Temperature and Pressure Dependence of the Reaction between Ethyl Radical and Molecular Oxygen: Experiments and Master Equation Simulations.

We have used laser-photolysis - photoionization mass-spectrometry to measure the rate coefficient for the reaction between ethyl radical and molecular oxygen as a function of temperature (190-801 K) and pressure (0.2-6 Torr) under pseudo-first-order conditions ([He] ≫ [O2] ≫ [C2H5•]). Multiple ethyl precursor, photolysis wavelength, reactor material, and coating combinations were used. We reinvestigated the temperature dependence of the title reaction's rate coefficient to resolve inconsistencies in existing data. The current results indicate that some literature values for the rate coefficient may indeed be slightly too large. The experimental work was complemented with master equation simulations. We used the current and some previous rate coefficient measurements to optimize the values of key parameters in the master equation model. After optimization, the model was able to reproduce experimental falloff curves and C2H4 + HO2• yields. We then used the model to perform simulations over wide temperature (200-1500 K) and pressure (10-4-102 bar) ranges and provide the results in PLOG format to facilitate their use in atmospheric and combustion models.

An experimental and computational study of the reaction between pent-3-en-2-yl radicals and oxygen molecules: switching from pure stabilisation to pure decomposition with increasing temperature.

We have used laser-photolysis-photoionization mass spectrometry, quantum chemical calculations, and master equation simulations to investigate the kinetics of the reaction between (E/Z)-pent-3-en-2-yl a resonance-stabilised hydrocarbon radical, and molecular oxygen. The time-resolved experiments were performed over a wide temperature range (240-750 K) at relatively low pressures (0.4-7 Torr) under pseudo-first-order conditions (excess [O2]). Helium bath gas was used in most experiments, but nitrogen was employed in a few measurements to investigate the effect of a heavier collider on the kinetics of the studied reaction. The experimental traces were directly used to optimise parameters in the master equation model using the recently implemented trace fitting feature in the MESMER program. At low temperatures (T < 300 K), the reaction proceeds by barrierless recombination reactions to form peroxyl adducts, and the radical traces are single-exponential. Between 326 K and 376 K, equilibration between the reactants and the peroxyl adducts is observed, and the radical traces are multi-exponential. Interestingly, at temperatures above 500 K, single-exponential decays were again observed, although the reaction is much slower than at low temperatures. The master equation simulations revealed that at both low and high temperatures, the radical decay rate is governed by a single eigenvalue. At low temperatures, this eigenvalue corresponds to recombination reactions, and at high temperatures to the phenomenological formation of bimolecular products. Between low and high temperatures (the exact temperature thresholds depend on [O2]), there is a region of avoided crossing in which the rate coefficient "jumps" from one eigencurve to the other. Although chemically significant eigenvalues are not well separated from internal energy relaxation eigenvalues at elevated temperatures (600 K at 0.01 bar, 850 K at 100 bar), we observed that many of the Bartis-Widom rate coefficients produced by the master equation model were valid up to 1500 K. Our simulations predict that the most important reaction channel at high temperatures is the formation of (E/Z)-penta-1,3-diene and hydroperoxyl. The experimentally constrained master equation model was used to simulate the title reaction over a wide range of conditions. To facilitate the use of our results in autoignition and combustion models, modified Arrhenius representations are given for the most important reaction channels.

An experimental and computational study of the reaction between 2-methylallyl radicals and oxygen molecules: optimizing master equation parameters with trace fitting.

We have investigated the reaction between 2-methylallyl radicals and oxygen molecules with experimental and computational methods. Kinetic experiments were conducted in a tubular laminar flow reactor using laser photolysis for radical production and photoionization mass spectrometry for detection. The reaction was investigated as a function of temperature (203-730 K) and pressure (0.2-9 torr) in helium and nitrogen bath gases. At low temperatures (T < 410 K), the reaction proceeds by a barrierless reaction to form 2-methylallylperoxyl. Equilibration of the peroxyl adduct and the reactants was observed between 350-410 K. Measurements were extended to even higher temperatures, up to 730 K, but no reaction could be observed. Master equation simulations of the reaction system were performed with the MESMER program. Kinetic parameters in the master equation model were optimized by direct fitting to time-resolved experimental 2-methylallyl traces. Trace fitting is a recently implemented novel feature in MESMER. The trace approach was compared with the more traditional approach where one uses experimental rate coefficients for parameter optimization. The optimized parameters yielded by the two approaches are very similar and do an excellent job at reproducing the experimental data. The optimized master equation model was then used to simulate the reaction under study over a wide temperature and pressure range, from 200 K and 0.01 bar to 1500 K and 100 bar. The simulations predict a small phenomenological rate coefficient under autoignition conditions; about 1 × 10-18 cm3 s-1 at 400 K and 5 × 10-16 cm3 s-1 at 1000 K. We provide modified Arrhenius expressions in PLOG format for the most important product channels to facilitate the use of our results in combustion models.

