Association Rate Coefficients Research Articles

Association reactions of nickel atoms with carbon monoxide and alkenes at room temperature: binding energy of NiC 2H 4

We report a kinetic study of association reactions of nickel atoms with carbon monoxide, ethene, propene and 1-butene in carbon dioxide buffer gas in the pressure range 1–600 Torr at 296 K. RRKM theory is used together with results of previous ab initio electronic structure calculations to interpret association rate coefficients in terms of binding energies of 1:1 complexes. The binding energy of NiC 2H 4 at 0 K is estimated as 35.5±5 kcal mol −1. The role of excited electronic states of the complexes in the association reactions is briefly discussed.

Gas phase atomic association reactions of Ar+, Kr+, and Xe+ at temperatures near 1 K

We report the measurement of gas phase three-body atomic ion–atom association reaction rate coefficients at temperatures near 1 K using a free jet flow reactor. The association rate coefficients for the reactions of Ar+ with Ar and Ne, Kr+ with Kr and Ar, and the reactions of Xe+ with Xe, Kr, and Ar have been measured. In all these cases except Ar++Ne, the rate at 1 K is in excess of 9×10−29 cm3/s and a strong negative temperature dependence is observed. These observations are discussed with respect to several microscopic descriptions of the association process.

A global view of radiative association as a function of product size: interstellar implications

Radiative association processes are currently thought to play a significant if not dominant role in the gas phase chemistry of dense interstellar clouds. Current detailed gas phase models of the chemistry of dense clouds contain large numbers of molecules comprising up to about 10 atoms. Radiative association will play a more important role when such models are extended to larger molecules because this process tends to increase in rate as the reactant size increases. Here, a current detailed theory of radiative association rate coefficients (the so-called phase space theory) is utilized to determine such rate coefficients for generic reactive systems ranging in size from fewer than 10 atoms to upwards of 100 atoms

A mathematical model for the computation of carboxyhaemoglobin in human blood as a function of exposure time

A mathematical model is developed for the carbon monoxide (CO) uptake by the blood by taking into account the molecular diffusion, convection, facilitated diffusion and the non-equilibrium kinetics of CO with haemoglobin. The overall rate for the combination of CO with haemoglobin is derived by including the dissociation of CO from carboxyhaemoglobin (COHb). The resulting coupled system of nonlinear partial differential equation with physiologically relevant initial, entrance and boundary conditions is solved numerically. A fixed point iterative technique is used to deal with nonlinearities. The concentration of COHb in the blood is computed as a function of exposure time and ambient CO concentration. The COHb levels computed from our model are in good agreement with those measured experimentally. Also, results computed from our model give better approximation to the experimental values compared with the results from other models. The time taken by the blood COHb to attain 95% of its equilibrium value is computed. The COHb concentration in the blood increases with the increase in ventilation rate, association rate coefficient of CO with haemoglobin and total haemoglobin content in the blood, and with the decrease in dissociation rate coefficient of CO with haemoglobin and mean capillary blood PO2. It is found that the COHb level in the blood is not affected significantly because of endogenous production of CO in the body under normal condition. However, the effect may be significant in the patients with haemolytic anaemia.

Theoretical approaches to reversible diffusion-influenced reactions: Monomer–excimer kinetics

Three formalisms that describe the influence of diffusion on the kinetics of the reversible reaction, A+B⇌AB, are discussed and compared. The simplest involves a modification of the irreversible rate equations of Smoluchowski theory; the second is based on a generalization of physically appealing convolution relations that hold rigorously for reversible reactions between isolated pairs, and the third can be obtained by using a superposition approximation to truncate the hierarchy of equations satisfied by the reactive reduced distribution functions. The various formalisms are developed to the point that their implementation requires knowledge only of the time-dependent irreversible association rate coefficient and the microscopic dissociation rate constant. All these approaches give the correct equilibrium concentrations at infinite time, have the same short-time behavior, reduce correctly when the dissociation rate is zero, and become equivalent in the reaction-controlled limit. However, none of them provides an exact treatment of the underlying many-particle diffusive model of the reaction. Some illustrative calculations are presented and the relative merits of these approaches are discussed. All three approaches predict that the relaxation of a small initial deviation of the concentrations from their equilibrium values is nonexponential, except, of course, in the reaction-controlled limit. With a view towards treating monomer–excimer kinetics, the formalisms are generalized to incorporate unimolecular decay pathways.

