A pulsed-laser photolysis/laser-induced fluorescence technique is employed in the determination of the pressure and temperature dependence of the reaction of CN with NO in the range from 207 to 740 K and for Ar bath gas pressures ranging from 30 to 900 Torr. A variational RRKM model coupled with a one-dimensional master equation treatment provides a reasonably satisfactory description of the available kinetic data for both the association and dissociation processes. The association kinetic data is best fit by a collisional energy transfer parameter, 〈ΔE〉down, for the simple exponential down model, which gradually increases from −35 cm-1 at 100 K to −500 cm-1 at 740 K. The expression 3.4 × 10-10 exp(120/T) cm3 s-1 reproduces the present theoretical estimates for the high-pressure rate constant in the range from 207 to 740 K.

The potential energy surface for the C6H5-H-2 system has been calculated with a modified Gaussian-2 method (G2M). The system includes the reactions C6H5 + H-2 reversible arrow C6H6 + H (1) and H + C6H6 reversible arrow C6H7 (2). The computed molecular parameters and energetics are employed to calculate the thermal rate constants for these reactions. For the direct abstraction reaction (1), the energy barrier was found to be 8.8 kcal/mol at our best G2M(rcc,MP2) level of theory, with the tunneling corrected transition-state-theory rate constant k(1) = 9.48 x 10(-20)T(2.43) exp(-3159/T) cm(3)/(molecule s) covering 300-5000 K. This result is consistent with scattered kinetic data available in the literature. For the addition reaction (2), the barrier was found to be 8.9 kcal/mol. The rate constant calculated by solving the master equation, with tunneling corrections based on the RRKM theory, gave k(2) = 5.27 x 10(-11) exp(-1605/T) cm(3)/(molecule s) at the high-pressure limit and 300 less than or equal to T less than or equal to 1000 K. In this temperature regime, where most addition kinetics have been measured, the calculated results between 1 and 100 Torr encompass all experimental data. k(2) was found to be strongly pressure dependent above room temperature. Additionally, the effects of isotope substitution and possible secondary reactions on reported experimental data have been discussed.

The kinetics of the reaction of phenyl radical with ethylene has been investigated with the cavity-ring-down method at six temperatures between 297 and 523 K under a constant pressure of 20 torr Ar. A test performed at 60-torr pressure revealed no noticeable change in the measured rate constant value. The second-order rate constant determined by directly monitoring the decay of the phenyl radical under excess ethylene concentration conditions could be effectively represented by the Arrhenius equation k”C2H4 = 10− 11.92± 0.35 exp (−2,250 ± 630/T) cm3/s where the errors represent one-standard deviation evaluated with the weighting factor wi = (kiσi)2. This low-temperature and comparatively high-pressure result can be satisfactorily correlated by means of the RRKM theory with the high-temperature (1000–1300 K) and low-pressure (1–10 mtorr) styrene formation data reported by Fahr and Stein (Ref. 15), k″C6H5C2H3 = 4.2 × 10−12exp(−3120/T)cm3/s. The result of our multichannel RRKM calculation based on the mechanism C6H5 + C2H4 a⇌ C6H5CH2CH2b→ C6H5C2H3 + Hc→ C6H5C2H4 +(M) suggests that the rate constant for the production of styrene under the conditions employed by Fahr and Stein (kb) is essentially the same as the total rate constant, k″C2H4 = kb + kc, because kb ⪢ kc at high temperatures (T > 1000 K) and low pressures (P < 20 torr). Under atmospheric combustion conditions, however, both kb and kc are comparable and strongly dependent on T and P. The total rate constant for the C6H5 + C2H4 reaction can be given by the following expression: k”C2H4 = 1.2×10− 17 T 1.62 exp (−1490/T) cm3/s for the temperature range 300–2000 K, effectively encompassing both sets of kinetic data.