An experimental and master-equation modeling study of the kinetics of the reaction between resonance-stabilized (CH3)2CCHCH2 radical and molecular oxygen.

The kinetics of the reaction between resonance-stabilized (CH3)2CCHCH2 radical (R) and O2 has been investigated using photoionization mass spectrometry, and master equation (ME) simulations were performed to support the experimental results. The kinetic measurements of the (CH3)2CCHCH2 + O2 reaction (1) were carried out at low helium bath-gas pressures (0.2-5.7 Torr) and over a wide temperature range (238-660 K). Under low temperature (238-298 K) conditions, the pressure-dependent bimolecular association reaction R + O2 → ROO determines kinetics, until at an intermediate temperature range (325-373 K) the ROO adduct becomes thermally unstable and increasingly dissociates back to the reactants with increasing temperature. The initial association of O2 with (CH3)2CCHCH2 radical occurs on two distinct sites: terminal 1(t) and non-terminal 1(nt) sites on R, leading to the barrierless formation of ROO(t) and ROO(nt) adducts, respectively. Important for autoignition modelling of olefinic compounds, bimolecular reaction channels appear to open for the R + O2 reaction at high temperatures (T > 500 K) and pressure-independent bimolecular rate coefficients of reaction (1) with a weak positive temperature dependence, (2.8-4.6) × 10-15 cm3 molecule-1 s-1, were measured in the temperature range of 500-660 K. At a temperature of 501 K, a product signal of reaction (1) was observed at m/z = 68, probably originating from isoprene. To explore the reaction mechanism of reaction (1), quantum chemical calculations and ME simulations were performed. According to the ME simulations, without any adjustment to energies, the most important and second most important product channels at the high temperatures are isoprene + HO2 (yield > 91%) and (2R/S)-3-methyl-1,2-epoxybut-3-ene + OH (yield < 8%). After modest adjustments to ROO(t) and ROO(nt) well-depths (∼0.7 kcal mol-1 each) and barrier height for the transition state associated with the kinetically most dominant channel, R + O2 → isoprene + HO2 (∼2.2 kcal mol-1), the ME model was able to reproduce the experimental findings. Modified Arrhenius expressions for the kinetically important reaction channels are enclosed to facilitate the use of current results in combustion models.

Assessing apathy in multiple sclerosis: Validation of the dimensional apathy scale and comparison with apathy evaluation scale.

BACKGROUND: Apathy is a predictor of cognitive decline in the course of multiple sclerosis (MS). Early identification of apathetic patients is relevant in clinical settings. OBJECTIVE: To assess applicability and psychometric properties of the self-rated version of the Dimensional Apathy Scale (DAS) in a large cohort of patients with MS and to compare its diagnosing accuracy with that of the Apathy Evaluation Scale (AES). METHODS: One hundred and twenty-four patients underwent clinical interview based on diagnostic criteria for apathy, DAS, AES, and assessment of depression, global cognitive functioning, and non-verbal intelligence. RESULTS: According to diagnostic criteria, apathy occurred in 33.4% of the patients. The DAS showed high consistency, and good convergent, discriminant and criterion validity. Factor analysis indicated a three-factor structure: executive, behavioural and emotional apathy. Unlike AES, no significant association between DAS score and severity of neurological disability (expressed by EDSS total score) was found, suggesting that the DAS might be less related to levels of disability. Receiver operating characteristics analyses, with clinical diagnostic criteria for apathy as the gold standard, revealed that a DAS score of 28/29 and an AES score of 35/36 were optimal cut-off values for identifying clinically relevant apathy. The two scales had similar diagnostic accuracy in the present sample. CONCLUSIONS: The DAS is a valid and reliable multidimensional tool to assess apathy in MS, with diagnostic accuracy similar to that of the AES. However, the DAS score appears to be less strongly related to neurological disability.

