39,919 research outputs found
Hohenpeissenberg Photochemical Experiment (HOPE 2000) : measurements and photostationary state calculations of OH and peroxy radicals
No full textMeasurements of OH, total peroxy radicals, non-methane hydrocarbons (NMHCs) and various other trace gases were made at the Meteorological Observatory Hohenpeissenberg in June 2000. The data from an intensive measurement period characterised by high solar insolation (18-21 June) are analysed. The maximum midday OH concentration ranged between 4.5x106 molecules cm-3 and 7.4x106 molecules cm-3. The maximum total ROx (ROx =OH+RO+HO2+RO2) mixing ratio increased from about 55 pptv on 18 June to nearly 70 pptv on 20 and 21 June. A total of 64 NMHCs, including isoprene and monoterpenes, were measured every 1 to 6 hours. The oxidation rate of the NMHCs by OH was calculated and reached a total of over 14x106 molecules cm-3 s-1 on two days. A simple photostationary state balance model was used to simulate the ambient OH and peroxy radical concentrations with the measured data as input. This approach was able to reproduce the main features of the diurnal profiles of both OH and peroxy radicals. The balance equations were used to test the effect of the assumptions made in this model. The results proved to be most sensitive to assumptions about the impact of unmeasured volatile organic compounds (VOC), e.g. formaldehyde (HCHO), and about the partitioning between HO2 and RO2. The measured OH concentration and peroxy radical mixing ratios were reproduced well by assuming the presence of 3 ppbv HCHO as a proxy for oxygenated hydrocarbons, and a HO2/ RO2 ratio between 1:1 and 1:2. The most important source of OH, and conversely the greatest sink for peroxy radicals, was the recycling of HO2 radicals to OH. This reaction was responsible for the recycling of more than 45x106 molecules cm-3 s-1 on two days. The most important sink for OH, and the largest source of peroxy radicals, was the oxidation of NMHCs, in particular, of isoprene and the monoterpenes
Formation of [b(n−1) + OH + H]+ Ion Structural Analogs by Solution-Phase Chemistry
No full textDerivatization of a variety of peptides by a method known to enhance anhydride formation is demonstrated by mass spectrometry to yield ions that have elemental composition and fragmentation properties identical to [b(n−1) + OH + H]+ ions formed by gas-phase rearrangement and fragmentation. The [b(n−1) + OH + H]+ ions formed by gas-phase rearrangement and fragmentation and the solution-phase [b(n−1) + OH + H]+ ion structural analogs formed by derivatization chemistry show two different forms of dissociation using multiple-collision CAD in a quadrupole ion trap and unimolecular decomposition in a TOF-TOF; one group yields identical product ions as a truncated form of the peptide with a free C-terminal carboxylic acid and fragments at the same activation energy; the other group fragments differently from the truncated peptide, being more resistant to fragmentation than the truncated peptide and yielding primarily the [b(n−2) + OH + H]+ product ion. Nonergodic electron capture dissociation MS/MS suggests that any structural differences between the specific-fragmenting [b(n−1) + OH + H]+ ions and the truncated peptide is at the C-terminus of the peptide. The specific-fragmentation can be readily observed by MSn experiments to occur in an iterative fashion, suggesting that the C-terminal structure of the original [b(n−1) + OH + H]+ ion is maintained after subsequent rearrangement and fragmentation events in peptides which fragment specifically. A mechanism for the formation of specific-fragmenting and nonspecific-fragmenting [b(n−1) + OH + H]+ ions is proposed
Laser-induced fluorescence study of OH in flat flames of 1–10 bar compared with resonance CARS experiments
No full textKohse-Höinghaus K, Meier U, Attal-Trétout B. Laser-induced fluorescence study of OH in flat flames of 1–10 bar compared with resonance CARS experiments. Applied Optics. 1990;29(10):1560-1569.Laser-induced fluorescence (LIF) measurements of OH were performed in flat stoichiometric CH4/air flames burning at 1, 3, 5, 7, and 9.6 bar, which had previously been investigated using OH resonance CARS. In the LIF study, line shape information and temperatures were extracted from excitation spectra; in addition, OH profiles as a function of height above the burner surface and an estimate of the OH concentration for the different flames were obtained. The perspectives and feasibility of quantitative fluorescence measurements in high pressure flames are discussed, particularly in comparison with the application of resonance CARS
