61 research outputs found
Two-Pulse Atomic Coherent Control (2PACC) Spectroscopy of Eley-Rideal Reactions. An Application of an Atom Laser
A spectroscopic application of the atom laser is suggested. The spectroscopy
termed 2PACC employs the coherent properties of matter-waves from a two pulse
atom laser. These waves are employed to control a gas-surface chemical
recombination reaction. The method is demonstrated for an Eley-Rideal reaction
of a hydrogen or alkali atom-laser pulse where the surface target is an
adsorbed hydrogen atom. The reaction yields either a hydrogen or alkali hydride
molecule. The desorbed gas phase molecular yield and its internal state is
shown to be controlled by the time and phase delay between two atom-laser
pulses. The calculation is based on solving the time-dependent Schrodinger
equation in a diabatic framework. The probability of desorption which is the
predicted 2PACC signal has been calculated as a function of the pulse
parameters.Comment: 29 pages and 11 figure
A Pulse Shaping Algorithm of a Coherent Matter Wave. Controlling Reaction Dynamics
A pulse shaping algorithm for a matter wave with the purpose of controlling a
binary reaction has been designed. The scheme is illustrated for an Eley-Rideal
reaction where an impinging matter-wave atom recombines with an adsorbed atom
on a metal surface. The wave function of the impinging atom is shaped such that
the desorbing molecule leaves the surface in a specific vibrational state.Comment: 4 pages, 5 figure
Atmospheric Fate of Methacrolein. 1. Peroxy Radical Isomerization Following Addition of OH and O_2
Peroxy radicals formed by addition of OH and O_2 to the olefinic carbon atoms in methacrolein react with NO to form methacrolein hydroxy nitrate and hydroxyacetone. We observe that the ratio of these two compounds, however, unexpectedly decreases as the lifetime of the peroxy radical increases. We propose that this results from an isomerization involving the 1,4-H-shift of the aldehydic hydrogen atom to the peroxy group. The inferred rate (0.5 ± 0.3 s^(–1) at T = 296 K) is consistent with estimates obtained from the potential energy surface determined by high level quantum calculations. The product, a hydroxy hydroperoxy carbonyl radical, decomposes rapidly, producing hydroxyacetone and re-forming OH. Simulations using a global chemical transport model suggest that most of the methacrolein hydroxy peroxy radicals formed in the atmosphere undergo isomerization and decomposition
Atmospheric Fate of Methacrolein. 2. Formation of Lactone and Implications for Organic Aerosol Production
We investigate the oxidation of methacryloylperoxy nitrate (MPAN) and methacrylicperoxy acid (MPAA) by the hydroxyl radical (OH) theoretically, using both density functional theory [B3LYP] and explicitly correlated coupled cluster theory [CCSD(T)-F12]. These two compounds are produced following the abstraction of a hydrogen atom from methacrolein (MACR) by the OH radical. We use a RRKM master equation analysis to estimate that the oxidation of MPAN leads to formation of hydroxymethyl–methyl-α-lactone (HMML) in high yield. HMML production follows a low potential energy path from both MPAN and MPAA following addition of OH (via elimination of the NO_3 and OH from MPAN and MPAA, respectively). We suggest that the subsequent heterogeneous phase chemistry of HMML may be the route to formation of 2-methylglyceric acid, a common component of organic aerosol produced in the oxidation of methacrolein. Oxidation of acrolein, a photo-oxidation product from 1,3-butadiene, is found to follow a similar route generating hydroxymethyl-α-lactone (HML)
Criegee Intermediates React with Ozone
We have investigated the reaction of the one-carbon stabilized Criegee intermediate (H_2COO, formaldehyde oxide) with ozone, theoretically, using high level coupled cluster ab initio methods. Key to the reactivity of the Criegee intermediate with ozone is the strongly exothermic formation of an intermediate consisting of five oxygen and one carbon atoms (H_2CO_5) in a six-membered ring structure. This intermediate proceeds via a spin-allowed route over two transition states with low energy barriers to form molecular oxygen and formaldehyde. The reaction may contribute to the loss of these biradicals in the atmosphere
