Room Temperature Rate Constants Research Articles

Kinetics of O3 with Ca+ and Its Higher Oxides CaOn+ (n = 1-3) and Updates to a Model of Meteoric Calcium in the Mesosphere and Lower Thermosphere.

The room-temperature rate constants and product branching fractions of CaOn+ (n = 0-3) + O3 are measured using a selected ion flow tube apparatus. Ca+ + O3 produces CaO+ + O2 with k = 9 ± 4 × 10-10 cm3 s-1, within uncertainty equal to the Langevin capture rate constant. This value is significantly larger than several literature values. Most likely, those values were underestimated due to the reformation of Ca+ from the sequential chemistry of higher calcium oxide cations with O3, as explored here. A rate constant of 8 ± 3 × 10-10 cm3 s-1 is recommended. Both CaO+ and CaO2+ react near the capture rate constant with ozone. The CaO+ reaction yields both CaO2+ + O2 (0.80 ± 0.15 branching) and Ca+ + 2O2. Similarly, the CaO2+ reaction yields both CaO3+ + O2 (0.85 ± 0.15 branching) and CaO+ + 2O2. CaO3+ + O3 yield CaO2+ + 2O2 at 2 ± 1 × 10-11 cm3 s-1, about 2% of the capture rate constant. The results are supported using density functional calculations and statistical modeling. In general, CaOn+ + O3 yield CaOn+1+ + O2, the expected oxidation. Some fraction of CaOn+1+ is produced with sufficient internal energy to further dissociate to CaOn-1+ + O2, yielding the same products as the oxidation of O3 by CaOn+. Mesospheric Ca and Ca+ concentrations are modeled as functions of day, latitude, and altitude using the Whole Atmosphere Community Climate Model (WACCM); incorporating the updated rate constants improves agreement with concentrations derived from lidar measurements.

Atmospheric chemistry of ketones: Reaction of OH radicals with 2-methyl-3-pentanone, 3-methyl-2-pentanone and 4-methyl-2-pentanone.

This work reports new kinetic and mechanistic information on the atmospheric chemistry of ketones. Both absolute and relative rate methods were used to determine the rate constants for OH reactions with 2-methyl-3-pentanone (2M3P), 3-methyl-2-pentanone (3M2P) and 4-methyl-2-pentanone (4M2P), three widely used compounds in the industry. This work constitutes the first temperature dependence study of the reactions of OH with 2M3P and 3M2P. The following rate constants values are recommended at 298 K (in 10-12 cm3 molecule-1 s-1): kOH+2M3P = 3.49 ± 0.5; kOH+3M2P = 6.02 ± 0.14 and kOH+4M2P = 11.02 ± 0.42. The following Arrhenius expressions (in units of cm3 molecule-1 s-1) adequately describe the measured rate constants for OH reactions with 2M3P and 3M2P in the temperature range 263-373 K: k2M3P = (2.33 ± 0.06) × 10-12 exp((127.4 ± 18.6)/T) and k3M2P = (1.05 ± 0.14) × 10-12 exp((537 ± 41)/T). Products studies from the reactions of OH with the investigated ketones were conducted in a 7.3 m3 simulation chamber using PTR-ToF-MS, UHPLC-MS and GC-MS. A series of short chain carbonyl compounds including formaldehyde, acetone, acetaldehyde, 2-butanone and 2-methypropanal were observed as products. Combining the yields of carbonyls measured with those estimated from the SAR method, we propose various mechanistic degradation schemes of the investigated ketones initiated by reaction with OH radicals.

Kinetics of CO+ and CO2+ with N and O atoms.

We have measured reaction rate constants for CO+ and CO2+ reacting with N and O atoms using a selected ion flow tube apparatus equipped with a microwave discharge atom source. Experimental work was supplemented by molecular structure calculations. Calculated pathways show the sensitivity of kinetic barriers to theoretical methods and imply that high-level ab initio methods are required for accurate energetics. We report room-temperature rate constants of 1.0 ± 0.4 × 10-11 cm3 s-1 and 4.0 ± 1.6 × 10-11 cm3 s-1 for the reactions of CO+ with N and O atoms, respectively, and 8.0 ± 3.0 × 10-12 cm3 s-1 and 2.0 ± 0.8 × 10-11 cm3 s-1 for the reactions of CO2+ with N and O atoms, respectively. The reaction of CO2+ + O is observed to yield O2+ exclusively. These values help resolve discrepancies in the literature and are important for modeling of the Martian atmosphere.

