Isotopic self-exchange reactions of water: evaluation of the rule of the geometric mean in liquid-vapor isotope partitioning

Deviations from the random distribution of hydrogen isotopes among isotopic species of liquid and vapor water (the rule of the geometric mean) were critically assessed theoretically and experimentally from the triple to critical point of water. A third-order polynomial equation of the classical near-critical expansion was used to accurately describe the liquid-vapor isotope fractionation of H2O and D2O on the basis of their equations of state. It was found that experimental data for the enthalpy of mixing of H2O-D2O can be used to calculate accurately the deviation from the rule of the geometric mean in liquid and vapor water, ln(KD(v)/KD(l)). A new equation obtained in this study shows that the value of ln(KD(v)/KD(l)) smoothly decreases from +0.009 to 0 with increasing temperature from the triple to critical temperature of water. In contrast, the equation available in the literature and that derived from mass spectrometric measurements of liquid-vapor partitioning of H2O and HDO show complex behavior, including maximum, minimum, and crossover.     

Totally inclusive cubic equation of state with five parameters for pure fluids

A totally inclusive cubic equation of state (cubic EOS) is proposed. Although, its form is fairly simple as compared with the present cubic equations, it can include all of them as special cases. The EOS has five parameters. By fitting the experimental critical isothermal for six typical substances combining the critical conditions, the generalized expressions for the five parameters at critical temperature are established. The temperature coefficients of the five parameters for 43 substances are determined by fitting the experimental data of vapor pressure and saturated liquid density. These coefficients are correlated with the critical compressibility factor and acentric factor to obtain the generalized expressions. The predicted saturated vapor pressure, saturated liquid density, critical isothermal and coexistence curve near the critical point show that the equation gives the best results when compared with the Redlich–Kwong–Soave (RKS) and Peng–Robinson (PR) EOS.     

Isotope fractionation factors of N2O diffusion

Isotopic signatures of N2O are increasingly used to constrain the total global flux and the relative contribution of nitrification and denitrification to N2O emissions. Interpretation of isotopic signatures of soil-emitted N2O can be complicated by the isotopic effects of gas diffusion. The aim of our study was to measure the isotopic fractionation factors of diffusion for the isotopologues of N2O and to estimate the potential effect of diffusive fractionation during N2O fluxes from soils using simple simulations. Diffusion experiments were conducted to monitor isotopic signatures of N2O in reservoirs that lost N2O by defined diffusive fluxes. Two different mathematical approaches were used to derive diffusive isotope fractionation factors for 18O (epsilon18O), average 15N (epsilonbulk) and 15N of the central (alpha(-)) and peripheral (beta(-)) position within the linear N2O molecule (epsilon15Nalpha, epsilon15Nbeta). The measured epsilon18O was -7.79 +/- 0.27 per thousand and thus higher than the theoretical value of -8.7 per thousand. Conversely, the measured epsilonbulk (-5.23 +/- 0.27 per thousand) was lower than the theoretical value (-4.4 per thousand). The measured site-specific 15N fractionation factors were not equal, giving a difference between epsilon15Nalpha and epsilon15Nbeta (epsilonSP) of 1.55 +/- 0.28 per thousand. Diffusive fluxes of the N2O isotopologues from the soil pore space to the atmosphere were simulated, showing that isotopic signatures of N2O source pools and emitted N2O can be substantially different during periods of non-steady state fluxes. Our results show that diffusive isotope fractionation should be taken into account when interpreting natural abundance isotopic signatures of N2O fluxes from soils.     

On the compatibility between vapor pressure data and the critical constants: Use of the van der Waals family of cubic equations of state to study the cases of 2-methoxyethanol and 2-ethoxyethanol

The performance of five cubic equations of state (EOSs) for the correlation of vapor-pressure data of 2-methoxyethanol and 2-ethoxyethanol are compared. The cubic EOSs considered are the van der Waals EOS, modified with Soave's approach (MvdW), the Soave-Redlich-Kwong (SRK), the Peng-Robinson (PR), and the two versions of the Peng-Robinson-Stryjek-Vera EOSs: PRSV and PRSV2. Three sets of critical constants for 2-methoxyethanol are considered and new acentric factor is proposed for this compound. New critical constants and acentric factor for 2-ethoxyethanol are proposed. Pure compound parameters for the PRSV and the PRSV2 EOSs for 2-methoxyethanol and 2-ethoxyethanol are evaluated using a genetic algorithm (GA) optimization technique. The results show the importance of using compatible information in vapor liquid equilibrium studies.     