Thermodynamic and Transport Properties of H<sub>2</sub>/H<sub>2</sub>O/NaB(OH)<sub>4</sub> Mixtures Using the Delft Force Field (DFF/B(OH)<sub>4</sub><sup>-</sup>)
No full textSodium borohydride (NaBH4) has a high hydrogen (H2 ) gravimetric capacity of 10.7 wt %. NaBH4 releases H2 through a hydrolysis reaction in which aqueous NaB(OH)4 is formed as a byproduct. NaB(OH)4 strongly influences the thermophysical properties of aqueous solutions (i.e., densities, viscosities, and electrical conductivities) and the hydrolysis reaction kinetics and conversion of NaBH4. Here, molecular dynamics (MD) simulations are performed to compute viscosities, electrical conductivities, and self-diffusivities of H2 , Na+, and B(OH)4- for a temperature and concentration range of 298-353 K and 0-5 mol NaB(OH)4/kg water, respectively. Continuous fractional component Monte Carlo (CFCMC) simulations are used to compute the solubilities of H2 and activities of water in aqueous NaB(OH)4 solutions for the same temperature and concentration range. A new force field is developed (Delft force field of B(OH)4-: DFF/B(OH)4-) in which B(OH)4- is modeled as a tetrahedral structure with a scaled charge of −0.85. The OH group in B(OH)4- is modeled as a single interaction site. This force field is based on TIP4P/2005 water and the Madrid-2019 Na+ force field. The MD simulations can accurately capture the densities and viscosities within 2.5% deviation from available experimental data at 298 K up to a concentration of 5 mol NaB(OH)4/kg water. The computed electrical conductivities deviate by ca. 10% from experimental data at 298 K for the same concentration range. Based on the molecular simulations results, engineering equations are developed for shear viscosities, self-diffusivities of H2, Na+, and B(OH)4-, and solubilities of H2, which can be used to design and model NaBH4 hydrolysis reactors.Engineering ThermodynamicsComplex Fluid ProcessingTeam Poulumi De
Search for direct CP violation in D0→h−h+ modes using semileptonic B decays
No full textA search for direct CP violation in D0 → h-h+ (where h = K or π) is presented using data corresponding to an integrated luminosity of 1.0 fb-1 collected in 2011 by LHCb in pp collisions at a centre-of-mass energy of 7 TeV. The analysis uses D0 mesons produced in inclusive semileptonic b-hadron decays to the D0μX final state, where the charge of the accompanying muon is used to tag the flavour of the D0 meson. The difference in the CP-violating asymmetries between the two decay channels is measured to be ΔACP = ACP(K-K+) - ACP(π-π+) = (0.49± 0.30 (stat) ± 0.14 (syst))%
Seasonal measurements of total OH reactivity emission rates from Norway spruce in 2011
No full textNumerous reactive volatile organic compounds (VOCs) are emitted into the atmosphere by vegetation. Most biogenic VOCs are highly reactive towards the atmosphere's most important oxidant, the hydroxyl (OH) radical. One way to investigate the chemical interplay between biosphere and atmosphere is through the measurement of total OH reactivity, the total loss rate of OH radicals. This study presents the first determination of total OH reactivity emission rates (measurements via the comparative reactivity method) based on a branch cuvette enclosure system mounted on a Norway spruce (Picea abies) throughout spring, summer and autumn 2011. In parallel VOC emission rates were monitored by a second proton-transfer-reaction mass spectrometer (PTR-MS), and total ozone (O3) loss rates were obtained inside the cuvette. Total OH reactivity emission rates were in general temperature and light dependent, showing strong diel cycles with highest values during daytime. Monoterpene emissions contributed most, accounting for 56–69% of the measured total OH reactivity flux in spring and early summer. However, during late summer and autumn the monoterpene contribution decreased to 11–16%. At this time, a large missing fraction of the total OH reactivity emission rate (70–84%) was found when compared to the VOC budget measured by PTR-MS. Total OH reactivity and missing total OH reactivity emission rates reached maximum values in late summer corresponding to the period of highest temperature. Total O3 loss rates within the closed cuvette showed similar diel profiles and comparable seasonality to the total OH reactivity fluxes.