Effect of Methyl Group Substitution on the Kinetics of Vinyl Radical + O2 Reaction.

The kinetics of (CH3)2CCH + O2 (1) and (CH3)2CCCH3 + O2 (2) reactions have been measured as a function of temperature (223-600 K) at low pressures (0.4-2 Torr) using a tubular laminar flow reactor coupled to a photoionization mass spectrometer (PIMS). These reactions are important for accurate modeling of unsaturated hydrocarbon combustion. Photolysis of a brominated precursor by a pulsed excimer laser radiation at 248 nm wavelength along the flow reactor axis was used for the production of radicals. The measured bimolecular rate coefficient of reaction 1 shows a negative temperature dependence over the temperature range 223-384 K and becomes temperature independent at higher temperatures. The bimolecular rate coefficient of reaction 2 exhibits a negative temperature dependence throughout the experimental temperature range. The bimolecular rate coefficients of reactions 1 and 2 are expected to be at the high-pressure limit under the current experimental conditions, and the following values are obtained at 298 K: k1(298 K) = (4.5 ± 0.5) × 10-12 cm3 s-1 and k2(298 K) = (8.9 ± 1.0) × 10-12 cm3 s-1. The observed products for reactions 1 and 2 were CH3COCH3 and CH3 + CH3COCH3, respectively. Substituting both ß-hydrogens in the vinyl radical (CH2CH) with methyl groups decreases the rate coefficient of the CH2CH + O2 reaction by about 50%. However, the rate coefficient of the triply substituted (CH3)2CCCH3 radical reaction with O2 is almost identical to the CH2CH + O2 rate coefficient under the covered temperature range.

Oxidation Kinetics and Thermodynamics of Resonance-Stabilized Radicals: The Pent-1-en-3-yl + O2 Reaction.

The kinetics and thermochemistry of the pent-1-en-3-yl radical reaction with molecular oxygen (CH2CHCHCH2CH3 + O2) has been studied by both experimental and computational methods. The bimolecular rate coefficient of the reaction was measured as a function of temperature (198-370 K) and pressure (0.2-4.5 Torr) using laser photolysis-photoionization mass-spectrometry. Quantum chemical calculations were used to explore the potential energy surface of the reaction, after which Rice-Ramsperger-Kassel-Marcus theory/master equation simulations were performed to investigate the reaction. The experimental data were used to adjust key parameters, such as well depths, in the master equation model within methodological uncertainties. The master equation simulations suggest that the formation rates of the two potential RO2 adducts are equal and that the reaction to QOOH is slower than for saturated hydrocarbons. The initial addition reaction, CH2CHCHCH2CH3 + O2, is found to be barrierless when accounting for multireference effects. This is in agreement with the current experimental data, as well as with past experimental data for the allyl + O2 reaction. Finally, we conducted numerical simulations of the pent-1-en-3-yl + O2 reaction system and observed significant amounts of penta-1,3-diene being formed under engine-relevant conditions.

Kinetics of the Methyl-Vinyl Radical + O2 Reactions Associated with Propene Oxidation.

The bimolecular rate coefficients of reactions CH3CCH2 + O2 (1) and cis/ trans-CH3CHCH + O2 (2a/3a) have been measured using a tubular laminar flow reactor coupled with a photoionization mass spectrometer (PIMS). These reactions are relevant in the combustion of propene. Pulsed excimer laser photolysis of a ketone or a bromide precursor molecule at 193 or 248 nm wavelength was used to produce radicals of interest homogeneously along the reactor. Time-resolved experiments were performed under pseudo-first-order conditions at low pressure (0.3-1.5 Torr) over the temperature range 220-660 K. The measured bimolecular rate coefficients were found to be independent of bath gas concentration. The bimolecular rate coefficients possess negative temperature dependence at low temperatures ( T < 420 K) and appear to be independent of temperature at high temperatures ( T > 420 K). Observed products of the reaction CH3CCH2 + O2 were CH3 and H2CO, while for the reaction cis/trans-CH3CHCH + O2, observed products were CH3CHO and HCO. Current results indicate that the reaction mechanism of both reactions is analogous to that of C2H3 + O2. Methyl substitution of the vinyl radical changes its reactivity toward O2 upward by ca. 50% if it involves the α-position and downward by ca. 30% if the methyl group takes either of the ß-positions, respectively.

