Fall-off Region Research Articles

Relative reactivity of chlorine atoms with NO, NO 2 and HCCl 2F at room temperature and atmospheric pressure

The reactions of chlorine atoms with NO, NO 2 and HCCl 2F have been studied at 294 K and 1 atm total pressure. The experiments involved steady-state photolysis of Cl 2-HCCL 2F-O 2-NO-NO 2-N 2 mixtures. Two sets of experiments were done, one in which NO removal was monitored by its chemiluminescent reaction with O 3, and one in which CClFO was monitored by in situ IR spectroscopy. The reactions studied were Cl + HCCl 2F → HCl + CCl 2F (1) Cl + No + M → ClNO + M (6) Cl + NO 2 + M → ClONO (or ClNO 2) + M (7) From the chemiluminescence experiments k 6/ k 1 was found to be (1.0 ± 0.2) × 10 −17 cm 3 with N 2 as a chaperone which leads to a value of k 1 = 1.0 × 10 −14 cm 3 molecule −1 s −1 based on the literature value for k 6 of 1.7 × 10 −32 × exp(+530/ T) cm 6 molecule −2 s −1 (where T is in kelvins). Also k 7/ k 6 was found to be 2.5 ± 1.0 which gives k 7 = 2.5 × 10 −31 cm 6 molecule −2s −1, which is considerably lower than the literature values of (7.2 – 16) × 10 −31 cm 6 molecule −2 s −1. Part of all of the discrepancy may reflect the possibility that k 7 is in its pressure fall-off region under our conditions of 1 atm pressure.

Kinetics of the reactions of isopropoxy radicals with NO, NO2, and O2

A fluorescence excitation spectrum of (CH3)2CHO (isopropoxy radical) is reported following photolysis of isopropyl nitrite at 355 nm. Rate constants for the reaction of isopropoxy with NO, NO2, and O2 have been measured as a function of pressure (1–50 Torr) and temperature (25–110°C) by monitoring isopropoxy radical concentrations using laser-induced fluorescence. We have obtained the following Arrhenius expressions for the reaction of isopropoxy with NO and O2 respectively: (1.22±0.28)×10−11 exp[(+0.62±0.14 kcal)/RT]cm2/s and (1.51±0.70)×10−14 exp[(−0.39±0.28)kcal/RT]cm3/s where the uncertainties represent 2σ. The results with NO2 are more complex, but indicate that reaction with NO2 proceeds more rapidly than with NO contrary to previous reports. The pressure dependence of the thermal decomposition of the isopropoxy radical was studied at 104 and 133°C over a 300 Torr range using nitrogen as a buffer gas. The reaction is in the fall-off region over the entire range. Upper limits for the reaction of isopropoxy with acetaldehyde, isobutane, ethylene, and trimethyl ethylene are reported. We have performed the first LIF study of the isopropoxy radical. Arrhenius parameters were measured for the reaction of i-PrO with O2, NO, NO2, using direct radical measurement techniques. All reactions are in their high-pressure limits at a few Torr of pressure. The rate constant for the reactions of i-PrO with NO and NO2 reactions exhibit a small negative activation energy. Studies of the i-PrO + NO2 reaction produce data which indicate that O(3P) reacts rapidly with i-PrO. Unimolecular decomposition studies of i-PrO indicate that the reaction is in the fall-off region between 1 and 300 Torr of N2 and the high-pressure limit is above 1 atmosphere of N2.

Thermal unimolecular decomposition of β-propiolactone (oxetan-2-one)

The thermal decomposition of oxetan-2-one has been investigated in the gas phase in the temperature range 262–322 °C. The reaction, which yields ethylene and carbon dioxide, is homogeneous and obeys first-order kinetics. There is a minor heterogeneous isomerization to yield acrylic acid. The decomposition is almost certainly a unimolecular process, which in the pressure range studied, ⩽ 6 Torr, is in the fall-off region. High-pressure rate constants were determined by extrapolation using three procedures that are discussed. The preferred method using plots of k–1 against p–0.61 yielded the Arrhenius equation logk/s–1= 14.86±0.30 – 180.46±3.20 kJ mol–1/RT ln10. Theoretical (RRKM) calculations are close to the experimental fall-off curves on the basis of the strong-collision assumption or with 〈ΔE〉d(stepladder model) 24 kJ mol–1, from which it is hardly distinguishable. Certainly it is necessary to assume very efficient intermolecular energy transfer. Comparison with the result from other studies suggests a concerted decomposition via an activated complex with zwitterionic character rather similar to those involved in cyclobutanone decompositions.

