Mark A. Blitz

Accelerated chemistry in the reaction between the hydroxyl radical and methanol at interstellar temperatures facilitated by tunnelling

Understanding the abundances of molecules in dense interstellar clouds requires knowledge of the rates of gas-phase reactions between uncharged species. However, because of the low temperatures within these clouds, reactions with an activation barrier were considered too slow to play an important role. Here we show that, despite the presence of a barrier, the rate coefficient for the reaction between the hydroxyl radical (OH) and methanol--one of the most abundant organic molecules in space--is almost two orders of magnitude larger at 63 K than previously measured at ∼200 K. We also observe the formation of the methoxy radical product, which was recently detected in space. These results are interpreted by the formation of a hydrogen-bonded complex that is sufficiently long-lived to undergo quantum-mechanical tunnelling to form products. We postulate that this tunnelling mechanism for the oxidation of organic molecules by OH is widespread in low-temperature interstellar environments.

Interception of excited vibrational quantum states by o2 in atmospheric association reactions

Vibrating in a Crowd High-vacuum molecular beam studies can probe the roles of specific vibrations and rotations on molecular reactivity with remarkably fine resolution. Glowacki et al. (p. 1066; see the Perspective by Tyndall) now show, through a combination of spectroscopy and theoretical modeling, that oxidation of acetylene under effectively atmospheric conditions proceeds in part through vibrationally excited intermediates prior to collisional randomization. Vibrationally excited reaction intermediates play a bigger role under atmospheric conditions than previously suspected. Bimolecular reactions in Earth’s atmosphere are generally assumed to proceed between reactants whose internal quantum states are fully thermally relaxed. Here, we highlight a dramatic role for vibrationally excited bimolecular reactants in the oxidation of acetylene. The reaction proceeds by preliminary adduct formation between the alkyne and OH radical, with subsequent O2 addition. Using a detailed theoretical model, we show that the product-branching ratio is determined by the excited vibrational quantum-state distribution of the adduct at the moment it reacts with O2. Experimentally, we found that under the simulated atmospheric conditions O2 intercepts ~25% of the excited adducts before their vibrational quantum states have fully relaxed. Analogous interception of excited-state radicals by O2 is likely common to a range of atmospheric reactions that proceed through peroxy complexes.

OH formation from CH3CO+O2: a convenient experimental marker for the acetyl radical

Abstract OH formation has been observed directly from the reaction between CH 3 CO and O 2 . This reaction proceeds via the formation of energised CH 3 CO(O 2 ), which can either dissociate to OH or be collisionally stabilized to CH 3 CO(O 2 ). The kinetics of OH formation were measured as a function of pressure at 295 and 213 K, giving rate coefficients in good agreement with the literature. The OH yield was also determined as a function of pressure enabling the reaction to be further quantified.

Comment on "Atmospheric hydroxyl radical production from electronically excited NO2 and H2O".

Li et al. (Reports, 21 March 2008, p. 1657) suggested that the reaction between electronically excited nitrogen dioxide and water vapor is an important atmospheric source of the hydroxyl radical. However, under conditions that better approximate the solar flux, we find no evidence for OH production from this reaction.

Site-Specific Rate Coefficients for Reaction of OH with Ethanol from 298 to 900 K

The rate coefficients for reactions of OH with ethanol and partially deuterated ethanols have been measured by laser flash photolysis/laser-induced fluorescence over the temperature range 298-523 K and 5-100 Torr of helium bath gas. The rate coefficient, k(1.1), for reaction of OH with C(2)H(5)OH is given by the expression k(1.1) = 1.06 × 10(-22)T(3.58) exp(1126/T) cm(3) molecule(-1) s(-1), and the values are in good agreement with previous literature. Site-specific rate coefficients were determined from the measured kinetic isotope effects. Over the temperature region 298-523 K abstraction from the hydroxyl site is a minor channel. The reaction is dominated by abstraction of the α hydrogens (92 ± 8)% at 298 K decreasing to (76 ± 9)% with the balance being abstraction at the β position where the errors are 2σ. At higher temperatures decomposition of the CH(2)CH(2)OH product from β abstraction complicates the kinetics. From 575 to 650 K, biexponential decays were observed, allowing estimates to be made for k(1.1) and the fractional production of CH(2)CH(2)OH. Above 650 K, decomposition of the CH(2)CH(2)OH product was fast on the time scale of the measured kinetics and removal of OH corresponds to reaction at the α and OH sites. The kinetics agree (within ±20%) with previous measurements. Evidence suggests that reaction at the OH site is significant at our higher temperatures: 47-53% at 865 K.