Experimental and Theoretical Study of the Kinetics of the OH + Propionaldehyde Reaction between 277 and 375 K at Low Pressure.

Measurements of the rate constant for the reaction of OH radicals with propionaldehyde as a function of temperature were performed using low-pressure discharge-flow tube techniques coupled with laser-induced fluorescence detection of OH radicals. The measured room-temperature rate constant of (1.51 ± 0.22) × 10(-11) cm(3) molecules(-1) s(-1) at 4 Torr was generally lower but in reasonable agreement with previous absolute and relative rate studies at higher pressures. Measurements as a function of temperature resulted in an Arrhenius expression of (2.3 ± 0.4) × 10(-11) exp[(-110 ± 50)/T] cm(3) molecules(-1) s(-1) between 277 and 375 K at 4 Torr. The observed temperature dependence at low pressure is in contrast to previous measurements of a negative temperature dependence at higher pressures. Ab initio calculations of the potential energy surface for this reaction suggest that the primary reaction pathway involves the formation of a hydrogen-bonded prereactive complex, which could account for the difference in the observed temperature dependence at lower and higher pressures.

Kinetics, Mechanism, and Secondary Organic Aerosol Yield of Aqueous Phase Photo-oxidation of α-Pinene Oxidation Products.

Formation of secondary organic aerosol (SOA) involves atmospheric oxidation of volatile organic compounds (VOCs), the majority of which are emitted from biogenic sources. Oxidation can occur not only in the gas-phase but also in atmospheric aqueous phases such as cloudwater and aerosol liquid water. This study explores for the first time the aqueous-phase OH oxidation chemistry of oxidation products of α-pinene, a major biogenic VOC species emitted to the atmosphere. The kinetics, reaction mechanisms, and formation of SOA compounds in the aqueous phase of two model compounds, cis-pinonic acid (PIN) and tricarballylic acid (TCA), were investigated in the laboratory; TCA was used as a surrogate for 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA), a known α-pinene oxidation product. Aerosol time-of-flight chemical ionization mass spectrometry (Aerosol-ToF-CIMS) was used to follow the kinetics and reaction mechanisms at the molecular level. Room-temperature second-order rate constants of PIN and TCA were determined to be 3.3 (± 0.5) × 10(9) and 3.1 (± 0.2) × 10(8) M(-1) s(-1), respectively, from which were estimated their condensed-phase atmospheric lifetimes. Aerosol-ToF-CIMS detected a large number of products leading to detailed reaction mechanisms for PIN and MBTCA. By monitoring the particle size distribution after drying, the amount of SOA material remaining in the particle phase was determined. An aqueous SOA yield of 40 to 60% was determined for PIN OH oxidation. Although recent laboratory studies have focused primarily on aqueous-phase processing of isoprene-related compounds, we demonstrate that aqueous formation of SOA materials also occurs from monoterpene oxidation products, thus representing an additional source of biogenically driven aerosol formation.

Exploring NH (X3Σ−) + D (2S) → N (4S) + HD (X1Σg+) reaction with time-dependent wave packet method

For the title reaction, we performed the time-dependent wave packet calculations over the collision energy range from 0 to 1.0eV on the double many-body expansion 4A″ potential energy surface reported by Poveda and Varandas (2005) and combined many-body expansion and neural network 4A″ potential energy surface reported by Zhai and Han (2011), respectively. We obtained the initial state-selected reaction probabilities, reaction cross-sections, and rate constants. The calculated room-temperature rate constants on both potential energy surfaces agree with the experimental estimation.

Selected-ion flow tube temperature-dependent measurements for the reactions of O₂⁺ with N atoms and N₂⁺ with O atoms.

