Kinetics of phenyl radical reactions studied by the cavity-ring-down method

C14H10 polycyclic aromatic hydrocarbon formation by acetylene addition to naphthalenyl radicals observed

The formation of polycyclic aromatic hydrocarbons (PAHs) during combustion has a substantial impact on environmental pollution and public health. The hydrogen-abstraction-acetylene-addition (HACA) mechanism is expected to be a significant source of larger PAHs containing more than two rings. In this study, the reactions of 1-naphthalenyl and 2-naphthalenyl radicals with acetylene (C2H2) are investigated using VUV photoionization time-of-flight mass spectrometry at 500 to 800 K, 15 to 50 torr, and reaction times up to 10 ms. Our experimental conditions allow us to probe the Bittner-Howard and modified Frenklach HACA routes, but not routes that require multiple radicals to drive the chemistry. The kinetic measurements are compared to a temperature-dependent kinetic model constructed using quantum chemistry calculations and accounting for chemical-activation and fall-off effects. We measure significant quantities of C14H10 (likely phenanthrene and anthracene), as well as 2-ethynylnaphthalene (C12H8), from the reaction of the 2-naphthalenyl radical with C2H2; these results are consistent with the predictions of the kinetic model and the HACA mechanism, but contradict a previous experimental study that indicated no C14H10 formation in the 2-naphthalenyl + C2H2 reaction. In the 1-naphthalenyl radical + C2H2 reaction system, the primary product measured is C12H8, consistent with the predicted formation of acenaphthylene via HACA. The present work provides direct experimental evidence that single-radical HACA can be an important mechanism for the formation of PAHs larger than naphthalene, validating a common assumption in combustion models.

Direct Kinetics and Product Measurement of Phenyl Radical + Ethylene

The phenyl + ethylene (C₆H₅ + C₂H₄) reaction network was explored experimentally and theoretically to understand the temperature dependence of the reaction kinetics and product distribution under various temperature and pressure conditions. The flash photolysis apparatus combining laser absorbance spectroscopy (LAS) and time-resolved molecular beam mass spectrometry (MBMS) was used to study reactions on the C₈H₉ potential energy surface (PES). In LAS experiments, 505.3 nm laser light selectively probed C₆H₅ decay, and we measured the total C₆H₅ consumption rate coefficients in the intermediate temperature region (400–800 K), which connects previous experiments performed in high-temperature (pyrolysis) and low-temperature (cavity-ring-down methods) regions. From the quantum chemistry calculations by Tokmakov and Lin using the G2M(RCC5)//B3LYP method, we constructed a kinetic model and estimated phenomenological pressure-dependent rate coefficients, k(T, P), with the Arkane package in the reaction mechanism generator. The MBMS experiments, performed at 600–800 K and 10–50 Torr, revealed three major product peaks: m/z = 105 (adducts, mostly 2-phenylethyl radical, but also 1-phenylethyl radical, ortho-ethyl phenyl radical, and a spiro-fused ring radical), 104 (styrene, co-product with a H atom), and 78 (benzene, co-product with C₂H₃ radical). Product branching ratios were predicted by the model and validated by experiments for the first time. At 600 K and 10 Torr, the yield ratio of the H-abstraction reaction (forming benzene + C₂H₃) is measured to be 1.1% and the H-loss channel (styrene + H) has a 2.5% yield ratio. The model predicts 1.0% for H-abstraction and 2.3% for H-loss, which is within the experimental error bars. The branching ratio and formation of styrene increase at high temperature due to the favored formally direct channel (1.0% at 600 K and 10 Torr, 5.8% at 800 K and 10 Torr in the model prediction) and the faster β-scission reactions of C₈H₉ isomers. The importance of pressure dependence in kinetics is verified by the increase in the yield of the stabilized adduct from radical addition from 80.2% (800 K, 10 Torr) to 88.9% (800 K, 50 Torr), at the expense of styrene + H. The pressure-dependent model developed in this work is well validated by the LAS and MBMS measurements and gives a complete picture of the C₆H₅ + C₂H₄ reaction.

