Methanediol from cloud-processed formaldehyde is only a minor source of atmospheric formic acid.

Atmospheric formic acid is severely underpredicted by models. A recent study proposed that this discrepancy can be resolved by abundant formic acid production from the reaction (1) between hydroxyl radical and methanediol derived from in-cloud formaldehyde processing and provided a chamber-experiment-derived rate constant, k1 = 7.5 × 10-12 cm3 s-1. High-level accuracy coupled cluster calculations in combination with E,J-resolved two-dimensional master equation analyses yield k1 = (2.4 ± 0.5) × 10-12 cm3 s-1 for relevant atmospheric conditions (T = 260-310 K and P = 0-1 atm). We attribute this significant discrepancy to HCOOH formation from other molecules in the chamber experiments. More importantly, we show that reversible aqueous processes result indirectly in the equilibration on a 10 min. time scale of the gas-phase reaction [Formula: see text] (2) with a HOCH2OH to HCHO ratio of only ca. 2%. Although HOCH2OH outgassing upon cloud evaporation typically increases this ratio by a factor of 1.5-5, as determined by numerical simulations, its in-cloud reprocessing is shown using a global model to strongly limit the gas-phase sink and the resulting production of formic acid. Based on the combined findings in this work, we derive a range of 1.2-8.5 Tg/y for the global HCOOH production from cloud-derived HOCH2OH reacting with OH. The best estimate, 3.3 Tg/y, is about 30 times less than recently reported. The theoretical equilibrium constant Keq (2) determined in this work also allows us to estimate the Henry's law constant of methanediol (8.1 × 105 M atm-1 at 280 K).

High-accuracy first-principles-based rate coefficients for the reaction of OH and CH3OOH.

The ˙OH-initiated oxidation of methyl hydroperoxide, which plays an important role in the atmospheric chemistry of methane, was theoretically characterized using high-accuracy composite amHEAT-345(Q) coupled-cluster calculations followed by a two-dimensional E,J resolved master equation analysis. The reaction is found to proceed through two distinct hydrogen-bonded pre-reactive complexes leading to two product channels, in accord with the experimental observations: (i) ˙OH + CH3OOH → CH3OO˙ + H2O with a yield of 0.8 ± 0.1, and (ii) ˙OH + CH3OOH → HCHO + ˙OH + H2O with a yield of 0.2 ± 0.1. The calculated reaction enthalpies are within 0.2 kcal mol-1 of the benchmark ATcT values. Overall thermal rate coefficients obtained from first principles are found to be in the low-pressure limit at atmospheric pressure; the total rate coefficient can be expressed over the T = 200-450 K range as k(T) = 5.0 × 10-12 × T-0.152 × exp(287/T) cm3 s-1, strongly supporting the experimental results of Vaghjiani and Ravishankara (J. Phys. Chem. 1989, 93, 1948), with which this expression agrees within ca. 15%. The current results show that (i) is the principal reaction channel and support the view that, due to its inherently fast transformations, CH3OOH is an important redistribution species for HOx˙ radicals in the Earth's atmosphere.

The CH(X2Π) + H2O reaction: two transition state kinetics.

The reaction of ground state methylidyne (CH) with water vapor (H2O) is theoretically re-investigated using high-level coupled cluster computations in combination with semi-classical transition state theory (SCTST) and two-dimensional master equation simulations. Insertion of CH into a H-O bond of H2O over a submerged barrier via a well-skipping mechanism yielding solely H and CH2O is characterized. The reaction kinetics is effectively determined by the formation of a pre-reaction van der Waals complex (PRC, HC-OH2) and its subsequent isomerization to activated CH2OH in competition with PRC re-dissociation. The tunneling effects are found to be minor, while variational effects in the PRC → CH2OH step are negligible. The calculated rate coefficient k(T) is nearly pressure-independent, but strongly depends on temperature with pronounced down-up behavior: a high value of 2 × 10-10 cm3 s-1 at 50 K, followed by a fairly steep decrease down to 8 × 10-12 cm3 s-1 at 900 K, but increasing again to 5 × 10-11 cm3 s-1 at 3500 K. Over the T-range of this work, k(T) can be expressed as: k(T, P = 0) = 2.31 × 10-11 (T/300 K)-1.615 exp(-38.45/T) cm3 s-1 for T = 50-400 K k(T, P = 0) = 1.15 × 10-12 (T/300 K)0.8637 exp(892.6/T) cm3 s-1 for T = 400-1000 K k(T, P = 0) = 4.57 × 10-15 (T/300 K)3.375 exp(3477.4/T) cm3 s-1 for T = 1000-3500 K.