Direct examination of H2O2 activation by a heme peroxidase

Described here is the application of oxygen isotope fractionation together with computational methods, to elucidate a mechanism of enzymatic H2O2 activation. Horseradish peroxidase (HRP) has been the subject of intensive experimental and computational studies, yet questions remain as to the reversibility of the O-O cleavage step. New insight is afforded by the competitive oxygen kinetic isotope effect (18O KIE) upon H2O2 consumption determined under turnover conditions. The 18O KIE is compared to isotope effects calculated for the O-O heterolysis transition state and potential intermediates using density functional theory. In addition, experiments in enriched water provide evidence for HRP-catalyzed scrambling of the 18O label into the unreacted H2O2. The results provide an unprecedented view of H2O2 activation by a heme peroxidase and challenge the assumption of rate-limiting O-O heterolysis.     

Oxygen kinetic isotope effects upon catalytic water oxidation by a monomeric ruthenium complex

Oxygen isotope fractionation is applied for the first time to probe the catalytic oxidation of water using a widely studied ruthenium complex, [Ru(II)(tpy)(bpy)(H(2)O)](ClO(4))(2) (bpy = 2,2'-bipyridine; tpy = 2,2';6",2"-terpyridine). Competitive oxygen-18 kinetic isotope effects ((18)O KIEs) derived from the ratio of (16,16)O(2) to (16,18)O(2) formed from natural-abundance water vary from 1.0132 ± 0.0005 to 1.0312 ± 0.0004. Experiments were conducted with cerium(IV) salts at low pH and a photogenerated ruthenium(III) tris(bipyridine) complex at neutral pH as the oxidants. The results are interpreted within the context of catalytic mechanisms using an adiabatic formalism to ensure the highest barriers for electron-transfer and proton-coupled electron-transfer steps. In view of these contributions, O-O bond formation is predicted to be irreversible and turnover-limiting. The reaction with the largest (18)O KIE exhibits the greatest degree of O-O coupling in the transition state. Smaller (18)O KIEs are observed due to multiple rate-limiting steps or transition-state structures which do not involve significant O-O motion. These findings provide benchmarks for systematizing mechanisms of O-O bond formation, the critical step in water oxidation by natural and synthetic catalysts. In addition, the measurements introduce a new tool for calibrating computational studies using relevant experimental data.     

On the electron-induced isotope fractionation in low temperature (32)O(2)/(36)O(2) ices--ozone as a case study

The formation of six ozone isotopomers and isotopologues, (16)O(16)O(16)O, (18)O(18)O(18)O, (16)O(16)O(18)O, (18)O(18)O(16)O, (16)O(18)O(16)O, and (18)O(16)O(18)O, has been studied in electron-irradiated solid oxygen (16)O(2) and (18)O(2) (1?∶?1) ices at 11 K. Significant isotope effects were found to exist which involved enrichment of (18)O-bearing ozone molecules. The heavy (18)O(18)O(18)O species is formed with a factor of about six higher than the corresponding (16)O(16)O(16)O isotopologue. Likewise, the heavy (18)O(18)O(16)O species is formed with abundances of a factor of three higher than the lighter (16)O(16)O(18)O counterpart. No isotope effect was observed in the production of (16)O(18)O(16)O versus(18)O(16)O(18)O. Such studies on the formation of distinct ozone isotopomers and isotopologues involving non-thermal, non-equilibrium chemistry by irradiation of oxygen ices with high energy electrons, as present in the magnetosphere of the giant planets Jupiter and Saturn, may suggest that similar mechanisms may contribute to the (18)O enrichment on the icy satellites of Jupiter and Saturn such as Ganymede, Rhea, and Dione. In such a Solar System environment, energetic particles from the magnetospheres of the giant planets may induce non-equilibrium reactions of suprathermal and/or electronically excited atoms under conditions, which are quite distinct from isotopic enrichments found in classical, thermal gas phase reactions.     

Deuterium and oxygen-18 determination of microliter quantities of a water sample using an automated equilibrator

We describe a modified version of the equilibration method and a correction algorithm for isotope ratio measurements of small quantities of water samples. The deltaD and the delta(18)O of the same water sample can both be analyzed using an automated equilibrator with sample sizes as small as 50 microL. Conventional equilibration techniques generally require water samples of several microL. That limitation is attributable mainly to changes in the isotope ratio ((18)O/(16)O or D/H) of water samples during isotopic exchange between the equilibration gas (CO(2) or H(2)) and water, and therefore the technique for microL quantities of water requires mass-balance correction using the water/gas (CO(2) or H(2)) mole ratio to correct this isotopic effect. We quantitatively evaluate factors controlling the variability of the isotopic effect due to sample size. Theoretical consideration shows that a simple linear equation corrects for the effects without determining parameters such as isotope fractionation factors and water/gas mole ratios. Precisions (1-sigma) of 50-microL meteoric water samples whose isotopic compositions of -1.4 to -396.2 per thousand for deltaD are +/-0.5 to +/-0.6 per thousand, and of -0.37 to -51.37 per thousand for delta(18)O are +/-0.01 to +/-0.11 per thousand.     