Association reactions at low pressure. IV. The HC3N+/HC3N system

The reactions between HC3N+ and HC3N, and between HC5N+ and HC3N have been examined at pressures from 1×10−7 to 1×10−3 Torr by ion cyclotron resonance mass spectrometry. The reaction between HC3N+ and HC3N has both a bimolecular reaction path and a termolecular reaction path. The overall bimolecular reaction rate coefficient was found to be 1.3×10−9 cm3 s−1. The primary product, HC5N+, represents 90% of the product ions, while the minor products HC6N+2 and H2C6N+2 each represent 5%. The termolecular association rate coefficient was 3.7×10−24 cm6 s−1 with He as the third body. From double resonance experiments the mean lifetime of the collision complex was determined to be 180 μs. HC5N+ was found to react with HC3N and form the adduct ion H2C8N+2 through both bimolecular and termolecular channels. The bimolecular rate coefficient was 5.0×10−10 cm3 s−1 and the termolecular rate coefficient was observed to be 1.2×10−22 cm6 s−1 with HC3N as the third body. With He as the stabilizing molecule, the termolecular rate coefficient was 6.0×10−24 cm6 s−1. The mean unimolecular lifetime of the collision complex was estimated to be ≥15 μs and the mean radiative lifetime for the radiative association channel was found to be ≥89 μs. HC5N+ was found to be formed with excess internal energy and did not react by bimolecular association until relaxed by several nonreactive collisions with HC3N.

The radiative association of He(+) and H

view Abstract Citations (60) References (5) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Radiative Association of He + and H Zygelman, B. ; Dalgarno, A. Abstract An accurate evaluation of the rate coefficient for the radiative association of He(+) and H to form HeH(+) is described. The rate coefficient at 10,000 K is 1 x 10 to the -15th cu cm/s. The new value leads to an enhanced production of HeH(+) in astrophysical plasmas. Publication: The Astrophysical Journal Pub Date: December 1990 DOI: 10.1086/169475 Bibcode: 1990ApJ...365..239Z Keywords: Association Reactions; Helium Ions; Hydrogen; Interstellar Chemistry; Interstellar Gas; Planetary Nebulae; Molecular Ions; Radiation Spectra; Stellar Envelopes; Astrophysics; INTERSTELLAR: MOLECULES; MOLECULAR PROCESSES; NEBULAE: PLANETARY full text sources ADS | data products SIMBAD (2)

The radiative association of N and O atoms

Calculations are carried out of the rate coefficient for the radiative association of ground state nitrogen oxygen atoms to form nitric oxide as a function of the temperature T. The rate coefficient is given by α(T) = 2.8(300/T)0.5 × 10−17 cm³ s−1. Detailed predictions are presented for the emission spectra accompanying the radiative association process. The widths of the emission profiles vary significantly with temperature.

Temperature dependence of the atomic three-body association reaction Ar+ + 2Ar .fwdarw. Ar2+ + Ar below 3 K

Absolute termolecular association rate coefficients for the reaction Ar{sup +} + 2Ar {yields} Ar{sub 2}{sup +} + Ar were measured at translational temperatures between 0.5 and 3 K by using the free jet flow rector technique. The values of k{sub 3} are found to be strongly temperature dependent and fit an inverse temperature rate law, k{sub 3} = 9.52 {times} 10{sup {minus}29}T{sup {minus}1.1} cm{sup 6} s{sup {minus}1}, incorporating data between 0.5 and 300 K. There is some evidence that this fit is dominated by the low-temperature limiting behavior and that the strict T{sup {minus}n} dependence with a constant value of n is currently unjustified for atomic ion-atom association processes.

