Partial Rate Constants Research Articles

Kinetics of N3+ and N4+ with N(4S), O(3P), and NO

Kinetics of the reactions of N3+ and N4+ with N(4S) and O(3P) are measured at room temperature using a flowing afterglow-selected ion flow tube apparatus. Oxygen atoms are produced by titrating NO against nitrogen atoms formed from a microwave discharge of N2. The reaction rate constants are generally well-described by assuming strict spin-conservation along with Langevin capture kinetics. N3+ + N occurs with k = 1.8 × 10−10 cm3 s−1, spin state counting indicates both doublet and quartet states of N2+ are formed. N4+ + N occurs with k = 2.2 × 10−10 cm3 s−1, yielding exclusively N3+. The N3+ + O reaction occurs with k = 2.5 × 10−10 cm3 s−1, yielding significant amounts of NO+ and N2+ products. N4+ + O, the only of these reactions without any spin-constraint, is the only reaction to occur near the Langevin capture rate with k = 5.6 × 10−10 cm3 s−1. The major product of the N4+ + O reaction is charge transfer to yield O+ (k = 4.1 × 10−10 cm3 s−1) with N2O+ (k = 7.7 × 10−11 cm3s−1) and probably NO + formed as minor products. The reaction of N4+ + NO minimally occurs with k < 6.5 × 10−12 cm3 s−1. A novel method of assessing uncertainties in rate constants derived from complicated chemical reaction networks is presented, yielding probability density curves as a function of each partial rate constant. The analysis obviates the need of assuming experimental errors of the rate constants are normally distributed.

(Invited, Digital Presentation) Langmuir-Type Formulation for Atomic-Order Surface Reactions of Reactant Gases on Si (100) and Ge (100) Surfaces

Accurate knowledge of atomically controlled processing for group IV semiconductors is very important for the fabrication of Si-based ultrasmall devices in ultralarge-scale integration [1]. In this review, the Langmuir-type formulation is performed for atomic-order surface reactions of reactant gases on group IV semiconductor (100) surface in ultraclean hot-wall low-pressure CVD. Now, assuming that one reactant molecule M is adsorbed at one free surface site with partial decomposition of M, total site density n0 on the surface is given by n 0 = Q s + Q Ms, (1)where Q s is density of free site; Q Ms, the site density where M is adsorbed.Then the surface adsorption velocity of Q Ms is given bydQ Ms/dt = k Ms P M Q s - k -Ms Q Ms = k Ms P M n 0 - (k Ms P M + k -M) Q Ms, (2)where k Ms and k -Ms is adsorption and desorption rate constants of M, respectively; P M, partial pressure of M. The integration of eq. (2) with Q Ms = 0 at t =0 gives Q Ms= [k Ms P M n 0/(k Ms P M + k -Ms)]{1 - exp[ - (k Ms P M + k -Ms)t]}. (3)The self-limited adsorption given by eq. (3) with fitted n 0 and k Ms (k -Ms = 0) fits the experimental data of CH4 on Si(100) at 500-600oC [2], SiH3CH3 on Si and Ge(100) at 450oC [3], B2H6 on hydrogen(H)-free Si (100) at 180oC [4], PH3 on Ge (100) at 300oC [5], SiH4 on H-free Ge (100) at 260oC [6]. It is mentioned that H-free surface is prepared by preheating at the higher temperature than around 400oC on Si (100) and 300oC on Ge (100) in Ar and at the higher temperature the self-limited amount is scarcely influenced even by H2 in the present condition. In the case of NH3 at 400oC on H-terminated Si (100) [7] and SiH4 at 260oC on H-terminated Ge (100) [6], it is proposed that the NH3 or SiH4 molecules are physically adsorbed self-limitedly at H-terminated sites. Then, total site density n H0 is given by n H0 = Q Hs + Q MHs + Q HMR, (4)where Q Hs is the site density of H-terminated site; Q HMs, the site density where M is adsorbed physically at H-terminated site; Q HMR, the site density where physically adsorbed M at H-terminated site decomposes partially. If the relationship between Q Hs and Q HMs is Q HMs=K HM Q Hs with thermal equilibrium constant K HM, the surface decomposition velocity of Q HMR is given bydQ HMR/dt = k HMR Q HMR = k HMR K HM P M(n H0 - Q HMR), (5)where k HMR is partial decomposition rate constant of adsorbed M.The integration of Eq. (5) with Q HMR = 0 at t =0 gives Q HMR=n H0(1 - exp { - [k HMR K HM P M/(1 + K HM P M)]t}). (6)The H-terminated surface is composed of H-free and H-terminated sites. For fraction x of the H-terminated sites, total site density is n 0 (1 - x) + n H0 x and the self-limited amount is given by Q Ms (1 - x) + Q HMR x.The self-limited adsorption/reaction for NH3 at 400oC on H-terminated Si (100) surface and SiH4 at 260oC on H-terminated Ge (100) surface given by eqs. (3) and (6) with x, n 0, n H0, k Ms, K HM and k HMR fits the experimental data. For the SiH4, SiH3CH3 or PH3 where total site density on the surface is equal to substrate surface atomic density n SSA, it is assumed that the site is the pair site such as Si-Si, Si-Ge or Ge-Ge pair. For CH4, B2H6 or NH3 where the total site density is equal to 2n SSA, it is proposed that the site is one bond of Si or Ge. In the PH3 on Si (100) [5], it is assumed that P atoms of maximum 2n SSA are adsorbed with self-limited reaction rate constant of PH3 at the P occupied sites where PH3 molecules have been adsorbed at Si-Si pair sites [8]. These results demonstrate that the atomic level surface reactions of reactant gases in the practical low-pressure CVD are explained based on the modified Langmuir-type model.