Autoxidation of Organic Compounds in the Atmosphere
We present a hypothesis that autoxidation (inter- and intramolecular hydrogen abstraction by peroxy radicals) plays an important role in the oxidation of organic compounds in the atmosphere, particularly organic matter associated with aerosol. In the laboratory, we determine the rate of this process at room temperature for a model system, 3-pentanone. We employ ab initio calculations to investigate H-shifts within a broader group of substituted organic compounds. We show that the rate of abstraction of hydrogen by peroxy radicals is largely determined by the thermochemistry of the nascent alkyl radicals and thus is highly influenced by neighboring substituents. As a result, autoxidation rates increase rapidly as oxygen-containing functional groups (carbonyl, hydroxy, and hydroperoxy) are added to organic compounds. This mechanism is consistent with formation of the multifunctional hydroperoxides and carbonyls often found in atmospheric aerosol particles
Supplementation of docosahexaenoic acid (DHA), vitamin D<sub>3</sub> and uridine in combination with six weeks of cognitive and motor training in prepubescent children: a pilot study
BACKGROUND: Learning and memory have been shown to be influenced by combination of dietary supplements and exercise in animal models, but there is little available evidence from human subjects. The aim of this pilot study was to investigate the effect of combining a motor- and cognitive exercise program with dietary supplementation consisting of 500 mg docosahexaenoic acid (DHA), 10 μg vitamin D3 and 1000 mg uridine (DDU-supplement) in 16 prepubescent children (age 8–11 years). METHODS: We designed a randomized, placebo-controlled, double-blinded study lasting 6 weeks in which DDU-supplement or placebo was ingested daily. During the intervention period, all children trained approximately 30 min 3 days/week using an internet-based cognitive and motor training program (Mitii). Prior to and post the intervention period dietary record, blood sampling, physical exercise tests and motor and cognitive tests were performed. RESULTS: Fourteen of the 16 children completed the intervention and ingested the supplement as required. 6 weeks DDU-supplementation resulted in a significant increase in the blood concentration of vitamin D2+3 and DHA (p = 0.023 and p < 0.001, respectively). Power calculation based on one of the cognitive tasks revealed a proper sample size of 26 children. CONCLUSION: All children showed improved performance in the trained motor- and cognitive tasks, but it was not possible to demonstrate any significant effects on the cognitive tests from the dietary supplementation. However, DDU-supplementation did result in increased blood concentration of DHA and vitamin D2+3.: TRIAL REGISTRATION: Clinical registration ID: NCT02426554 (clinical Trial.gov). January 2015 retrospectively registered
Rapid Hydrogen Shift Scrambling in Hydroperoxy-Substituted Organic Peroxy Radicals
Using quantum mechanical calculations, we have investigated hydrogen shift (H-shift) reactions in peroxy radicals derived from the atmospheric oxidation of 1-pentene (CH_2═CHCH_2CH_2CH_3) and its monosubstituted derivatives. We investigate the peroxy radicals, HOCH_2CH(OO)CR_1HCH_2CH_3, HOCH_2CH(OO)CH_2CR_1HCH_3, and HOCH_2CH(OO)CH_2CH_2CR_1H_2, where the substituent R_1 is an alcoholic (OH), a hydroperoxy (OOH), or a methoxy (OCH_3) group. For peroxy radicals with an OOH substituent, the H-shift reaction from the hydrogen atom on the OOH group to the OO group is extremely fast. We find that the rate constants of this type of H-shift reactions are greater than 10^3 s^(–1) for both the forward and the reverse reactions. It leads to the formation of two different radical isomers that react through different reaction mechanisms and yield different products. These very fast H-shift reactions are much faster than the reactions with NO and HO_2 under most atmospheric conditions and must be included in the atmospheric modeling of volatile organic compounds where hydroperoxy peroxy radicals are formed
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