Psychometric properties of the Italian version of the multifactorial memory questionnaire for adults and the elderly.

Reliable and valid metamemory measures are needed to assess subjective memory complaints that can be distinct from objective memory performance. The Multifactorial Memory Questionnaire (MMQ) evaluates dimensions of subjective memory functioning such as frequency of memory problems (Ability), affect related to memory abilities (Contentment), and strategy use in everyday life (Strategy). To examine the psychometric properties of the Italian version of the MMQ, six hundred Italian healthy individuals (aged 25-91 years) completed MMQ, a questionnaire assessing metacognition (Cognitive Failures Questionnaire, CFQ) and two batteries assessing cognitive global status (Montreal Cognitive Assessment, MoCA; Mini Mental State Examination, MMSE). MMQ was easy to administer, acceptable, and had good test-retest reliability (r for the total MMQ score 0.95), and internal consistency (Cronbach's α for the total MMQ score = 0.83). An exploratory factor analysis provided a four-factor solution: "Ability" (α = 0.99), "Contentment" (α = 0.91), "External Strategies" (α = 0.85) and "Internal Strategies" (α = 0.78) factors. MMQ total score and MMQ-Ability factor score showed good convergent validity when compared to CFQ score (r rho ≥ 0.51), whereas MMQ total score and the four MMQ factors showed good divergent validity when compared to MoCA and MMSE score (r rho ≤ 0.27). Demographic variables significantly influenced MMQ total score and most subscale scores. From the derived linear equations, we computed correction factors for raw scores and percentile distribution of adjusted scores. The Italian version of MMQ is reliable and valid to assess dimensions of metamemory in adult and elderly subjects.

Kinetics of Several Oxygen-Containing Carbon-Centered Free Radical Reactions with Nitric Oxide.

Kinetics of four carbon-centered, oxygen-containing free radical reactions with nitric oxide (NO) were investigated as a function of temperature at a few Torr pressure of helium, employing flow tube reactors coupled to a laser-photolysis/resonance-gas-discharge-lamp photoionization mass spectrometer (LP-RPIMS). Rate coefficients were directly determined from radical (R) decay signals under pseudo-first-order conditions ([R]0 ≪ [NO]). The obtained rate coefficients showed negative temperature dependences, typical for a radical-radical association process, and can be represented by the following parametrizations (all in units of cm(3) molecule(-1) s(-1)): k(CH2OH + NO) = (4.76 × 10(-21)) × (T/300 K)(15.92) × exp[50700/(RT)] (T = 266-363 K, p = 0.79-3.44 Torr); k(CH3CHOH + NO) = (1.27 × 10(-16)) × (T/300 K)(6.81) × exp[28700/(RT)] (T = 241-363 K, p = 0.52-3.43 Torr); k(CH3OCH2 + NO) = (3.58 ± 0.12) × 10(-12) × (T/300 K)(-3.17±0.14) (T = 221-363 K, p = 0.50-0.80 Torr); k(T)3 = 9.62 × 10(-11) × (T/300 K)(-5.99) × exp[-7100/(RT)] (T = 221-473 K, p = 1.41-2.95 Torr), with the uncertainties given as standard errors of the fits and the overall uncertainties estimated as ±20%. The rate of CH3OCH2 + NO reaction was measured in two density ranges due to its observed considerable pressure dependence, which was not found in the studied hydroxyalkyl reactions. In addition, the CH3CO + NO rate coefficient was determined at two temperatures resulting in k298K(CH3CO + NO) = (5.6 ± 2.8) × 10(-13) cm(3) molecule(-1) s(-1). No products were found during these experiments, reasons for which are briefly discussed.

CH2NH2 + O2 and CH3CHNH2 + O2 reaction kinetics: photoionization mass spectrometry experiments and master equation calculations.

