Raimo S. Timonen

Kinetics of the reactions of CH2Br and CH2I radicals with molecular oxygen at atmospheric temperatures.

The kinetics of the reactions of CH2Br and CH2I radicals with O2 have been studied in direct measurements using a tubular flow reactor coupled to a photoionization mass spectrometer. The radicals have been homogeneously generated by pulsed laser photolysis of appropriate precursors at 193 or 248 nm. Decays of radical concentrations have been monitored in time-resolved measurements to obtain the reaction rate coefficients under pseudo-first-order conditions with the amount of O2 being in large excess over radical concentrations. No buffer gas density dependence was observed for the CH2I + O2 reaction in the range 0.2-15 x 10(17) cm(-3) of He at 298 K. In this same density range the CH2Br + O2 reaction was obtained to be in the third-body and fall-off area. Measured bimolecular rate coefficient of the CH2I + O2 reaction is found to depend on temperature as k(CH2I + O2)=(1.39 +/- 0.01)x 10(-12)(T/300 K)(-1.55 +/- 0.06) cm3 s(-1)(220-450 K). Obtained primary products of this reaction are I atom and IO radical and the yield of I-atom is significant. The rate coefficient and temperature dependence of the CH2Br + O2 reaction in the third-body region is k(CH2Br + O2+ He)=(1.2 +/- 0.2)x 10(-30)(T/300 K)(-4.8 +/- 0.3) cm6 s(-1)(241-363 K), which was obtained by fitting the complete data set simultaneously to a Troe expression with the F(cent) value of 0.4. Estimated overall uncertainties in the measured reaction rate coefficients are about +/-25%.

Kinetics of the reactions of vinyl radicals with molecular oxygen and chlorine at temperatures 200–362 K

The kinetics of the C2H3 + O2 and C2H3 + Cl2 reactions have been studied in direct measurements at temperatures between 200–362 K using a tubular flow reactor coupled to a photoionization mass spectrometer (PIMS). The vinyl radicals were homogeneously generated by the pulsed laser photolysis of methyl vinyl ketone at 193 nm. The subsequent decays of the radical concentrations were monitored in time-resolved measurements to obtain the reaction rate coefficients under pseudo-first-order conditions. Reaction products identified were HCO and H2CO for the oxygen reaction and C2H3Cl for the chlorine reaction, respectively. The rate coefficients of both reactions were independent of the bath gases (He or N2) and pressures within the experimental range, 0.13–0.53 kPa, and can be expressed by the Arrhenius equations k(C2H3 + O2) = (4.62 ± 0.40) × 10−12exp(1.41 ± 0.18 kJ mol−1/RT) cm3 molecule−1 s−1 and k(C2H3 + Cl2) = (4.64 ± 0.59) × 10−12exp(3.12 ± 0.27 kJ mol−1/RT) cm3 molecule−1 s−1, where uncertainties are one standard deviation. These experimental results obtained by using our new apparatus are in good agreement with previous direct measurements.

Kinetics of the reactions of alkyl radicals (CH3, C2H5, i-C3H7, and t-C4H9) with molecular bromine

The gas-phase kinetics of the reactions of four alkyl radicals (CH{sub 3}, C{sub 2}H{sub 5}, i-C{sub 3}H{sub 7}, and t-C{sub 4}H{sub 9}) with molecular bromine have been studied over the temperature range 296-532 K. The reactions were isolated for quantitative study in a heatable tubular rector coupled to a photoionization mass spectrometer. Radicals were homogeneously generated in the reactor by pulsed photolysis of suitable precursor molecules at 193 or 248 nm. The subsequent decays of the radical concentration in the presence of different Br{sub 2} concentrations were monitored in time-resolved experiments. Rate constants were obtained at five temperatures.

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.

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 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.

Formation of vibrationally excited OH radicals in the O(1D)+H2S reaction

The rate constant, macroscopic branching ratio and vibrational distribution for the reaction O( 1 D) + H 2 S→OH + SH were measured. Temporal dependencies of the OH radical concentration in different ro-vibronic states were monitored by the LIF technique after laser flash photolysis of ozone which generated O( 1 D) atoms. The total rate constant was found to be (2.2±0.5 ) × 10 −10 cm −3 molecule −1 s −1 . The relative fraction of channel producing OH and SH radicals is 0.26 ± 0.04. The vibrational distribution of OH radicals P (ν=0): P (ν= 1) : P (ν=2) : P (ν=3) : P (ν=4) : P (ν=5) = 0.19:0.21:0.26:0.2:0.11:0.03 is similar to that for the reaction of O ( 1 D) atoms with H 2 , CH 4 and NH 3 which indicates the similar dynamics of these reactions.

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 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.