From benzene to naphthalene: direct measurement of reactions and intermediates of phenyl radicals and acetylene

First-time measurement of time evolution of the main products and critical intermediates on phenyl HACA pathways with a validated pressure-dependent model.

Model-Based, Closed-Loop Control of PZT Creep for Cavity Ring-Down Spectroscopy

Cavity ring-down spectrometers typically employ a PZT stack to modulate the cavity transmission spectrum. While PZTs ease instrument complexity and aid measurement sensitivity, PZT hysteresis hinders the implementation of cavity-length-stabilized, data-acquisition routines. Once the cavity length is stabilized, the cavity's free spectral range imparts extreme linearity and precision to the measured spectrum's wavelength axis. Methods such as frequency-stabilized cavity ring-down spectroscopy have successfully mitigated PZT hysteresis, but their complexity limits commercial applications. Described herein is a single-laser, model-based, closed-loop method for cavity length control.

Low temperature kinetics of unstable radical reactions

Recent advances in Earth and satellite based observations of molecules in interstellar environments and in planetary atmospheres have highlighted the lack of information regarding many important gas-phase formation mechanisms involving neutral species at low temperatures. Whilst significant progress has been made towards a better understanding of radical-molecule reactions in these regions, the inherent difficulties involved in the investigation of reactions between two unstable radical species have hindered progress in this area. This perspective article provides a brief review of the most common techniques applied to study radical-radical reactions below room temperature, before outlining the developments in our laboratory that have allowed us to extend such measurements to temperatures relevant to astrochemical environments. These developments will be discussed with particular emphasis on our recent investigations of the reactions of atomic nitrogen with diatomic radicals.

Pound-Drever-Hall-locked, frequency-stabilized cavity ring-down spectrometer

We describe a high sensitivity and high spectral resolution laser absorption spectrometer based upon the frequency-stabilized cavity ring-down spectroscopy (FS-CRDS) technique. We used the Pound-Drever-Hall (PDH) method to lock the probe laser to the high-finesse ring-down cavity. We show that the concomitant narrowing of the probe laser line width leads to dramatically increased ring-down event acquisition rates (up to 14.3 kHz), improved spectrum signal-to-noise ratios for weak O(2) absorption spectra at λ = 687 nm and substantial increase in spectrum acquisition rates compared to implementations of FS-CRDS that do not incorporate high-bandwidth locking techniques. The minimum detectable absorption coefficient and the noise-equivalent absorption coefficient for the spectrometer are about 2×10(-10) cm(-1) and 7.5×10(-11) cm(-1)Hz(-1/2), respectively.

Rotationally resolved infrared spectroscopy of a jet-cooled phenyl radical in the gas phase

The first high-resolution IR spectra of a jet-cooled phenyl radical are reported, obtained via direct absorption laser spectroscopy in a slit-jet discharge supersonic expansion. The observed A-type band arises from fundamental excitation of the out-of-phase symmetric CH stretch mode (nu19) of b2 symmetry. Unambiguous spectral assignment of the rotational structure to the phenyl radical is facilitated by comparison with precision 2-line combination differences from Fourier transform microwave and direct absorption mm-wave measurements on the ground state [R. J. McMahon et al., Astrophys. J., 2003, 590, L61]. Least-squares fits to an asymmetric top Hamiltonian permit the upper-state rotational constants to be obtained. The corresponding gas-phase vibrational band origin at 3071.8904 (10) cm(-1) is in remarkably good agreement with previous matrix isolation studies [A. V. Friderichsen et al., J. Am. Chem. Soc., 2001, 123, 1977], and indicates only a relatively minor red shift (approximately 0.9 cm(-1)) between the gas and Ar matrix phase environment. Such studies offer considerable promise for further high resolution IR study of other aromatic radical species of particular relevance to combustion phenomena and interstellar chemistry.

Feasibility of destruction of gaseous benzene with dielectric barrier discharge

Destruction of gaseous benzene (C(6)H(6)) by dielectric barrier discharge (DBD) was studied in both laboratory-scale and scale-up DBD systems. The effects of input power, gas flow rate as well as initial concentration on benzene decomposition and energy yield were investigated. In addition, qualitative analysis on byproducts and relatively detailed discussion on mechanisms were also presented in this paper. At last, we systematically illustrated the feasibility of benzene removal with DBD on basis of three aspects: estimation of treatment cost per unit volume, comparison with other plasmas, and problems existed in DBD system. The results will help impel actual application of DBD on waste gas containing benzene.

