Minor Channel Research Articles

Secondary organic aerosol (SOA) formation from reaction of isoprene with nitrate radicals (NO<sub>3</sub>)

Abstract. Secondary organic aerosol (SOA) formation from the reaction of isoprene with nitrate radicals (NO3) is investigated in the Caltech indoor chambers. Experiments are performed in the dark and under dry conditions (RH&lt10%) using N2O5 as a source of NO3 radicals. For an initial isoprene concentration of 18.4 to 101.6 ppb, the SOA yield (defined as the ratio of the mass of organic aerosol formed to the mass of parent hydrocarbon reacted) ranges from 4.3% to 23.8%. By examining the time evolutions of gas-phase intermediate products and aerosol volume in real time, we are able to constrain the chemistry that leads to the formation of low-volatility products. Although the formation of ROOR from the reaction of two peroxy radicals (RO2) has generally been considered as a minor channel, based on the gas-phase and aerosol-phase data it appears that RO2+RO2 reaction (self reaction or cross-reaction) in the gas phase yielding ROOR products is a dominant SOA formation pathway. A wide array of organic nitrates and peroxides are identified in the aerosol formed and mechanisms for SOA formation are proposed. Using a uniform SOA yield of 10% (corresponding to Mo≅10 μg m−3), it is estimated that ~2 to 3 Tg yr−1 of SOA results from isoprene+NO3. The extent to which the results from this study can be applied to conditions in the atmosphere depends on the fate of peroxy radicals in the nighttime troposphere.

Impact of the new HNO<sub>3</sub>-forming channel of the HO<sub>2</sub>+NO reaction on tropospheric HNO<sub>3</sub>, NO<sub>x</sub>, HO<sub>x</sub> and ozone

Abstract. We have studied the impact of the recently observed reaction NO+HO2→HNO3 on atmospheric chemistry. A pressure and temperature-dependent parameterisation of this minor channel of the NO+HO2→NO2+OH reaction has been included in both a 2-D stratosphere-troposphere model and a 3-D tropospheric chemical transport model (CTM). Significant effects on the nitrogen species and hydroxyl radical concentrations are found throughout the troposphere, with the largest percentage changes occurring in the tropical upper troposphere (UT). Including the reaction leads to a reduction in NOx everywhere in the troposphere, with the largest decrease of 25% in the tropical and Southern Hemisphere UT. The tropical UT also has a corresponding large increase in HNO3 of 25%. OH decreases throughout the troposphere with the largest reduction of over 20% in the tropical UT. The mean global decrease in OH is around 13%, which is very large compared to the impact that typical photochemical revisions have on this modelled quantity. This OH decrease leads to an increase in CH4 lifetime of 5%. Due to the impact of decreased NOx on the OH:HO2 partitioning, modelled HO2 actually increases in the tropical UT on including the new reaction. The impact on tropospheric ozone is a decrease in the range 5 to 12%, with the largest impact in the tropics and Southern Hemisphere. Comparison with observations shows that in the region of largest changes, i.e. the tropical UT, the inclusion of the new reaction tends to degrade the model agreement. Elsewhere the model comparisons are not able to critically assess the impact of including this reaction. Only small changes are calculated in the minor species distributions in the stratosphere.

Acetaldehyde photochemistry on TiO 2(1 1 0)

The ultraviolet (UV) photon induced decomposition of acetaldehyde adsorbed on the oxidized rutile TiO 2(1 1 0) surface was studied with photon stimulated desorption (PSD) and thermal programmed desorption (TPD). Acetaldehyde desorbs molecularly from TiO 2(1 1 0) with minor decomposition channels yielding butene on the reduced TiO 2 surface and acetate on the oxidized TiO 2 surface. Acetaldehyde adsorbed on oxidized TiO 2(1 1 0) undergoes a facile thermal reaction to form a photoactive acetaldehyde–oxygen complex. UV irradiation of the acetaldehyde–oxygen complex initiated photofragmentation of the complex resulting in the ejection of methyl radical into gas phase and conversion of the surface bound fragment to formate.