Kinetics study of the reactions IO + NO2 + M .fwdarw. IONO2 + M, IO + IO .fwdarw. products, and atomic iodine + ozone .fwdarw. iodine oxide (IO) + molecular oxygen

A photochemical modulation technique was employed to investigate the title reactions. IO radicals were monitored in absorption at 426.9 nm where the absorption cross section at this wavelength was determined to be (2.2 +/- 0.5) x 10/sup -17/ cm/sup 2/ molecule/sup -1/. The reaction IO + NO/sub 2/ + M ..-->.. IONO/sub 2/ + M, studied between 35 and 404 torr at 277 K, was found to be in the falloff region between second- and third-order kinetics for this pressure range. The self-reaction IO + IO ..-->.. products was investigated between 10 and 100 torr at 303 K and between 25 and 404 torr at 277 K. The reaction appeared to proceed by at least two pathways, one pressure dependent and the other pressure independent. The results also allowed an estimate of (9.6 +/- 3.0) x 10/sup -13/ cm/sup 3/ molecule/sup -1/ s/sup -1/ for the rate coefficient of the reaction I + O/sub 3/ ..-->.. IO + O/sub 2/ at 26 torr and 303 K. 16 references, 10 figures, 4 tables.

Collaborative shock tube studies of benzene pyrolysis

Three independent shock tube techniques, time-of-flight mass spectrometry (TOF), atomic resonance absorption spectroscopy (ARAS), and laser-schlieren density gradient measurements (LS), were employed to investigate the thermal decomposition of benzene (C6H6 or C6D6) in dilute inert gas mixtures over a wide range of reactant concentration (3–21,000 ppm), total pressure (0.2–3 atm) and temperature (1515–2500 K). The latter two methods were used to derive rate constants for the dissociation of C6H6→C6H5+H and C6H5→C2H2+C4H3 and the overall rate of benzene decomposition that occurred primarily via a direct ring rupture process C6H6→C2H2+C4H4. The ring rupture step was shown to be in the fall-off region. Rate constants subject to fall-off effects were adjusted for particular reaction conditions and used in a mechanism which included steps for acetylene pyrolysis to model the major species, C6H6, C2H2, C4H2, recorded by the TOF experiments, D atom production by ARAS, and density gradient profiles by LS.

The thermal dissociation of ethane. A study of the reaction over an extended pressure range

The rates of dissociation of ethane into two methyl radicals have been determined by measuring the rate of formation of methane at 862 K at pressures in the range 0.01–30 Torr. The results coupled with earlier measurements provide rate coefficients for the dissociation reaction over a pressure region where the reaction order varies from ca. 1.9 to 1.1. The data have been fitted to an RRKM model over this range. There are indications of the occurrence at the lower end of the pressure range of a heterogeneous process leading to methane formation, which prevented the extension of the investigation to even lower pressures. Application of the results obtained from this study to a recently proposed expression for rate coefficients of unimolecular reactions in the fall-off region shows that there is good agreement in this case between theory and experiment.

ChemInform Abstract: THERMAL DECOMPOSITION OF METHYL NITRITE IN SHOCK WAVES STUDIED BY LASER PROBING

The unimolecular decomposition of methyl nitrite in the temperature range 680–955 K and pressure range 0.64 to 2.0 atm has been studied in shock-tube experiments employing real-time absorption of CW CO laser radiation by the NO product. Computer kinetic modeling using a set of 23 reactions shows that NO product is relatively unreactive. Its initial rate of production can be used to yield directly the unimolecular rate constant, which in the fall-off region, can be represented by the second-order rate coefficient in the Arrhenius form: A RRKM model calculation, assuming a loose CH3ONO≠ complex with two degrees of free internal rotation, gives good agreement with the experimental rate constants.