Oxidation of halobenzenes by OH radical produced radiolytically in aqueous solution

The radiolytic oxidation of aromatic compounds in dilute aqueous solution is due to the electrophilic strength of the •OH that results from the radiolysis of water. This radical is preferentially added to the electron-rich centers in aromatic rings. Therefore, studies of the charge distribution of the hydroxycyclohexadienyl radicals produced by the addition of the •OH to the aromatics can provide information about their charge distribution. In the present work, the radiolytic oxidation reaction of fluorobenzene, chlorobenzene, bromobenzene, and iodobenzene with the •OH in diluted aqueous solutions was studied. The identification and quantification of the radiolytic products thus formed were carried out utilizing HPLC. The yields found for fluorobenzene, chlorobenzene, bromobenzene, and iodobenzene were 0.609, 0.527, 0.508, and 0.501 μmol/J, respectively. The electron density distribution in fluorobenzene, chlorobenzene, and bromobenzene was in the following order: for para > ortho > meta » ipso; however, in iodobenzene, there is a higher electron density in the "ortho" position than in the "para" position. From the total reaction rates for the halobenzenes with the •OH, the partial rate constants were obtained for the reaction of the •OH with each of the positions in the halobenzenes.

Channel-resolved rotationally dependent predissociation rate constants reveal the state-to-state dissociation dynamics of carbon monoxide in electronically excited states.

Rotational dependence of the total predissociation rate constants deduced from linewidth measurements in spectroscopic studies have often been used to predict the possible electronic coupling schemes in the photodissociation process of carbon monoxide (CO), while the intrinsic multi-channel characteristics of CO photodissociation make the prediction unreliable and sometimes misleading conclusions could be reached. Here, we demonstrate for the first time in the Rydberg 4p(2) and 5p(0) complexes region of 13C16O that absolute partial predissociation rate constants into each individual channel and their dependences on the rotational quantum numbers can be obtained through branching ratio measurements in combination with the total predissociation rate constants reported in spectroscopic studies. These channel-resolved rotationally dependent predissociation rate constants are found to unambiguously reveal the detailed state-to-state photodissociation dynamics of CO, which is not available from either the branching ratio or the total predissociation rate constant measurements individually.

Modelling polyurethane synthesis rates on the example of heteropolyaddition of 2,4-toluene diisocyanate (TDI) and 1,4-butanediol (BD)

This paper investigates the effects of increase of reaction mixture viscosity on the kinetics of linear polymer creation in a bulk heteropolyaddition process of 2,4-toluene diisocyanate (TDI) and 1,4-butanediol (BD). The paper presents a method for solving a system of bulk polyaddition of 2,4-toluene diisocyanate and 1,4-butanediol process balance equations, allowing the determination of the process kinetic parameters. Determination of polymerisation reaction kinetic parameters was also made possible by the use of the so-called partial reaction rate constant. Such an approach enabled a significant simplification of the mathematical expressions describing the heteropolyaddition process and provided an opportunity to associate kinetic parameters with the average molar mass of the mixture and, thus, with the viscosity. The method presented herein facilitates an analysis of the linear polymers heteropolyaddition process.