Temperature dependence of termolecular association reactions N2+ + 2N2 .fwdarw. N4+ + N2 and O2+ + 2O2 .fwdarw. O4+ + O2 occurring in free jet expansions below 20 K

A new experimental technique that combines supersonic expansions and laser ionization mass spectrometry has been used to measure the termolecular association rate coefficients, k{sub 3}, for the gas-phase reactions N{sub 2}{sup +} + 2N{sub 2} {yields} N{sub 4}{sup +} + N{sub 2} and O{sub 2}{sup +} + 2O{sub 2} {yields} O{sub 4}{sup +} + O{sub 2} below 20 K. The continuity of the proposed inverse temperature law of k{sub 3} = CT{sup -n} was observed down to 4 K for both reactions. Statistical analysis of our results along with the higher temperature data obtained by other authors gives the values of 1.92 {plus minus} 0.15 and 1.86 {plus minus} 0.15 for n for the reactions of N{sub 2}{sup +} and O{sub 2}{sup +}, respectively. The results of phase space calculations are presented for these reactions and excellent agreement between theory and experiment is obtained.

Association reactions with state selected ions at meV collision energies: CO+ (v=0,j) + 2 CO ? (CO) 2 + +CO

Relative three-body association rate coefficients for the title reaction have been measured as a function of the rotational statej of the CO+ ion at collision energies below 1 meV. A significant decrease of the dimer formation has been observed whenj increases from 0 to 7 (Erot=0−13.7 meV). Multiphoton ionization (MPI) is used to create the ions in the high pressure region of a pulsed supersonic expansion of pure CO. This method allows both the rather precise determination of the rotational population of the neutrals (〈Erot〉 ∼1.5 meV) and the preparation of the ions exclusively in the vibrational ground state and in well defined rotational states. Due to its very steep energy dependence the ternary association reaction can be “switched off” within a few tenths of a millimeter using an acceleration field of 1–3 V/cm. The short effective interaction time of less than 250 ns leads to almost single collision conditions. i.e. we perform state selective studies. The kinetics and kinematics (mean kinetic energy, competition between relaxation and association) are analyzed numerically. A consistent picture results from the assumption that the ternary rate coefficient depends only on the total energy of the complex irrespective of whether its origin is rotation or translation.

Can interstellar H2S be formed via gas-phase reactions? Calculations concerning the rates of the ternary and radiative association reactions between HS+ and H2.

Based on laboratory work involving the ternary association reaction of HS+ and H2 at 80 K, we have estimated the rate of the analogous radiative association reaction under interstellar conditions. Both the ternary and radiative association reactions appear to occur via a mechanism in which the electronic spin of the H3S+ complex changes before the complex is stabilized. Although this spin change is of low probability, it leads to a radiative association rate coefficient at 80 K of 7 x 10(-16) cm3 s-1 if radiative stabilization occurs at a rate of 10(3) s-1. This value of the radiative association rate coefficient at 80 K is large enough to lead to the observed abundance of H2S in the ambient ridge source in Orion.

