system. Both self- and mutual diffusion coefficients in nanopores are much lower than that of the bulk, and they ever decrease as the pore width decreases. Nevertheless, the self- and mutual diffusion coefficients increase as the mole fraction of Ar increases, and as expected, increase as the temperature increases. The self-diffusion coefficients of mixtures both in the bulk and in the nanopores are predicted by the Carman model and by the molecular cluster model.     

Thermodynamic Properties of Tetraphenyltetrahydroxycyclotetrasiloxane,Octaphenyltetrahydroxytricyclooctasiloxane, Octaphenylpentacyclosilsesquioxane,and Polyphenylsilsesquioxane in the Range 0-300 K

The temperature dependences of the heat capacities of crystalline tetraphenyltetrahydroxycyclotetrasiloxane, octaphenyltetrahydroxytricyclooctasiloxane, and octaphenylpentacyclosilsesquioxane and of glassy polyphenylsilsesquioxane were measured in the range 6-300 K with an adiabatic vacuum calorimeter, with an accuracy of 0.3%. From these data, the thermodynamic functions C ⁰ _p (T), H ⁰(T) - H ⁰(0), S ⁰(T) - S ⁰(0), and G ⁰(T) - H ⁰(0) of these substances were calculated for the range 0-300 K. The standard entropies of their formation from elements at 298.15 K, _f S ⁰, and the entropies of mutual transformations of these substances in the range 0-298.15 K were calculated.     

Electron attachment to SF5X compounds: SF5C6H5, SF5C2H3, S2F10, and SF5Br, 300-550 K

Rate constants and ion product channels have been measured for electron attachment to four SF5 compounds, SF5C6H5, SF5C2H3, S2F10, and SF5Br, and these data are compared to earlier results for SF6, SF5Cl, and SF5CF3. The present rate constants range over a factor of 600 in magnitude. Rate constants measured in this work at 300 K are 9.9+/-3.0x10(-8) (SF5C6H5), 7.3+/-1.8x10(-9) (SF5C2H3), 6.5+/-2.5x10(-10) (S2F10), and 3.8+/-2.0x10(-10) (SF5Br), all in cm3 s-1 units. SF5- was the sole ionic product observed for 300-550 K, though in the case of S2F10 it cannot be ascertained whether the minor SF4- and SF6- products observed in the mass spectra are due to attachment to S2F10 or to impurities. G3(MP2) electronic structure calculations (G2 for SF5Br) have been carried out for the neutrals and anions of these species, primarily to determine electron affinities and the energetics of possible attachment reaction channels. Electron affinities were calculated to be 0.88 (SF5C6H5), 0.70 (SF5C2H3), 2.95 (S2F10), and 2.73 eV (SF5Br). An anticorrelation is found for the Arrhenius A-factor with exothermicity for SF5- production for the seven molecules listed above. The Arrhenius activation energy was found to be anticorrelated with the bond strength of the parent ion.     

Unimolecular rate coefficients for very weak collisions

A known result for low-pressure thermal unimolecular rate coefficients in the presence of very weak colliders is that these coefficients depend on the detailed energy-transfer rate constants only through the total collision frequency and the average energy transferred per collision. It is shown that this result holds at all pressures both for thermal reactions and for reactions following chemical activation.     

Correlation of the mutual diffusion coefficients of binary liquid mixtures

A correlative UNIDIF model for the mutual diffusion coefficients of binary liquid mixtures is developed using statistical thermodynamics and the absolute reaction rate theory. In this model, a mole fraction average of the logarithm of the pure-component limiting diffusion coefficients is taken as a reference term. The model expresses the excess part of the diffusion coefficient relative to this reference term in a form similar to that of a UNIQUAC equation which comprises two parts due to the combinatorial and residual contributions. The combinatorial part depends on the molecular sizes and shapes. The residual part includes two binary interaction parameters, which are obtained from data regression, for each binary mixture. Mutual diffusion coefficients of nonpolar+nonpolar, nonpolar+polar and polar+polar fluid mixtures are correlated in this study. Optimal binary interaction parameters are presented. Correlation results using the UNIDIF model for mutual diffusion coefficient are satisfactory and are superior to those from other methods.     

Spectra of propylperoxy radicals and rate constants for mutual interaction

The flash photolyses of azo-n-propane and azoisopropane in the presence of oxygen have been studied by kinetic spectroscopy. The transient absorption spectra observed in the region of 210–290 nm are assigned to the n-propylperoxy and isopropylperoxy radicals. For the n-propylperoxy radical, ε_max = 1148 ± 29 L/mol cm at 242.5 nm and for the isopropylperoxy radical, ε_max = 1273 ± 75 L/mol cm at 240 nm. The rate constants for the mutual reactions (7) 2RO₂· → products were measured to be k₇ = (2.0 ± 0.2) X 10⁸ L/mol s for the n-propylperoxy radical and k₇ = (7.8 ± 2.2) X 10⁵ L/mol s for the isopropylperoxy radical.     

Ion-solvent and ion-ion interactions of phosphomolybdic acid in aqueous solution of catechol at 298.15, 308.15, and 318.15 K

Apparent molar volume (V _Ø) and viscosity B-coefficients were measured for phosphomolybdicacid in aqueous solution of catechol from solution density (ρ) and viscosity (η) at 298.15, 308.15, and 318.15 K at various solute concentrations. The experimental density data were evaluated by Masson equation and the derived data were interpreted in terms of ion-solvent and ion-ion interactions. The viscosity data have been analyzed using Jones-Dole equation and the derived parameters, B and A, have been interpreted in terms of ion-solvent and ion-ion interactions respectively. The structure-making or breaking capacity of the solute under investigation has been discussed in terms of sign of (δ ² V _Ø ^o /δT ²) _P . The activation parameters of viscous flow were determined and discussed by application of transition state theory.