Kinetics and Products of the Reaction of OH Radicals with ClNO from 220 to 940 K.

The kinetics and products of the reaction of OH radicals with ClNO have been studied in a flow reactor coupled with an electron impact ionization mass spectrometer at nearly 2 Torr total pressure of helium and over a wide temperature range, T = 220-940 K. The rate constant of the reaction OH + ClNO → products was determined under pseudo-first order conditions, monitoring the kinetics of OH consumption in excess of ClNO: k1 = 1.48 × 10-18 × T2.12 exp(146/T) cm3 molecule-1 s-1 (uncertainty of 15%). HOCl, Cl, and HONO were observed as the reaction products. As a result of quantitative detection of HOCl and Cl, the partial rate constants of the HOCl + NO and Cl + HONO forming reaction pathways were determined in the temperature range 220-940 K: k1a = 3.64 × 10-18 × T1.99 exp(-114/T) and k1b = 4.71 × 10-18 × T1.74 exp(246/T) cm3 molecule-1 s-1 (uncertainty of 20%). The dynamics of the title reaction and, in particular, non-Arrhenius behavior observed for both k1a and k1b in a wide temperature range, seems to be an interesting topic for theoretical research.

Mesoscale Structures in the Adlayer of A-B2 Heterogeneous Catalysis.

This article explores the adsorbate distributions in the adlayer for a model A-B2 system of heterogeneous catalysis, i.e., A + 1/2B2 → AB, via kinetic Monte Carlo (KMC) simulations. In comparison with our previous work on the A-B model (Sun, F.; Huang, W.; Li, J. Structural characteristics of the adlayer in heterogeneous catalysis. Chem. Eng. Sci. 2016, 153, 87-92), species B2 here brings about significant new features due to its special site requirement during adsorption and desorption and a different stoichiometric ratio in reactions. The effects of various kinetic processes on the adsorbate distribution are found to be similar to those in the A-B system; that is, both desorption and diffusion (besides adsorption) processes contribute to the adlayer uniformity while reactions account for clustering. However, desorption exhibits a stronger role than diffusion in homogenizing the adlayer, which is opposite to the finding in the previous A-B model. Under a fixed partial pressure, different reaction and desorption rate constants can lead to steady states with different dominant species, which has not been observed in the A-B system. The regime of species B poisoning shrinks as well, leading to the spreading of the coexisting regime, in comparison with the A-B model.

Modelling polycarbonate synthesis rates on the example of bulk heteropolyaddition of diphenyl carbonate and bisphenol A diglycidyl ether

This paper investigates the effects of increased reaction mixture viscosity on the kinetics of linear polymer creation in a bulk polyaddition process of diphenyl carbonate and bisphenol A diglycidyl ether. The paper presents a method for solving a system of bulk polyaddition of diphenyl carbonate and bisphenol A diglycidyl ether process balance equations, allowing the determinatiof the process kinetic parameters. Determination of polymerisation reaction kinetic parameters was also made possible by the use of the so-called partial reaction rate constant. Such an approach enabled a significant simplification of the mathematical expressions describing the heteropolyaddition process and provided an opportunity to associate kinetic parameters with the average molar mass of the mixture and, thus, with the viscosity. The method presented herein facilitates an analysis of the linear polymers heteropolyaddition process.

Dissociative recombination of molecular ions in the He–Ne plasma. Partial rate constants of atoms formation in the 2p 53d and 2p 54d configurations

A spectroscopic study of population processes was carried out for states of the neon atom of the 2p53d configuration in the discharge afterglow in helium with a small admixture of neon ([Ne]/[He] ≈ 10–5) at a pressure of 38 Torr. Based on the comparative analysis of time dependences of the spectral line intensities of the Ne I 2p53d → 2p53d and 2p54d → 2p53d transitions and ratio of ion densities [Ne2+ ]/[HeNe+], it has been found that the formation of excited Ne(2p53d) atoms in the afterglow caused by two dissociative recombination processes: HeNe+ + e → Ne(2p53d) + Ne and Ne2+ + e → Ne(2p53d) + Ne. Distributions of population fluxes of the 2p53d levels \(\Gamma _{3{d_t}}^{HeN{e^ + }}\) and \(\Gamma _{3{d_t}}^{Ne_2^ + }\)—were obtained for each of these processes.