Total OH reactivity fluxes were also compared to emissions from needle storage pools predicted by a temperature-only-dependent algorithm. Deviations of total OH reactivity fluxes from the temperature-only-dependent emission algorithm were observed for occasions of mechanical and heat stress. While for mechanical stress, induced by strong wind, measured VOCs could explain total OH reactivity emissions, during heat stress they could not. The temperature-driven algorithm matched the diel variation of total OH reactivity emission rates much better in spring than in summer, indicating a different production and emission scheme for summer and early autumn. During these times, unmeasured and possibly unknown primary biogenic emissions contributed significantly to the observed total OH reactivity flux
Preparation of LGe(Se)OH: A germanium analogue of a selenocarboxylic acid (L=H[C(CMe)(NAr)](2), Ar=2,6-iPr(2)C(6)H(3))
No full textLGe(Se)OH (2) (L = HC[(CMe)(NAr)](2), Ar = 2,6-iPr(2)C(6)H(3)) was obtained by oxidative addition of red selenium to LGeOH (1). Compound 2 was characterized by IR, multinuclear NMR, EI-MS, and single-crystal X-ray diffraction. Furthermore, theoretical calculations of the acid strength (pK(a)) of compound 2 and LGe(S)OH were carried out by means of density functional theory (DFT)
(A,B,C) triplet of infrared OH bands of zeolitic H-complexes
No full textThe phenomenon of the (A,B,C) triplet of infrared OH bands at similar to 2800, similar to 2400 and similar to 1700 cm(-1), well-known for strong X-OH ... Y molecular H-complexes in solutions, liquids, and solids, is studied for the first time for surface H-complexes using CD3CN and CCl3CN adsorption on deuterated H-ZSM5 and H-FeSil zeolites. A direct experimental proof is given that the minimum between the A and B bands of D- complexes occurs at nearly exactly the 2 delta(OD) in-plane bending overtone frequency of the perturbed OD group. This verifies the resonance theory of the (A,B) doublet by Claydon and Sheppard. In reference to zeolites this means that the similar to 2800 and similar to 2400 cm(-1) OH bands recently found in adsorption of many basic molecules on zeolitic OH groups are actually pseudobands, caused by the subdivision of the very broad v(OH) +/- kv(OH ... Y) superposition band of the perturbed OH groups by Evans transmission window at the 2 delta(OH) similar to 2600 cm(-1) frequenc
Protein expression, selective isotopic labeling, and analysis of hyperfine-shifted NMR signals of Anabaena 7120 vegetative [2Fe-2S]ferredoxin
No full textTwo alternative T7 RNA promoter/polymerase systems have been employed for the heterologous expression of a plant-type [2Fe-2S]ferredoxin, Anabaena 7120 vegetative ferredoxin, in Escherichia coli at high levels (similar to 20 mg/liter of culture). One system was used when N-15-labeling the ferredoxin uniformly by growing E. coli with (NH4Cl)-N-15 as the nitrogen source; the other was used in conjunction with auxotrophic host strains to enrich the protein selectively by incorporating H-2-, C-13-, and N-15-labeled amino acids. The labeled ferredoxin samples were studied by H-1, H-2, C-13 and N-15 NMR spectroscopy. Results from H-1 and H-2 NMR studies of samples containing [H-2(alpha)]Cys, [H-2(beta 2,beta 3)]Cys, [C-13(beta)]-Cys, and [N-15]Cys have confirmed previous cysteinyl proton resonance assignments (L. Skjeldal, W. M. Westler, B.-H. Oh, A. M. Krezel, H. M., Holden, B. L. Jacobson, I. Rayment, and J. L. Markley (1991) Biochemistry 30, 7363-7368). All four C-13 NMR peaks arising from the four cysteinyl beta-carbons and all four N-15 NMR peaks from the four cysteinyl nitrogens were resolved in spectra of both the oxidized and reduced ferredoxins. The nitrogen resonance of Cys(46), which is located in a unique (Ala-Cys) dipeptide, was assigned by detection of C-13(i)-N-15(i+1) coupling in a ferredoxin sample with incorporated [C-13']Ala and [N-15]Cys. The nitrogen signal of Cys(41) was assigned tentatively on the basis of its chemical shift and T-1 relaxation time. The cysteinyl beta-carbon resonances in the reduced state have been assigned to individual residues on the basis of correlations with their (previously assigned) beta-protons. The