ADVERTISEMENT RETURN TO ISSUEPREVFeaturesNEXTProbing the Chemical Kinetics of Air PollutionWhen used with care, kinetic studies can produce valuable results despite data uncertainties and mechanistic surprises.Sidney W. BensonCite this: Environ. Sci. Technol. 2002, 36, 1, 28A–32APublication Date (Web):January 1, 2002Publication History Published online1 January 2002Published inissue 1 January 2002https://pubs.acs.org/doi/10.1021/es022146zhttps://doi.org/10.1021/es022146znewsACS Publications. This publication is available under these Terms of Use. Request reuse permissions This publication is free to access through this site. Learn MoreArticle Views1292Altmetric-Citations3LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail PDF (65 MB) Get e-Alertsclose SUBJECTS:Air pollution,Kinetics Get e-Alerts

The kinetics of the reaction H + Cl-2 --> HCl + Cl has been studied in the very low-pressure reactor (VLPR) system at 298 K using two different H atom generation sources. The first one is the microwave decomposition of HCl giving k(1) = (8.66 +/- 0.18) x 10(-12) cm(3)/(molecule-s). No traces of HCl(nu) excited product side reaction could be found in the system due to the much longer residence time in the reactor than the spontaneous decay of excited HCl. The second H atom source is the microwave decomposition of H-2. This technique produces 5% H-2(nu) which initiates the reaction Cl + H-2(nu) -->(2) HCl + H, hydrogen atom recovery and the quenching reaction, H + H-2(nu) -->(tr) H-2 + H. They are identified by differences in the HCl and Cl yields, as well as from the extra H-2 consumption. Rate constants k(2) = (2.88 +/- 0.13) x 10(-11) and k(tr) = (2.95 +/- 0.17) x 12(-12) cm(3)/(molecule-s) are measured along with excellent mass balances between reactant consumption and product formation. Consideration of the high value found for k(tr) leads to the conclusion that its mechanism is by atom transfer rather than collisional deactivation. Both rate constants agree well with the corresponding A-factors reported for thermal reactions indicating that the vibrational excitation energy of H-2(nu) provides the activation energies of the thermal reactions.

The bimolecular reaction: C2H5 + Cl-2 (4)(-->) C2H5Cl + Cl is studied in the very low pressure reactor (VLPR) system at 298 K starting with Cl/C2H6/Cl-2 mixtures. Mass conservation is found to be 97+/-3% for the overall chemical change in the system. From the C2H5Cl product formation kinetics. k(4) = (1.05 +/- 0.05) . 10(-12) cm(3)/(molecule-s) is derived. Two independent estimates of the A factor gives A(4) = 1.3 . 10(-12) cm(3)/(molecule-s). These parameters yield a positive activation energy of 0.13 kcal/mol in contrast with recently reported negative activation energy values. Some side reactions of small extent are initiated by H atom contamination in the system. The measurement of CH3Cl product formed from CH3 + Cl-2 (11)(-->) CH3Cl + Cl gives k(11) = (3.4 +/- 0.3) . 10(-12) cm(3)/(molecule-s) with an estimated A factor of A(11) = 5.0 . 10(-12) cm(3)/(molecules). The source of CH3 radicals is the side reaction H+C2H5 --> 2CH(3).

The bimolecular reaction H + Cl-2 --> HCl + Cl (1) is studied in the very low-pressure reactor (VLPR) system at room temperature. Excellent mass balances are found between the Cl-2 consumption and both HCl as well as Cl product formation. This indicates that only reaction 1 occurs in the system. Its rate constant is k(1) = (0.96 +/- 0.04) x 10(-11) cm(3)/(molecule.s) at 298 K. Two independent estimates of the A factor give a value log A(1)(298 K) = -9.80 +/- 0.15 (units of cm(3)/(molecules)). Together with the measured rate constant this yields an activation energy of 1.7 +/- 0.2 kcal/mol. All experimental data support a bent transition state for reaction 1.

The bimolecular reaction Cl + C2H5Cl --> HCl + C2H4Cl (1) is studied in the very-low-pressure reactor (VLPR) system at room temperature. Detailed mass spectrometric analysis of simultaneous reactant consumption and product formation rates indicates that the overall mechanism also includes two disproportionation reactions: Cl + C2H4Cl --> HCl + C2H3Cl (2) and 2C(2)H(4)Cl --> C2H5Cl + C2H3Cl (3). Rate constants obtained for these reactions are: k(1) = (8.07 +/- 0.36) x 10(-12), k(2) = (1.18 +/- 0.06) x 10(-11), and k(3) = (1.83 +/- 0.15) x 10(-12) cm(3)/(molecule-s). The decrease in the reactivity of C2H5Cl compared with C2H6 reactivity in reaction type (1) correlates well with the electronegativity of the Cl substituent. The decrease points to a dipole-dipole repulsion in the transition state. Preliminary measurement of the reaction of C2H4Cl radical with Cl-2 yields a value of k(5) = (1.7 + 1.0) x 10(-13) cm(3)/(molecule-s).