The Thermal Decomposition of CF3I in Ar

Abstract The thermal decomposition of CF3I behind reflected shock waves was studied by means of the UV absorption over the temperature range of 935–1235 K and in the density range of 6.3×10−6–3.8×10−6 mol cm−3. The decomposition rate was in the fall-off region, and the main process was the C-I bond fission. The rate constants corresponding to the limiting high- and low-pressure regions were determined to be; k ∞=1014.67exp(−27200⁄T)s−1 and; k 0⁄[Ar]=1014.71exp(−15000⁄T)cm3mol−1s−1.

Pressure and temperature dependence of the reaction nitrogen dioxide + nitrogen trioxide + M .fwdarw. nitrogen pentoxide (N2O5) + M

The kinetics of the reaction NO/sub 2/ + NO/sub 3/ + M ..-->.. N/sub 2/O/sub 5/ + M (1) were studied with flash photolysis/visible absorption by monitoring the disappearance of NO/sub 3/ under pseudo-first-order conditions over the temperature range 236-358 K and pressure range 20-700 torr of M (helium and nitrogen). Reaction 1 was found to be in the falloff region between second- and third-order kinetics in this range. Falloff parameters were obtained by fitting the experimental data to the falloff equation of Troe and co-workers. When one uses a published temperature-dependent broadening factor (F/sub c/), the data are described by k/sub 0/(N/sub 2/) = 4.5 x 10/sup -30/ cm/sub 6/ molecule/sup -1/ s/sup -1/ and k/sub infinity/ - 1.65 x 10/sup -12/ cm/sup 3/ molecule/sup -1/ s/sup -1/ at 298 K with T dependencies of T/sup -3.4/ and T/sup -0.4/ for k/sub 0/ and K/sub infinity/, respectively. This expression was used in conjunction with recent measurements of the rate constant for N/sub 2/O/sub 5/ thermal decomposition to obtain an improved measurement ..delta..H/sub f//sup 0/ (298) for NO/sub 3/, which was determined to be 17.3 +/- 0.7 kcal mol/sup -1/. From these data the recommended equilibrium constant expression is K/submore » eq/ = k/sub 1//K/sub -1/ = 2.31 x 10/sup -8/ exp(11350/T) (atm/sup -1/). 31 references, 6 figures, 2 tables.« less

Kinetics of the reaction OH + SO2 + M .fwdarw. HOSO2 + M. Temperature and pressure dependence in the fall-off region

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTKinetics of the reaction OH + SO2 + M .fwdarw. HOSO2 + M. Temperature and pressure dependence in the fall-off regionP. H. Wine, R. J. Thompson, A. R. Ravishankara, D. H. Semmes, C. A. Gump, A. Torabi, and J. M. NicovichCite this: J. Phys. Chem. 1984, 88, 10, 2095–2104Publication Date (Print):May 1, 1984Publication History Published online1 May 2002Published inissue 1 May 1984https://doi.org/10.1021/j150654a031RIGHTS & PERMISSIONSArticle Views405Altmetric-Citations51LEARN 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 InReddit PDF (1 MB) Supporting Info (1)»Supporting Information Supporting Information Get e-Alerts

Rate constant for the reaction of the CFCl 2 radical with oxygen in the pressure range 0.2–12 Torr at 298 K

The rate constant for the reaction of CFCl 2 with oxygen is measured in the pressure range 0.2–12 Torr using pulsed-laser photolysis and time-resolved mass spectrometry. CFCl 2 radicals are generated by photolysis of CFCl 3 at 193 nm. The reaction kinetics are recorded by monitoring the build-up of the CFCl 2O 2 radical concentration. The reaction is in its fall-off region, and the parameters of the relation for the treatment of the fall-off are for M = N 2: k(0) = (5.0 ± 0.8) × 10 −30 cm 6 molecule −2 s −1. k(∞) = (6.0 ± 1.0) × 10 −12 cm 3 molecule −1 s −1. This value of k(∞) is consistent with results obtained at low pressure taking F c = 0.6, but the uncertainty in the high-pressure limit is much higher. The results are compared to measurements performed with CH 3 and CF 3. Estimates of the relative third-body efficiencies of He and N 2 are given for CFCl 2 and CF 3.