The kinetics of C6H5 reactions with c-C5H10, c-C6H12, C-C7H14, c-C8H16, and CCl4 have been studied by means of the cavity-ring-down technique in the temperature range 297-523 K. The rates of these reactions were monitored by measuring either the decay of C6H5 at 504.8 nm or the formation of C6H5O2 at 496.4 nm (or 510 nm) when a small, constant amount of O-2 was added to the slow-flowing reaction mixtures. Our absolute rate constants determined at several specific temperatures compare reasonably well with those measured in solution by either a direct probing (as for CCl4) or an indirect, relative rate method (as for the cycloalkanes). The effects of temperature on the reactions with CCl4, C-C5H10, and c-C6H12 have been investigated and our results give rise to the following Arrhenius equations: k(CCl4) = 10(-11.70 +/- 0.05) exp[(-1379 +/- 56)/T]; k(c-C5H10) = 10(-11.35 +/- 0.08) exp[(-2039 +/- 66)/T]; k(c-C6H12) = 10(-11.10 +/- 0.24) exp[(-1913 +/- 191)/T], where k(x)'s are given in units of cm(3)/molecules. Our averaged value of k(CCl4), at 333 K, 3.40 x 10(-14) cm(3)/molecules, was used to evaluate many rate constants for C6H5 reactions with hydrocarbons (RH) of interest to combustion using previously determined k(RH)/k(CCl4) ratios in solution.

The association of C6H5O with NO was studied with the cavity-ring-down method by directly monitoring the decay of C6H5O in the presence of varying, excess amounts of NO. The bimolecular rate constant determined in the temperature range 297-373 K can be effectively represented by k1 = 10(-12.12+/-0.24)e(194+/-185)/T cm3 molecule-1 with a negative activation energy of 0.8 kcal mol-1 (1 kcal = 4.184 kJ). In order to understand better the mechanism of the reaction, ab initio molecular orbital calculations were also carried out at the MP4(SDQ)/6-31G* level of theory using the HF optimized geometries. The molecular structures and energetics of five C6H5N1O2 isomers were calculated. Among them, the most likely and stable association product, phenyl nitrite (C6H5ONO), was found to be 17 kcal mol-1 below the reactants, C6H5O + NO. Combining the measured rate constant and the calculated equilibrium constant for the association reaction, C6H5O + NO = C6H5ONO the rate constant for the unimolecular decomposition of C6H5ONO was obtained as k-1 = 4.6 x 10(15)E-8580/T s-1. The relatively large frequency factor suggests that a loose transition state was involved in the reaction, akin to those of its alkyl analogs (RONO, R = CH3, C2H5, etc.).

The absolute rate constant for the C6H5 + CCl4 reaction, an important reference process employed in numerous kinetic studies in solution, has been measured for the first time with the cavity-ring-down method in the gas phase at temperatures between 298 and 523 K. A weighted least-squares analysis of our gas-phase kinetic data gives k(CCl4) = 10(9.10+/-0.08) exp[-1397 +/- 83)/T] M(-1) s(-1). Our values obtained at 298, 318, and 333 K compare reasonably well with those measured in solution by means of relative product formation and transient kinetic determination.

Absolute rate constants were measured for the reaction CN + CH2O over the temperature range 297-673 K and CN + 1,3,5-trioxane over the range of 297-600 K by the laser photolysis/laser induced fluorescence technique. The rate constants for these reactions can be effectively represented, in units of cm3/s, by: k(CH2O) = 2.82 x 10(-19) T2.72 exp(718/T), and k(1,3,5-trioxane) = 1.39 x 10(-23) T4.26 exp(1333/T), respectively. Transition state theory calculations were able to fit the temperature dependence of the CN + CH2O rates relatively well. We attempted to correlate the CN reaction rate with CH2O and other molecules which occur through simple abstraction with the corresponding OH reaction rates, yielding only a qualitative linear correlation for a majority of the processes. The reactions which deviated significantly from linearity include those which contain strong dipoles, highlighting the significant role long-range attractive forces play in CN and OH reactions. Using a simple electrostatic potential, cross-sections were determined for reactions with CN. No linear correlation was found between the calculated and experimental cross sections for the majority of the reactions studied. (C) 1993 John Wiley & Sons, Inc.