The photolysis of α-hydroperoxycarbonyls.

In this work, we theoretically elucidated the mechanism and predicted the major products of the photolysis of α-hydroperoxycarbonyls, known to be products of the atmospheric oxidation of biogenic volatile organic compounds (BVOC) and components of secondary organic aerosol (SOA) in rural and remote areas. Using 2-hydroperoxypropanal OCHCH(OOH)CH3 as a model compound, we show that the likely major photolysis mechanism is a fast 1,5 H-shift in the initially excited singlet S1 state followed by spontaneous elimination of singlet oxygen to yield an enol HOCH[double bond, length as m-dash]CHCH3, while intersystem crossing (ISC) to the triplet T1 state and C-C scission into HC˙O + HOOC˙HCH3 followed by expulsion of a hydroxyl radical from the unstable HOOC˙HCH3 is another product channel. The direct S1 reaction was found to occur at such a high rate that the quantum yield in atmospheric conditions is expected to approach unity. In the atmosphere, the enol should generally react with OH radicals or tautomerize into the more stable carbonyl O[double bond, length as m-dash]CH-CH2CH3. Vinylalcohol is shown to be a major end product of the photolysis of hydroperoxyacetaldehyde, an isoprene oxidation product. Taking into account also the important enhancement of the absorption cross sections over those of the constituent monofunctional compounds as observed for the analogous ß-ketohydroperoxides, (F. Jorand et al., J. Photochem. Photobiol. A: Chem., 2000, 134, 119-125) the atmospheric photolysis rate of α-hydroperoxycarbonyls was estimated to be in the range of (1 to 5) × 10-4 s-1, generally faster than the rate of their OH reactions.

Theoretically derived mechanisms of HPALD photolysis in isoprene oxidation.

In this work we identified and theoretically quantified two photolysis mechanisms of HPALDs (hydroperoxy aldehydes) that result from the isomerization of peroxy radicals in the atmospheric oxidation of isoprene at low/moderate NOx. As a first photolysis mechanism, we show that a fraction of the initially excited S1-state HPALDs isomerizes by a near-barrierless 1,5 H-shift at a rate approaching 1012 s-1 - competing with the ∼equally fast intersystem crossing to the T2 triplet state - forming an unstable biradical that spontaneously expels an OH (hydroxyl) radical. A second mechanism is shown to proceed through the activated T2 triplet biradical - formed from S1 - undergoing a concerted ring-closure and OH-expulsion, yielding an oxiranyl-type co-product radical that quickly ring-opens to enoxy radicals. In both mechanisms, subsequent chemistry of the co-product radicals yields additional first-generation OH. The combined HPALD-photolysis quantum yield by these two mechanisms - which may not be the only photolysis routes - is estimated at 0.55 and the quantum yield of OH generation at 0.9, in fair accordance with experimental data on an HPALD proxy (Wolfe et al., Phys. Chem. Chem. Phys., 2012, 14, 7276-7286).

The reaction of methyl peroxy and hydroxyl radicals as a major source of atmospheric methanol.