Two carbon centered amino radical (CH2NH2 and CH3CHNH2) reactions with O2 were scrutinized by means of laboratory gas kinetics experiments together with quantum chemical computations and master equation modeling. In the experiments, laser photolysis of alkylamine compounds at 193 nm was used for the radical production and photoionization mass spectrometry was employed for the time-resolved detection of the reactants and products. The investigations were performed in a tubular, uncoated borosilicate glass flow reactor. The rate coefficients obtained were high, ranging from 2.4 × 10(-11) to 3.5 × 10(-11) cm(3) molecule(-1) s(-1) in the CH2NH2 + O2 reaction and from 5.5 × 10(-11) to 7.5 × 10(-11) cm(3) molecule(-1) s(-1) in the CH3CHNH2 + O2 reaction, showed negative temperature dependence with no dependence on the helium bath gas pressure (0.5 to 2.5 Torr He). The measured rate coefficients can be expressed as a function of temperature with: k(CH2NH2 + O2) = (2.89 ± 0.13) × 10(-11) (T/300 K)(-(1.10±0.47)) cm(3) molecule(-1) s(-1) (267-363 K) and k(CH3CHNH2 + O2) = (5.92 ± 0.23) × 10(-11) (T/300 K)(-(0.50±0.42)) cm(3) molecule(-1) s(-1) (241-363 K). The reaction paths and mechanisms were characterized using quantum chemical calculations and master equation modeling. Master equation computations, constrained by experimental kinetic results, were employed to model pressure-dependencies of the reactions. The constrained modeling results reproduce the experimentally observed negative temperature dependence and the dominant CH2NH imine production in the CH2NH2 + O2 reaction at the low pressures of the present laboratory investigation. In the CH3CHNH2 + O2 reaction, similar qualitative behavior was observed both in the rate coefficients and in the product formation, although the fine details of the mechanism were observed to change according to the different energetics in this system. In conclusion, the constrained modeling results predict significant imine + HO2 production for both reactions even at atmospheric pressure.

Kinetics of several oxygenated carbon-centered free radical reactions with NO2.

Five oxygenated carbon-centered free radical reactions with nitrogen dioxide (NO2) have been studied in direct time-resolved measurements. Experiments were conducted in a temperature-controlled flow tube reactor coupled to a 193 nm exciplex laser photolysis and a resonance gas lamp photoionization mass spectrometer. Reactions were investigated under pseudofirst-order conditions, with the NO2 concentrations of the experiments in great excess over the initial radical concentrations ([R]0 << [NO2]). The study consists of the three isomeric C2H5O radicals (CH3CHOH, CH2CH2OH, and CH3OCH2), and the CH2OH and CH3CO radical reactions with NO2 and, hence, includes the three smallest hydroxyalkyl radical species (CH2OH, CH2CH2OH, and CH3CHOH). The obtained rate coefficients are high with the temperature-dependent rate coefficients given by a formula k(T) = k300K × (T/300 K)(-n) as (in units of cm(3) molecule(-1) s(-1)): k(CH2OH + NO2) = (8.95 ± 2.70) × 10(-11) × (T/300 K)(-0.54±0.27) (T = 298-363 K), k(CH2CH2OH + NO2) = (5.99 ± 1.80) × 10(-11) × (T/300 K)(-1.49±0.45)(T = 241-363 K), k(CH3CHOH + NO2) = (7.48 ± 2.24) × 10(-11) × (T/300 K)(-1.36±0.41) (T = 266-363 K), k(CH3OCH2 + NO2) = (7.85 ± 2.36) × 10(-11) × (T/300 K)(-0.93±0.28) (T = 243-363 K), and k(CH3CO + NO2) = (2.87 ± 0.57) × 10(-11) × (T/300 K)(-2.45±0.49) (T = 241-363 K), where the uncertainties refer to the estimated overall uncertainties of the values obtained. The determined rate coefficients show negative temperature dependence with no apparent bath gas pressure dependence under the current experimental conditions (241-363 K and about 1-3 Torr helium). This behavior is typical for a radical-radical addition mechanism with no potential energy barrier above the energy of the separated reactants in the entrance channel of the reaction. Unfortunately the absence of detected product signals prevented gaining deeper insight into the reaction mechanism.

Gas phase kinetics and equilibrium of allyl radical reactions with NO and NO2.