The absolute rate constant for the reaction of phenyl radical with acetylene has been measured at 20 torr total pressure in the temperature range of 297 to 523 K using the cavity-ring-down technique. These new kinetic data could be quantitatively correlated with the data obtained earlier with a relative rate method under low-pressure (10(-3)-10(-2) torr) and high-temperature (1000-1330 K) conditions. These kinetic data were analyzed in terms of the RRKM theory employing the thermochemical and molecular structure data computed with the BAC-MP4 technique.The calculated results reveal that the total rate constant for the C6H5 + C2H2 reaction (k(t)) is pressure-independent, whereas those for the formation of C6H5C2H (k(b)) and the C6H5C2H2 adduct (k(c)) are strongly pressure-dependent. A least-squares analysis of the calculated values for 300-2000 K at the atmospheric pressure of N2 or Ar can be given byk(b) = 9.5 x 10(-42)T9.33 exp(-1,713/T) k(c) = 1.8 x 10(-7)T-1.63 exp(-2,711/T)andk(t) = 4.1 x 10(-18)T1.77 exp(-1,152/T),all in units of cm3/s. The latter equation effectively represents the two sets of experimental data. (C) 1994 John Wiley & Sons, Inc.

The rate constants for the C6H5 + NO --> C6H5NO reaction have been measured with the cavity-ring-down method between 298 and 500 K to be k = 10(-11.35+/-0.06) exp[(+433+/-111)/T] cm(3)/s. The effect of pressure was examined at 388 K bysextupling of the total pressure from 20 to 120 Torr, with no significant increase in the measured rate constants within the scatter of the data. this is consistent with the k/k(infinity) ratio predicted for 388 K using the known literature value of k(infinity) for the unimolecular decomposition of nitrosobenzene. Out result for C6H5 + NO compares reasonably with the values for other R(.) + NO association reactions.

The rate constant for the reaction of NH2 with NO has been measured between 297 and 673 K using the cavity-ring-down technique to monitor the disappearance of the NH2 radical. The measured bimolecular rate constant can be effectively represented by the expression k(II) = (2.2 +/- 0.7) X 10(-12) exp[525 +/- 80)/T] cm(3)/s, which agrees reasonably well with the results of several other recent measurements employing various diagnostic methods. A multichannel RRKM calculation has been carried out to account for the observed negative temperature dependence and the product branching ratio, OH/H2O, based on Walch's recent potential energy surface data for various transition states and stable intermediates leading to the formation of the OH and H2O products. The predicted temperature dependencies agree reasonably well with experimental observations. We have also performed kinetic modeling using a set of of reactions involving H, NH3, NH2, NO, and their anticipated products. The result of the modeling aided by sensitivity analysis suggests that the unknown ''third channel'' responsible for the decline of the ([OH] + [H2O])/[NH2](0) ratio at high temperatures (ref 23) may result from secondary reactions which produce neither OH nor H2O. These reactions include NH2 + H --> NH + H-2 and NH2 + NH2 --> NH + NH3.