Methyl peroxy, a key radical in tropospheric chemistry, was recently shown to react with the hydroxyl radical at an unexpectedly high rate. Here, the molecular reaction mechanisms are elucidated using high-level quantum chemical methodologies and statistical rate theory. Formation of activated methylhydrotrioxide, followed by dissociation into methoxy and hydroperoxy radicals, is found to be the main reaction pathway, whereas methylhydrotrioxide stabilization and methanol formation (from activated and stabilized methylhydrotrioxide) are viable minor channels. Criegee intermediate formation is found to be negligible. Given the theoretical uncertainties, useful constraints on the yields are provided by atmospheric methanol measurements. Using a global chemistry-transport model, we show that the only explanation for the high observed methanol abundances over remote oceans is the title reaction with an overall methanol yield of ∼30%, consistent with the theoretical estimates given their uncertainties. This makes the title reaction a major methanol source (115 Tg per year), comparable to global terrestrial emissions.

Fast (E)-(Z) Isomerization Mechanisms of Substituted Allyloxy Radicals in Isoprene Oxidation.

Unusually rapid (E) ⇌ (Z) isomerization mechanisms are proposed and theoretically quantified for substituted allyloxy radicals, R'RC═CH-CH2O(•), with R and R' alkyl or oxygenated substituents, termed below ß,γ-enoxy radicals. These conversions are shown to occur by a sequence of (i) ring closure to nearly isoergic oxiranyl-C(•)RR' radicals, (ii) internal rotation of the oxiranyl-moiety over 180°, and (iii) oxiranyl-ring reopening to yield the (E) ⇌ (Z)-isomerized oxy radicals. The barriers for all three steps were computed at the CCSD(T)/aug-cc-pVTZ//QCISD/6-311(d,p) level of theory to be only ≈5 ± 2 kcal mol(-1), and the rate constants at 298 K for the overall reactions were evaluated using transition-state theory to be in the range of 10(8)-10(9) s(-1). Specifically, and of relevance to the isoprene oxidation mechanism, it is predicted that the (E)-δ-hydroxy-isoprenyloxy radicals resulting from isoprene oxidation at high NO levels should isomerize to their (Z)-analogues at a rate of about 1.5 × 10(9) s(-1), much faster than the competing 1,5-H shift that was proposed earlier as the major fate of these (E)-oxy radicals ( Dibble, T. S. J. Phys. Chem. A 2002, 106, 6643-6650 ). It is concluded that under high-NO conditions the (E)- and (Z)-δ-hydroxy-isoprenylperoxy precursors should yield identical and therefore indistinguishable C5-hydroxycarbonyls as main products.

Atmospheric Vinyl Alcohol to Acetaldehyde Tautomerization Revisited.

The atmospheric oxidation of vinyl alcohol (VA) produced by photoisomerization of acetaldehyde (AA) is thought to be a source of formic acid (FA). Nevertheless, a recent theoretical study predicted a high rate coefficient k1(298 K) of ≈10(-14) cm(3) molecule(-1) s(-1) for the FA-catalyzed tautomerization reaction 1 of VA back into AA, which suggests that FA buffers its own production from VA. However, the unusually high frequency factor implied by that study prompted us to reinvestigate reaction 1 . On the basis of a high-level ab initio potential energy profile, we first established that transition state theory is applicable, and derived a k1(298 K) of only ≈2 × 10(-20) cm(3) molecule(-1) s(-1), concluding that the reaction is negligible. Instead, we propose and rationalize another important VA sink: its uptake by aqueous aerosol and cloud droplets followed by fast liquid-phase tautomerization to AA; global modeling puts the average lifetime by this sink at a few hours, similar to oxidation by OH.

Hydroxyl radical recycling in isoprene oxidation driven by hydrogen bonding and hydrogen tunneling: the upgraded LIM1 mechanism.