Using group additivity values to estimate Delta(vap)H degrees(298) and the Kistiakowsky equation to estimate both Delta(vap)S degrees(T-b) and Delta(vap)H degrees(T-b) at the boiling point (T-b), it is shown how an average value of Delta(vap)C(p)degrees can be obtained which can then be used to calculate values of Delta(vap)S degrees(298). This latter can then be used in conjunction with S degrees(g,298) to calculate S degrees(l,298). Alternatively, where values of S degrees(l,298) are available but not SO(g,298) the latter can be calculated. The method applies to regular liquids, even those with relatively large dipoles but not to H-bonded liquids. The accuracy of estimated values of S degrees(l,298) are 0.45 +/- 0.16 cal/(mol K) with a maximum deviation of 1.0 cal/(mol K) for an assortment of 14 selected compounds and 0.3 +/- 0.12 for another 17 liquids for which groups are not available but Delta(vap)H degrees(298) and Delta(vap)S degrees(298) are. Here the largest deviation is 1.9 cal/(mol K). Calculated values of C(p)degrees(l,298) are much less accurate, +/-3 cal/(mol K) with a maximum deviation of 9.0 cal/(mol K). It is also shown that the best average value of C(p)degrees to use in calculating changes in Delta H degrees and Delta S degrees in a specified temperature interval, T-1 to T-2, is the arithmetic mean of the initial and final values, [C(p)degrees(T-1) + C(p)degrees(T-2)]/2. Changes are recommended in some of the group values for calculating Delta(vap)H degrees(298) and also in the group O-(C)(2) for calculating gas-phase entropies of ethers and for N-(C)(2)(H) for calculating entropies of secondary amines.

A large number of reactions of the type R-. + HX and R-. + X-2 have been reported as having negative activation energies (X = I, Br, Cl). These reactions have none of the behavior of reactions that are expected to have negative activation energies. It is shown that they must be simple metathesis reactions having a single transition state, (R . H . X)double dagger or (R . X . X)double dagger. It is concluded that the negative activation energies must be artifacts of the experimental techniques employed. Some of what appear to be simple metathesis reactions but which proceed via atom + radical recombination have had rate constants reported, close to the collision limit. When examined from a collisional point of view, it is shown that they require collision diameters from 8 to 25 Angstrom, far in excess of any known long-range interaction at these distances between neutral species. Again, artifacts of the experimental methods may be responsible.

Temperature coefficients for the kinetics of reaction Cl + HBr → HCl(v,j) + Br have been measured in the range 228−368 K using the very low-pressure reactor (VLPR) system. This experimental technique permits measurement of both the reactant consumption and product formation rates under second-order conditions and achieves mass conservation of 98 ± 2% for the overall chemical change in the reactor. Thermalization of vibrationally and rotationally excited HCl product is well attained in this system. A simple Arrhenius function describes the thermal rate constant as k = (1.99 ± 0.10) × 10-11 exp[−(710 ± 29)/RT] cm3/(molecule s). The measured activation energy is in good agreement with Ea = 0.72 kcal/mol obtained from total energy balancing of energy partition among the reaction products, while the A factor indicates a bent configuration of the transition state.

Current data on ΔfH°298 of alkyl fluorides are reexamined from the point of view of internal consistency and new values are recommended for all of them. In particular, fairly large discrepancies are suggested for current values of ΔfH°298 of t-BuF, n-PrF, i-PrF, and EtF.

The reaction H + HI -->(1) H-2 + I was studied at 298 K and millitorr pressures employing the ''Very Low Pressure Reactor'' (VLPR) kinetic technique. H-atoms were generated by dissociating H-2 molecules (of a H-2/Ar mixture) in a microwave discharge cavity that preceded the very low pressure well-mixed reaction vessel. Quadrupole mass spectrometry was used to analyze molecules and atoms. The mass signal intensities of I and HI were measured at both 20 and 40 eV ionizing potentials while those of H and H-2 were measured at 40 eV due to the very weak signal of these species at lower ionization potentials. Three different exit flow orifices were utilized in the reported VLPR experiments of about 2, 3, and 5 mm inner diameter to vary the species concentration under steady-state reaction conditions. A rate constant of k(1) = (2.1 +/- 0.2) x 10(-11) cm(3)/molecule.s was determined for the forward reaction at 298 K, which lies between the two previously reported values directly measured at 298 K. Satisfactory mass balance relations were obtained for the iodine atoms (from the HI and I species) which were better than 90% for most of the experiments. The value of the reported rate constant (k,) is 14.3% higher than the value measured by Umemoto et al. [6], and 33.3% lower than the value measured by Lorenz et al. [4]. Based on this comparison, the activation energy E-1 of the forward reaction probably lies between those two previously reported values of 580 and 720 cal/mol. Transition State Calculations of A(1) and A(2) for the reaction of H + I-2 -->(2) HI + I are in good agreement with the data on both reactions and suggest an activation energy of about 500 +/- 100 cal/mol for E-2. (C) 1997 John Wiley & Sons, Inc.

