Search

Collisional quenching of electronically excited states by inert gases is a fundamental physical process. For reactive excited species such as singlet methylene, 1CH2, the competition between relaxation and reaction has important implications in practical systems such as combustion. The gateway model has previously been applied to the relaxation of 1CH2 by inert gases [U. Bley and F. Temps, J. Chem. Phys. 98, 1058 (1993)]. In this model, gateway states with mixed singlet and triplet character allow conversion between the two electronic states. The gateway model makes very specific predictions about the relative relaxation rates of ortho and para quantum states of methylene at low temperatures; relaxation from para gateway states leads to faster deactivation independent of the nature of the collision partner. Experimental data are reported here which for the first time confirm these predictions at low temperatures for helium. However, it was found that in contrast with the model predictions, the magnitude of the effect decreases with increasing size of the collision partner. It is proposed that the attractive potential energy surface for larger colliders allows alternative gateway states to contribute to relaxation removing the dominance of the para gateway states. [ABSTRACT FROM AUTHOR]

Recent, direct studies have shown that several reactions of stabilized Criegee intermediates (SCI) are significantly faster than indicated by earlier indirect measurements. The reaction of SCI with SO2 may contribute to atmospheric sulfate production, but there are uncertainties in the mechanism of the reaction of the C1 Criegee intermediate, CH2OO, with SO2. The reactions of C1, CH2OO, and C2, CH3CHOO, Criegee intermediates with SO2 have been studied by generating stabilized Criegee intermediates by laser flash photolysis (LFP) of RI2/O2 (R = CH2 or CH3CH) mixtures with the reactions being followed by photoionization mass spectrometry (PIMS). PIMS has been used to determine the rate coefficient for the reaction of CH3CHI with O2, k = (8.6 ± 2.2) × 10−12 cm3 molecule−1 s−1 at 295 K and 2 Torr (He). The yield of the C2 Criegee intermediate under these conditions is 0.86 ± 0.11. All errors in the abstract are a combination of statistical at the 1σ level and an estimated systematic contribution. For the CH2OO + SO2 reaction, additional LFP experiments were performed monitoring CH2OO by time-resolved broadband UV absorption spectroscopy (TRUVAS). The following rate coefficients have been determined at room temperature ((295 ± 2) K):EQNTEXT DISPLAY="DISPLAYED"CH2OO + SO2: k = (3.74 ± 0.43) × 10−11 cm3 molecule−1 s−1 (LFP/PIMS),/EQNTEXTEQNTEXT DISPLAY="DISPLAYED"k = (3.87 ± 0.45) × 10−11 cm3 molecule−1 s−1 (LFP/TRUVAS)/EQNTEXTEQNTEXT DISPLAY="DISPLAYED"CH3CHOO + SO2: k = (1.7 ± 0.3) × 10−11 cm3 molecule−1 s−1 (LFP/PIMS)/EQNTEXTLFP/PIMS also allows for the direction observation of CH3CHO production from the reaction of CH3CHOO with SO2, suggesting that SO3 is the co-product. For the reaction of CH2OO with SO2 there is no evidence of any variation in reaction mechanism with [SO2] as had been suggested in an earlier publication (Chhantyal-Pun et al., Phys. Chem. Chem. Phys., 2015, 17, 3617). A mean value of k = (3.76 ± 0.14) × 10−11 cm3 molecule−1 s−1 for the CH2OO + SO2 reaction is recommended from this and previous studies. The atmospheric implications of the results are briefly discussed. [ABSTRACT FROM AUTHOR]

Ab initio theory has been used to identify the pre-reaction complex in the atmospherically important reaction between OH + SO2, (R1), where the binding energy of the pre-reaction complex was determined to be 7.2 kJ mol−1. Using reaction rate theory, implemented with the master equation package MESMER, the effects of this complex on the kinetics of R1 at temperatures above 250 K have been investigated. From simulations and fitting to the experimental kinetic data, it is clear that the influence of this pre-reaction complex is negligible and that the kinetics are controlled by the inner transition-state that leads to the product, HOSO2. While the effect of this complex on the thermal kinetics is small it potentially provides an efficient route to remove energy from vibrationally excited OH. The fitting to the past experimental data reveals that this inner transition-state is submerged with a barrier −0.25 kJ mol−1 below the entrance channel, which is outside the range predicted from the best theoretical calculations. The data fitting also yielded ΔR1H0K equal to −(109 ± 5.6) kJ mol−11 and a more precise expression for k∞1(T), (5.95 ± 0.83) × 10−13× (T/298)−0.11±0.27. [ABSTRACT FROM AUTHOR]