Determination of OH Number Densities Outside of a Platinum Catalyst Using Cavity Ringdown Spectroscopy

It is demonstrated that cavity ringdown spectroscopy (CRDS) can be used to probe reaction intermediates desorbing from the surface during a heterogeneous catalytic reaction and provide information valuable in understanding the reaction kinetics. During water formation from H2 and O2, desorbed OH molecules outside of a polycrystalline platinum catalyst were quantified as a function of the relative hydrogen concentration, alphaH2 using CRDS. The temperature of the catalyst was 1500 K, the total pressure was 26 Pa, and the flow was set to 100 sccm. At a distance of 6.5 mm from the Pt catalyst, the maximum OH concentration was found to be 1.5+/-0.2x10(12) cm(-3) at an alphaH2 value of 10%, and the rotational temperature was determined to be 775+/-24 K. The desorbed OH molecules were also probed using laser-induced fluorescence (LIF), and the alphaH2-dependent OH abundance was compared with the CRDS results. The relative concentration of OH probed with LIF appeared to be lower at alphaH2=30-50% compared to what was determined by CRDS. The observed discrepancy is suggested to be due to electronic quenching, as was indicated by a shorter fluorescence lifetime at alphaH2=30% compared to at alphaH2=10%.

Experimental and Computational Studies of the Phenyl Radical Reaction with Allene

The kinetics for the gas-phase reaction of phenyl radicals with allene has been measured by cavity ring-down spectrometry (CRDS), and the mechanism and initial product branching have been elucidated with the help of quantum-chemical calculations. The absolute rate constant measured by the CRDS technique can be expressed by the following Arrhenius equation: kallene (T=301-421 K)=(4.07+/-0.38)x10(11) exp[-(1865+/-85)/T] cm3 mol(-1) s(-1). Theoretical calculations, employing high level G2M energetic and IRCMax(RCCSD(T)//B3LYP-DFT) molecular parameters, indicate that under our experimental conditions the most preferable reaction channel is the addition of phenyl radicals to the terminal carbon atoms in allene. Predicted total rate constants agree with the experimental values within 40%. Calculated total and branching rate constants are provided for high-T kinetic modeling.

A Gas-Phase Kinetic Study of the Reaction between Bromine Monoxide and Methylperoxy Radicals at Atmospheric Temperatures

The rate constant of the reaction of BrO with CH(3)O(2) was determined to be k1 = (6.2 +/- 2.5) x 10(-12) cm3 molecule(-1) s(-1) at 298 K and 100-200 Torr of O2 diluent. Quoted uncertainty was two standard deviations. No significant pressure dependence of the rate constants was observed at 100-200 Torr total pressure of N2 or O2 diluents. Temperature dependence of the rate constants was further investigated over the range 233-333 K, and an Arrhenius type expression was obtained for k1 = 4.6 x 10(-13) exp[(798 +/- 76)/T] cm3 molecule(-1) s(-1). The product branching ratios were evaluated and the atmospheric implications were discussed.

Reaction of phenylperoxy radicals with NO2 at 298 K

In the present work, phenylperoxy radicals were generated by stationary 254 nm photolysis of iodobenzene and nitrosobenzene in the presence of O(2) and NO(2) at 298 K and a total pressure of 1 bar (M = N(2)). Experiments were performed on time scales of seconds or minutes in a temperature controlled photoreactor made of quartz (v = 209 L). Major gas phase products identified and quantified in situ by long-path IR absorption include N(2)O(5), NO, HONO, HNO(3), CO, and o-nitrophenol. In addition, evidence is presented for the formation of an aerosol consisting of p-nitrophenol. The occurrence of N(2)O(5) as a major product in both reaction systems, the strong loss of NO(2) in the iodobenzene system and the comparison of measured product distributions with the results of numerical model calculations suggest that the reaction C(6)H(5)O(2) + NO(2) --> C(6)H(5)O + NO(3), k(5)occurs in both photolysis systems, a major part of the NO(3) being scavenged as N(2)O(5). The results of ab initio calculations imply that proceeds via a short-lived peroxynitrate intermediate. In the photolysis of nitrosobenzene-NO(2)-O(2)-N(2) mixtures, NO and NO(2) compete for C(6)H(5)O(2) radicals. Comparison of measured and modelled product distributions allows to set a lower limit of k(5) > 1 x 10(-12) cm(3) molecule(-1) s(-1) at 298 K. This lower limit is consistent with the assumption that k(5) is equal to the high pressure recombination rate constant of RO(2) + NO(2) --> RO(2)NO(2) reactions, i.e. with k(5) approximately 7 x 10(-12) cm(3) molecule(-1) s(-1) at 298 K, 1bar.