Sedimentary evolution of the Late Eocene Vernet lacustrine system (South-Central Pyrenees). Tectono-climatic control in an alluvial-lacustrine piggyback basin

A complete third-order tectono-sedimentary cycle filled the Vernet lacustrine piggyback basin during the late Eocene. The depositional sequence can be subdivided into five units or systems tracts with distinct architectural frameworks, which developed a progressive unconformity. Sedimentary infilling of the piggyback basin began with retrogradational alluvial fans and fluvio-alluvial deposits being deposited, arranged in two thinning- and fining-upward cycles during the uplifting and emplacement of the southernmost Pyrenean thrusts, and with three thickening- and coarsening-upward progradational floodplain and lacustrine cycles, during the quiescence of the southernmost thrusts, at the same time as the emplacement of several northernmost thrusts of the southern Pyrenees. The facies associations identified represent different depositional environments in a sheetflow-dominated system, including playa-lake deposits, floodplain deposits, sheetflow fluvial deposits and sheet deltas in ephemeral to shallow lacustrine settings. Fourth and fifth order arid/semi-arid climatic cycles can be recognized, driven either by orbital obliquity and precessional forcing (Milankovitch cyclicity), insolation cycles or possibly subprecessional autogenic mechanisms related to the emplacement of northern thrusts and the consequent changes in the drainage basin. Arid periods are recorded by sheetflood fluvial, ephemeral lacustrine or terminal splay deposits, with abundant medium-to-coarse-grained sandstone packages in red-to-grey mudstones, a product of unconfined or low-confined sheetflows or hyperconcentrated flows, with bedload and suspended load, during flood episodes, at times of lake lowstand. Semi-arid periods are recorded as relative highstand lacustrine deposits, including grey-to-white mudstones deposited in prodelta environments and fine-grained sheet sandstone units sedimented in a delta front (with minor channels and extensive sheet lobes) and prodelta environments in shallow lakes, from sheetflood events.

F5SN(H)Xe+; a Rare Example of Xenon Bonded to sp3-Hybridized Nitrogen; Synthesis and Structural Characterization of [F5SN(H)Xe][AsF6

The salt [F5SN(H)Xe][AsF6] has been synthesized by the reaction of [F5SNH3][AsF6] with XeF2 in anhydrous HF (aHF) and BrF5 solvents and by solvolysis of [F3S triple bond NXeF][AsF6] in aHF. Both F5SN(H)Xe(+) and F5SNH3(+) have been characterized by (129)Xe, (19)F, and (1)H NMR spectroscopy in aHF (-20 degrees C) and BrF5 (supercooled to -70 degrees C). The yellow [F5SN(H)Xe][AsF6] salt was crystallized from aHF at -20 degrees C and characterized by Raman spectroscopy at -45 degrees C and by single-crystal X-ray diffraction at -173 degrees C. The Xe-N bond length (2.069(4) A) of the F5SN(H)Xe(+) cation is among the shortest Xe-N bonds presently known. The cation interacts with the AsF6(-) anion by means of a Xe---F-As bridge in which the Xe---F distance (2.634(3) A) is significantly less than the sum of the Xe and F van der Waals radii (3.63 A) and the AsF6(-) anion is significantly distorted from Oh symmetry. The (19)F and (129)Xe NMR spectra established that the [F5SN(H)Xe][AsF6] ion pair is dissociated in aHF and BrF5 solvents. The F5SN(H)Xe(+) cation decomposes by HF solvolysis to F5SNH3(+) and XeF2, followed by solvolysis of F5SNH3(+) to SF6 and NH4(+). A minor decomposition channel leads to small quantities of F5SNF2. The colorless salt, [F5SNH3][AsF6], was synthesized by the HF solvolysis of F3S triple bond NAsF5 and was crystallized from aHF at -35 degrees C. The salt was characterized by Raman spectroscopy at -160 degrees C, and its unit cell parameters were determined by low-temperature X-ray diffraction. Electronic structure calculations using MP2 and DFT methods were used to calculate the gas-phase geometries, charges, bond orders, and valencies as well as the vibrational frequencies of F 5SNH3(+) and F5SN(H)Xe(+) and to aid in the assignment of their experimental vibrational frequencies. In addition to F5TeN(H)Xe(+), the F5SN(H)Xe(+) cation provides the only other example of xenon bonded to an sp (3)-hybridized nitrogen center that has been synthesized and structurally characterized. These cations represent the strongest Xe-N bonds that are presently known.