The article “Methanol dimer formation drastically enhances hydrogen abstraction from methanol by OH at low temperature” proposes a dimer mediated mechanism in order to explain the large low temperature rate coefficients for the OH + methanol reaction measured by several groups. It is demonstrated here theoretically that under the conditions of these low temperature experiments, there are insufficient dimers formed for the proposed new mechanism to apply. Experimental evidence is also presented to show that dimerization of the methanol reagent does not influence the rate coefficients reported under the conditions of methanol concentration used for the kinetics studies. It is also emphasised that the low temperature experiments have been performed using both the Laval nozzle expansion and flow-tube methods, with good agreement found for the rate coefficients measured using these two distinct techniques. [ABSTRACT FROM AUTHOR]

Objective: To determine how prenatal ultrasound measurements of dividing membrane thickness correlate with postnatal histological measurements and chorionicity in twin gestations.Methods: This was a prospective, longitudinal cohort study of twin gestations. Dividing membrane thickness was measured by transabdominal ultrasound, with the insonation beam both parallel and perpendicular to the membrane, in the second or third trimester, depending on when care was established. Ultrasound examinations were performed every 4 weeks following initial assessment until delivery. Measurements of membrane thickness from the first ultrasound examination were compared with histological measurements after delivery.Results: A total of 45 twin pregnancies (32 dichorionic, 13 monochorionic) were included. Mean gestational age at initial ultrasound examination was 24.1 ± 7.3 weeks. Parallel ultrasound measurements of membrane thickness were 1.6 ± 0.8 mm for monochorionic and 2.5 ± 0.9 mm for dichorionic gestations (P = 0.001). Perpendicular ultrasound measurements were 1.6 ± 0.3 mm for monochorionic and 2.2 ± 0.8 mm for dichorionic gestations (P = 0.009). Inter- and intraobserver reliability of ultrasound measurements were 0.847 and 0.950, respectively. Parallel and perpendicular ultrasound measurements correlated better with each other (R = 0.807, P < 0.001) than with histological measurements of membrane thickness (Rparallel  = 0.538, P < 0.001; Rperpendicular  = 0.529, P < 0.001). Receiver-operating characteristics curve analyses to predict histological membrane thickness > 50th percentile resulted in an area under the curve (AUC) of 0.828 for parallel (P < 0.001) and 0.874 for perpendicular (P < 0.001) measurements with a cut-off value of 1.9 mm for both approaches. The AUCs for parallel and perpendicular measurements to predict dichorionicity were 0.892 (P < 0.001) and 0.823 (P < 0.001) with cut-off values of 1.9 and 1.8 mm, respectively.Conclusion: Prenatal ultrasound measurement of twin dividing membrane thickness is positively correlated with postnatal histological measurement. Dichorionicity can be determined by a magnified dividing membrane thickness ≥ 1.9 mm. Measurements with the ultrasound beam parallel to the dividing membrane may be more accurate than perpendicular measurements. Copyright © 2016 ISUOG. Published by John Wiley & Sons Ltd. [ABSTRACT FROM AUTHOR]

The OH initiated gas-phase chemistry of several amines that are potential candidates for use in post-combustion carbon capture (PCCC) plants have been studied by laser flash photolysis with OH monitored by laser induced fluorescence. The rate coefficients for the reaction of OH with N-methylethanolamine (MMEA) and N,N-dimethylethanolamine (DMEA) have been measured as a function of temperature (∼300–500 K): UGRAPHIC DISPLAY="INLINE" ID="UGT1" SRC="UGT1"/, UGRAPHIC DISPLAY="INLINE" ID="UGT2" SRC="UGT2"/. The results for DMEA lie between previous values. This is the first kinetic study of the OH + MMEA reaction. At low pressures in the presence of oxygen, OH is recycled in the DMEA reaction as has been observed for other tertiary amines. Branching ratios for OH abstraction with MEA, DMEA and MMEA are dominated by abstraction from the αCH2 group. Abstraction from N–H is determined to be 0.38 ± 0.06 for MEA and 0.52 ± 0.06 for MMEA at 298 K. The impact of these studies has been assessed by using a modified chemical box model to calculate downwind concentrations of nitramines and nitrosamine formed in the photo-oxidation of MEA. Under clear sky conditions, the simulations suggest that current safe guidelines for nitramines may be significantly exceeded with predicted MEA emission rates. [ABSTRACT FROM AUTHOR]