The kinetics of the C6H5 + O-2 reaction has been studied with the cavity-ring-down (CRD) method by monitoring the rate of C6H5O2 radical formation at 496.4 nm. The second-order rate constants measured under the conditions, 20 Torr less than or equal to P less than or equal to 80 Torr (Ar), 297 K less than or equal to T less than or equal to 473 K, were found to be pressure-independent, with a sm negative activation of 0.32 kcal/mol. A least-squares analysis of a dozen sets of data obtained by two different kinetic evaluation methods gives rise to k(O2) = 10(-11.00+/-0.08) exp[(+161 +/- 66)/T] cm(3)/molecule.s where the errors represent one-standard deviations, evaluated with the weighting factors w(i) = (k(i)/sigma(i))(2). Under the conditions employed in the present work, the C6H5 + O-2 reaction takes place primarily by the addition-stabilization process C6H5 + O-2 <-> C6H5O2 dagger (+M) --> C6H5O2. A search carried out at 575.4 nm for the production of C6H5O, which could be conveniently and sensitively detected by the CRD method, failed to detect the radical. This finding is fully consistent with the apparent large activation energy for the formation of C6H5O from the C6H5 + O-2 reaction (6 kcal/mol) recently reported by Frank and co-workers (ref 30) using the atomic resonance absorption-shock tube method at T > 900 K.

The two-laser pump-probe technique has been used to study the kinetics of the reaction of CN radicals with C3H6, CD3C2H3, C3D6, C2H3CN, C3H4 and C4H6 at temperatures between 297 and 740 K. CN was generaged by 248-nm photolysis of ICN. Laser-induced fluorescence of the CN radical has been used for its detection by CN (B←X) excitation. The values for the rate constants, given in units of cm3/s, are reported as: k(C3H6)=10−9.88±0.10exp(+244±96.3/T), k(C3H3D3) = 10−9.76±0.04 exp (+143.4±35.1/T), k(C3D6)=10−9.87±0.05exp(+231.4±47.01/T, k(C2H3CN)=10−10.52±0.02exp(+103.6±20.3/T), k(C3H4)=10−9.58±0.08 ×exp(+167.4±73.7/T), k(C4H6)=10−9.59±0.04exp(+169.2±33.1/T). The absolute rates of CN reactions with CH3CHCH2, CD3CHCH2 and CD3CDCD2 are essentially the same and are somewhat faster than that of the CN+C2H4 reaction. This suggests that the CN+C3H6 reaction occurs primarily by addition to the unsaturated bond and the CH3 group enhances the addition process slightly. The rate of the CN+CH2CHCN reaction, however, was found to be a factor of six slower than that of CN+C2H4, indicating a substantial electron withdrawing effect of the CN group in vinyl cyanide which results in the reduction in the addition rate. The rates for CN reaction with CH2CCH2 and CH2CHCHCH2 are approximately the same and are twice that of CN+C2H4.

The rate constants for the reaction Of C6H5 with HBr and DBr have been measured with the cavity-ring-down method in the temperature range of 297 to 523 K and 297 to 500 K, respectively. These rate constants can be effectively represented, in units of cm3/s, by k(HBr) = 10(-10.40+/-0.24) exp[(-554+/-208)/T] and k(DBr) = 10(-10.36+/-0.17) exp[(-612+/-151)/T].Both activation energies are similar and positive, contrary to those of alkyl radical reactions, all of which exhibit negative temperature dependencies. The difference, as pointed out before [1], could be accounted for by the electron-withdrawing effect of the phenyl vis-a-vis the electron-donating ability of the alkyls. (C) 1994 John Wiley & Sons, Inc.

The kinetics of the CN reaction with CH4 and CD4 were studied by laser induced fluorescence in the temperature range of 183 to 740 K. The measured rates may effectively be expressed, in units of cm3/s, by kCH4 = 5.15 × 10−16 T1.53 exp(−504/T) and kCD4 = 8.51 × 10−19 T2.38 exp(−403/T). Both reactions increased monotopically with temperature and exhibited a significant isotope effect. Variational TST calculations were performed for these reactions in which the energies of the transition state were calculated at several points along the reaction coordinate using the BAC-MP4 method. It was determined that as the temperature was increased, the position of the transition state moved towards smaller values of the NC:HCH3(DCD3) reaction coordinates for both CH4 and CD4 reactions. This affected both the zero-point energies and entropies of reactions. Good agreement was found between the calculated and experimental activation energies of the reactions.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTKinetics of phenyl radical reactions studied by the cavity-ring-down methodT. Yu and M. C. LinCite this: J. Am. Chem. Soc. 1993, 115, 10, 4371–4372Publication Date (Print):May 1, 1993Publication History Published online1 May 2002Published inissue 1 May 1993https://pubs.acs.org/doi/10.1021/ja00063a069https://doi.org/10.1021/ja00063a069research-articleACS PublicationsRequest reuse permissionsArticle Views613Altmetric-Citations136LEARN 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 optionsGet e-AlertscloseSupporting Info (2)»Supporting Information Supporting Information Get e-Alerts