Experimental and theoretical study of oxidative addition reaction of nickel atom to O–H bond of water

The reaction of atomic nickel with water in the gas phase has been investigated by kinetic studies under static pressure conditions near room temperature, and by accurate quantum chemical calculations. Experimental and theoretical results are consistent with a reaction mechanism involving formation of a weakly bound nickel–water adduct, which may react further by oxidative addition of nickel to the O–H bond of water to form the insertion product HNiOH. Experimental estimates of reaction energetics have been made by using unimolecular reaction theory calculations to model rate coefficients obtained by fitting kinetic data to a simple rate equations model. These experimental estimates are in agreement with the theoretical results, and indicate that the insertion product is bound by at least 20–25 kcal/mol, relative to nickel plus water. There is also agreement that the barrier to oxidative addition is no greater than 1–2 kcal/mol, and may be smaller. This theoretical result was obtained only at the highest ...

Experimental and Modeling Studies of the Pressure and Temperature Dependences of the Kinetics and the OH Yields in the Acetyl + O2 Reaction

The acetyl + O(2) reaction has been studied by observing the time dependence of OH by laser-induced fluorescence (LIF) and by electronic structure/master equation analysis. The experimental OH time profiles were analyzed to obtain the kinetics of the acetyl + O(2) reaction and the relative OH yields over the temperature range of 213-500 K in helium at pressures in the range of 5-600 Torr. More limited measurements were made in N(2) and for CD(3)CO + O(2). The relative OH yields were converted into absolute yields by assuming that the OH yield at zero pressure is unity. Electronic structure calculations of the stationary points of the potential energy surface were used with a master equation analysis to fit the experimental data in He using the high-pressure limiting rate coefficient for the reaction, k(∞)(T), and the energy transfer parameter, (ΔE(d)), as variable parameters. The best-fit parameters obtained are k(∞) = 6.2 × 10(-12) cm(-3) molecule(-1) s(-1), independent of temperature over the experimental range, and (ΔE(d))(He) = 160(T/298 K) cm(-1). The fits in N(2), using the same k(∞)(T), gave (ΔE(d))(N(2)) = 270(T/298 K) cm(-1). The rate coefficients for formation of OH and CH(3)C(O)O(2) are provided in parametrized form, based on modified Troe expressions, from the best-fit master equation calculations, over the pressure and temperature ranges of 1 ≤ p/Torr ≤ 1.5 × 10(5) and 200 ≤ T/K ≤ 1000 for He and N(2) as the bath gas. The minor channels, leading to HO(2) + CH(2)CO and CH(2)C(O)OOH, generally have yields <1% over this range.

Laser induced fluorescence studies of the reactions of O(1D2) with N2, O2, N2O, CH4, H2, CO2, Ar, Kr and n-C4H10