The rates of the reactions of ethyl radicals with HBr (k(7)) and with Br atoms (k(8)) have been measured in the temperature range 228-368 K at millitorr pressures using the very low pressure reactor (VLPR) technique. The Arrhenius function for the H atom abstraction reaction is found to be k(7)=(1.43+/-0.06) x 10(-12) exp-[-(444 +/- 26)/RT] cm(3)/(molecule s), while the ethyl radical disproportionation with Br atom shows no temperature dependence. Its average value over the entire temperature range is k(8)=(1.18+/-0.05) x 10(-11) cm(3)/(molecule s). Reaction 7 is significantly slower than has been reported in the only other two direct measurements, both finding a negative activation energy for k(7) of from -1.0 to -1.1 kcal/mol. The small positive activation energy found in this work for k(7) fits standard models for H atom metathesis. Combination with all known kinetic information for ethane bromination gives an average reaction enthalpy of Delta H-7(0)=13.0+/-0.2 kcal/mol using both the second-and third-law thermochemical calculations. It sets the heat of ethyl radical formation to Delta(f)H(0)(C2H5)=28.40+/-0.25 kcal/mol and the bond dissociation enthalpy, DH0-(C2H5-H)=100.5+/-0.3 kcal/mol.

The thermochemistry and kinetics of the initiation reactions involved in the oxidation of unsaturated fuels are explored. The thermochemistry of intermediate radicals, diradicals, and molecular species involved are estimated using group additivity with some assistance from bond additivity. Kinetic parameters are estimated with the techniques of thermochemical kinetics. In the case of acetylene, estimated rate constants are in excellent agreement with experimental results on the induction period and name speed,It is shown that the route initiated by O-2 addition to an unsaturated carbon atom to produce a 1,4 diradical is faster than any other path available to form radicals capable of propagating a chain. The 1,4 diradicals so produced can generally cyclize to form a dioxetane which exothermically opens to a dialdehyde which is the ultimate radical source. Below 1000 K unsaturates will always initiate oxidation faster than saturated fuels. (C) 1996 John Wiley & Sons, inc.

Recent studies from one laboratory over the past decade of the equilibrium reaction[GRAPHICS]for seven different radicals have led to discrepancies in six cases in Delta H-1 degrees of From 3 to 7 kcal/mol when compared to estimates from group additivity. These studies have utilized flash photolysis to generate R(.) radicals and have measured the decay of [R(.)] by mass spectrometry using near threshold photoionization. This decay was fitted to a double-exponential rate expression, Only one additional reaction was introduced in addition to 1 and -1, namely, the wall decay (w) of R(.) radicals, measured in the absence of O-2. Reexamination of the differential equations has shown that they are not compatible with the reported data and in fact lead to negative values of K-1 and k(1), It has not been possible using the equation given by the authors of the papers under consideration to derive K-1 from k(w) and the exponents of the double-exponential decay. It is shown that HO2. elimination is important for EtO(2)(.), iPrO(2)(.), and tBuO(2)(.) at the temperatures used, If this is the only additional reaction, it is further shown that the authors' values of Delta H-1 are still not compatible with their equations.

From comparisons of the differences in heats of vaporization of alcohols (ROH) and their analogous hydrocarbons (RCH(3)) which turned out to be 6.1 +/- 0.1 kcal/mol at 298 K it was concluded that this difference represents the contribution of the hydrogen bond to the value of Delta H-vap(alcohol, 298 K), It is further concluded that alcohols are self associated in pure liquids in cyclic clusters each of four alcohols. These conclusions are supported by extensive, earlier studies of PVT relations in vapors, heat capacities, and IR spectra in solutions. Group additivity tables of Ducros et al, an shown to be in excellent agreement with directly measured Delta H-vap(298 K) for both alcohols and alkanes. Similar analysis of amines shows a much weaker H-bond with a lower limit of about 2.2 kcal/mol between amines so that amines exist mostly as monomers. Scattered data on diols of lesser accuracy suggest compact sandwich structures with the hydrocarbon tying together two cyclic tetramer rings of H-bonded oxygen atoms.