ChemInform Abstract: THERMAL UNIMOLECULAR DECOMPOSITION OF β‐BUTYROLACTONE (4‐METHYLOXETAN‐2‐ONE)

The thermal decomposition of β-butyrolactone has been studied in the temperature range 209–250 °C. At each temperature, runs were carried out from 10 Torr to < 0.1 Torr. In an aged reaction vessel the decomposition was homogeneous and yielded propene and carbon dioxide as the only products. All the experimental evidence indicated that the reaction was a true unimolecular decomposition which at 10 Torr was close to its high-pressure limit.The Arrhenius equation corresponding to this limit is log (k∞/s–1)= 14.39 ± 0.10–(163.4 ± 1.0) kJ mol–1/RT ln 10. Arrhenius parameters are also reported for the reaction some way into the fall-off region. The experimental fall-off curves are compared with those calculated from RRKM theory and some discussion about the deviations between theory and experiment is presented. The mechanism of the reaction is considered and the results obtained in this work compared with those reported for β-propiolactone.

Temperature dependence of intermolecular vibrational energy transfer in the two-channel cyclopropane-1,1-d2 system. Neon bath gasa)

The competitive, two-channel cyclopropane-1, 1-d2 isomerization to propylene has been used for the study of the temperature dependence of vibrational energy transfer efficiency. This note reports work with Ne bath gas at 823 and 1050 K. The fall-off region was k/k∞≊0.01. The average amount of energy &amp;lt;ΔE≳ transferred per collision with Ne from activated cyclopropane-1, 1-d2 molecules declines with temperature rise, from 575 cm−1 at 823 K to 275 cm−1 at 1050 K. The relative efficiency of collisions β̄ω declines from 0.062 at 823 K to 0.012 at 1050 K.

Thermal unimolecular decomposition of β-butyrolactone (4-methyloxetan-2-one)

The thermal decomposition of β-butyrolactone has been studied in the temperature range 209–250 °C. At each temperature, runs were carried out from 10 Torr to < 0.1 Torr. In an aged reaction vessel the decomposition was homogeneous and yielded propene and carbon dioxide as the only products. All the experimental evidence indicated that the reaction was a true unimolecular decomposition which at 10 Torr was close to its high-pressure limit.The Arrhenius equation corresponding to this limit is log (k∞/s–1)= 14.39 ± 0.10–(163.4 ± 1.0) kJ mol–1/RT ln 10. Arrhenius parameters are also reported for the reaction some way into the fall-off region. The experimental fall-off curves are compared with those calculated from RRKM theory and some discussion about the deviations between theory and experiment is presented. The mechanism of the reaction is considered and the results obtained in this work compared with those reported for β-propiolactone.

The Monte Carlo simulation of energy transfer processes in thermal unimolecular reactions. III. The simulation of pressure dependent behaviour

A Monte Carlo solution to the ro-vibrational master equation for the low pressure unimolecular decomposition of a polyatomic molecule presented recently has been extended to include the simulation of pressure dependent behaviour. The calculations concentrate on two molecular properties which can influence the pressure dependence of a unimolecular reaction, namely adiabatic rotations and the random lifetime assumption. The results from the calculations involving adiabatic rotations show a qualitative agreement with those presented by other authors. However, an examination of the form taken by the random lifetime assumption in RRKM theory reveals that the use of the microcanonical rate constant to give an average lifetime for an activated molecule. does not provide an adequate description of the entire distribution of lifetimes. In particular, calculated results in the fall-off region are found to be sensitive to details of the short lifetime tail of the distribution.

Pulsed laser photolysis–long path laser absorption kinetics study of the reaction of methylperoxy radicals with NO2

The technique of pulsed laser photolysis–long path laser absorption is employed to study the kinetics of the reaction CH3O2+NO2+N2 k1→CH3O2NO2+N2 over the temperature range 253–353 K. CH3O2 is prepared by 193 nm (ArF) laser photolysis of azomethane in the presence of O2. The temporal profile of [CH3O2] is followed by measurement of the time resolved attenuation of a frequency doubled argon ion laser beam (257 nm). A path length of 860 cm is employed to obtain initial absorptions of (typically) 4%, corresponding to a CH3O2 concentration of 2×1013 cm−3. At all temperatures, k1 is found to increase monotonically but nonlinearly with increasing N2 pressure (76–722 Torr) indicating that Reaction (1) proceeds predominantly via an addition mechanism and that the reaction is in the falloff region between second and third order kinetics. The measured values of k1 are compared with results of previous investigations and their atmospheric implications discussed.