beta-carbon resonance from Cys(46) in the oxidized state has been assigned by its correlation with the corresponding resonance in the reduced state; this was accomplished by following the progressive air oxidation of a protein sample reduced by dithionite in the presence of methyl viologen. The spin-lattice relaxation times of the beta-carbons of the two cysteines coordinated to Fe(III) were similar in the oxidized and reduced states. This suggests that the antiferromagnetic coupling present in the reduced cluster has little influence on the electronic relaxation time of the Fe(III). Studies of the temperature dependence of the H-1, C-13, and N-15 signals of the cysteinyl ligands to the [2Fe-2S] cluster show that the slope of the temperature dependence (Delta delta/Delta T-1) can be different for different atom types within a given residue. For example, in the reduced ferredoxin, although Delta delta/Delta T-1 is positive for Cys(49) H-1(beta 2) and H-1(beta 3), it is negative for Cys(49) C-13(beta). Although Delta delta/Delta T-1 is negative for protons of cysteines ligated to Fe(II) and positive for protons of cysteines ligated to Fe(III), it is positive for all the cysteinyl nitrogens. Nearly complete assignments for the spin system of Arg(42) were derived from NMR studies of three selectively labeled samples: ferredoxin incorporating [U-N-15]Arg, [26% U-C-13]Arg, and [H-2(alpha,beta 2,beta 3)]Arg. The resonance arising from the backbone amide nitrogen exhibited an unusual chemical shift at 201.6 ppm in the oxidized state but was unresolved in the reduced state. The NMR results indicate that the hydrogen bond observed between the Arg(42) backbone nitrogen and a sulfide of the iron-sulfur cluster in the X-ray structure of the oxidized ferredoxin crystal (W. R. Rypniewski, W. R. Breiter, M. M. Benning, G. Wesenberg, B.-H. Oh, J. L. Markley, I. Rayment, and H. M. Holden (1991) Biochemistry 30, 4126-4131) is present in solution in both the oxidized and reduced forms of the protein. The results show that the noncysteinyl, hyperfine-shifted peak (peak ''K'') in the spectrum of the reduced ferredoxin does not arise from H-1(alpha) of Arg(42) as previously postulated. (C) 1995 Academic Press, Inc
What can 14 CO measurements tell us about OH?
No full textThe possible use of 14CO measurements to constrain hydroxyl radical (OH) concentrations in the atmosphere is investigated. 14CO is mainly produced in the upper atmosphere from cosmic radiation. Measurements of 14CO at the surface show lower concentrations compared to the upper atmospheric source region, which is the result of oxidation by OH. In this paper, the sensitivity of 14CO mixing ratio surface measurements to the 3-D OH distribution is assessed with the TM5 model. Simulated 14CO mixing ratios agree within a few molecules 14CO cm¿3 (STP) with existing measurements at five locations worldwide. The simulated cosmogenic 14CO distribution appears mainly sensitive to the assumed upper atmospheric 14C source function, and to a lesser extend to model resolution. As a next step, the sensitivity of 14CO measurements to OH is calculated with the adjoint TM5 model. The results indicate that 14CO measurements taken in the tropics are sensitive to OH in a spatially confined region that varies strongly over time due to meteorological variability. Given measurements with an accuracy of 0.5 molecules 14CO cm¿3 STP, a good characterization of the cosmogenic 14CO fraction, and assuming perfect transport modeling, a single 14CO measurement may constrain OH to 0.2¿0.3×106 molecules OH cm¿3 on time scales of 6 months and spatial scales of 70×70 degrees (latitude×longitude) between the surface and 500 hPa. The sensitivity of 14CO measurements to high latitude OH is about a factor of five higher. This is in contrast with methyl chloroform (MCF) measurements, which show the highest sensitivity to tropical OH, mainly due to the temperature dependent rate constant of the MCF¿OH reaction. A logical next step will be the analysis of existing 14CO measurements in an inverse modeling framework. This paper presents the required mathematical framework for such an analysis
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