Absolute rate constants were obtained for CN radical reactions with HCN and C2N2 employing the method of laser photolysis/laser induced fluorescence. The rate constants were found to be temperature dependent in the range 300–740 K and pressure independent in the range 100–600 Torr. The rates for CN+HCN may be described well, in units of cm3/s, by k(HCN)=2.50×10−17T1.71 exp(−770/T), which includes the shock tube results of Szekely et al. at 3000 K [Int. J. Chem. Kinetics 15, 1237 (1983)]. The measured rates for CN+C2N2 may be described well by k(C2N2)=2.19×10−21T2.70 exp(−325/T). Rice–Ramsperger–Kassel–Marcus (RRKM) theory calculations employing transition state parameters predicted by the BAC-MP4 method were able to account for the effects of temperature and pressure on both reactions.

The rates of CN reactions with C2H2, C2H4, and C4H4 (vinylacetylene) have been measured by the method of laser photolysis/laser-induced fluorescence. These rates were well fit by the following three-parameter Arrhenius expressions (in units of cm3/s): k(C2H2) = 5.67 x 10(-9) T-0.55 exp(-4.0/T), k(C2H4) = 1.72 x 10(-10) T-0.035 esp(160/T), k(C4H4) = 1.07 x 10(-7) T-0.82 exp(-228/T). Though the olefinic and acetylenic C-H bond strengths differ significantly in C2H2 and C2H4, the reaction rates in C2H2 and C2H4 are very similar. This indicates that the reactions most likely occur by addition to unsaturated carbon-carbon bonds. The rate for CN + C4H4 is approximately the sum of k(C2H2) and k(C2H4) indicating that the double and the triple bond in C4H4 are equally reactive as in C2H2 and C2H4. Conventional TST-RRKM results were unable to account for the slight negative temperature dependence of the CN + C2H2 and CN + C2H4 reaction rates. However, the approximate method introduced by Forst for calculating the specific rate constants for the decomposition of the CNC2H2 and CNC2H4 adducts yielded correct temperature dependences.

The kinetics of the reaction of CN radicals with four molecules containing only secondary CH bonds, cyclopropane (c-C3H6), cyclopentane (c-C5H10), cyclohexane (c-C6H12) and cyclooctane (c-C8H16), have been studied with the two-laser pump-probe laser photolysis/laser induced fluorescence technique in the temperature range 219 to 740 K. The rate constant for CN + c-C3H6 was found to increase monotonically with temperature. However, a small negative temperature dependene of the rate was observed for CN + c-C5H10, c-C6H12, and c-C8H16. The rate constants for these reactions can be effectively represented, in units of cm3/s, by kc-C3H6 = 1.26 × 10−15 T1.50 exp(174/T), kc-C5H10 = 10−9.76±0.06 exp(40±19/T), kc-C6H12 = 10−9.67±0.08 exp(69±12/T), and kc-C8H16 = 10−9.56±0.05 exp(73±13/T). The reactivity per secondary CH bond for c-C3H6 is significantly smaller than that for c-C5H10, c-C6H12, and c-C8H16, the latter three of which have essentially the same reactivity. This is primarily due to the significant differences between the CH bond strength in c-C3H6 and the other cyclocompounds. The small negative temperature dependences in the larger cycloalkanes observed here are attributed to long-range dipole-induced-dipole attractive interactions.