The pressure dependence of ion–molecule association rate coefficients

Two new techniques are presented for calculating the pressure dependence of ion/molecule association rates: (i) a semiclassical method of incorporating the effect of the hindered dipole rotation into an RRKM calculation of the microscopic rate coefficients k(E,J), and (ii) a solution for the master equation for unimolecular dissociation and recombination reactions, which incorporates angular momentum (J) conservation and is applicable when the moments of inertia of reactant and activated complex differ by a large amount. These techniques provide the optimal currently available means for calculating the pressure dependence of rate coefficients for ion–molecule reactions, which are highly sensitive to J-conservation effects. The method may be used for reliable estimates and fitting of experimental fall-off data. The new technique shows that Troe’s solution of the low-pressure J-conserving master equation is accurate for the nonequilibrium population distribution but overestimates (by up to a factor of 2) the rate coefficient; extensions to Troe’s method to provide more accurate analytical approximations are derived. Application is presented to the association of CH3+ with HCN. The experimental pressure dependence, which cannot be fitted without proper accounting for J conservation, is fitted using the new treatment with values of ∼125 cm−1 for the average downward energy transfer for both rotational and internal energies. The new J-conservation method is also applicable to reactions of neutrals where the moment of inertia of the activated complex is more than six times that of the stable reactant.

Deduction of low density limit to rate coefficient for tertiary association of ions and polar molecules from measurements

The low density limit k0 to the rate coefficient for tertiary association of ions and polar molecules is related through a double integral to an entity z1 or z2 that involves only the temperature, the reduced mass of the colliding pair, the polarizability and dipole moment of the neutral, and k(exp)[M], where k(exp) is the measured rate coefficient at ambient gas density [M]. The results are parametrized so that k0 may easily be found from laboratory data (irrespective of whether or not third order kinetics prevail).

Association reactions at low pressure. I. Collision-stabilized association below 1 micron

Rate coefficients for collision-stabilized association of a protonated ion AH + and its parent molecule A to form the proton bound dimer A 2H + were measured in an ion cyclotron resonance (ICR) cell at room temperature for the systems where A  dimethyl ether and acetonitrile. Third-order rate coefficients are reported for the dimethyl ether system with the third bodies (CH 3) 2O, Ar, N 2, Ne, He, and for the acetonitrile system with the third bodies CH 3CN, N 2, Ne, and He. A selected ion flow tube (SIFT) study on these same systems provides evidence that the rate coefficient, k f, for (A 2H +)* formation may be less than the collision rate, k c. The lifetimes of the collision complex (A 2H +)* are ≥ 0.5 × 10 −6 s. A consequence of k f < k c is that derivation of the average lifetime of (A 2H +)* with respect to unimolecular decomposition from the third-order rate coefficient may generally lead to values that are too small.

SIFDT studies of some CH +3 ternary association reactions

Following our previous detailed studies of CH + 3 association reactions in a VT-SIFT under truly thermal conditions, we have studied some of the same reactions under non-thermal conditions in a variable-temperature selected ion flow drift tube (VT-SIFDT). The ternary association rate coefficients, k 3, for the reactions of CH + 3 with N 2 and CO in helium carrier gas (and hence with helium atoms as the stabilizing third bodies) at temperatures of variously 80 and 300 K are observed to vary according to the power law k 3 ≈ E −1.5 c, where E c is the CH + 3/He centre-of-mass energy. For the CH + 3 association reaction with H 2, k 3 varies as k 3 ≈ E −1.5 r, where E r is the CH + 3/H 2 centre-of-mass energy. These experimental results are in accordance with a simple theoretical model we have constructed based on the theoretical ideas of Bates and of Herbst, which describe the temperature dependence of k 3 in terms of the number of rotational degrees of freedom in the separated reactants. For the CH + 3 +CO reaction at the higher values of E c and at a carrier gas temperature of 300 K, a more rapid decrease in k 3 occurs than is predicted by our simple model and this is attributed to the onset of vibrational excitation of the CH + 3 ions. This latter phenomenon is expected to be more important for polyatomic ions, which possess low-energy vibrational states and some experimental evidence is presented in support of this.