The Thermal Unimolecular Decomposition of Methyl Chloride behind Shock Waves

Abstract The thermal decomposition of methyl chloride diluted in Ar has been studied behind shock waves over the temperature range between 1680 and 2430 K and the total density range of 2.0×10−6–3.5×10−5 mol cm−3. The decomposition rate was monitored by means of the UV absorption of the CH3 radical produced. The initiation reaction was found to be the C–Cl bond fission; the process proceeded in the fall-off region under the present experimental conditions. From the fall-off data, low- and high-pressure rate constants (k0/[Ar] and k∞) were obtained by the application of a refined RRKM theory involving a weak collision effect. Thus, the Arrhenius expressions were given as: &amp; k_0/[Ar]=10^15.56 exp(-247 kJ mol^-1/RT) cm^3 mol^-1 s^-1, &amp; k_∞=10^13.86 exp(-383 kJ mol^-1/RT) s^-1. The collision efficiency factor, βc, was obtained as 0.02 at the mean temperature of the experiment by comparing the low-pressure rate constant with the strong collision-rate constant. The high-pressure rate constant was well explained by the maximum free-energy criterion presented by Quack and Troe.

Kinetics studies of the reactions of methyldioxy with nitric oxide, nitrogen dioxide, and methyldioxy at 298 K

The flash photolysis/ultraviolet absorption technique was used to measure the rate constants for the reactions CH/sub 3/O/sub 2/ + NO ..-->.. CH/sub 3/O + NO/sub 2/ (1), CH/sub 3/O/sub 2/ + NO/sub 2/ + M ..-->.. CH/sub 3/O/sub 2/NO/sub 2/ + M (2), and CH/sub 3/O/sub 2/ + CH/sub 3/O/sub 2/ ..-->.. products (3) at 298 K over the pressure range 50 to 700 torr. Values for k/sub 1/ and k/sub 3/ were determined to be (7.1 +- 1.4) x 10/sup -12/ cm/sup 3/ molecule/sup -1/s/sup -1/ and (3.6 +- 0.7) x 10/sup -13/ cm/sup 3/ molecule/sup -1/s/sup -1/, respectively, where k/sub 3/ is defined by the relation -d(CH/sub 3/O/sub 2/)/dt = 2k/sub 3/(CH/sub 3/O/sub 2/)/sup 2/; k/sub 2/ was found to vary strongly with pressure, indicating that the reaction occurs primarily by addition and is in the falloff region between second- and third-order kinetics. Experimentally determined parameters describing the shape of the falloff curve for k/sub 2/ were in reasonable agreement with those obtained by using theoretical methods developed by Troe and co-workers. An upper limit of 7 x 10/sup -18/ cm/sup 3/ molecule/sup -1/s/sup -1/ was determined for the rate constant for the reaction CH/sub 3/O/sub 2/ +more » CO ..-->.. products.« less

Ozone photolysis: determination of the O( 3P) quantum yield at 266 nm

Direct production of O( 3P) from the photodissociation of O 3 at 266 nm has been observed by time-resolved resonance fluorescence following laser flash photolysis. The quantum yield for O( 3P) production was determined to be 0.12 = 0.02 at this wavelength. This result confirms the qualitative observations of two previous photofragment studies and establishes an absolute value of Φ(O 1D)) = 0.88. This value has been used as a basis for normalizing relative quantum yields in the falloff region, and interpreting O( 1D) kinetics studies.

Laser flash photolysis of ozone: O( 1D) quantum yields in the fall-off region 297–325 nm

The wavelength dependence of the quantum yield for O( 1D) production from ozone photolysis has been determined between 297.5 nm and 325 nm in order to resolve serious discrepancies among previous studies. The result of this investigation are compared to earlier work by calculating atmospheric production rate constants for O( 1D). It is found that for the purpose of calculating this rate constant, there is now good agreement among three studies at 298 K. Furthermore, it appears that previous data on the temperature dependence of the O( 1D) quantum yield fall-off is adequate for determining the vertical profile of the O( 1D) production rate constant. Several experimental difficulties associated with using NO* 2 chemiluminescence to monitor O( 1D) have been identified.