Experimental and theoretical investigation of electron attachment to SF5Cl

Thermal electron attachment to SF(5)Cl has been studied with the flowing afterglow Langmuir probe technique. The rate coefficient is moderate, 4.8(+/-1.2)x10(-8) cm(3) s(-1), and invariant with temperature over the temperature range of 300-550 K. The reaction is dissociative, forming mainly SF(5) (-)+Cl. Minor yields of Cl(-) and FCl(-) were also found. The yields of the minor channels increase slightly with temperature. Statistical unimolecular rate modeling is employed to elucidate the character of the dissociation pathways and to support the assumption that the dissociations involve the formation of metastable anionic SF(5)Cl(-).

Half-metallic L21 structures with (001) planar insertions

A number of L21 phase alloys (composition X2YZ) are half-metallic. Although this structure is typically described in terms of a fcc Bravais lattice with a four atom basis, it can be viewed more simply as a variant of bcc or B2 in which planes of X2 alternate with planes of YZ along the 001 direction. Using ab initio electronic structure calculations, we have investigated planar insertions along 001 into the L21 structure. For most scenarios, insertion of single or double atomic layers of Cr into Co2MnGe or Co2MnSi did not destroy the half-metallic property. One insertion of a Cr layer into Co2MnGe was observed to increase the gap. In fact, we observed that for a large number of insertions using various transition metals or combinations of transition metals and nontransition metals, the band gap in the minority channel at the Fermi energy remains. An ad hoc rule that seems to partially capture the tendency to form half-metals can be formulated as: “001 planar insertions that can plausibly yield eight down spin electrons on the X2 layer and four down spin electrons on the YZ layer yield half-metals.”

GRID‐MD—A tool for massive simulation of protein channels

We present here a fast method for the exploration of channels in proteins based on molecular dynamics simulations of probe particles in a discrete grid space defined by an ensemble of protein conformations obtained either experimentally or by out-of-the-grid atomistic molecular dynamics simulations. The method is able to provide millisecond-long trajectories with a small computational effort, requires no human intervention in defining possible exit pathways and can detect both major and minor channels, giving a correct balance to the relative flux between them. The Grid-Molecular-Dynamics approach is then a suitable method for massive exploration of channels in proteins, even of those with unknown functional annotation.

Theoretical study on the OH + CH3NHC(O)OCH3 reaction

The multiple-channel reactions OH + CH3NHC(O)OCH3 --> products are investigated by direct dynamics method. The optimized geometries, frequencies, and minimum energy path are all obtained at the MP2/6-311+G(d,p) level, and energetic information is further refined by the BMC-CCSD (single-point) method. The rate constants for every reaction channels, R1, R2, R3, and R4, are calculated by canonical variational transition state theory with small-curvature tunneling correction over the temperature range 200-1000 K. The total rate constants are in good agreement with the available experimental data and the two-parameter expression k(T) = 3.95 x 10(-12) exp(15.41/T) cm3 molecule(-1) s(-1) over the temperature range 200-1000 K is given. Our calculations indicate that hydrogen abstraction channels R1 and R2 are the major channels due to the smaller barrier height among four channels considered, and the other two channels to yield CH3NC(O)OCH3 + H2O and CH3NHC(O)(OH)OCH3 + H2O are minor channels over the whole temperature range.

Competition of electron transfer, dissociation, and bond-forming processes in the reaction of the CO22+ dication with neutral CO2

The bimolecular reactivity of the CO(2)(2+) dication with neutral CO(2) is investigated using triple quadrupole and ion-ion coincidence mass spectrometry. Crucial for product analysis is the use of appropriate isotope labelling in the quadrupole experiments in order to distinguish the different reactive pathways. The main reaction corresponds to single-electron transfer from the neutral reagent to the dication, i.e. CO(2)(2+) + CO(2) --> 2CO(2)(+); this process is exothermic by almost 10 eV, if ground state monocations are formed. Interestingly, the results indicate that the CO(2)(+) ion formed when the dication accepts an electron dissociates far more readily than the CO(2)(+) ion formed from the neutral CO(2) molecule. This differentiation of the two CO(2)(+) products is rationalized by showing that the population of the key dissociative states of the CO(2)(+) monocation will be favoured from the CO(2)(2+) dication rather than from neutral CO(2). In addition, two bond-forming reactions are observed as minor channels, one of which leads to CO(+) and O(2)(+) as ionic products and the other affords a long-lived C(2)O(3)(2+) dication.