Density functional theory analysis of molybdenum isotope fractionation

Analytical studies have found an enrichment of the lighter Mo isotopes in oxic marine sediments compared to seawater, with isotope fractionation factors of -1.7 to -2.0 per thousand for Delta97/95Mosediment-seawater. These data place constraints on the possible identities of dissolved and adsorbed species because the equilibrium isotope fractionation depends on the energy differences between the isotopomers of the adsorbed species, minor dissolved species, and the dominant solution species, MoO42-. Adsorption likely involves molybdic acid, whose structure is indicated by previous studies to be MoO3(H2O)3. Here we used DFT calculations of vibrational frequencies to determine the isotope fractionation factors versus MoO42-. The results indicate that isotope equilibration of MoO42- with MoO3(H2O)3, yielding Delta97/95Momolybdic acid-molybdate=-1.33 per thousand, is most likely responsible for the isotope fractionation of Mo between oxic sediments and seawater. The difference between the calculated value of Delta97/95Momolybdic acid-molybdate for MoO3(H2O)3 and the value observed in natural sediments and experiments is probably due to effects of solvation and adsorption onto the manganese oxyhydroxide surface.     

Photo-oxidation of water by molecular oxygen: isotope exchange and isotope effects

Photolysis of (17,18)O-labeled water in the presence of molecular oxygen is accompanied by transfer of (17)O and (18)O isotopes from water to oxygen, demonstrating that photoinduced oxidation of water does occur. The reaction exhibits the following isotope effect: oxidation of H(2)(17)O is faster by 2.6% (in the Earth's magnetic field) and by 6.0% (in the field 0.5 T) than that of H(2)(18)O. The effect is supposed to arise in the two spin-selective, isotope-sorting reactions-recombination and disproportionation-in the pairs of encountering HO(2) radicals. The former is spin allowed from the singlet state; the latter occurs only in the triplet one. Nuclear spin sorting produced by these reactions proceeds in opposite directions with the dominating contribution of recombination, which provides observable (17)O/(18)O isotope fractionation in favor of magnetic isotope (17)O. Neither isotope exchange nor the reaction itself occurs in the dark.     

High precision measurements of 17O/16O and 18O/16O ratios in H2O

We have optimized the method of water fluorination using the solid reagent CoF3 to produce O2. This allows isotope ratio measurements by dual-inlet mass spectrometry with very high precision of 0.01 to 0.03/1000 for both delta17O and delta18O. Using this method, delta17O and delta18O of atmospheric O2 were determined as 12.08 and 23.88/1000 vs. VSMOW, respectively. Likewise, delta17O and delta18O of GISP were -13.12 and -24.73/1000, and for SLAP they were -29.48 and -55.11/1000 vs. VSMOW, respectively. Analysis of these data in a ln(delta17O + 1) vs. ln(delta18O + 1) plot yields a line with a regression coefficient (lambda) of 0.5279 +/- 0.0001 (R2 = 0.999999). We also determined the fractionation factors 17alpha and 18alpha in liquid-vapor equilibrium, and found that the ratio ln 17alpha/ln 18alpha is constant (0.529 +/- 0.001) over the temperature range 11.4 to 41.5 degrees C.     

A new cubic equation of state for reservoir fluids

A new three-parameter cubic equation of state is developed with special attention to the application for reservoir fluids. One parameter is taken temperature dependent and others are held constant. The EOS parameters were evaluated by minimizing saturated liquid density deviation from experimental values and satisfying the equilibrium condition of equality of fugacities simultaneously. Then, these parameters were fitted against reduced temperature and Pitzer acentric factor. For calculating the thermodynamic properties of a pure component, this equation of state requires the critical temperature, the critical pressure, the acentric factor and the experimental critical compressibility of the substance. Using this equation of state, saturated liquid density, saturated vapor density and vapor pressure of pure components, especially near the critical point, are calculated accurately. The average absolute deviations of the predicted saturated liquid density, saturated vapor density and vapor pressure of pure components are 1.4%, 1.19% and 2.11%, respectively. Some thermodynamic properties of substances have also been predicted in this work.     