The Leuven isoprene mechanism, proposed earlier to aid in rationalizing the unexpectedly high hydroxyl radical (OH) concentrations in isoprene-rich, low-nitric-oxide (NO) regions ( Peeters ; et al. Phys. Chem. Chem. Phys . 2009 , 11 , 5935 ), is presented in an upgraded and extended version, LIM1. The kinetics of the crucial reactions in the proposed isoprene-peroxy radical interconversion and isomerization pathways are re-evaluated theoretically, on the basis of energy barriers computed at the much higher CCSD(T)/aug-cc-pVTZ//QCISD/6-311G(d,p) level of theory, and using multiconformer partition functions obtained at the M06-2X/6-311++G(3df,2p) level that, different from the B3LYP level used in our earlier work, accounts for the crucial London dispersion effects in the H-bonded systems involved. The steady-state fraction of the specific Z-δ-OH-peroxy radical isomers/conformers that can isomerize by a 1,6-H shift is shown to be largely governed by hydrogen-bond strengths, whereas their isomerization itself is found to occur quasi-exclusively by hydrogen atom tunneling. The isomer-specific Z-δ-OH-peroxy 1,6-H-shift rate coefficients are predicted to be of the order of 1 s(-1) at 298 K, but the experimentally accessible bulk rate coefficients, which have to be clearly distinguished from the former, are 2 orders of magnitude lower due to the very low Z-δ-OH-peroxy steady-state fractions that are only around or below 0.01 at low to moderate NO and depend on the peroxy lifetime. Two pathways subsequent to the peroxy radical 1,6-H shift are identified, the earlier predicted route yielding the photolabile hydroperoxy-methylbutenals (HPALDs), and a second, about equally important path, to dihydroperoxy-carbonyl peroxy radicals (di-HPCARP). Taking this into account, our predicted bulk peroxy isomerization rate coefficients are about a factor 1.8 higher than the available experimental results for HPALD production ( Crounse ; et al. Phys. Chem. Chem. Phys. 2011 , 13 , 13607 ), which is within the respective uncertainty margins. We also show that the experimental temperature dependence of the HPALD production rates as well as the observed kinetic isotope effect for per-deuterated isoprene support quantitatively our theoretical peroxy interconversion rates. Global modeling implementing LIM1 indicates that on average about 28% of the isoprene peroxys react via the 1,6-H-shift isomerization route, representing 100-150 Tg carbon per year. The fast photolysis of HPALDs we proposed earlier as primary OH regeneration mechanism ( Peeters and Muller . Phys. Chem. Chem. Phys . 2010 , 12 , 14227 ) found already experimental confirmation ( Wolfe ; et al. Phys. Chem. Chem. Phys. 2012 , 14 , 7276 ); based on further theoretical work in progress, reaction schemes are presented of the oxy coproduct radicals from HPALD photolysis and of the di-HPCARP radicals from the second pathway following peroxy isomerization that are both expected to initiate considerable additional OH recycling.

Absolute rate coefficient of the gas-phase reaction between hydroxyl radical (OH) and hydroxyacetone: investigating the effects of temperature and pressure.

The rate coefficient (k1) of the reaction between hydroxyl radical and hydroxyacetone, which remained so far controversial, was determined over the temperature range 290-500 K using pulsed-laser photolysis coupled to pulsed-laser induced fluorescence (PLP-PLIF). Hydroxyl radical was generated by pulsed photolysis of H2O2 at 248 nm. The results show that at a pressure of 50 Torr He, the rate coefficient obeys a negative temperature dependence k1(T) = (1.77 ± 0.19) × 10(-12) exp((353 ± 36)/T) cm(3) molecule(-1) s(-1) for temperatures between 290 and 380 K, in good agreement with the results of Dillon et al. (Phys. Chem. Chem. Phys. 2006, 8, 236) at 60 Torr He. However, always at 50 Torr He but for the higher temperature range 410-500 K, a positive temperature dependence was found: k1(T) = (1.14 ± 0.25) × 10(-11) exp(-(378 ± 102)/T) cm(3) molecule(-1) s(-1), close to the expression obtained by Baasandorj et al. (J. Phys. Chem. A 2009, 113, 10495) for pressures of 2 and 5 Torr He but at lower temperatures, 280-360 K, where their k1(T) values are well below these of Dillon et al. and of this work. Moreover, the rate coefficient k1(301 K) determined as a function of pressure, from 10 to 70 Torr He, shows a pronounced decrease once the pressure is below ∼40 Torr He, thus explaining the disparity between the higher-pressure data of Dillon et al. and the lower-pressure results of Baasandorj et al. The pressure dependence of k1 and of its temperature-dependence below ∼400 K is rationalized by the reaction proceeding via a hydrogen-bonded prereactive complex (PRC) and a submerged transition state, such that at high pressures collisionally thermalized PRCs contribute additional reactive flux over and through the submerged barrier. The high-pressure rate coefficient data both of Dillon et al. and of this work over the combined range 230-500 K can be represented by the theory-based expression k1(T) = 5.3 × 10(-20) × T(2.6) exp(1100/T) cm(3) molecule(-1) s(-1).