The temperature variation of rate constants has been measured for the gas phase reactions of the oxycation O2(+) with N atoms and of N2(+) with O atoms from 120 to 400 K using a variable temperature-selected ion flow tube. Measured room temperature rate constants, 0.75 × 10(-10) cm(3) s(-1) (±30%) for O2(+) with N and 1.4 × 10(-10) cm(3) s(-1) (±30%) for N2(+) with O, are in agreement with previously reported values. A temperature dependence of T(-0.7(±0.3)) is observed for the O2(+) + N reaction; however, the N2(+) + O reaction is found to be independent of temperature. Calculations at varying levels of theory were used in tandem with experiments to evaluate likely pathways in potential energy surfaces for the reactions of concern.

Dynamics of the O + ClO reaction: reactive and vibrational relaxation processes.

Classical trajectories have been integrated to study the O + ClO reaction, both reactive and vibrational energy transfer processes, for the range of temperatures 100 ≤ T/K ≤ 500 using momentum Gaussian binning. The employed potential energy surface is the recently proposed single-sheeted double many-body expansion potential energy surface for the (2)A" ground-state of ClO2 based on multireference ab initio data. A capture-type regime with a room-temperature rate constant of (17.8 ± 0.5) × 10(-12) cm(3) s(-1) and temperature dependence of k(T/K)/cm(3) s(-1) = 22.4 × 10(-12) × T(-0.81) exp(-39.2/T) has been found. Although the value reported here is half of the experimental and recommended one, tentative explanations are given. Other dynamical attributes are also examined for the title reaction, with state-to-all and state-to-state vibrational relaxation and excitation rate constants reported for temperatures of relevance in stratospheric chemistry.

Theoretical investigation of intersystem crossing between the ã¹A₁ and X³B₁ states of CH₂ induced by collisions with helium.

Collisional energy transfer between the ground (X³B₁) and first excited (ã¹A₁) states of CH2 is facilitated by strong mixing of the rare pairs of accidentally degenerate rotational levels in the ground vibrational manifold of the [Formula: see text] state and the (020) and (030) excited bending vibrational manifolds of the X state. The simplest model for this process involves coherent mixing of the scattering T-matrix elements associated with collisional transitions within the unmixed ã and X states. From previous calculations in our group, we have determined cross sections and room-temperature rate constants for intersystem crossing of CH2 by collision with He. These are used in simulations of the time dependence of the energy flow, both within and between the X and ã vibronic manifolds. Relaxation proceeds through three steps: (a) rapid equilibration of the two mixed-pair levels, (b) fast relaxation within the ã state, and (c) slower relaxation among the levels of the X state. Collisional transfer between the fine-structure levels of the triplet (X) state is very slow.

Photochemical properties of trans-1-chloro-3,3,3-trifluoropropene (trans-CHCl═CHCF3): OH reaction rate constant, UV and IR absorption spectra, global warming potential, and ozone depletion potential.

Measurements of the rate constant for the gas-phase reactions of OH radicals with trans-1-chloro-3,3,3-trifluoropropene (trans-CHCl═CHCF3) were performed using a flash photolysis resonance-fluorescence technique over the temperature range 220-370 K. The reaction rate constant exhibits a noticeable curvature of the temperature dependence in the Arrhenius plot, which can be represented by the following expression: kt-CFP (220-370 K) = 1.025 × 10(-13) × (T/298)(2.29) exp(+384/T) cm(3)molecule(-1) s(-1). The room-temperature rate constant was determined to be kt-CFP (298 K) = (3.29 ± 0.10) × 10(-13) cm(3) molecule(-1) s(-1), where the uncertainty includes both two standard errors (statistical) and the estimated systematic error. For atmospheric modeling purposes, the rate constant below room temperature can be represented by the following expression: kt-CFP (220-298 K) = (7.20 ± 0.46) × 10(-13) exp[-(237 ± 16)/T] cm(3) molecule(-1) s(-1). There was no difference observed between the rate constants determined at 4 kPa (30 Torr) and 40 kPa (300 Torr) at both 298 and 370 K. The UV and IR absorption cross sections of this compound were measured at room temperature. The atmospheric lifetime, global warming potential, and ozone depletion potential of trans-CHCl═CHCF3 were estimated.