New approximate limiting laws for activity and rate coefficients of ions in solution

A new approach is present for concentration effects on ionic reactions in solutions of small electrolyte concentrations. It is assumed that the mutual Coulomb interaction is felt by an ion pair only when their separation is smalller than the “Coulombic horizon”, R av, namely when there is no intervening ion. This leads to an approximate but simple and parameterless, formula for activity coefficients, with a cube root dependence on concentration in this regime. Steady-state diffusional rate coefficients are calculated to and from the sphere at R av. Comparison made with experimental activity coefficients of strong and weak electrolytes shows that the upper validity limit of the new formula exceeds that of the Debye-Hückel limiting law. The diffusional rate constant from R av gives a reasonable description of the concentration dependence of the ionic association rate coefficients, for concentrations where the Brönsted-Bjerrum limiting expression ceases to apply.

Competition between binary ion molecule reactions and ternary association reactions of CH+3 with NH3 in He buffer gas in the pressure range 0.2–1.7 Torr

The binary reaction of CH+3 with NH3 as well as their ternary association have been investigated in the regime from 0.047 to 0.2 eV, KEcm, using the Innsbruck selected ion drift tube (SIDT). In this energy regime the association rate coefficient declines from 4×10−26 to 6×10−27 cm6 s−1 and the lifetimes of the collision complex (CH+3−NH3)* range from ∼10−7 to ∼10−8 s.

Can gas phase reactions produce complex oxygen-containing molecules in dense interstellar clouds? - A revision of some important radiative association rate coefficients

view Abstract Citations (55) References (34) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Can Gas Phase Reactions Produce Complex Oxygen-containing Molecules in Dense Interstellar Clouds? A Revision of Some Important Radiative Association Rate Coefficients Herbst, Eric Abstract In light of new laboratory measurements and theoretical techniques, previously suggested gas phase syntheses of dimethyl ether (CH3OCH3), ethyl alcohol (C2H5OH), and acetaldehyde (CH3CHO) in dense interstellar clouds have been reexamined. Several radiative association rate coefficients leading to the precursor ions to these species are now calculated to be significantly smaller than previous estimates. On the other hand, a new and rapid synthesis of the precursor ion to acetaldehyde appears to be viable. The diminution in the values of selected radiative association rate coefficients raises the possibility that normal gas phase syntheses under ambient dense cloud conditions may not account fully for the observed abundances of ethyl alcohol and, especially, dimethyl ether. Publication: The Astrophysical Journal Pub Date: February 1987 DOI: 10.1086/165026 Bibcode: 1987ApJ...313..867H Keywords: Association Reactions; Interstellar Chemistry; Molecular Clouds; Reaction Kinetics; Vapor Phases; Acetaldehyde; Ethers; Ethyl Alcohol; Methyl Compounds; Oxygen; Astrophysics; INTERSTELLAR: MOLECULES; MOLECULAR PROCESSES full text sources ADS | data products SIMBAD (2)

Ion/molecule association reactions. A study of the temperature dependence of the reactions CO + + CO + M → (CO) 2+ + M for M = CO, Ne, and He, and NO + + 2 NO → (NO) 2+ + NO. Experiment and theory

The temperature dependence of the third-order kinetics of the association reactions CO + + CO + M → (CO) 2 + + M and NO + + 2 NO → (NO) 2 + + NO have been investigated as well as the equilibrium NO + + 2 NO ⇌ (NO) 2 + + NO using a low-energy drift technique. Thermal reaction rate coefficients were obtained over the temperature range of 80–530 K, using Ne, He, and CO as buffer gases for the CO system. The experimental data reported here are compared with literature data as well as with the predictions of phase-space theory calculations. Good agreement was found for the pure CO system between the present results, previous experiments, and theoretical calculations. Less good agreement was found when He was used as the buffer gas. In particular, the smaller temperature dependence of the association rate coefficient found experimentally in this case was not reproduced in the calculated results. The available experimental temperature range for the NO system was 200–530 K. Above 450 K, equilibrium is reached. Below 450 K, the temperature dependence of the termolecular rate constant was determined. Results in the NO system indicate the need to include explicitly the back dissociation reaction in the kinetic analysis. An exact treatment of this effect has been developed and details of this correction are discussed.