First direct kinetic measurement of i-C4H5 (CH2CHCCH2) + O2 reaction: Toward quantitative understanding of aromatic ring formation chemistry

The kinetics of the i-C₄H₅ (buta-1,3-dien-2-yl) radical reaction with molecular oxygen has been measured over a wide temperature range (275–852 K) at low pressures (0.8–3 Torr) in direct, time-resolved experiments. The measurements were performed using a laminar flow reactor coupled to photoionization mass spectrometer (PIMS), and laser photolysis of either chloroprene (2-chlorobuta-1,3-diene) or isoprene was used to produce the resonantly stabilized i-C₄H₅ radical. Under the experimental conditions, the measured bimolecular rate coefficient of i-C₄H₅ + O₂ reaction is independent of bath gas density and exhibits weak, negative temperature dependency, and can be described by the expression k₃ = (1.45 ± 0.05) × 10^?12 × (T/298 K)^{?(0.13±0.05)} cm³ s^?1. The measured bimolecular rate coefficient is surprisingly fast for a resonantly stabilized radical. Under combustion conditions, the reactions of i-C₄H₅ radical with ethylene and acetylene are believed to play an important role in forming the first aromatic ring. However, the current measurements show that i-C₄H₅ + O₂ reaction is significantly faster under combustion conditions than previous estimations suggest and, consequently, inhibits the soot forming propensity of i-C₄H₅ radicals. The bimolecular rate coefficient estimates used for the i-C₄H₅ + O₂ reaction in recent combustion simulations show significant variation and are up to two orders of magnitude slower than the current, measured value. All estimates, in contrast to our measurements, predict a positive temperature dependency. The observed products for the i-C₄H₅ + O₂ reaction were formaldehyde and ketene. This is in agreement with the one theoretical study available for i-C₄H₅ + O₂ reaction, which predicts the main bimolecular product channels to be H₂CO + C₂H₃ + CO and H₂CCO + CH₂CHO.     

Ultrasonically determined thickness of long cortical bones: Three-dimensional simulations of in vitro experiments

It was reported in a previous study that simulated guided wave axial transmission velocities on two-dimensional (2D) numerically reproduced geometry of long bones predicted moderately real in vitro ultrasound data on the same bone samples. It was also shown that fitting of ultrasound velocity with simple analytical model yielded a precise estimate (UTh) for true cortical bone thickness. This current study expands the 2D bone model into three dimensions (3D). To this end, wave velocities and UTh were determined from experiments and from time-domain finite-difference simulations of wave propagation, both performed on a collection of 10 human radii (29 measurement sites). A 3D numerical bone model was developed with tuneable fixed material properties and individualized geometry based on X-ray computed tomography reconstructions of real bones. Simulated UTh data were in good accordance (root-mean-square error was 0.40 mm; r(2)=0.79, p<0.001) with true cortical thickness, and hence the measured phase velocity can be well estimated by using a simple analytical inversion model also in 3D. Prediction of in vitro data was improved significantly (by 10% units) and the upgraded bone model thus explained most of the variability (up to 95% when sites were carefully matched) observed in in vitro ultrasound data.     

Interaction of ethyl alcohol vapor with sulfuric acid solutions

We investigated the uptake of ethyl alcohol (ethanol) vapor by sulfuric acid solutions over the range approximately 40 to approximately 80 wt % H2SO4 and temperatures of 193-273 K. Laboratory studies used a fast flow-tube reactor coupled to an electron-impact ionization mass spectrometer for detection of ethanol and reaction products. The uptake coefficients (gamma) were measured and found to vary from 0.019 to 0.072, depending upon the acid composition and temperature. At concentrations greater than approximately 70 wt % and in dilute solutions colder than 220 K, the gamma values approached approximately 0.07. We also determined the effective solubility constant of ethanol in approximately 40 wt % H2SO4 in the temperature range 203-223 K. The potential implications to the budget of ethanol in the global troposphere are briefly discussed.     

Kinetic (T = 201-298 K) and equilibrium (T = 320-420 K) measurements of the C3H5 + O2 ⇆ C3H5O2 reaction

The kinetics and equilibrium of the allyl radical reaction with molecular oxygen have been studied in direct measurements using temperature-controlled tubular flow reactor coupled to a laser photolysis/photoionization mass spectrometer. In low-temperature experiments (T = 201-298 K), association kinetics were observed, and the measured time-resolved C(3)H(5) radical signals decayed exponentially to the signal background. In this range, the determined rate coefficients exhibited a negative temperature dependence and were observed to depend on the carrier-gas (He) pressure {p = 0.4-36 Torr, [He] = (1.7-118.0) × 10(16) cm(-3)}. The bimolecular rate coefficients obtained vary in the range (0.88-11.6) × 10(-13) cm(3) s(-1). In higher-temperature experiments (T = 320-420 K), the C(3)H(5) radical signal did not decay to the signal background, indicating equilibration of the reaction. By measuring the radical decay rate under these conditions as a function of temperature and following typical second- and third-law procedures, plotting the resulting ln K(p) values versus 1/T in a modified van't Hoff plot, the thermochemical parameters of the reaction were extracted. The second-law treatment resulted in values of ΔH(298)° = -78.3 ± 1.1 kJ mol(-1) and ΔS(298)° = -129.9 ± 3.1 J mol(-1) K(-1), with the uncertainties given as one standard error. When results from a previous investigation were taken into account and the third-law method was applied, the reaction enthalpy was determined as ΔH(298)° = -75.6 ± 2.3 kJ mol(-1).     