Temperature dependences for the reactions of O2− and O− with N and O atoms in a selected-ion flow tube instrument

Rate constants for the reactions of O2(-) and O(-) with N and O atoms have been measured for the first time as a function of temperature from 173 to 500 K for O(-) reactions and 173 to 400 K for O2(-) reactions. Room temperature rate constants for O2(-) reacting with N and O are 3.1 × 10(-10) and 1.7 × 10(-10) cm(3) s(-1), respectively, and the corresponding O(-) rate constants are 1.7 × 10(-10) and 1.5 × 10(-10) cm(3) s(-1), in good agreement with previous values. Temperature dependences are about T(-1.7) for both O2(-) reactions and T(-0.6) and T(-1.3) for the reactions of O(-) with N and O, respectively. Branching for the O2(-) reaction with N is found to predominantly form O(-) (>85%) in contrast to previous measurements, which reported NO2 + e(-) as the main channel. Calculations point to the present results being correct. The potential energy surface for this reaction was calculated using density functional theory, coupled cluster with singles, doubles (triples), complete active space self-consistent field, and complete active space second-order perturbation methods and is found to be quite complex, with agreement between the calculated surface and the observed kinetic data only possible through the inclusion of dynamical correlation.

The kinetics of the gas-phase reactions of selected monoterpenes and cyclo-alkenes with ozone and the NO3 radical

The relative rate method has been used to measure the room-temperature rate constants for the gas-phase reactions of ozone and NO3 with selected monoterpenes and cyclo-alkenes with structural similarities to monoterpenes. Measurements were carried out at 298 ± 2 K and 760 ± 10 Torr. The following rate constants (in units of 10−18 cm3 molecule−1 s−1) were obtained for the reaction with ozone: methyl cyclohexene (132 ± 17), terpinolene (1290 ± 360), ethylidene cyclohexane (223 ± 57), norbornene (860 ± 240), t-butyl isopropylidene cyclohexane (1500 ± 460), cyclopentene (543 ± 94), cyclohexene (81 ± 18), cyclooctene (451 ± 66), dicyclopentadiene (1460 ± 170) and α-pinene (107 ± 13). For the reaction with NO3 the rate constants obtained (in units of 10−12 cm3 molecule−1 s−1) were: methyl cyclohexene (7.92 ± 0.95), terpinolene (47.9 ± 4.0), ethylidene cyclohexane (4.30 ± 0.24), norbornene (0.266 ± 0.029), cyclohexene (0.540 ± 0.017), cyclooctene (0.513 ± 0.029), dicyclopentadiene (1.20 ± 0.10) and α-pinene (5.17 ± 0.62). Errors are quoted as the root mean square of the statistical error (95% confidence) and the quoted error in the rate constant for the reference compound. Combining these results with previous studies, new recommendations for the rate constants are presented. Molecular orbital energies were calculated for each alkene and the kinetic data are discussed in terms of the deviation from the structure–activity relationship obtained from the rate constants for a series of simple alkenes. Lifetimes with respect to key initiators of atmospheric oxidation have been calculated suggesting that the studied reactions play dominant roles in the night-time removal of these compounds from the atmosphere.

Exploring the Reactions of Fe+ and FeO+ with NO and NO2

We report for the first time temperature dependences (from 300 to 600 K) of the reactions of Fe(+) and FeO(+) with NO and NO(2). Both ions react quickly with NO(2), and their rate constants have weak negative temperature dependences. The former is consistent with the calculated energy profile along the Fe(+) + NO(2) reaction coordinate. Ground state Fe(+) reacts with NO(2) to produce only FeO(+), while FeO(+) reacts with NO(2) to produce NO(+) exclusively. Certain source conditions produce excited Fe(+), as evidenced by production of primary NO(+), which is endothermic with the ground state by 0.35 eV. The room temperature rate constants are in agreement with previous values. For the reactions of Fe(+) and FeO(+) with NO, we find an upper limit of <1.0 × 10(-12) cm(3) s(-1) for both rate constants, in contrast to a previous report of a rate constant of ∼1.7 × 10(-11) cm(3) s(-1) for Fe(+) + NO. Because this is an endothermic process, the prior report interpreted the reaction as a termolecular process involving two NO molecules; instead, we show that the previous results were likely due to an NO(2) impurity. Implications for other metal cation reactions which have been speculated to occur by the termolecular mechanism are discussed.