Experimental and theoretical study of the reaction of the ethynyl radical with nitrous oxide, C2H + N2O.

We investigated the rate constants and reaction mechanism of the gas phase reaction between the ethynyl radical and nitrous oxide (C(2)H + N(2)O) using both experimental methods and electronic structure calculations. A pulsed-laser photolysis/chemiluminescence technique was used to determine the absolute rate coefficient over the temperature range 570 K to 836 K. In this experimental temperature range, the measured temperature dependence of the overall rate constants can be expressed as: k(T) (C(2)H + N(2)O) = 2.93 × 10(-11) exp((-4000 ± 1100) K/T) cm(3) s(-1) (95% statistical confidence). Portions of the C(2)H + N(2)O potential energy surface (PES), containing low-energy pathways, were constructed using the composite G3B3 method. A multi-step reaction route leading to the products HCCO + N(2) is clearly preferred. The high selectivity between product channels favouring N(2) formation occurs very early. The pathway corresponds to the addition of the terminal C atom of C(2)H to the terminal N atom of N(2)O. Refined calculations using the coupled-cluster theory whose electronic energies were extrapolated to the complete basis set limit CCSD(T)/CBS led to an energy barrier of 6.0 kcal mol(-1) for the entrance channel. The overall rate constant was also determined by application of transition-state theory and Rice-Ramsperger-Kassel-Marcus (RRKM) statistical analyses to the PES. The computed rate constants have similar temperature dependence to the experimental values, though were somewhat lower.

Unusually fast 1,6-h shifts of enolic hydrogens in peroxy radicals: formation of the first-generation C2 and C3 carbonyls in the oxidation of isoprene.

In a theoretical investigation using the CBS-QB3//UB3LYP/6-31+G** method supported by higher-level computations such as CBS-QB3//UQCISD/6-31+G**, the 1,6-H shifts of the enolic hydrogen in peroxy radicals of the type Z-HO-CH═CH-CH(2)-OO(•) were found to face exceptionally low energy barriers of only about 11 kcal mol(-1)--i.e., 6-9 kcal mol(-1) lower than the barriers for similar shifts of alkane hydrogens--such that they can proceed at unequaled rates of order 10(5) to 10(6) s(-1) at ambient temperatures. The unusually low barriers for enolic 1,6-H shifts in peroxy radicals, characterized here for the first time to our knowledge, are rationalized. As cases in point, the secondary peroxy radicals Z-HO-CH═C(CH(3))-CH(OO(•))-CH(2)OH (case A) and Z-HO-CH═CH-C(CH(3))(OO(•))-CH(2)OH (case B) derived from the primary Z-δ-hydroxy-peroxy radicals in the oxidation of isoprene, are predicted to undergo 1,6-H shifts of their enolic hydrogens at TST-calculated rates in the range 270-320 K of k(T)(A) = 5.4 × 10(-4) × T(5.04) × exp(-1990/T) s(-1) and k(T)(B) = 109 × T(3.13) × exp(-3420/T) s(-1), respectively, i.e., 2.0 × 10(6) and 6.2 × 10(4) s(-1), respectively, at 298 K, far outrunning in all relevant atmospheric and laboratory conditions their reactions with NO proposed earlier as their dominant pathways (Dibble J. Phys. Chem. A 2004, 108, 2199). These fast enolic-H shifts are shown to provide the explanation for the first-generation formation of methylglyoxal + glycolaldehyde, and glyoxal + hydroxyacetone in the oxidation of isoprene under high-NO conditions, recently determined by several groups. However, under moderate- and low-NO atmospheric conditions, the fast interconversion and equilibration of the various thermally labile, initial peroxy conformers/isomers from isoprene and the isomerization of the initial Z-δ-hydroxy-peroxy radicals, both recently proposed by us (Peeters et al. Phys. Chem. Chem. Phys. 2009, 11, 5935), are expected to substantially reduce the yields of the small carbonyls at issue.