Ratecoefficients for the CH3+ CH3reaction,over the temperature range 300–900 K, have been corrected forerrors in the absorption coefficients used in the original publication(Slagleet al., J. Phys. Chem.1988, 92, 2455−2462). These corrections necessitated the developmentof a detailed model of the B̃2A1′(3s)–X̃2A2″ transition inCH3and its validation against both low temperature andhigh temperature experimental absorption cross sections. A masterequation (ME) model was developed, using a local linearization ofthe second-order decay, which allows the use of standard matrix diagonalizationmethods for the determination of the rate coefficients for CH3+ CH3. The ME model utilized inverse Laplace transformationto link the microcanonical rate constants for dissociation of C2H6to the limiting high pressure rate coefficientfor association, k∞(T); it was used to fit the experimental rate coefficients using theLevenberg–Marquardt algorithm to minimize χ2calculated from the differences between experimental and calculatedrate coefficients. Parameters for both k∞(T) and for energy transfer ⟨ΔE⟩down(T) were variedand optimized in the fitting procedure. A wide range of experimentaldata were fitted, covering the temperature range 300–2000 K.A high pressure limit of k∞(T) = 5.76 × 10–11(T/298 K)−0.34cm3molecule–1s–1was obtained, which agrees well with the bestavailable theoretical expression. [ABSTRACT FROM AUTHOR]

Highlights: [•] ArF pulsed excimer photolysis laser and IR tunable diode probe laser. [•] The modelling of the reactions CH3 +CH3ICl and CH3I+CH3ICl. [•] HCl yield is nonlinear decreasing with increasing the [CH3I]. [•] HCl yield is nonlinear decreasing with increasing the [CH3] radical. [Copyright &y& Elsevier]

The kinetics of the reactions of the hydroxyl radical (OH) with acetone and dimethyl ether (DME) have been studied between 63–148 K and at a range of pressures using laser-flash photolysis coupled with laser induced fluorescence detection of OH in a pulsed Laval nozzle apparatus. For acetone, a large negative temperature dependence was observed, with the rate coefficient increasing from k1 = (1.6 ± 0.8) × 10−12 cm3 molecule−1 s−1 at 148 K to (1.0 ± 0.1) × 10−10 cm3 molecule−1 s−1 at 79 K, and also increasing with pressure. For DME, a similar behaviour was found, with the rate coefficient increasing from k2 = (3.1 ± 0.5) × 10−12 cm3 molecule−1 s−1 at 138 K to (1.7 ± 0.1) × 10−11 cm3 molecule−1 s−1 at 63 K, and also increasing with pressure. The temperature and pressure dependence of the experimental rate coefficients are rationalised for both reactions by the formation and subsequent stabilisation of a hydrogen bonded complex, with a non-zero rate coefficient extrapolated to zero pressure supportive of quantum mechanical tunnelling on the timescale of the experiments leading to products. In the case of DME, experiments performed in the presence of O2 provide additional evidence that the yield of the CH3OCH2 abstraction product, which can recycle OH in the presence of O2, is ≥50%. The experimental data are modelled using the MESMER (Master Equation Solver for Multi Energy Well Reactions) code which includes a treatment of quantum mechanical tunnelling, and uses energies and structures of transition states and complexes calculated by ab initio methods. Good agreement is seen between experiment and theory, with MESMER being able to reproduce for both reactions the temperature behaviour between ∼70–800 K and the pressure dependence observed at ∼80 K. At the limit of zero pressure, the model predicts a rate coefficient of ∼10−11 cm3 molecule−1 s−1 for the reaction of OH with acetone at 20 K, providing evidence that the reaction can proceed quickly in those regions of space where both species have been observed. The results and modelling build considerably on our previous experimental study performed under a much more limited range of conditions (Shannon et al., Phys. Chem. Chem. Phys., 2010, 12, 13511–13514). [ABSTRACT FROM AUTHOR]