Rate constants for the reactions of Cl atoms with a series of C6–C10 cycloalkanes and cycloketones at 297 ± 2 K

AbstractRate constants for the reactions of Cl atoms with cycloheptane, cyclooctane, cyclodecane, cyclohexanone, cycloheptanone, cyclooctanone, and cyclodecanone have been measured at 297 ± 2 K and atmospheric pressure of air using a relative rate method. n‐Butane, with a rate constant of 2.05 × 10−10 cm3 molecule−1 s−1, was used as the reference compound, and the rate constants obtained (in units of 10−10 cm3 molecule−1 s−1) were cycloheptane, 4.22 ± 0.15; cyclooctane, 4.57 ± 0.15; cyclodecane, 5.13 ± 0.15; cyclohexanone, 1.79 ± 0.06; cycloheptanone, 2.46 ± 0.07; cyclooctanone, 2.97 ± 0.09; and cyclodecanone, 3.65 ± 0.15, where the indicated errors are two least‐squares standard deviations and do not include uncertainties in the rate constant for the reference compound n‐butane. Room temperature rate constants for the C5–C10 cycloketones indicate that the CH2 groups adjacent to the carbonyl group are almost totally deactivated toward H‐atom abstraction by Cl atoms, and this also applies to acyclic ketones. A previous structure–reactivity relationship for Cl + alkanes has been extended to include acyclic and cyclic ketones. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 45: 52–58, 2013

Laser-induced fluorescence study of spin–orbit quenching of Cl(2P1/2) by H2, D2, and HD gases

Employing ultraviolet laser flash photolysis in combination with pulsed vacuum ultraviolet laser induced-fluorescence spectroscopy room-temperature rate constants kQ,H2=(5.8±0.6)×10−11cm3s−1, kQ,D2=(5.6±0.6)×10−12cm3s−1, and kQ,HD=(2.7±0.3)×10−11cm3s−1 for electronically non-adiabatic quenching of spin–orbit excited Cl(2P1/2) atoms in collisions with H2, D2, and HD gases were determined, which allow for comparison with results of recent quantum-scattering calculations [14,15].

A tunable diode laser absorption spectrometer for formaldehyde atmospheric measurements validated by simulation chamber instrumentation

Abstract A tunable diode laser absorption spectrometer (TDLAS) for formaldehyde atmospheric measurements has been set up and validated through comparison experiments with a Fourier transform infrared spectrometer (FT-IR) in a simulation chamber. Formaldehyde was generated in situ in the chamber from reaction of ethene with ozone. Three HCHO ro-vibrational line intensities (at 2909.71, 2912.09 and 2914.46 cm −1) possibly used by TDLAS were calibrated by FT-IR spectra simultaneously recorded in the 1600-3200 cm −1 domain during ethene ozonolysis, enabling the on-line deduction of the varying concentration for HCHO in formation. The experimental line intensities values inferred confirmed the calculated ones from the updated HITRAN database. In addition, the feasibility of stratospheric in situ HCHO measurements using the 2912.09 cm −1 line was demonstrated. The TDLAS performances were also assessed, leading to a 2s detection limit of 88 ppt in volume mixing ratio with a response time of 60 sec at 30 Torr and 294 K for 112 m optical path. As part of this work, the room-temperature rate constant of this reaction and the HCHO formation yield were found to be in excellent agreement with the compiled literature data.

Molecular size dependent falloff rate constants for the recombination reactions of alkyl radicals with O2 and implications for simplified kinetics of alkylperoxy radicals