Mechanism of the catalytic deperoxidation of tert-butylhydroperoxide with cobalt(II) acetylacetonate.

The Co(II)/Co(III)-induced decomposition of hydroperoxides is an important reaction in many industrial processes and is referred to as deperoxidation. In the first step of the so-called Haber-Weiss cycle, alkoxyl radicals and Co(III)-OH species are generated upon the reaction of the Co(II) ion with ROOH. The catalytic cycle is closed upon the regeneration of the Co(II) ion through the reaction of the Co(III)-OH species with a second ROOH molecule, thus producing one equivalent of the peroxyl radicals. Herein, the deperoxidation of tert-butylhydroperoxide by dissolved cobalt(II) acetylacetonate is studied by using UV/Vis spectroscopy in situ with a noninteracting solvent, namely, cyclohexane. Kinetic information extracted from experiments, together with quantum-chemical calculations, led to new mechanistic hypotheses. Even under anaerobic conditions, the Haber-Weiss cycle initiates a radical-chain destruction of ROOH propagated by both alkoxyl and peroxyl radicals. This chain mechanism rationalizes the high deperoxidation rates, which are directly proportional to the cobalt concentration up to approximately 75 µM at 333 K. However, at higher cobalt concentrations, a remarkable decrease of the rate is observed. The hypothesis put forward herein is that this remarkable autoinhibition effect could be explained by the hitherto overlooked chain termination of two Co(III)--OH species. The direct competition between the first-order Haber-Weiss initiation and the second-order termination can indeed explain this peculiar kinetic behavior of this homogeneous deperoxidation system.

HO(x) radical regeneration in isoprene oxidation via peroxy radical isomerisations. II: experimental evidence and global impact.

A consistent body of experimental evidence from work of other groups is presented in support of the novel, theoretically based, isoprene oxidation mechanism we recently proposed to rationalize the unexpectedly high OH concentrations observed over areas with high isoprene emissions. Some explicit or implicit criticisms on the new mechanism are addressed. A particular photochemical mechanism is newly proposed for the OH-regenerating photolysis of the crucial hydroperoxy-methyl-butenals (HPALDs), formed by isomerisation of the initial isoprene hydroxy-peroxy radicals, that rationalizes a quantum yield close to 1. A similar photolysis mechanism of the resulting photolabile peroxy-acid-aldehydes (PACALDs) is shown to generate ample additional OH. Global modeling demonstrates the major importance of the new chemistry for the oxidizing capacity of the atmosphere over continents. The globally averaged yield of the HPALDs in the oxidation of isoprene by OH is estimated to be of the order of 0.6. The isomerisation reactions of isoprene peroxy radicals are found to result in modelled [OH] increases in the planetary boundary layer by up to a factor of 3, in agreement with the reported observations as in the Amazon basin.

HOx regeneration in the oxidation of isoprene III: theoretical study of the key isomerisation of the Z-δ-hydroxy-peroxy isoprene radicals.