Direct observation and reactions of Cl3 radical

The broad absorption of Cl3 radical was observed between 1150 and 1350 nm using cavity ring-down spectroscopy at 213-265 K and 50-200 Torr with He, N2, Ar, or SF6 diluents. The absorption intensity of Cl3 increased at lower temperature and higher pressure. SF6 was the most efficient diluent gas. The temperature dependent equilibrium constants for Cl3 formation from Cl+Cl2 were theoretically calculated at the MP4SDQ6-311+G(d) level. Observed decay time profiles of Cl3 and the pressure dependence of Cl3 formation are explained by the equilibrium reaction and a decay reaction of Cl+Cl3.

Kinetic Study of IO Radical with RO2 (R = CH3, C2H5, and CF3) Using Cavity Ring-Down Spectroscopy

The reactions of iodine monoxide radical, IO, with alkyl peroxide radicals, RO(2) (R = CH(3), C(2)H(5), and CF(3)), have been studied using cavity ring-down spectroscopy. The rate constant of the reaction of IO with CH(3)O(2) was determined to be (7.0 +/- 3.0) x 10(-11) cm(3) molecule(-1) s(-1) at 298 K and 100 Torr of N(2) diluent. The quoted uncertainty is two standard deviations. No significant pressure dependence of the rate constant was observed at 30-130 Torr total pressure of N(2) diluent. The temperature dependence of the rate constants was also studied at 213-298 K. The upper limit of the branching ratio of OIO radical formation from IO + CH(3)O(2) was estimated to be <0.1. The reaction rate constants of IO + C(2)H(5)O(2) and IO + CF(3)O(2) were determined to be (14 +/- 6) x 10(-11) and (6.3 +/- 2.7) x 10(-11) cm(3) molecule(-1) s(-1) at 298 K, 100 Torr of N(2) diluent, respectively. The upper limit of the reaction rate constant of IO with CH(3)I was <4 x 10(-14) cm(3) molecule(-1) s(-1).

High-resolution IR cavity ring-down spectroscopy of jet-cooled free radicals and other species

Initial spectral results are reported from a newly constructed cavity ringdown spectrometer. The apparatus incorporates a slit-jet expansion, with or without a discharge, to produce cold sample molecules. High spectral resolution in both the near- and mid-IR is obtained by using stimulated Raman scattering of the pulsed amplified output of a cw Ti:Sa ring laser. Molecular spectra presented include the electronic near-IR transitions a (1)Delta(g)(-)<-- X (3)Sigma(g)(-) of O(2) and B (3)Pi(g)<-- A (3)Sigma(u)(+) of metastable N(2) and vibrational overtones of H(2)O (polyad 2) and the OH radical. Fundamental vibrational transitions of CH(3) (nu(3)) in the mid-IR are also observed. This apparatus has demonstrated the potential for obtaining high-resolution spectra of both reactive and non-reactive species throughout the entire IR region.

