OH yields from the CH3CO+O-2 reaction using an internal standard

Laser flash photolysis of CH3C(O)OH at 248 nm was used to create equal zero time yields of CH3CO and OH. The absolute OH yield from the CH3CO + O2 (+M) reaction was determined by following the OH temporal profile using the zero time OH concentration as an internal standard. The OH yield from CH3CO + O2 (+M) was observed to decrease with increasing pressure with an extrapolated zero pressure yield close to unity (1.1 ± 0.2, quoted uncertainties correspond to 95% confidence limits). The results are in quantitative agreement with those obtained from 248 nm acetone photolysis in the presence of O2

The gas-phase reaction of NH2 with formaldehyde (CH2O) is not a source of formamide (NH2CHO) in interstellar environments

The first experimental study of the low-temperature kinetics of the gas-phase reaction of NH2 with formaldehyde (CH2O) has been performed. This reaction has previously been suggested as a source of formamide (NH2CHO) in interstellar environments. A pulsed Laval nozzle equipped with laser-flash photolysis and laser-induced fluorescence spectroscopy was used to create and monitor the temporal decay of NH2 in the presence of CH2O. No loss of NH2 could be observed via reaction with CH2O and we place an upper-limit on the rate coefficient of <6x10-12 cm3 molecule-1 s-1 at 34K. Ab initio calculations of the potential energy surface were combined with RRKM calculations to predict a rate coefficient of 6.2x10-14 cm3 molecule-1 s-1 at 35K, consistent with the experimental results. The presence of a significant barrier, 18 kJ mol-1, for the formation of formamide as a product, means that only the H-abstraction channel producing NH3 + CHO, in which the transfer of an H-atom can occur by quantum mechanical tunnelling through a 23 kJ mol-1 barrier, is open at low temperatures. These results are in contrast with a recent theoretical study which suggested that the reaction could proceed without a barrier and was therefore a viable route to gas-phase formamide formation. The calculated rate coefficients were used in an astrochemical model which demonstrated that this reaction produces only negligible amounts of gas-phase formamide under interstellar and circumstellar conditions. The reaction of NH2 with CH2O is therefore not an important source of formamide at low temperatures in interstellar environments.Comment: Manuscript, 14 pages, 4 figures. Supporting Information, 8 pages, 2 figures. Accepted for publication in The Astrophysical Journal Letter

Time-Resolved Measurements and Master Equation Modelling of the Unimolecular Decomposition of CH3OCH2

The rate coefficient for the unimolecular decomposition of CH3OCH2,k(1), has been measured in time-resolved experiments by monitoring the HCHO product. CH3OCH2 was rapidly and cleanly generated by 248 nm excimer photolysis of oxalyl chloride, (ClCO)(2), in an excess of CH3OCH3, and an excimer pumped dye laser tuned to 353.16 nm was used to probe HCHO via laser induced fluorescence. k(1)(T,p) was measured over the ranges: 573-673 K and 0.1-4.3 x 10(18) molecule cm(-3) with a helium bath gas. In addition, some experiments were carried out with nitrogen as the bath gas. Ab initio calculations on CH3OCH2 decomposition were carried out and a transition-state for decomposition to CH3 and H2CO was identified. This information was used in a master equation rate calculation, using the MESMER code, where the zero-point-energy corrected barrier to reaction, Delta E-0,E-1, and the energy transfer parameters, x T-n, were the adjusted parameters to best fit the experimental data, with helium as the buffer gas. The data were combined with earlier measurements by Loucks and Laidler (Can J. Chem. 1967, 45, 2767), with dimethyl ether as the third body, reinterpreted using current literature for the rate coefficient for recombination of CH3OCH2. This analysis returned Delta E-0,E-1 = (112.3 +/- 0.6) kJ mol(-1), and leads to k(1)(infinity)(T) = 2.9 x 10(12) (T/300)(2)(.5) exp(-106.8 kJ mol(-1)/RT). Using this model, limited experiments with nitrogen as the bath gas allowed N-2 energy transfer parameters to be identified and then further MESMER simulations were carried out, where N-2 was the buffer gas, to generate k(1)(T,p) over a wide range of conditions: 300-1000 K and N-2 = 10(12) -10(25) molecule cm(-3). The resulting k(1)(T,p) has been parameterized using a Troe-expression, so that they can be readily be incorporated into combustion models. In addition, k(1)(T,p) has been parametrized using PLOG for the buffer gases, He, CH3OCH3 and N-2.Peer reviewe

Dynamically Driven Evolution of the Interstellar Medium in M51

We report the highest-fidelity observations of the spiral galaxy M51 in CO emission, revealing the evolution of giant molecular clouds (GMCs) vis-a-vis the large-scale galactic structure and dynamics. The most massive GMCs (so-called GMAs) are first assembled and then broken up as the gas flow through the spiral arms. The GMAs and their H2 molecules are not fully dissociated into atomic gas as predicted in stellar feedback scenarios, but are fragmented into smaller GMCs upon leaving the spiral arms. The remnants of GMAs are detected as the chains of GMCs that emerge from the spiral arms into interarm regions. The kinematic shear within the spiral arms is sufficient to unbind the GMAs against self-gravity. We conclude that the evolution of GMCs is driven by large-scale galactic dynamics --their coagulation into GMAs is due to spiral arm streaming motions upon entering the arms, followed by fragmentation due to shear as they leave the arms on the downstream side. In M51, the majority of the gas remains molecular from arm entry through the inter-arm region and into the next spiral arm passage.Comment: 6 pages, including 3 figures. Accepted, ApJ