Allyl radical reactions with NO and NO(2) were studied in direct, time-resolved experiments in a temperature controlled tubular flow reactor connected to a laser photolysis/photoionization mass spectrometer (LP-PIMS). In the C(3)H(5) + NO reaction 1 , a dependence on the bath gas density was observed in the determined rate coefficients and pressure falloff parametrizations were performed. The obtained rate coefficients vary between 0.30-14.2 × 10(-12) cm(3) s(-1) (T = 188-363 K, p = 0.39-23.78 Torr He) and possess a negative temperature dependence. The rate coefficients of the C(3)H(5) + NO(2) reaction 2 did not show a dependence on the bath gas density in the range used (p = 0.47-3.38 Torr, T = 201-363 K), and they can be expressed as a function of temperature with k(C(3)H(5) + NO(2)) = (3.97 ± 0.84) × 10(-11) × (T/300 K) (-1.55±0.05) cm(3) s(-1). In the C(3)H(5) + NO reaction, above 410 K the observed C(3)H(5) radical signal did not decay to the signal background, indicating equilibrium between C(3)H(5) + NO and C(3)H(5)NO. This allowed the C(3)H(5) + NO ⇄ C(3)H(5)NO equilibrium to be studied and the equilibrium constants of the reaction between 414 and 500 K to be determined. With the standard second- and third-law analysis, the enthalpy and entropy of the C(3)H(5) + NO ⇄ C(3)H(5)NO reaction were obtained. Combined with the calculated standard entropy of reaction (ΔS°(298) = 137.2 J mol(-1)K(-1)), the third-law analysis resulted in ΔH°(298) = 102.4 ± 3.2 kJ mol(-1) for the C(3)H(5)-NO bond dissociation enthalpy.

Kinetic (T = 201-298 K) and equilibrium (T = 320-420 K) measurements of the C3H5 + O2 ⇆ C3H5O2 reaction.

The kinetics and equilibrium of the allyl radical reaction with molecular oxygen have been studied in direct measurements using temperature-controlled tubular flow reactor coupled to a laser photolysis/photoionization mass spectrometer. In low-temperature experiments (T = 201-298 K), association kinetics were observed, and the measured time-resolved C(3)H(5) radical signals decayed exponentially to the signal background. In this range, the determined rate coefficients exhibited a negative temperature dependence and were observed to depend on the carrier-gas (He) pressure {p = 0.4-36 Torr, [He] = (1.7-118.0) × 10(16) cm(-3)}. The bimolecular rate coefficients obtained vary in the range (0.88-11.6) × 10(-13) cm(3) s(-1). In higher-temperature experiments (T = 320-420 K), the C(3)H(5) radical signal did not decay to the signal background, indicating equilibration of the reaction. By measuring the radical decay rate under these conditions as a function of temperature and following typical second- and third-law procedures, plotting the resulting ln K(p) values versus 1/T in a modified van't Hoff plot, the thermochemical parameters of the reaction were extracted. The second-law treatment resulted in values of ΔH(298)° = -78.3 ± 1.1 kJ mol(-1) and ΔS(298)° = -129.9 ± 3.1 J mol(-1) K(-1), with the uncertainties given as one standard error. When results from a previous investigation were taken into account and the third-law method was applied, the reaction enthalpy was determined as ΔH(298)° = -75.6 ± 2.3 kJ mol(-1).

Kinetics of the reactions of CH2Cl, CH3CHCl, and CH3CCl2 radicals with Cl2 in the temperature range 191-363 K.

The kinetics of three chlorinated free radical reactions with Cl(2) have been studied in direct time-resolved measurements. Radicals were produced in low initial concentrations by pulsed laser photolysis at 193 nm, and the subsequent decays of the radical concentrations were measured under pseudo-first-order conditions using photoionization mass spectrometer (PIMS). The bimolecular rate coefficients of the CH(3)CHCl + Cl(2) reaction obtained from the current measurements exhibit negative temperature dependence and can be expressed by the equation k(CH(3)CHCl + Cl(2)) = ((3.02 +/- 0.14) x 10(-12))(T/300 K)(-1.89+/-0.19) cm(3) molecule(-1) s(-1) (1.7-5.4 Torr, 191-363 K). For the CH(3)CCl(2) + Cl(2) reaction the current results could be fitted with the equation k(CH(3)CCl(2) + Cl(2)) = ((1.23 +/- 0.02) x 10(-13))(T/300 K)(-0.26+/-0.10) cm(3) molecule(-1) s(-1) (3.9-5.1 Torr, 240-363 K). The measured rate coefficients for the CH(2)Cl + Cl(2) reaction plotted as a function of temperature show a minimum at about T = 240 K: first decreasing with increasing temperature and then, above the limit, increasing with temperature. The determined reaction rate coefficients can be expressed as k(CH(2)Cl + Cl(2)) = ((2.11 +/- 1.29) x 10(-14)) exp(773 +/- 183 K/T)(T/300 K)(3.26+/-0.67) cm(3) molecule(-1) s(-1) (4.0-5.6 Torr, 201-363 K). The rate coefficients for the CH(3)CCl(2) + Cl(2) and CH(2)Cl + Cl(2) reactions can be combined with previous results to obtain: k(combined)(CH(3)CCl(2) + Cl(2)) = ((4.72 +/- 1.66) x 10(-15)) exp(971 +/- 106 K/T)(T/300 K)(3.07+/-0.23) cm(3) molecule(-1) s(-1) (3.1-7.4 Torr, 240-873 K) and k(combined)(CH(2)Cl + Cl(2)) = ((5.18 +/- 1.06) x 10(-14)) exp(525 +/- 63 K/T)(T/300 K)(2.52+/-0.13) cm(3) molecule(-1) s(-1) (1.8-5.6 Torr, 201-873 K). All the uncertainties given refer only to the 1sigma statistical uncertainties obtained from the fitting, and the estimated overall uncertainty in the determined bimolecular rate coefficients is about +/-15%.

