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Negative Temperature Dependence (negative + temperature_dependence)
Selected AbstractsMeasurements of the kinetics of the OH + ,-pinene and OH + ,-pinene reactions at low pressureINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 5 2002B. Chuong The rate constants for the OH + ,-pinene and OH + ,-pinene reactions have been measured in 5 Torr of He using discharge-flow systems coupled with resonance fluorescence and laser-induced fluorescence detection of the OH radical. At room temperature, the measured effective bimolecular rate constant for the OH + ,-pinene reaction was (6.08 ± 0.24) × 10,11 cm3 molecule,1 s,1. These results are in excellent agreement with previous absolute measurements of this rate constant, but are approximately 13% greater than the value currently recommended for atmospheric modeling. The measured effective bimolecular rate constant for the OH + ,-pinene reaction at room temperature was (7.72 ± 0.44) × 10,11 cm3 molecule,1 s,1, in excellent agreement with previous measurements and current recommendations. Above 300 K, the effective bimolecular rate constants for these reactions display a negative temperature dependence suggesting that OH addition dominates the reaction mechanisms under these conditions. This negative temperature dependence is larger than that observed at higher pressures. The measured rate constants for the OH + ,-pinene and OH + ,-pinene reactions are in good agreement with established reactivity trends relating the rate constant for OH + alkene reactions with the ionization potential of the alkene when ab initio calculated energies for the highest occupied molecular orbital are used as surrogates for the ionization potentials for ,- and ,-pinene. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 300,308, 2002 [source] Ab initio direct dynamics studies on the reactions of chlorine atom with CH3,nFnCH2OH (n = 1,3)JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 16 2007Ying Wang Abstract The hydrogen abstraction reactions of Cl atom with a series of fluorinated alcohols, i.e., CH3,nFnCH2OH + Cl (n = 1,3) (R1,R3) have been studied systematically by ab initio direct dynamics method and the canonical variational transition state theory (CVT). The potential energy surface information is calculated at the MP2/6-311G(d,p) level. Energies along the minimum energy paths are improved by a series of single-point calculations at the higher modified GAUSSIAN-2 (G2M) level of theory. Theoretical analysis shows that three kinds of hydrogen atoms can be abstracted from the reactants CH2FCH2OH and CHF2CH2OH, and for CF3CH2OH, two possible pathways are found. The rate constants for each reaction channel are evaluated by CVT with the small-curvature tunneling correction (SCT) over a wide range of temperature from 200 to 2000 K. The calculated CVT/SCT rate constants are in good agreement with the available experimental values for the reactions CHF2CH2OH + Cl and CF3CH2OH + Cl. However, for the reaction CH2FCH2OH + Cl, there is negative temperature dependence below 500 K, which is different from the experimental fitted. It is shown that in the low temperature ranges, the three reactions all proceed predominantly via H-abstraction from the methylene positions, and with the increase of the temperature the H-abstraction channels from the fluorinated-methyl positions should be taken into account, while the H-abstraction channels from the hydroxyl groups are negligible over the whole temperature ranges. Also, the reactivity decreases substantially with fluorine substitution at the methyl position of alcohol. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2007 [source] CH3CH2SCH3,+,OH radicals: temperature-dependent rate coefficient and product identification under atmospheric pressure of air,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 10 2010Gabriela Oksdath-Mansilla Abstract Relative rate coefficients have been determined for the gas-phase reaction of hydroxyl (OH) radicals with ethyl methyl sulfide (EMS) using isobutene as a reference compound. The experiments were performed in a 1080,L quartz glass photoreactor in the temperature range of 286,313,K at a total pressure of 760,±,10,Torr synthetic air using in situ FTIR absorption spectroscopy to monitor the concentration-time behaviors of reactants and products. OH radicals were produced by the 254,nm photolysis of hydrogen peroxide (H2O2). The kinetic data obtained were used to derive the following Arrhenius expression valid in the temperature range of 286,313,K (in units of cm3,molecule,1,s,1): The rate coefficient displays a negative temperature dependence and low pre-exponential factor which supports the existence of an addition mechanism for the reaction involving reversible OH-adduct formation. The results are compared with previous data of other sulfides from the literature and are rationalized in terms of structure,reactivity relationships. Additionally, product identification of the title reaction was performed for the first time by the FTIR technique under atmospheric conditions. Sulfur dioxide, formaldehyde, and formic acid were observed as degradation products in agreement with the two possible reaction channels (addition/abstraction). Copyright © 2010 John Wiley & Sons, Ltd. [source] Atomic structure and positron lifetime in the metallic glass Zr55Cu30Ni5Al10PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2007K. Sugita Abstract Zr-based metallic glasses have superior characteristics such as mechanical strength, corrosion resistance and precision casting ability. From positron lifetime measurements, a negative temperature dependence of the mean positron lifetime above room temperature was reported and ascribed to the presence of shallow traps. However, the trapping sites remain unknown under the present circumstances. To get a further understanding of the experimental positron lifetime value, the positron density distribution and the lifetime in the Zr-based metallic glass Zr55Cu30Ni5Al10 have been calculated. The calculation shows that the positrons are annihilated inhomogeneously and most positrons are annihilated preferentially around Cu/Al. These results indicate that the positron lifetime not exactly reflects the total free volume. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Kinetic isotope effects for the H2 + C2H , C2H2 + H reaction based on the ab initio calculations and a global potential energy surfaceINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 5 2010Liping Ju In the present paper, kinetic isotope effects of the title reaction are studied with canonical variational transition state theory on the modified Wang Bowman (MWB) potential energy surface (PES) (Chem Phys Lett 2005, 409, 249) and the ab initio calculations at the quadratic configuration interaction (QCISD (T, full))/aug-cc-pVTZ//QCISD (full)/cc-pVTZ level. The calculated rate constants for the isotopic variants of this title reaction on the MWB PES have good agreement with those of the present ab initio calculations over the temperature range of 20,5000 K for the forward reactions and 800,5000 K for the reverse reactions, respectively. In particular, the forward rate constants for the title reaction and its isotopically substituted reactions have negative temperature dependences at about 40 K. Rate expressions are presented for all the studied reactions. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 289,298, 2010 [source] | ||||||||||