Simplified Representation of Partial and Total Rate Constants of Complex-Forming Bimolecular Reactions.

The temperature and pressure dependence of partial and total rate constants of complex-forming bimolecular reactions are investigated with the goal to obtain simplified and compact rate constant expressions suitable for data compilations. The transition of the reactions from low pressure chemical activation to high pressure association character is analyzed. The two processes are modeled separately first by solving master equations, leading to "inverse" and "normal" falloff curves, respectively, and allowing for a compact representation of the separated rate constants. It is shown that broadening factors of the two falloff curves are different, and those of chemical activation often approaching unity. Coupling of the two separate processes then is modeled in a simplified manner. Finally, thermal redissociation of the adducts formed by association is accounted for.

Formation of molecular ions by radiative association of cold trapped atoms and ions

Radiative emission during cold collisions between trapped laser-cooled Rb atoms and alkaline-earth ions (Ca+, Sr+, Ba+) and Yb+, and between Li and Yb+, are studied theoretically, using accurate effective core potential based quantum chemistry calculations of potential energy curves and transition dipole moments of the related molecular ions. Radiative association of molecular ions is predicted to occur for all systems with a cross section two to ten times larger than the radiative charge transfer one. Partial and total rate constants are also calculated and compared to available experiments. Narrow shape resonances are expected, which could be detectable at low temperature with an experimental resolution at the limit of the present standards. Vibrational distributions are also calculated, showing that the final molecular ions are not created in their ground state level.

Quantification of the ambident electrophilicities of halogen-substituted quinones.

Kinetics and mechanisms of the reactions of p-quinone, 2,5-dichloro-p-quinone, 2,3,4,5-tetrachloro-p-quinone (chloranil), 2,3,4,5-tetrafluoro-p-quinone (fluoranil), and 3,4,5,6-tetrachloro-o-quinone with π-nucleophiles (siloxyalkenes, enamines) and amines have been investigated. Products arising from nucleophilic attack at all conceivable sites, that is, at C and O of the carbonyl groups (pathways a, b) as well as at halogenated and nonhalogenated conjugate positions (pathways c, d), were observed. The partial rate constants for the C-attack pathways (a, c, d), which are derived from the photometrically determined second-order rate constants and the product ratios followed the linear free energy relationship log k (20 °C) = sN(E + N) ( Mayr , H.; J. Am. Chem. Soc. 2001 , 123 , 9500 - 9512 ). It was, therefore, possible to calculate the electrophilicity parameters E of the different positions of the quinones from log k (20 °C) and the N and sN parameters of the nucleophilic reaction partners, which have previously been derived from their reactions with benzhydrylium ions. Almost all rate constants for the C-attack pathways (a, c, d) were considerably larger than those calculated for the corresponding SET processes, indicating the operation of polar mechanisms. SET mechanisms may only account for the formation of the products formed via O-attack. With the E parameters determined in this work, it is now possible to predict rate constants for the reactions of these quinones with a large variety of nucleophiles and, thus, envisage unprecedented reactions of quinones.

Kinetic study of formation/dissociation of Cu(II) and Zn(II) complexes of cyclen macrocyclic ligand with pendant thiol group