Spatial arrangement of fold types in the Zagros Simply Folded Belt, Iran, indicated by landform morphology and drainage pattern characteristics

Please click here to download the map associated with this article. The spatial arrangement of fold types within the Zagros Simply Folded Belt was analysed using satellite images, a digital elevation model and a digital drainage network. Distinct fold geometries, principally fault-bend folds and detachment folds, can be identified by characteristic interactions with streams flowing SW from the High Zagros Mountains into the Persian Gulf. In addition, the morphology of the landforms is reected in the minor channel patterns across the folds. This morphology can also be categorised using geomorphic indices, L/W ratio and symmetry index. The distribution of fold types is shown for the region N27°-N30°, E50°-E54° at a scale of 1:750,000. Anomalously long, high-aspect ratio folds, coincident with topographic steps, were inferred to be fault-bend folds, crossed by multiple wind gaps, overlying major thrust faults. These faults formed sequentially as the deformation front migrated from the collision zone towards the SW, causing diversion of stream channels. Movement up thrust ramps created fault-bend folds behind which serial detachment folding developed in the cover.

Real-space pseudopotential calculations of spin-dependent electron transport in magnetic molecular junctions

A real-space pseudopotential approach is developed to calculate the spin-dependent transport in nanoscale junctions. Our method is based on self-consistent solution of the Kohn-Sham equation of density functional theory with asymptotic boundary conditions. This method is applied to a simple magnetic molecule, the Sc dimer, bridging nonmagnetic, planar jellium electrodes for a series of molecule-lead spacing. We find that the spin-dependent conductance within this formalism is rather robust over a wide range of electronic coupling parameters. The minority channel of parallel-aligned ${\mathrm{Sc}}_{2}$ produces a fairly stable conductance of roughly half of a quantum unit $({e}^{2}∕h)$. Other systems show sensitive dependence on the coupling strength. Atomic origins of the dependence are discussed.

Selective Oxidation of Crotyl Alcohol over Pd(111)

The selective oxidation of crotyl alcohol has been explored over a Pd(111) model catalyst. At low temperatures, the alcohol adsorbs intact with the CdC bond parallel to the surface. Activation likely proceeds through an allyl alkoxide intermediate that follows two distinct reaction channels. Over the clean surface, �90% of the alcohol oxidizes to surface bound crotonaldehyde above 200 K, which subsequently all decarbonylates to propene and CO at room temperature. The minor reaction channel involves C-O scission to 2-butene and water. While some of these undesired reactively formed alkene products desorb around 300 K, the majority dehydrogenate to irreversibly bound carbon above 380 K. This latter decomposition pathway is unlikely to be important at the low temperatures utilized in liquid-phase crotyl alcohol oxidation over supported palladium catalysts. Adsorbed CO persists until 430 K and is likely responsible for site-blocking and deactivation of dispersed metallic Pd clusters. Coadsorbed oxygen suppresses crotonaldehyde decarbonylation and promotes its release from the surface.

Dissociative recombination of H+(H2O)3 and D+(D2O)3 water cluster ions with electrons: Cross sections and branching ratios

Dissociative recombination (DR) of the water cluster ions H(+)(H(2)O)(3) and D(+)(D(2)O)(3) with electrons has been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). For the first time, absolute DR cross sections have been measured for H(+)(H(2)O)(3) in the energy range of 0.001-0.8 eV, and relative cross sections have been measured for D(+)(D(2)O)(3) in the energy range of 0.001-1.0 eV. The DR cross sections for H(+)(H(2)O)(3) are larger than previously observed for H(+)(H(2)O)(n) (n=1,2), which is in agreement with the previously observed trend indicating that the DR rate coefficient increases with size of the water cluster ion. Branching ratios have been determined for the dominating product channels. Dissociative recombination of H(+)(H(2)O)(3) mainly results in the formation of 3H(2)O+H (probability of 0.95+/-0.05) and with a possible minor channel resulting in 2H(2)O+OH+H(2) (0.05+/-0.05). The dominating channels for DR of D(+)(D(2)O)(3) are 3D(2)O+D (0.88+/-0.03) and 2D(2)O+OD+D(2) (0.09+/-0.02). The branching ratios are comparable to earlier DR results for H(+)(H(2)O)(2) and D(+)(D(2)O)(2), which gave 2X(2)O+X (X=H,D) with a probability of over 0.9.