Reanalysis of Rate Data for the Reaction CH₃ + CH₃ → C₂H₆ Using Revised Cross Sections and a Linearized Second-Order Master Equation

Rate coefficients for the CH3 + CH3 reaction, over the temperature range 300-900 K, have been corrected for errors in the absorption coefficients used in the original publication ( Slagle et al., J. Phys. Chem. 1988 , 92 , 2455 - 2462 ). These corrections necessitated the development of a detailed model of the B̃(2)A1' (3s)-X̃(2)A2″ transition in CH3 and its validation against both low temperature and high temperature experimental absorption cross sections. A master equation (ME) model was developed, using a local linearization of the second-order decay, which allows the use of standard matrix diagonalization methods for the determination of the rate coefficients for CH3 + CH3. The ME model utilized inverse Laplace transformation to link the microcanonical rate constants for dissociation of C2H6 to the limiting high pressure rate coefficient for association, k∞(T); it was used to fit the experimental rate coefficients using the Levenberg-Marquardt algorithm to minimize χ(2) calculated from the differences between experimental and calculated rate coefficients. Parameters for both k∞(T) and for energy transfer ⟨ΔE⟩down(T) were varied and optimized in the fitting procedure. A wide range of experimental data were fitted, covering the temperature range 300-2000 K. A high pressure limit of k∞(T) = 5.76 × 10(-11)(T/298 K)(-0.34) cm(3) molecule(-1) s(-1) was obtained, which agrees well with the best available theoretical expression

Genotypic determinants of fluoroquinolone and macrolide resistance in Neisseria gonorrhoeae.

Background:High rates of antimicrobial resistance (AMR) in Neisseria gonorrhoeae hinder effective treatment, but molecular AMR diagnostics may help address the challenge. This study aimed to appraise the literature for resistance-associated genotypic markers linked to fluoroquinolones and macrolides, to identify and review their use in diagnostics. Methods: Medline and EMBASE databases were searched and data pooled to evaluate associations between genotype and phenotypic resistance. The minimum inhibitory concentration (MIC) cut-offs were ≤ 0.06 mg L-1 for non-resistance to ciprofloxacin and ≤ 0.5 mg L-1 for non-resistance to azithromycin. Results: Diagnostic accuracy estimates were limited by data availability and reporting. It was found that: 1) S91 and D95 mutations in the GyrA protein independently predicted ciprofloxacin resistance and, used together, gave 98.6% (95% confidence interval (CI) 98.0-99.0%) sensitivity and 91.4% (95%CI 88.6-93.7%) specificity; 2) the number of 23S rRNA gene alleles with C2611T or A2059G mutations was highly correlated with azithromycin resistance, with mutation in any allele giving a sensitivity and specificity of 66.1% (95%CI 62.1-70.0%) and 98.9% (95%CI 97.5-99.5%) respectively. Estimated negative (NPV) and positive predictive values (PPV) for a 23S rRNA diagnostic were 98.6% (95%CI 96.8-99.4%) and 71.5% (95%CI 68.0-74.8%) respectively; 3) mutation at amino acid position G45 in the MtrR protein independently predicted azithromycin resistance; however, when combined with 23S rRNA, did not improve the PPV or NPV. Conclusions: Viable candidates for markers of resistance detection for incorporation into diagnostics were demonstrated. Such tests may enhance antibiotic stewardship and treatment options

Kinetic Energy Decay Rates of Supersonic and Super-Alfvenic Turbulence in Star-Forming Clouds

We present numerical studies of compressible, decaying turbulence, with and without magnetic fields, with initial rms Alfven and Mach numbers ranging up to five, and apply the results to the question of the support of star-forming interstellar clouds of molecular gas. We find that, in 1D, magnetized turbulence actually decays faster than unmagnetized turbulence. In all the regimes that we have studied 3D turbulence-super-Alfvenic, supersonic, sub-Alfvenic, and subsonic-the kinetic energy decays as (t-t0)^(-x), with 0.85 < x < 1.2. We compared results from two entirely different algorithms in the unmagnetized case, and have performed extensive resolution studies in all cases, reaching resolutions of 256^3 zones or 350,000 particles. We conclude that the observed long lifetimes and supersonic motions in molecular clouds must be due to external driving, as undriven turbulence decays far too fast to explain the observations.Comment: Submitted to Phys. Rev. Letters, 29 Nov. 1997. 10 pages, 2 figures, also available from http://www.mpia-hd.mpg.de/theory/preprints.html#maclo

Control of star formation by supersonic turbulence

Understanding the formation of stars in galaxies is central to much of modern astrophysics. For several decades it has been thought that stellar birth is primarily controlled by the interplay between gravity and magnetostatic support, modulated by ambipolar diffusion. Recently, however, both observational and numerical work has begun to suggest that support by supersonic turbulence rather than magnetic fields controls star formation. In this review we outline a new theory of star formation relying on the control by turbulence. We demonstrate that although supersonic turbulence can provide global support, it nevertheless produces density enhancements that allow local collapse. Inefficient, isolated star formation is a hallmark of turbulent support, while efficient, clustered star formation occurs in its absence. The consequences of this theory are then explored for both local star formation and galactic scale star formation. (ABSTRACT ABBREVIATED)Comment: Invited review for "Reviews of Modern Physics", 87 pages including 28 figures, in pres