Experimental and Computational Studies of the Phenyl Radical Reaction with Propyne

The kinetics for the gas-phase reaction of phenyl radical with propyne has been measured by cavity ring-down spectrometry (CRDS), and the mechanism and initial product branching have been elucidated with the help of quantum chemical calculations. Absolute rate constants measured by the CRDS technique can be expressed by the following Arrhenius equation: (k/cm(3) mol(-1) s(-1)): k(propyne)(T=301-428 K)=(3.68+/-0.92) x 10(11)exp[-(1685+/-80)/T]. The experiment is unable to distinguish between the possible reactive channels, but theory indicates that phenyl radicals preferably add to the unsaturated terminal carbon atom in propyne under our experimental conditions. Theoretical kinetic calculations, employing high-level G2M(RCC, RMP2) and G3 energetic and IRCMax(RCCSD(T)//B3LYP-DFT) molecular parameters, reproduce the total experimental rate constants within a factor of three. Calculated total and branching rate constants are provided for high-T kinetic modeling. Addition reactions of phenyl to C3H4 are estimated to be less important molecular-growth pathways in high-T conditions (T>1000 K) in comparison to the C6H5 + C2H2 reaction.

Computational Study on the Mechanism and Rate Constant for the C6H5 + C6H5NO Reaction

The reaction mechanism of C6H5 + C6H5NO involving four product channels on the doublet-state potential energy surface has been studied at the B3LYP/6-31+G(d, p) level of theory. The first reaction channel occurs by barrierless association forming (C6H5)2NO (biphenyl nitroxide), which can undergo isomerization and decomposition. The second channel takes place by substitution reaction producing C12H10 (biphenyl) and NO. The third and fourth channels involve direct hydrogen abstraction reactions producing C6H4NO + C6H6 and C6H5NOH + C6H4, respectively. Bimolecular rate constants of the above four product channels have been calculated in the temperature range 300-2000 K by the microcanonical Rice-Ramsperger-Kassel-Marcus theory and/or variational transition-state theory. The result shows the dominant reactions are channel 1 at lower temperatures (T < 800 K) and channel 3 at higher temperatures (T > 800 K). The total rate constant at 7 Torr He is predicted to be k(t) = 3.94 x 10(21) T(-3.09) exp(-699/T) for 300-500 K, 2.09 x 10(20) T(-3.56) exp(2315/T) for 500-1000 K, and 1.51 x 10(2) T(3.30) exp(-3043/T) for 1000-2000 K (in units of cm3 mol(-1) s(-1)), agreeing reasonably with the experimental data within their reported errors. The heats of formation of key products including biphenyl nitroxide, hydroxyl phenyl amino radical, and N-hydroxyl carbazole have been estimated.

Direct Observation of Adduct Formation of Alkyl and Aromatic Iodides with Cl Atoms Using Cavity Ring-Down Spectroscopy

The reactions of Cl atoms with RI (R = n-C3H7, n-C4H9, cyclo-C6H11, C6H5, C6F5, and p-CH3C6H4) have been studied using cavity ring-down spectroscopy at a temperature range of 233-313 K and at 100 Torr total pressure of N2 diluent. Visible absorption spectra of the RI-Cl adducts were recorded at 440-520 nm at 263 K. The yields of the adducts were temperature-dependent. There was no discernible reaction of the adducts in the presence of 100 Torr of O2 at 263 K. Theoretical calculations were performed for C4H9I-Cl and C6H5I-Cl for quantitative explanation of the absorption spectra and the strength of the I-Cl bonds in the charge-transfer complexes. Evidence for the adduct formation following the reaction of Cl with C6H5Br was sought but not found at 440 and 520 nm.

Validation of the Extended Simultaneous Kinetics and Ringdown Model by Measurements of the Reaction NH2 + NO

The determination of rate constants for fast chemical reactions from nonexponential cavity ringdown profiles requires a consideration of the interfering laser bandwidth effect that arises if the line width of the ringdown probe laser exceeds the absorption line width of the detected species. The deconvolution of the kinetics and the bandwidth effect can be accomplished with the extended simultaneous kinetics and ringdown (eSKaR) model presented by Guo et al. (Guo, et al. Phys. Chem. Chem. Phys. 2003, 5, 4622). We present a detailed validation of this eSKaR model by a corresponding investigation of the well-known rate constant for the reaction NH2 + NO. Line profiles of the pulsed ringdown probe laser and the NH2 absorption line were determined from forward modeling of experimental ringdown profiles and verified by narrow-bandwidth laser absorption measurements. In addition, the rate constant for the title reaction was evaluated using the eSKaR model and also by means of a conventional pump-probe approach with variable time delays between the photolysis (pump) and ringdown (probe) laser pulses. The resulting room temperature rate constant for the NH2 + NO reaction, k1= (8.5 +/- 1.0) x 10(12) cm(3) mol(-1) s(-1), and the room temperature pressure broadening coefficient of NH2, = 2.27 GHz/bar, measured on the A2A1<-- X2B1 transition at wavelengths around lambda = 597 nm, were found to be in excellent agreement with the available literature data.