New Insights into the Reaction Mechanisms of Phenylium Ions with Benzene

The chemistry of phenylium (benzen-1-ylium) cations with benzene is investigated by using a guided ion beam tandem mass spectrometer. The main ionic products from the reaction of C6H5+ with C6H6 are observed at m/z 155 (covalent adduct C12H11+), 154 (C12H10+), 153 (C12H9+), 129 (C10H9+), and 115 (C9H7+). We propose a mechanism according to which channels at m/z 154-115 are formed by elimination of stable neutral molecules (such as H2, C2H2, C3H4) from the collision complex C12H11+, for which the most plausible structure is protonated biphenyl. The proposed mechanism is demonstrated by using partial isotopical labeling of reagents to look for possible H/D atom scrambling. Almost the same ions are produced when benzene is chemically ionized at atmospheric pressure in an APCI source from which oxygen is excluded. Because an ion trap analyzer is coupled to this source, tandem MS experiments can be performed, allowing structural details to be established. Moreover, the use of partially deuterated reagents has allowed the detection of minor reactive channels resulting from charge exchange and H-/D- hydride-transfer processes. Theoretical calculations show that the most stable structure for ions at m/z 129 C10H9+ is that of protonated naphthalene, resulting from the loss of an acetylene molecule by the condensation product, with a reaction exothermicity of 1.27 eV. We have found a possible barrierless pathway for such a channel that might be viable for the synthesis of naphthalene, the smallest PAH, even at low collision energies and therefore would be of particular astrochemical relevance.

Photochemical Reaction Pathways of Carbon Tetrabromide in Solution Probed by Picosecond X-ray Diffraction

We report a liquid-phase time-resolved X-ray diffraction study that resolves the molecular structures of the short-lived intermediates formed in the photodissociation of tetrabromomethane in methanol. Time-resolved X-ray diffraction can detect all chemical species simultaneously, and the diffraction signal from each chemical species can be quantitatively calculated from molecular structures and compared with experimental data with high accuracy and precision. The photochemistry of carbon tetrahalides has long been explored to describe their reactions in the natural environment due to its relevance to ozone depletion. Excited with an ultraviolet optical pulse, the complicated photodissociation dynamics of CBr4 was followed in a wide temporal range from picoseconds up to microseconds and associated rate coefficients were determined by analyzing time-resolved diffraction patterns accumulated from 100 ps X-ray pulses. The homolytic cleavage of one C-Br bond in the parent CBr4 molecule yields the CBr3 and Br radicals, which escape from the solvent cage and combine nongeminately to form C2Br6 and Br2, respectively. C2Br6 eventually decays to give C2Br4 and Br2 as final stable products. Our diffraction data at the current signal-to-noise ratio could not provide any evidence for the geminate recombination of the CBr3 and Br radicals to form the Br2CBr-Br isomer or the solvated ion pair, implying that their formation is a minor channel compared with those observed clearly by time-resolved diffraction in this work.

Theoretical studies on the reactions of hydroxyl radicals with trimethylsilane and tetramethylsilane

The multiple-channel reactions OH + SiH(CH3)3 → products (R1) and the single-channel reaction OH + Si(CH3)4 → Si(CH3)3CH2 + H2O (R2) have been studied by means of the direct dynamics method at the BMC-CCSD//MP2/6-311+G(2d,2p) level. The optimized geometries, frequencies and minimum energy path are all obtained at the MP2/6-311+G(2d,2p) levels, and energy information is further refined by the BMC-CCSD (single-point) level. The rate constants for every reaction channels are calculated by canonical variational transition states theory (CVT) with small-curvature tunneling (SCT) contributions over the temperature range 200–2,000 K. The theoretical total rate constants are in good agreement with the available experimental data, and the three-parameter expression k1 = 2.53×10−21 T3.14 exp(1, 352.86/T), k2 = 6.00 × 10−19 T2.54 exp(−106.11/T) (in unit of cm3 molecule−1 s−1) over the temperature range 200–2,000 K are given. Our calculations indicate that at the low temperature range, for reaction R1, H-abstraction is favored for the SiH group, while the abstraction from the CH3 group is a minor channel.