As a sequel to our communication on a proposed new isoprene oxidation mechanism aiming to rationalize the unexpectedly high OH and HO(2) levels observed in isoprene-rich areas (J. Peeters, T. L. Nguyen, L. Vereecken, Phys. Chem. Chem. Phys. 2009, 11, 5935), we report herein the detailed quantum chemical and statistical kinetics characterization of the crucial 1,6-H shifts in the two Z-δ-hydroxy-peroxy radicals from isoprene. Geometries, energies and vibration frequencies of all conformers of the reactant radicals and transition states are computed at the B3LYP/6-31+G(d,p) level of theory and the energies of the lowest-lying conformers are then refined at various higher levels of theory, including CBS-QB3, IRCMax(CBS-QB3//B3LYP) and CBS-APNO. The rate coefficients over a wide temperature range are calculated using multi-conformer transition state theory with WKB tunneling factors evaluated for the barrier shape found by CBS-QB3//B3LYP IRC analyses. The WKB tunneling factors for these allyl-stabilisation-assisted reactions are about 25 at ambient temperatures. The rate coefficients can be represented by Arrhenius expressions over the 250-350 K range: k(T)=1.4×10(9) exp(-6380/T) s(-1) for the Z-1-OH-4-OO(·)-isoprene radical, and k(T)=0.72×10(9) exp(-5520/T) s(-1) for Z-1-OH-4-OO(·)-isoprene. With the k(1,6-H) of order 1 s(-1) at ambient temperatures, these isomerisations can compete with and even outrun the traditional peroxy reactions at low and moderate NO levels. The importance of these reactions as key processes in the newly proposed, OH-regenerating isoprene oxidation scheme is discussed.

Kinetics of O(1D) + H2O and O(1D) + H2: absolute rate coefficients and O(3P) yields between 227 and 453 K.

The rate coefficients for the crucial atmospheric reactions of O((1)D) with H(2)O and H(2), k(1) and k(2), were measured over a wide temperature range using O((1)D) detection based on the chemiluminescence reaction of O((1)D) with C(2)H. Analyzing the decays of the chemiluminescence intensities yielded a value for k(1)(T) of (1.70 x 10(-10)exp[36 K/T]) cm(3) s(-1). Multiplying or dividing k(1)(T) by a factor f(T) = 1.04 exp(5.59(|1 K/T- 1/287|)), gives the 95% confidence limits; our new determination, in good agreement with previous studies, further reduces the uncertainty in k(1). An extended study of k(2) yielded a temperature independent rate constant of (1.35 +/- 0.05) x 10(-10) cm(3) s(-1). This precise value, based on an extended set of determinations with very low scatter, is significantly larger than the current recommendations, as were two other recent k(2) determinations. Secondly, the fractions of O((1)D) quenched to O((3)P) by H(2)O and H(2), k(1b)/k(1) and k(2b)/k(2), were precisely determined from fits to chemiluminescence decays. A temperature-independent value for k(1b)/k(1) of 0.010 +/- 0.003 was found. For the quenching fraction k(2b)/k(2) a value of 0.007 +/- 0.007 was obtained at room temperature. Both determinations are significantly smaller than values and upper limits from previous studies.

Identification of a series of substituted 2-piperazinyl-5-pyrimidylhydroxamic acids as potent histone deacetylase inhibitors.

Pursuing our efforts in designing 5-pyrimidylhydroxamic acid anti-cancer agents, we have identified a new series of potent histone deacetylase (HDAC) inhibitors. These compounds exhibit enzymatic HDAC inhibiting properties with IC(50) values in the nanomolar range and inhibit tumor cell proliferation at similar levels. Good solubility, moderate bioavailability, and promising in vivo activity in xenograft model made this series of compounds interesting starting points to design new potent HDAC inhibitors.

The gas-phase ozonolysis of beta-caryophyllene (C(15)H(24)). Part I: an experimental study.