Observation of Adducts in the Reaction of Cl Atoms with XCH2I (X = H, CH3, Cl, Br, I) Using Cavity Ring-Down Spectroscopy

The reactions of Cl atoms with XCH2I (X = H, CH3, Cl, Br, I) have been studied using cavity ring-down spectroscopy in 25-125 Torr total pressure of N2 diluent at 250 K. Formation of the XCH2I-Cl adduct is the dominant channel in all reactions. The visible absorption spectrum of the XCH2I-Cl adduct was recorded at 405-632 nm. Absorption cross-sections at 435 nm are as follows (in units of 10(-18) cm2 molecule(-1)): 12 for CH3I, 21 for CH3CH2I, 3.7 for CH2ICl, 7.1 for CH2IBr, and 3.7 for CH2I2. Rate constants for the reaction of Cl with CH3I were determined from rise profiles of the CH3I-Cl adduct. k(Cl + CH3I) increases from (0.4 +/- 0.1) x 10(-11) at 25 Torr to (2.0 +/- 0.3) x 10(-11) cm3 molecule(-1) s(-1) at 125 Torr of N2 diluent. There is no discernible reaction of the CH3I-Cl adduct with 5-10 Torr of O2. Evidence for the formation of an adduct following the reaction of Cl atoms with CF3I and CH3Br was sought but not found. Absorption attributable to the formation of the XCH2I-Cl adduct following the reaction of Cl atoms with XCH2I (X = H, CH3, Br, I) was measured as a function of temperature over the range 250-320 K.

Enhancement of the cavity ringdown effect based on electromagnetically induced transparency

We show that the unique absorption and dispersion properties of the electromagnetically induced transparency can be used effectively to relax the conditions for observing the cavity ringdown effect (CRE), which can be useful in applications of CRE in ultrasensitive detection of chemical species. A more straightforward and simple method is used to model the interesting CRE.

Gas-phase mechanism for dioxin formation

We consider the formation of dioxin in the gas phase through the combination of phenoxy radicals in the context of a recently published scenario regarding the possibility of forming highly chlorinated benzenes in poorly mixed systems. It is demonstrated that the production of free chlorine in fuel lean combustion systems and subsequent mixing and quenching with fuel rich mixtures leads to chlorine atoms as the main reactive radicals. Under such conditions high concentrations of chlorinated phenoxy radicals can be formed. This leads to the formation of concentrations of dioxins that are consistent with recent measurements of Sidhu et al. [Combust. Flame 100 (1995) 11]. It suggests that there may be a gas-phase channel for the dioxin formation in addition to the usually assumed surface catalyzed process.

Relative rates and approximate rate constants for inter- and intramolecular hydrogen transfer reactions of polymer-bound radicals

Measurements of relative rates and rate constants for inter- and intramolecular hydrogen transfer reactions of polymer-bound radicals are reported. The relative rate of reaction of resin-bound primary alkyl radical with tributyltin hydride is about 2 times slower than that of the benchmark reaction in solution. The data do not reveal whether this is due to a reduced rate constant or a lower concentration of tin hydride in the resin phase. Yet the difference between solid and solution reactions is small enough to be neglected, and it appears that rate constants measured in solution can be applied directly to resin-bound radicals. A resin-bound aryl radical abstracts a hydrogen atom rapidly (k = 3 x 10(6) s(-1)) from its own polymer backbone and linker, and a simplified view of the resin as a "solvent" is suggested for predicting such effects with other polymers and linkers. Rapid cyclizations of resin-bound aryl radicals will be possible, but slower cyclizations and most bimolecular reactions will be difficult due to the competing polymer/linker hydrogen transfer.