Kinetics of the R + NO2 reactions (R = i-C3H7, n-C3H7, s-C4H9, and t-C4H9) in the temperature range 201-489 K.

The bimolecular rate coefficients of four alkyl radical reactions with NO(2) have been measured in direct time-resolved experiments. Reactions were studied under pseudo-first-order conditions in a temperature-controlled tubular flow reactor coupled to a laser photolysis/photoionization mass spectrometer (LP-PIMS). The measured reaction rate coefficients are independent of helium bath gas pressure within the experimental ranges covered and exhibit negative temperature dependence. For i-C(3)H(7) + NO(2) and t-C(4)H(9) + NO(2) reactions, the dependence of ordinate (logarithm of reaction rate coefficients) on abscissa (1/T or log(T)) was nonlinear. The obtained results (in cm(3) s(-1)) can be expressed by the following equations: k(n-C(3)H(7) + NO(2)) = ((4.34 +/- 0.08) x 10(-11)) (T/300 K)(-0.14+/-0.08) (203-473 K, 1-7 Torr), k(i-C(3)H(7) + NO(2)) = ((3.66 +/- 2.54) x 10(-12)) exp(656 +/- 201 K/T)(T/300 K)(1.26+/-0.68) (220-489 K, 1-11 Torr), k(s-C(4)H(9) + NO(2)) = ((4.99 +/- 0.16) x 10(-11))(T/300 K)(-1.74+/-0.12) (241-485 K, 2 - 12 Torr) and k(t-C(4)H(9) + NO(2)) = ((8.64 +/- 4.61) x 10(-12)) exp(413 +/- 154 K/T)(T/300 K)(0.51+/-0.55) (201-480 K, 2-11 Torr), where the uncertainties shown refer only to the 1 standard deviations obtained from the fitting procedure. The estimated overall uncertainty in the determined bimolecular rate coefficients is about +/-20%.

Kinetics of the reactions of CH3CH2, CH3CHCl, and CH3CCl2 radicals with NO2 in the temperature range 221-363 K.

The gas-phase kinetics of three ethyl radical reactions with NO(2) have been studied in direct measurements using a laser photolysis/photoionization mass spectrometer (LP-PIMS) coupled to a temperature controlled tubular flow reactor. Reactions were studied under pseudo-first-order conditions with NO(2) always in large excess over initial radical concentrations. All the measured rate coefficients exhibit a negative temperature dependence, which becomes stronger as the chlorine substitution in the alpha-carbon of the ethyl radical increases. No pressure dependence of the rate coefficients was observed within the experimental range covered (0.5-6 Torr). The obtained results can be expressed conveniently as follows: k(CH(3)CH(2) + NO(2)) = (4.33 +/- 0.13) x 10(-11) (T/300 K)(-0.34 +/- 0.22) cm(3) s(-1) (221-365 K), k(CH(3)CHCl + NO(2)) = (2.38 +/- 0.10) x 10(-11) (T/300 K)(-1.27 +/- 0.26) cm(3) s(-1) (221-363 K), and k(CH(3)CCl(2) + NO(2)) = (1.01 +/- 0.02) x 10(-11) (T/300 K)(-1.65 +/- 0.19) cm(3) s(-1) (248-363 K), where the given error limits are the 1sigma statistical uncertainties of the plots of log k against log(T/300 K). Overall uncertainties in the measured rate coefficients were estimated to be +/-20%. The observed reactivity toward NO(2) decreases with increasing chlorine substitution at the radical site as was expected with respect to our previous measurements of chlorine containing methyl radical reactions with NO(2). A potential reason for the observed reactivity differences is briefly discussed, and a possible reaction mechanism is presented.