The rates of the reactions of ethyl radicals with HBr (k(7)) and with Pr atoms (k(8)) have been measured at 298 K and millitorr pressures using the Very Low Pressure Reactor (VLPR) technique. The rate constants at 298 K are the following: k(7) = (6.67 +/- 0.14) x 10-(13) cm(3)/(molecule(.)s) and k(8) = (1.19 +/- 0.04) x 10-(11) cm(3)/(molecule(.)s). Reaction 7 is a factor of about 14 times slower than had been reported in the only other two direct measurements made (Nicovich, J. M. et al. J. Phys. Chem. 1991, 95, 9890. Seakins, P. W. et al. J. Phys Chem. 1992, 98, 9847) which also reported a negative activation energy for k(7) of from -0.8 to -1.1 kcal/mol. Using broadly accepted thermochemistry for reaction 7 and reported values for the reverse reaction, it is shown that all reported data give a positive activation energy for k(7).

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The shock tube data of Ogura [5] on the pyrolysis of C2H2/NO mixtures (1100-1500 K) is shown to be consistent with a simple mechanism whereby radicals are both initiated and terminated by NO (Scheme I). The scheme accounts for the rate of formation of the main product, vinylacetylene (VA), the lesser products CO and HCN and a very minor product, propionitrile. It is also shown to be consistent with other studies below 900 K and observation at 300 K on the reactions of vinyl radicals with NO. The substantial inhibition of vinyl acetylene formation by 5% NO makes untenable any substantial role of vinylidene in the C2H2 pyrolysis above 1000 degrees K.The reaction of NO with acetylene is an efficient source of HCN. It appears to be a general reaction of NO with substituted acetylenes and below 900 K a mechanism is presented to account for the production of acrylonitrile (AN) from the reaction of NO with VA.Thermochemical data are estimated on Delta(f)H degrees(298) and S degrees(298) for some alkyl-NO, vinyl NO, and acetylene NO compounds and radicals and some new and some revised group values are estimated for estimating Delta(f)H degrees(298) of derivatives of hydroxyl amines, imines, and isoxazolines. (C) 1994 John Wiley & Sons, Inc.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTThermochemistry of CF3O Radical and CF3OHSidney W. BensonCite this: J. Phys. Chem. 1994, 98, 8, 2216Publication Date (Print):February 1, 1994Publication History Published online1 May 2002Published inissue 1 February 1994https://pubs.acs.org/doi/10.1021/j100059a042https://doi.org/10.1021/j100059a042research-articleACS PublicationsRequest reuse permissionsArticle Views55Altmetric-Citations17LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access options Get e-Alerts

An estimate of the entropy change in the attachment of a Cl atom to benzene together with the reported equilibrium constant led to a DELTAH-degrees for the reaction and a DELTA(f)H-degrees (Ph-H:Cl) which coincides with that estimated for the chlorocyclohexadienyl radical. Kinetic considerations rule out the latter as a direct participant in chlorination. Instead its role appears to be that of a carrier for reversibly bound Cl atoms. It is in rapid cage equilibrium with the much more weakly bound pi-complex which probably has never been seen spectroscopically but which still remains the most probable agent responsible for the increased selectivity of chlorination. Pyridine which has been observed32 to be some 4-fold more selective than benzene in chlorination is shown to form a stronger complex (by 2.6 kcal/mol). However, this >N-Cl bonded complex is also not the selective chlorinating agent but rather again the more loosely bound pi-complex in equilibrium with it. Effective cage concentrations of substrate RH and benzene around nascent and free Cl atoms are different as well as being different from bulk solution concentrations. An outline is given of an empirical method to estimate the thermochemistry of Cl and HCl and other gases in various solvents as a function of concentration.

Ethyl radicals formed in the reaction of C2H6 + Cl are allowed to react with molecular oxygen in a very low pressure reactor (VLPR) experimental flow system over the temperature range of 243-368 K. Mass spectrometric analysis of reactants and products made possible the determination of rate constants (cm3/(molecules)) of all major reaction steps. Mass balances for C, H, and Cl are good to +/-4% on average. The elementary steps are the following: C2H5 + O2 --> HO2 + C2H4, k6 = (1.42 +/- 0.38) x 10(-17) exp[(5064 +/- 154)/RT], measured independently from recording C2H5 consumption or C2H4 formation rates; 2HO2 --> H2O2, k7 = (4.50 +/- 0.56) x 10(-13) exp[(1064 +/- 77)/RT]; C2H5 + HO2 --> H2O2 + C2H4, k8a = (2.98 +/- 0.11) x 10(-12); Cl + HO2 --> HCl + O2, k9 = (4.45 +/- 0.06) x 10(-11). Activation energies are given in cal/mol. Reactions 8a and 9 show no change in the temperature range of measurements, while reactions 6 and 7 both have negative temperature dependence. The radical oxidation reaction 6 is suggested to occur via excited ethylperoxy and 2-hydroperoxyethyl radical formations as consecutive reversible steps.