7‐(2‐Oxoalkoxy)coumarins: Synthesis and Anti‐Inflammatory Activity of a Series of Substituted Coumarins

A series of 7‐(2‐oxoalkoxy)coumarins have been synthesized by conjugating substituted 7‐hydroxycoumarins with different chloroketones. The anti‐inflammatory properties of 7‐(2‐oxoalkoxy)coumarins were studied in LPS‐induced inflammatory response in J774 macrophages. Western blot was used to determine the expression of iNOS and COX‐2, NO was determined by measuring its metabolite nitrite by Griess reaction and IL‐6 was measured by ELISA. Seventeen of the studied compounds inhibited NO and IL‐6 production over 50% at 100 μM concentrations. IC₅₀ values of the best inhibitors were 21 μM/24 μM (NO/IL‐6) for compound 12 and 30 μM/10 μM (NO/IL‐6) for compound 20 . The main result was that the substitution with 7‐(2‐oxoalkoxy) group improved the anti‐inflammatory properties of most of the investigated 7‐hydroxycoumarins.     

An Experimental Study of the Kinetics of the Reactions of Isopropyl,sec‐Butyl,and tert‐Butyl Radicals with Molecular Chlorine at Low Pressures (0.5–7.0 Torr) in the Temperature Range 190–480 K

In this work, we have measured the rate coefficients of the reactions of isopropyl (propan‐2‐yl), sec‐butyl (butan‐2‐yl), and tert‐butyl (2‐methylpropan‐2‐yl) radicals with molecular chlorine as a function of temperature (190–480 K). The experiments were done in a tubular laminar flow reactor coupled to a photoionization quadrupole mass spectrometer employing a gas‐discharge lamp for ionization. The radicals were homogeneously produced in the reactor by photolyzing suitable precursor molecules with 193‐nm pulsed exciplex laser radiation. The bimolecular rate coefficients were obtained by monitoring the radical decay signals in real time under pseudo–first‐order conditions. The rate coefficients of all three reactions showed negative temperature dependence. The bath gas used in the experiments was helium, and the rate coefficients appeared to be independent of the helium concentrations employed ([2.4–14] × 10¹⁶ cm^?3) for all three reactions. The rate coefficients of the reactions can be approximated in the studied temperature range by the following parameterizations: We estimate that the overall uncertainties of the measured rate coefficients are ±20%. We were able to observe 2‐chloropropane (i‐C₃H₇Cl) product for the i‐C₃H₇^● + Cl₂ reaction. No products were observed for the other two reactions, and the reasons for this are briefly discussed in the text.     

Kinetics of the reactions of C2H5, n‐C3H7, and n‐C4H9 radicals with Cl2 at the temperature range 190–360 K

The kinetics of the C₂H₅ + Cl₂, n‐C₃H₇ + Cl₂, and n‐C₄H₉ + Cl₂ reactions has been studied at temperatures between 190 and 360 K using laser photolysis/photoionization mass spectrometry. Decays of radical concentrations have been monitored in time‐resolved measurements to obtain reaction rate coefficients under pseudo‐first‐order conditions. The bimolecular rate coefficients of all three reactions are independent of the helium bath gas pressure within the experimental range (0.5–5 Torr) and are found to depend on the temperature as follows (ranges are given in parenthesis): k(C₂H₅ + Cl₂) = (1.45 ± 0.04) × 10^?11 (T/300 K)^{?1.73 ± 0.09} cm³ molecule^?1 s^?1 (190–359 K), k(n‐C₃H₇ + Cl₂) = (1.88 ± 0.06) × 10^?11 (T/300 K)^{?1.57 ± 0.14} cm³ molecule^?1 s^?1 (204–363 K), and k(n‐C₄H₉ + Cl₂) = (2.21 ± 0.07) × 10^?11 (T/300 K)^{?2.38 ± 0.14} cm³ molecule^?1 s^?1 (202–359 K), with the uncertainties given as one‐standard deviations. Estimated overall uncertainties in the measured bimolecular reaction rate coefficients are ±20%. Current results are generally in good agreement with previous experiments. However, one former measurement for the bimolecular rate coefficient of C₂H₅ + Cl₂ reaction, derived at 298 K using the very low pressure reactor method, is significantly lower than obtained in this work and in previous determinations. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 614–619, 2007