Rate constants and hydrogen isotope substitution effects in the CH3 + HCl and CH3 + Cl2 reactions.

The kinetics of the CH3 + Cl2 (k2a) and CD3 + Cl2 (k2b) reactions were studied over the temperature range 188-500 K using laser photolysis-photoionization mass spectrometry. The rate constants of these reactions are independent of the bath gas pressure within the experimental range, 0.6-5.1 Torr (He). The rate constants were fitted by the modified Arrhenius expression, k2a = 1.7 x 10(-13)(T/300 K)(2.52)exp(5520 J mol(-1)/RT) and k2b = 2.9 x 10(-13)(T/300 K)(1.84)exp(4770 J mol(-1)/RT) cm(3) molecule(-1) s(-1). The results for reaction 2a are in good agreement with the previous determinations performed at and above ambient temperature. Rate constants of the CH3 + Cl2 and CD3 + Cl2 reactions obtained in this work exhibit minima at about 270-300 K. The rate constants have positive temperature dependences above the minima, and negative below. Deuterium substitution increases the rate constant, in particular at low temperatures, where the effect reaches ca. 45% at 188 K. These observations are quantitatively rationalized in terms of stationary points on a potential energy surface based on QCISD/6-311G(d,p) geometries and frequencies, combined with CCSD(T) energies extrapolated to the complete basis set limit. 1D tunneling as well as the possibility of the negative energies of the transition state are incorporated into a transition state theory analysis, an approach which also accounts for prior experiments on the CH3 + HCl system and its various deuterated isotopic substitutions [Eskola, A. J.; Seetula, J. A.; Timonen, R. S. Chem. Phys. 2006, 331, 26].

Kinetics of the reactions of CH2I, CH2Br, and CHBrCl radicals with NO2 in the temperature range 220-360 K.

The kinetics of the CH2I + NO2, CH2Br + NO2, and CHBrCl + NO2 reactions have been studied at temperatures between 220 and 360 K using laser photolysis/photoionization mass spectrometry. Decays of radical concentrations have been monitored in time-resolved measurements to obtain reaction rate coefficients under pseudo-first-order conditions. The bimolecular rate coefficients of all three reactions are independent of the bath gas (He or N2) and pressure within the experimental range (2-6 Torr) and are found to depend on temperature as follows: k(CH2I + NO2) = (2.18 +/- 0.07) x 10(-11) (T / 300 K)(-1.45) (+/- 0.22) cm3 molecule(-1) s(-1) (220-363 K), k(CH2Br + NO2) = (1.76 +/- 0.03) x 10(-11) (T/300 K)(-0.86) (+/- 0.09) cm3 molecule(-1) s(-1) (221-363 K), and k(CHBrCl + NO2) = (8.81 +/- 0.28) x 10(-12) (T/300 K)(-1.55) (+/- 0.34) cm3 molecule(-1) s(-1) (267-363 K), with the uncertainties given as one-standard deviations. Estimated overall uncertainties in the measured bimolecular reaction rate coefficients are about +/-25%. In the CH2I + NO2 and CH2Br + NO2 reactions, the observed product is formaldehyde. For the CHBrCl + NO2 reaction, the product observed is CHClO. In addition, I atom and iodonitromethane (CH2INO2) or iodomethyl nitrite (CH2IONO) formations have been detected for the CH2I + NO2 reaction.

Interaction of ethyl alcohol vapor with sulfuric acid solutions.

We investigated the uptake of ethyl alcohol (ethanol) vapor by sulfuric acid solutions over the range approximately 40 to approximately 80 wt % H2SO4 and temperatures of 193-273 K. Laboratory studies used a fast flow-tube reactor coupled to an electron-impact ionization mass spectrometer for detection of ethanol and reaction products. The uptake coefficients (gamma) were measured and found to vary from 0.019 to 0.072, depending upon the acid composition and temperature. At concentrations greater than approximately 70 wt % and in dilute solutions colder than 220 K, the gamma values approached approximately 0.07. We also determined the effective solubility constant of ethanol in approximately 40 wt % H2SO4 in the temperature range 203-223 K. The potential implications to the budget of ethanol in the global troposphere are briefly discussed.