Prediction of gas condensate properties by Esmaeilzadeh–Roshanfekr equation of state

The Esmaeilzadeh–Roshanfekr (ER) equation of state (EOS) is used to predict the PVT properties of gas condensate reservoir fluids. Three gas condensate fluid samples taken from three wells in a real field in Iran, referred here as SA1, SA4 and SA8, as well as five samples from literature have been used to check the validity of the ER EOS in calculating the PVT properties of gas condensate mixtures. Some experiments such as constant composition expansion (CCE), constant volume depletion (CVD) and dew point pressures are carried out on these samples. In order to have an unbiased comparison between the ER and the Peng–Robinson (PR) equation of state, van der Waals mixing rules are used without using any adjustable parameters (k_ij = 0). Also, no pure component parameters are adjusted. The critical properties and acentric factor for plus-fraction are estimated by the Kesler–Lee, Pedersen et al. and Riazi–Daubert characterization methods. The results of dew point pressure calculations show that the ER EOS has smaller error than the PR EOS. For some mixtures, relative volume, gas compressibility factor and condensate drop-out in CVD and CCE test were also predicted. Comparison results between experimental and calculated data indicate that the ER EOS has smaller error than the PR EOS. The total average absolute deviation was found to be 0.82% and 2.97% for calculating gas compressibility factor and gas specific gravity in CVD test. Also, the total average absolute deviation was found to be 2.06% and 3.42% for calculating gas compressibility factor and relative volume in CCE test.     

Vapor-liquid equilibria simulation and an equation of state contribution for dipole-quadrupole interactions

A systematic investigation on vapor-liquid equilibria (VLEs) of dipolar and quadrupolar fluids is carried out by molecular simulation to develop a new Helmholtz energy contribution for equations of state (EOSs). Twelve two-center Lennard-Jones plus point dipole and point quadrupole model fluids (2CLJDQ) are studied for different reduced dipolar moments micro*2=6 or 12, reduced quadrupolar moments Q*2=2 or 4 and reduced elongations L*=0, 0.505, or 1. Temperatures cover a wide range from about 55% to 95% of the critical temperature of each fluid. The NpT+test particle method is used for the calculation of vapor pressure, saturated densities, and saturated enthalpies. Critical data and the acentric factor are obtained from fits to the simulation data. On the basis of this data, an EOS contribution for the dipole-quadrupole cross-interactions of nonspherical molecules is developed. The expression is based on a third-order perturbation theory, and the model constants are adjusted to the present 2CLJDQ simulation results. When applied to mixtures, the model is found to be in excellent agreement with results from simulation and experiment. The new EOS contribution is also compatible with segment-based EOS, such as the various forms of the statistical associating fluid theory EOS.     

Kinetics and mechanism of hydrogen-atom abstraction from rhodium hydrides by alkyl radicals in aqueous solutions

The kinetics of the reaction of benzyl radicals with [L(1)(H(2)O)RhH{D}](2+) (L(1)=1,4,8,11-tetraazacyclotetradecane) were studied directly by laser-flash photolysis. The rate constants for the two isotopologues, k=(9.3±0.6) × 10(7) M(-1) s(-1) (H) and (6.2±0.3) × 10(7) M(-1) s(-1) (D), lead to a kinetic isotope effect k(H)/k(D)=1.5±0.1. The same value was obtained from the relative yields of PhCH(3) and PhCH(2)D in a reaction of benzyl radicals with a mixture of rhodium hydride and deuteride. Similarly, the reaction of methyl radicals with {[L(1)(H(2)O)RhH](2+) + [L(1)(H(2)O)RhD](2+)} produced a mixture of CH(4) and CH(3)D that yielded k(H)/k(D)=1.42±0.07. The observed small normal isotope effects in both reactions are consistent with reduced sensitivity to isotopic substitution in very fast hydrogen-atom abstraction reactions. These data disprove a literature report claiming much slower kinetics and an inverse kinetic isotope effect for the reaction of methyl radicals with hydrides of L(1)Rh.     