Laboratory studies have revealed that alkenederived RO2 and longer chain alkane-derived RO2 (> C3) radicals rapidly convert to HO2 and then to OH in the presence of NO in a fluorescence assay by gas expansion (FAGE) detection cell (Fuchs et al., 2011). Three different FAGE cells that have been used to make ambient measurements of OH and HO2 in the University of Leeds ground-based instrument have been assessed to determine the sensitivity of each cell, when operating in HO2 detection mode, to RO2 radicals. The sensitivity to this interference was found to be highly dependent on cell design and operating parameters. Under the operating conditions employed, during fieldwork undertaken in the Borneo rainforest in 2008, an OH yield of 17% was experimentally determined for both ethene- and isoprene-derived RO2 radicals. The high pumping capacity of this system, resulting in a short residence time in the cell, coupled with poor mixing of NO into the ambient air-stream for the titration of HO2 to OH effectively minimised this potential interference. An OH yield of 46% was observed for ethene-derived RO2 radicals when a smaller detection cell was used, in which the mixing of NO into the ambient air was improved and the cell residence times were much longer. For a newly developed ROxLIF cell, used for detection of HO2 and RO2 radicals an OH yield of 95% was observed for ethene-derived RO2 radicals, when running in HO2 mode. In experiments in which conditions ensured the conversion of RO2 to OH were complete, the yields of OH from a range of different RO2 species agreed well with model predictions based on the Master Chemical Mechanism version 3.2. For ethene and isoprene-derived RO2 species, the relative sensitivity of FAGE was found to be close to that for HO2, with an OH yield of 100% and 92 %, respectively. For the longer chain or cyclic alkane-derived RO2 radicals (> C3), model predicted OH yields were highly dependent upon temperature. A model predicted OH yield of 74% at 298K and 36% at 255K were calculated for cyclohexane-derived RO2 radicals, and an experimental yield of 38% was observed indicating that the temperature within the cell was below ambient owing to the supersonic expansion of the airstream in the low pressure cell. These findings suggest that observations of HO2 by some LIF instruments worldwide may be higher than the true value if the instruments were sensitive to these RO2 species. If this is the case, it becomes necessary to compare atmospheric chemistry model simulations to HO"2 observations, where HO"2 = [HO"2] +Σiαi [RO2i], andαi is the mean fractional contribution of the RO2 species that interfere (RO2i). This methodology, however, relies on model simulations of speciated RO2 radicals, as instrumentation to make speciated RO2 measurements does not currently exist. Here we present an approach that enables the concentration of HO2 and RO2i to be selectively determined by varying the concentration of NO injected into a FAGE cell. Measurements of [HO2] and [RO2i] taken in London are presented. [ABSTRACT FROM AUTHOR]

The formation of atmospherically relevant iodine oxides IxOy (x = 1,…,3, y = 1,…,7) has been studied experimentally using time-of-flight mass spectrometry combined with a soft ionisation source, complemented with ab initio electronic structure calculations of ionisation potentials and bond energies at a high level of theory presented in detail in the accompanying paper (Galvez et al., 2013). For the first time, direct experimental evidence of the I2Oy (y = 1,…,5) molecules in the gas phase has been obtained. These chemical species are observed alongside their precursors (IO and OIO) in experiments where large amounts of aerosol are also generated. The measured relative concentrations of the IxOy molecules and their dependence on ozone concentration have been investigated by using chemical modelling and rate theory calculations. It is concluded that I2O4 is the most plausible candidate to initiate nucleation, while the contribution of I2O5 in the initial steps is likely to be marginal. The absence of large I3Oy (y = 3,…,6) peaks in the mass spectra and the high stability of the I2O4–I2O4 dimer indicate that dimerisation of I2O4 is the key step in iodine oxide particle nucleation. [ABSTRACT FROM AUTHOR]

Objective: To determine whether 17-a hydroxyprogesterone (17-OHPC) alters tumor necrosis factor-a (TNF-a) production and the expression of cyclooxygenase type 2 (COX-2) in myometrium exposed to lipopolysaccharide (LPS). Study Design: Lower segment myometrial biopsies were obtained from non-laboring patients at term. Tissues were cultured in serum-free media with 17-OHPC (1 mM) and LPS (1 mg/ml), either alone or in combination. At 24 h, the production of tumor necrosis factor-a (TNF-a) and the expression of COX-2 was determined using enzyme linked immunosorbent assay and real-time (RT-PCR). Statistical analysis was performed using non-parametric testing. A P-value of <0.05 was considered significant. Result: 17-OHPC had no effect on TNF-a production and COX-2 expression when compared with untreated myometrial explants (P¼0.61 and P¼0.95). LPS induced production of TNF-a (P¼0.03) and expression of COX-2 (P¼0.02). Treatment with 17-OHPC did not block LPS-induced TNF-a production (P¼0.37) or COX-2 expression (P¼0.12). Conclusion: In this pilot study, 17-OHPC did not affect the production of TNF-a or COX-2 expression in human myometrium. [ABSTRACT FROM AUTHOR]