The kinetic properties of macrocyclic ligand 2-[1,4,7,10–tetraazacyclododecane-1-yl]-ethanethiol (L1, DOSH) were studied with Cu(II) and Zn(II) metal ions, using conventional molecular absorption spectroscopy in pH regions 2.5–4.7 and 4.3–6.0 for Cu(II) and Zn(II), respectively. The obtained partial rate constants, measured at 25°C and ionic strength I=0.1M (KCl), are slightly lower than for parent cyclen (1,4,7,10-tetraazacyclododecane) macrocyclic ligand supporting the hypothesis that the thiol pendant arm is not directly involved in the formation of these metal complexes. The study of the dissociation of the copper(II)-L1 complex in the presence of perchloric acid and at higher temperatures (75–95°C) showed that this complex is more kinetically inert than the copper(II) complex with cyclen, indicating that the presence of the thiol pendant arm plays a key role as protecting group of copper(II) ion. On contrary, the dissociation of zinc(II) complex in presence of copper(II) ion measured at laboratory temperature (25°C) and in 0.1M-solution of hydrochloric acid is going via the metal-exchange reaction mechanism. Altogether, these findings show that L1 ligand presents favorable kinetic properties for further conjugation with biomolecules, aiming at their use in medicinal chemistry.

Electron attachment to CF3 and CF3Br at temperatures up to 890 K: Experimental test of the kinetic modeling approach

Thermal rate constants and product branching fractions for electron attachment to CF3Br and the CF3 radical have been measured over the temperature range 300-890 K, the upper limit being restricted by thermal decomposition of CF3Br. Both measurements were made in Flowing Afterglow Langmuir Probe apparatuses; the CF3Br measurement was made using standard techniques, and the CF3 measurement using the Variable Electron and Neutral Density Attachment Mass Spectrometry technique. Attachment to CF3Br proceeds exclusively by the dissociative channel yielding Br(-), with a rate constant increasing from 1.1 × 10(-8) cm(3) s(-1) at 300 K to 5.3 × 10(-8) cm(3) s(-1) at 890 K, somewhat lower than previous data at temperatures up to 777 K. CF3 attachment proceeds through competition between associative attachment yielding CF3 (-) and dissociative attachment yielding F(-). Prior data up to 600 K showed the rate constant monotonically increasing, with the partial rate constant of the dissociative channel following Arrhenius behavior; however, extrapolation of the data using a recently proposed kinetic modeling approach predicted the rate constant to turn over at higher temperatures, despite being only ~5% of the collision rate. The current data agree well with the previous kinetic modeling extrapolation, providing a demonstration of the predictive capabilities of the approach.

Reactivity of C-H bonds in cyclohexanone and 1-tert-butylperoxycyclohexanol toward the tert-butylperoxyl radical

The kinetics of oxygen uptake and the composition of the cyclohexanone oxidation products in the azobisisobutyronitrile-initiated oxidation of cyclohexanone in the presence of tert-butyl hydroperoxide have been investigated by the Howard-Ingold method. The partial rate constants of the reaction of the tert-butylperoxyl radical with the C-H bonds of cyclohexanol and 1-tert-butylperoxycyclohexanole at 333 K have been determined. The carbonyl group of cyclohexanone activates the C-H bonds in the 2- and 6-positions (α) and deactivates the C-H bonds in the 3- and 5-positions (β) compared to the C-H bonds in the 4-position (γ), whose reactivity is similar to that of the methylenic C-H bonds in cyclohexane. Evaluation of the joint effect of the hydroxyl and tert-butylperoxyl groups in 1-tert-butylperoxycyclohexanol suggests a considerable deactivation of the C-H bonds in the 2- and 6-positions (β) and, to a lesser extent, in the 3- and 5-positions (γ).

Reactivity of the C-H bonds of methyl hexanoate toward the tert-butylperoxy radical

The kinetics of oxygen uptake and the kinetics of accumulation of methyl hexanoate oxidation products in the presence of tert-butyl hydroperoxide have been investigated to determine the partial rate constants for the interaction of the tert-butylperoxy radical with C-H bonds of the ester at 373 K. The ester group deactivates the C-H bonds in the 2- and 3-positions and does not deactivate the same bonds in the 5-position, which are similar in reactivity to the methylenic C-H bonds of hydrocarbons. The C-H bonds of the methoxyl group of methyl hexanoate are substantially more reactive than the C-H bonds of the methyl group.

The dissociative recombination of HeNe+ ions with electrons: The partial rate constants for the formation of atoms in the 2p 53p configuration

A simple experiment transparent for interpretation is presented in which afterglow of a discharge in helium with a small admixture of neon was studied. The intensities of the spectral lines of the Ne* atom at the stage of plasma decay were analyzed to find the conditions under which the dissociative recombination of heteronuclear ions HeNe+ + e → He + Ne* played a predominant role in the formation of part of the after-glow spectrum. The absolute values of partial coefficients responsible for the formation of excited neon atoms in the 2p 53p configuration were determined.