The gas phase reaction of ozone with beta-caryophyllene was investigated in a static glass reactor at 750 Torr and 296 K under various experimental conditions. The reactants and gas phase products were monitored by FTIR-spectroscopy and proton-transfer-reaction mass spectrometry (PTR-MS). Aerosol formation was monitored with a scanning mobility particle sizer (SMPS) and particulate products analysed by liquid chromatography/mass spectrometry (HPLC-MS). The different reactivity of the two double bonds in beta-caryophyllene was probed by experiments with different ratios of reactants. An average rate coefficient at 295 K for the first-generation products was determined as 1.1 x 10(-16) cm(3) molecule(-1) s(-1). Using cyclohexane as scavenger, an OH-radical yield of (10.4 +/- 2.3)% was determined for the ozonolysis of the more reactive internal double bond, whereas the average OH-radical yield for the ozonolysis of the first-generation products was found to be (16.4 +/- 3.6)%. Measured gas phase products are CO, CO(2) and HCHO with average yields of (2.0 +/- 1.8)%, (3.8 +/- 2.8)% and (7.7 +/- 4.0)%, respectively for the more reactive internal double bond and (5.5 +/- 4.8)%, (8.2 +/- 2.8)% and (60 +/- 6)%, respectively from ozonolysis of the less reactive double bond of the first-generation products. The residual FTIR spectra indicate the formation of an internal secondary ozonide of beta-caryophyllene. From experiments using HCOOH as a Criegee intermediate (CI) scavenger, it was concluded that at least 60% of the formed CI are collisionally stabilized. The aerosol yield in the ozonolysis of beta-caryophyllene was estimated from the measured particle size distributions. In the absence of a CI scavenger the yield ranged between 5 and 24%, depending on the aerosol mass. The yield increases with addition of water vapour or with higher concentrations of formic acid. In the presence of HCHO, lower aerosol yields were observed. This suggests that HCOOH adds to a Criegee intermediate to form a low-volatility compound responsible for aerosol formation. The underlying reaction mechanisms are discussed and compared with the results from the accompanying theoretical paper.

A temperature dependence kinetic study of O(1D) + CH4: overall rate coefficient and product yields.

Using a recently-developed chemiluminescence technique for monitoring O(1D), the rate coefficient, k1, of the important atmospheric reaction O(1D) + CH4 --> products has been determined over a wide temperature range, 227 to 450 K. The rate coefficient was shown to be independent of temperature, having a value of (1.91 +/- 0.08) x 10(-10) cm3 s(-1); the quoted uncertainties are with 95% confidence. This highly precise value, based on an extended set of determinations with very low scatter, is significantly greater, 26%, than current recommended values. Secondly, the fraction of O(1D) quenched to O(3P) by CH4, k(1q)/k1, was precisely determined from chemiluminescence decays over the temperature range 236 to 340 K. A temperature independent value for k(1q)/k1 of 0.002 +/- 0.003 was found. Finally, LIF detection of OH has been applied to accurately determine the product branching fraction to OH of O(1D) + CH4 at room temperature. Our value, k(1a)/k1 = 0.76 +/- 0.08 (95% confidence), is in line with recent determinations by other groups.

Origin of byproducts during the catalytic autoxidation of cyclohexane.

The formation of byproducts during the Co(acac)2 and Cr(acac)3-catalyzed cyclohexane autoxidation is compared with the noncatalyzed thermal process. CoII ions seem to cause only a moderate perturbation of the reaction mechanism, causing a fast conversion of the cyclohexyl hydroperoxide via a redox cycle, rather than via abstraction of the alphaH-atom by chain carrying peroxyl radicals. Nevertheless, both the radical propagation and the CoII-induced decomposition of the hydroperoxide cause the formation of cyclohexoxy radicals that are partially transformed to 6-hydroxyhexanoic acid, the major primary byproduct for these systems. However, during the CoII-catalyzed reaction, the concentration of cyclohexanone increases much faster than that of the hydroperoxide, causing the ketone to take over the role of dominant byproduct source. A mechanism for the conversion of cyclohexanone to ring-opened byproducts is put forward. Cr(acac)3 seems to catalyze additional reactions, some of them probably leading directly to byproducts. Indeed, the evolution of (by)products is significantly different from the CoII-catalyzed and the thermal systems, in the sense that they all seem to be primary in origin.