Diffusivity fractionations of H2(16)O/H2(17)O and H2(16)O/H2(18)O in air and their implications for isotope hydrology

We have determined the isotope effects of (17)O and (18)O substitution of (16)O in H(2)O on molecular diffusivities of water vapor in air by the use of evaporation experiments. The derived diffusion fractionation coefficients (17)alpha(diff) and (18)alpha(diff) are 1.0146 +/- 0.0002 and 1.0283 +/- 0.0003, respectively. We also determined, for the first time, the ratio ln((17)alpha(diff))/ln((18)alpha(diff)) as 0.5185 +/- 0.0002. This ratio, which is in excellent agreement with the theoretical value of 0.5184, is significantly smaller than the ratio in vapor-liquid equilibrium (0.529). We show how this new experimental information gives rise to (17)O excess in meteoric water, and how it can be applied in isotope hydrology.     

Comparison of rovibronic density of asymmetric versus symmetric NO2 isotopologues at dissociation threshold: broken symmetry effects

We have measured the rovibronic densities of four symmetric (C2v) and two asymmetric (Cs) isotopologues of nitrogen dioxide just below their photodissociation threshold. At dissociation threshold and under jet conditions the laser-induced fluorescence abruptly disappears because the dissociation into NO(2pi(1/2)) + O(3P2) is much faster than the radiative decay. As a consequence, in a narrow energy range below D0, the highest bound rovibronic energy levels of J=1/2 and J=3/2 can be observed and sorted. A statistical analysis of the corresponding rovibronic density, energy spacing, and rovibronic transition intensities has been made. The observed intensity distributions are in agreement with the Porter-Thomas distribution. This distribution allows one to estimate the number of missing levels, and therefore to determine and compare the rovibronic and the vibronic densities. The four symmetric NO2 isotopologues, 16O14N16O, 18O14N18O, 16O15N16O, and 18O15N18O, have, respectively, a sum of J=1/2 and J=3/2 rovibronic densities of 18+/-0.8, 18.3+/-1.4, 18.4+/-2.7, and 19.8+/-3.5 cm(-1), while for the two asymmetric isotopologues, 18O14N16O and 18O15N16O, the corresponding densities are 20.9+/-4.5 and 23.6+/-5.6 cm(-1). The corresponding vibronic densities are in agreement only if we include both the merging of symmetry species (from those of C2v to those of Cs) and the contribution of the long-range tail(s) of the potential-energy surface along the dissociation coordinate. The effects of isotopic substitution on dissociation rates and the possible relation to mass-independent isotopic fractionation are discussed.     

A new high-temperature multinuclear-magnetic-resonance probe and the self-diffusion of light and heavy water in sub- and supercritical conditions

A high-resolution nuclear-magnetic-resonance probe (500 MHz for 1H) has been developed for multinuclear pulsed-field-gradient spin-echo diffusion measurements at high temperatures up to 400 degrees C. The convection effect on the self-diffusion measurement is minimized by achieving the homogeneous temperature distributions of +/-1 and +/-2 degrees C, respectively, at 250 and 400 degrees C. The high temperature homogeneity is attained by using the solid-state heating system composed of a ceramic (AlN) with high thermal conductivity comparable with that of metal aluminium. The self-diffusion coefficients D for light (1H2O) and heavy (2H2O) water are distinguishably measured at subcritical temperatures of 30-350 degrees C with intervals of 10-25 degrees C on the liquid-vapor coexisting curve and at a supercritical temperature of 400 degrees C as a function of water density between 0.071 and 0.251 gcm3. The D value obtained for 1H2O is 10%-20% smaller than those previously reported because of the absence of the convection effect. At 400 degrees C, the D value for 1H2O is increased by a factor of 3.7 as the water density is reduced from 0.251 to 0.071 gcm3. The isotope ratio D(1H2O)D(2H2O) decreases from 1.23 to approximately 1.0 as the temperature increases from 30 to 400 degrees C. The linear hydrodynamic relationship between the self-diffusion coefficient divided by the temperature and the inverse viscosity does not hold. The effective hydrodynamic radius of water is not constant but increases with the temperature elevation in subcritical water.     

Surface Tension of Liquid Organic Acids: An Artificial Neural Network Model

An artificial neural network model is proposed for the surface tension of liquid organic fatty acids covering a wide temperature range. A set of 2051 data collected for 98 acids (including carboxylic, aliphatic, and polyfunctional) was considered for the training, testing, and prediction of the resulting network model. Different architectures were explored, with the final choice giving the best results, in which the input layer has the reduced temperature (temperature divided by the critical point temperature), boiling temperature, and acentric factor as an independent variable, a 41-neuron hidden layer, and an output layer consisting of one neuron. The overall absolute percentage deviation is 1.33%, and the maximum percentage deviation is 14.53%. These results constitute a major improvement over the accuracy obtained using corresponding-states correlations from the literature.