AbstractThe recombination reactions of prototypical alkyl radicals (R) with O2, R + O2 → RO2, have been investigated theoretically by using the variational transition state theory and Rice–Ramsperger–Kassel–Marcus theory/master equation calculations based on the CASPT2(7,5)/aug‐cc‐pVDZ//B3LYP/6‐311G(d,p) potential energy curves and B3LYP/6‐311G(d,p) geometries and vibrational frequencies. The calculated high‐pressure limiting rate constants well reproduced the experimental room temperature rate constants for ethyl, i‐propyl, n‐butyl, s‐butyl, and t‐butyl radicals and were nearly the same for the same class of alkyl radicals, namely for primary (ethyl and n‐butyl) and for secondary (i‐propyl and s‐butyl) radicals. Hypothetical falloff calculations ignoring the subsequent dissociation/isomerization reactions of RO2 indicated that the low‐pressure limiting (LPL) rate constants increase monotonically and systematically with the size of the molecule and are almost identical for the same‐sized C4 alkyl radicals, n‐butyl, s‐butyl, and t‐butyl. With the extended calculations for 1‐hexyl (C6H13) and 2,4,4‐trimethylpentyl (C8H17)+ O2 based on the B3LYP/6‐311G(d,p) calculations and estimated potential energy curves, class‐specific molecular size dependent falloff rate expression was proposed. The analysis of the unexpected behavior of the LPL rate constants for large alkyl radicals at high temperatures suggested the collapse of the assumption of the steady‐state dissociation or the Lindemann–Hinshelwood type of the mechanism. The analysis of the dissociation/recombination steady state suggested near‐Boltzmann distribution of RO2 in partial equilibrium with R + O2, which implies the possible simplification of the kinetic modeling by using the high‐pressure limiting rate constants for the subsequent reactions of RO2. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 44: 59–74, 2012

Theoretical investigation of rotationally inelastic collisions of CH2(ã) with helium

Rotationally inelastic collisions of the CH(2) molecule in its ã(1)A(1) electronic state have been investigated. We have determined a potential energy surface (PES) for the interaction of rigid CH(2)(ã), frozen at its equilibrium geometry, with a helium atom, using a coupled-cluster method that includes all single and double excitations, as well as perturbative contributions of connected triple excitations [RSSCD(T)]. The PES is quite anisotropic, due to lack of electron density in the unoccupied CH(2) non-bonding orbital perpendicular to the molecular plane. Quantum scattering calculations have been carried out to compute state-to-state rotational energy transfer and elastic depolarization cross sections at collision energies up to 2400 cm(-1). These cross sections were thermally averaged to derive room-temperature rate constants. The total removal and elastic depolarization rate constants for the ortho k(a) = 1 levels agree well with recent experimental measurements by Hall, Sears, and their co-workers. We observe a strong even-odd alternation in the magnitude of the total rate constants which we attribute to the asymmetry splitting of the k(a) = 1 levels.

Room temperature lifetime of triplet excitons in fluorescent host/guest systems

We studied the triplet decay dynamics in the fluorescent polymer–polymer host/guest system F8BT/MEH-PPV and in the small molecule host/guest system Alq3/DCM. The triplet lifetime was determined by a highly sensitive time-resolved measurement of the triplet state absorption via a pump and probe experiment in a waveguide structure. We were able to identify triplet–triplet annihilation as a major decay mechanism of triplet states in the pristine amorphous organic materials at room temperature (rate constant: kTT≈10−12cm3s−1). On the contrary, the host/guest systems showed a significantly increased triplet lifetime caused by a reduced triplet–triplet annihilation rate which we attribute to a substantially lowered mobility of the localized triplet excitons. Our results are expected to be of paramount importance for organic light emitting devices based on fluorescent emitters (e.g. organic lasers and organic light emitting diodes).

Ion Chemistry of VX Surrogates and Ion Energetics Properties of VX: New Suggestions for VX Chemical Ionization Mass Spectrometry Detection

Room temperature rate constants and product ion branching ratios have been measured for the reactions of numerous positive and negative ions with VX chemical warfare agent surrogates representing the amine (triethylamine) and organophosphonate (diethyl methythiomethylphosphonate (DEMTMP)) portions of VX. The measurements have been supplemented by theoretical calculations of the proton affinity, fluoride affinity, and ionization potential of VX and the simulants. The results show that many proton transfer reactions are rapid and that the proton affinity of VX is near the top of the scale. Many proton transfer agents should detect VX selectively and sensitively in chemical ionization mass spectrometers. Charge transfer with NO(+) should also be sensitive and selective since the ionization potential of VX is small. The surrogate studies confirm these trends. Limits of detection for commercial and research grade CIMS instruments are estimated at 80 pptv and 5 ppqv, respectively.