A laser flash photolysis–laser-induced fluorescence (LIF) technique has been used to study the kinetics of the reactions of electronically excited oxygen atoms, O(1D2), with N2, O2, N2O, CH4, H2, CO2, Ar, Kr and n-C4H10 over the temperature range 195–673 K. The majority of studies employed direct detection of O(1D2) atoms using vacuum ultraviolet LIF at 115.2 nm, whereas some studies employed LIF detection of OH generated from reaction of O(1D2) with a H atom donor species as a marker for O(1D2). The bimolecular rate coefficient for reaction with N2 (kN2) is well described by the Arrhenius expression kN2 = (2.2 ± 0.3) × 10−11exp{(118 ± 21)/T} (95% confidence level), in good agreement with two other new studies reported in this issue, but giving significantly higher values of kN2 than previously measured, with important implications for production rates of OH and NO radicals in the atmosphere. At 295 K the following rate coefficients were obtained (in units of cm3 molecule−1 s−1, 95% confidence level including estimated systematic errors): kN2 = (3.06 ± 0.25) × 10−11, kO2 = (3.8 ± 0.4) × 10−11, kN2O = (1.07 ± 0.1) × 10−10, kCH4 = (1.4 ± 0.2) × 10−10, kH2 = (1.5 ± 0.1) × 10−10, kCO2 = (1.4 ± 0.1) × 10−10, kAr = (8 ± 3) × 10−13, kKr = (9 ± 1) × 10−12 and kn-C4H10 = (4.55 ± 0.2) × 10−10, in good agreement with the new studies reported in this issue, and with previous measurements, where available. An analysis of the correlation between the cross-section for O(1D2) removal and the ionisation potential of the collision partner suggests at least two mechanisms operate for the removal of O(1D2).

A Multidimensional Study of the Reaction CH2I+O2: Products and Atmospheric Implications

The CH(2)I+O(2) reaction has been studied using laser flash photolysis followed by absorption spectroscopy, laser-induced fluorescence spectroscopy and mass spectrometry. The rates of formation of IO and CH(2)O were found to be dependent upon the concentration of CH(2)I(2) under pseudo-first-order conditions ([O(2)]≫[CH(2)I(2)]), demonstrating that IO and CH(2)O are not formed directly from the title reaction, in contrast to recent investigations by Enami et al. It is proposed that the reaction proceeds via the formation of the peroxy radical species CH(2)IO(2), which undergoes self-reaction to form CH(2)IO, and which decomposes to CH(2)O+I, and that in laboratory systems IO is formed via the reaction I+CH(2)IO(2). The absorption spectrum of a species assigned to CH(2)IO(2) was observed in the range 310-400 nm with a maximum absorption at 327.2 nm of σ≥1.7×10(-18) cm(2) molecule(-1). A modelling study enabled the room temperature rate coefficients for the CH(2)IO(2)+CH(2)IO(2) self-reaction and the I+CH(2)IO(2) reaction to be confined within the ranges (6-12)×10(-11) cm(3) molecule(-1) s(-1), and (1-2)×10(-11) cm(3) molecule(-1) s(-1), respectively. In the atmosphere, CH(2)IO(2) will slowly react with other radicals to release iodine atoms, which can then form IO via reaction with ozone. Slow formation of IO means that lower concentrations are formed, which leads to a lower propensity to form particles as the precursor molecule OIO forms at a rate which is dependent on the square of the IO concentration.

Time-resolved studies of the reactions of gas-phase dimethylsilylene

We report rate constants for the reactions of SiMe2 with SiH4, MeSiH3, Me2SiH2, Me3SiH, Me4Si and pentamethyldisilane obtained using laser flash photolysis/laser absorption techniques. ArF exciplex laser (193 nm) photolysis of pentamethyldisilane was used to generate SiMe2 radicals in the gas phase and the transient absorption of 457.9 nm radiation from an Ar+ laser was monitored in real time. Both time-resolved and end product analysis experiments are described, confirming that the transient species observed is the SiMe2 radical. When normalised for the number of Si-H bonds in the reactant molecules, the rate constants for SiMe2 + SiH4, MeSiH3, Me2SiH2 and Me3SiH show a systematic increase with increasing methyl group substitution. We discuss possible explanations for this correlation. An improved estimate of ΔHf0(SiMe2, 298 K.) = 26 ± 2 kcal mol−1 is obtained by combining these results with those from previous kinetic studies.