First report on the kinetics of the uncatalyzed esterification of cellulose under homogeneous reaction conditions: a rationale for the effect of carboxylic acid anhydride chain-length on the degree of biopolymer substitution

The kinetics of the homogeneous acylation of microcrystalline cellulose, MCC, with carboxylic acid anhydrides with different acyl chain-length (Nc; ethanoic to hexanoic) in LiCl/N,N-dimethylacetamide have been studied by conductivity measurements from 65 to 85 °C. We have employed cyclohexylmethanol, CHM, and trans-1,2-cyclohexanediol, CHD, as model compounds for the hydroxyl groups of the anhydroglucose unit of cellulose. The ratios of rate constants of acylation of primary (CHM; Prim-OH) and secondary (CHD; Sec-OH) groups have been employed, after correction, in order to split the overall rate constants of the reaction of MCC into contributions from the discrete OH groups. For the model compounds, we have found that k(Prim-OH)/k(Sec-OH) > 1, akin to reactions of cellulose under heterogeneous conditions; this ratio increases as a function of increasing Nc. The overall, and partial rate constants of the acylation of MCC decrease from ethanoic- to butanoic-anhydride and then increase for pentanoic- and hexanoic anhydride, due to subtle changes in- and compensations of the enthalpy and entropy of activation.

Dynamics and mechanism of the non-adiabatic transitions from the ungerade I2(D0+u) state induced by collisions with rare gas atoms

The stepwise three-photon two-color laser excitation scheme is used for selective population of the first-tier ion-pair D0(+)(u) state of molecular iodine. Collection and analysis of the luminescence after the excitation of the v(D) = 6, 8, 13 and 18 vibronic levels of the D state in the pure iodine vapor and the gas-phase mixtures with He, Ar and Xe provide the total and, whenever possible, partial rate constants for the collision-induced non-adiabatic transitions to the other ion-pair states of the first tier. Comparison with the analogous data obtained previously for the non-adiabatic transitions from the E0(+)(g) state reveals the similarity between two cases. For He, the D ↔ E transitions are preferable, whereas for Ar and Xe transitions to the D' and β states dominate at v(D) = 6, 8 and 13, in accord with the statistical considerations. Efficient population of the δ state at v(D) = 18 in Ar and Xe is the most prominent non-statistical feature observed. The vibrational product state distributions for the D → E transitions are also obtained. In contrast to the previously studied E → D transition, they show significant positive vibronic energy transfer. The measurements for He and Ar are accompanied by the quantum scattering calculations that reproduce well the main qualitative features of the experimental results.

Dynamics and mechanism of the E→D, D′, β, γ, and δ nonadiabatic transitions induced in molecular iodine by collisions with CF4 and SF6 molecules

Nonadiabatic transitions among the first-tier ion-pair states of the iodine molecule in collisions with CF(4) and SF(6) partners are investigated by detecting the luminescence following the optical-optical double resonance excitation of the E0(g) (+)-state to the vibrational levels v(E)=8, 13, and 19. Total and partial rate constants, as well as vibrational product state distributions, are determined. It is found that electronic energy transfer in all channels is predominantly assisted by excitation of the dipole-allowed nu(3) and nu(4) modes of the partner. The measurements are accompanied by quantum scattering calculations that implement a close coupling treatment for the electronic and vibrational degrees of freedom and combine diatomics-in-molecule and long-range models for diabatic potential energy surfaces and coupling matrix elements. The analysis of experimental and theoretical data shows that the transitions without excitation of the partner are due to short-range couplings, whereas the vibrational excitation of the partner in the D0(u) (+) channel originates from the long-range coupling of two transition dipole moments: electronic of the iodine molecule and vibrational of the partner. Unexpectedly efficient excitations of the partner in the other ion-pair states, which are not coupled to the initial E0(g) (+)-state by the transition dipole, are interpreted within the postcollision mechanism. Qualitatively, this implies that during a single collision the long-range nonadiabatic transitions to D, nu(3) and D, nu(4) channels are followed by secondary short-range transitions without changing the state of the partner.