The reaction of chlorine atoms with alkyl iodides can play a role in the chemistry of the marine boundary layer. Previous studies have shown that at room temperature the reaction takes place viaa complex mechanism including adduct formation. For the Cl + ethyl iodide reaction results on the thermodynamics of adduct formation and on the product yields are inconsistent. The kinetics of the reaction Cl + C2H5I have been studied by the direct observation of the HCl product in real time flash photolysis/IR absorption experiments as a function of temperature from 273 to 450 K. At temperatures above 375 K kinetic measurements confirm a direct process and the rate coefficient determined (4.85 ± 0.55) × 10−11exp((−363 ± 51)/T) cm3molecule−1s−1is in good agreement with previous direct determinations. Product yield studies have also been undertaken by comparing the HCl signal from Cl + C2H5I with that from a calibration reaction which shows that HCl is the sole product of the reaction at these temperatures. Yield studies with selectively deuterated ethyl iodide demonstrate that abstraction occurs predominantly from the α site, with the selectivity decreasing with temperature. Extrapolation of the yield data to 298 K predicts an α : β ratio of 0.68 : 0.32. At temperatures between 273 and 325 K a biexponential growth was observed for the HCl signal consistent with adduct formation. Analysis of the HCl time profiles allowed the extractions of the forward and reverse rate coefficients for adduct formation and hence the calculations of the thermodynamic properties of adduct formation. A third law analysis yields a value of ΔrH= (−54 ± 4) kJ mol−1. The value of ΔrHis in good agreement with a previous third law determination (J. J. Orlando, C. A. Piety, J. M. Nicovich, M. L. McKee, P. H. Wine, J. Phys. Chem. A, 2005, 109, 6659). [ABSTRACT FROM AUTHOR]

Previous studies have shown a significant OH yield from the reaction of RCO radicals (generated from the photolysis of corresponding ketone) with oxygen below total pressures of 200 Torr. The potential of these reactions as a source of OH radicals for flash photolytic kinetic studies is investigated. The viability of the method was tested by measuring rate coefficients for the reaction of OH with ethanol using both acetone/O2 mixtures and t-butyl hydroperoxide photolysis. The results (with statistical errors at the 2σ level) are in excellent agreement with each other (kEtOH(acetone) = (5.87 ± 0.34) × 10-18T2 exp((515 ± 21)K/T) cm3 molecule-1 s-1 and kEtOH (t-butyl hydroperoxide) = (5.27 ± 0.34) × 10-18T2 exp((557 ± 20)K/T) cm3 molecule-1 s-1) and with the IUPAC recommendation. The reaction of OH with methyl ethyl ketone (2-butanone) has also been investigated using a similar technique. The results show a strong non-Arrhenius temperature dependence, k = (3.84 ± 0.12) × 10-24× T4 × exp((1038 ± 11)/t). The merits of the ketone/oxygen OH source are contrasted with other established precursors. A major advantage of the technique is the ability to cleanly generate OD without the potential for isotopic scrambling prior to photolysis. © 2008 Wiley Periodicals, Inc. 40: 504–514, 2008 [ABSTRACT FROM AUTHOR]

BACKGROUND: The use of MgSO(4) is one of numerous treatment options available during exacerbations of asthma. While the efficacy of therapy with IV MgSO(4) has been demonstrated, little is known about inhaled MgSO(4). OBJECTIVES: A systematic review of the literature was performed to examine the effect of inhaled MgSO(4) in the treatment of patients with asthma exacerbations in the emergency department. METHODS: Randomized controlled trials were eligible for inclusion and were identified from the Cochrane Airways Group 'Asthma and Wheez*' register, which consists of a combined search of the EMBASE, CENTRAL, MEDLINE, and CINAHL databases and the manual searching of 20 key respiratory journals. Reference lists of published studies were searched, and a review of the gray literature was also performed. Studies were included if patients had been treated with nebulized MgSO(4) alone or in combination with beta(2)-agonists and were compared to the use of beta(2)-agonists alone or with an inactive control substance. Trial selection, data extraction, and methodological quality were assessed by two independent reviewers. The results from fixed-effects models are presented as standardized mean differences (SMDs) for pulmonary functions and the relative risks (RRs) for hospital admission. Both are displayed with their 95% confidence intervals (CIs). RESULTS: Six trials involving 296 patients were included. There was a non-significant increase [corrected] in pulmonary function between patients whose treatments included nebulized MgSO(4) and those whose treatments [corrected] did not (SMD, 0.22; 95% CI, -0.02 to 0.47 [corrected] five studies); there was also a trend toward reduced [corrected] hospitalizations in patients whose treatments included nebulized MgSO(4) (RR, 0.67; 95% CI, 0.41 to 1.09; four studies). Subgroup analyses demonstrated that lung function improvement was similar in adult patients and in those patients who received nebulized MgSO(4) in addition to a beta(2)-agonist. CONCLUSIONS: The use of nebulized MgSO(4), particularly in addition to a beta(2)-agonist, in the treatment of an acute asthma exacerbation appears to produce benefits with respect to improved pulmonary function and may reduce the number of hospital admissions. [ABSTRACT FROM AUTHOR]

Review: 255 refs. [ABSTRACT FROM AUTHOR]