Dispersed fluorescence spectroscopy of 2-hydroxypyridine and its cyclically hydrogen-bonded water clusters in the gas phase: An examination of occurrence of excited-state proton transfer

The fluorescence excitation and dispersed fluorescence spectra of 2-pyridone (2PY), 2-hydroxypyridine (2HP), and 2HP(H 2O) n ( n = 1, 2) clusters in the electronic excited π–π ∗ state have been measured in a supersonic free jet expansion. The photoexcitation of various vibronic bands in the excited state of 2HP(H 2O) n ( n = 1, 2) does not provide fluorescence from 2PY(H 2O) n ( n = 1, 2) that can be formed by the excited state intermolecular proton transfer (ESPT), suggesting that ESPT is a minor decay channel from the excited state of 2HP(H 2O) n ( n = 1, 2).

A sedimentação proterozóica (esteniana-toniana) na borda oeste do Craton Amazônico

The Serra dos Pacaas Novos, located in the part of Rondonia, near the town of Guajara Mirim, and inserted in the Amazonian Craton, is built up mainly by conglomerates and sandstones of the Upper Proterozoic Palmeiral Formation. The facies analysis allows to distinguish six lithofacies: (1) massive or crudely bedded ortoconglomerate (Gm); (2) sandstone with horizontal bedding (Sh); (3) trough cross-bedded sandstone (St); (4) planar crossbedded sandstones (Sp); (5) sigmoidal cross-bedded (Ss) and (6) massive sandstones (Sm). The conglomerate facies was interpreted as longitudinal bars or stacked sheets formed during events of high discharge. The interbedded lenses of sandstone, on the other hand, was deposited during low water in minor channels. This interpretation points to a fluvial style with a braided channel patern similar to the Scott type. However, it is uncertain if this depositional pattern may be related to a proximal fluvial system or to a alluvial fan.The provenance of the Palmeiral deposits is shown by their different types of lithic fragments including quartz, quartzite, sandstone and volcanic rocks, which only in part are derived from the crystalline basement. Rock fragments from the nearby Jamari Complex (basement) were not found.The Palmeiral Formation went through several diagenetic processes which include quartz overgrowths, formation of kaolinite, illitization of kaolinite and pressure solution well documented by cavities in gravels. In the Serra of Pacaas Novos, the Palmeiral Formation is confined to a graben structure which, possibly during Cenozoic, suffered relief inversion leading to the present morphological configuration of the graben. The structural analysis resulted in the separation of three strutural sectors.

The Surgical Outcome of Total Urogenital Mobilization for Cloacal Repair

Total urogenital mobilization is deemed to preserve the urogenital sinus blood supply and avoid ischemic complications. We report our experience with this technique for cloacal repair. We retrospectively reviewed all consecutive cases of persistent cloaca managed by total urogenital mobilization via a posterior sagittal approach by a single pediatric urologist between 1998 and 2003. During this period 22 girls (12 with a common channel longer than 3 cm) underwent total urogenital mobilization for cloaca repair (1 redo) at a median age of 10 months (range 2 to 102). A total of 15 procedures (68%) could be completed by a perineal approach only. Four patients with a long common channel required additional maneuvers after mobilization to complete the reconstruction. Urethral stenosis was observed in 2 patients after urethral separation from the vagina (1) and common channel retubularization (1). One child with a perineal hemangioma required a redo posterior sagittal anorectoplasty for complete vaginal and anal closure. In addition, 1 case of urethrovaginal fistula was diagnosed on cystovaginoscopy but was asymptomatic, and 1 child with a minor residual common channel underwent urethral revision to allow easier intermittent catheterization. With a median followup of 48 months (range 11 to 162) the latest examination involving endoscopy with anesthesia showed a good result in 17 patients, a tight introitus that might require further surgery in 3 and a minimal residual common channel of 0.5 cm in 2. Total urogenital mobilization is an effective technique for repairing short and long common channels, and a low surgical complication rate can be anticipated.