Ravi X. Fernandes

Reaction Kinetics of Hydrogen Abstraction Reactions by Hydroperoxyl Radical from 2-Methyltetrahydrofuran and 2,5-Dimethyltetrahydrofuran

Highly accurate rate parameters for H-abstraction reactions by HO2 radicals are needed for development of predictive chemical kinetic models for ignition. In this article, we report the rate coefficients for reaction of hydroperoxyl radical (HO2) with 2-methyltetrahydrofuran (MTHF) and 2,5-dimethyltetrahydrofuran (DMTHF) computed employing CBS-QB3 and CCSD(T)/cc-pVTZ//B3LYP/cc-pVTZ level of theory in the temperature range of 500-2000 K. Conventional transition state theory (CTST) with hindered rotor approximation for low frequency torsional modes and RRHO (rigid-rotor harmonic oscillator) approximation for all other vibrational modes is employed to evaluate the high pressure rate constants as a function of temperature. Rate constant of each individual hydrogen abstraction channel is taken into account to calculate the overall rate constant. Three-parameter Arrhenius expressions have been obtained by fitting to the computed rate constants of all abstraction channels between 500 and 2000 K. Eight transition states have been identified for MTHF and four for slightly more stable trans-DMTHF. Intrinsic reaction coordinates (IRC) calculations were performed to verify the connectivity of all the transition states (TSs) with reactants and products. One dimensional Eckarts asymmetrical method has been used to calculate quantum mechanical tunneling effect. Results of the theoretically calculated rate coefficients indicate that the hydrogen abstraction by HO2 from the C2 carbon of both MTHF and DMTHF is the most dominant path among all reaction pathways attributed to its lowest barrier height. The total rate coefficients of the MTHF and DMTHF with HO2 at CCSD(T)/cc-pVTZ//B3LYP/cc-pVTZ level of theory are k(T) = 8.60T(3.54) exp(-8.92/RT) and k(T)= 3.17T(3.63) exp(-6.59/RT) cm(3) mol(-1) s(-1), respectively. At both the level of theories, the predicted total abstraction rate constant for DMTHF is found to be higher as compared to that of MTHF over an entire temperature range of investigation. The overall rate constant calculated at CCSD(T)/cc-pVTZ//B3LYP/cc-pVTZ level of theory is lower by 1.43 and 3.44 times at 2000 K than the CBS-QB3 level for MTHF and DMTHF, respectively.

Theoretical Investigation of Intramolecular Hydrogen Shift Reactions in 3-Methyltetrahydrofuran (3-MTHF) Oxidation.

3-Methyltetrahydrofuran (3-MTHF) is proposed to be a promising fuel component among the cyclic oxygenated species. To have detailed insight of its combustion kinetics, intramolecular hydrogen shift reactions for the ROO to QOOH reaction class are studied for eight ROO isomers of 3-MTHF. Rate constants of all possible reaction paths that involve formation of cyclic transition states are computed by employing the CBS-QB3 composite method. A Pitzer-Gwinn-like approximation has been applied for the internal rotations in reactants, products, and transition states for the accurate treatment of hindered rotors. Calculated relative barrier heights highlight that the most favorable reaction channel proceeds via a six membered transition state, which is consistent with the computed rate constants. Comparing total rate constants in ROO isomers of 3-MTHF with the corresponding isomers of methylcyclopentane depicts faster kinetics in 3-MTHF than methylcyclopentane reflecting the effect of ring oxygen on the intramolecular hydrogen shift reactions.

The pyrolysis of 2-, 3-, and 4-methylbenzyl radicals behind shock waves

The thermal decomposition of 2-, 3-, and 4-methylbenzyl radicals was studied behind reflected shock waves. α -Bromo- ortho ( meta - and para )-xylenes were used as precursors for a series of experiments with temperatures ranging from 1150 to 1600 K and pressures between 1.5 and 4 bar. Mixtures of 1–5.5 ppm of the precursor diluted in argon were used for the investigations. Initiated by the fast thermal dissociation of the precursor CH 3 C 6 H 4 CH 2 Br→CH 3 C 6 H 4 CH 2 +Br (R3) the methylbenzyl radicals subsequently decomposed. 2-methylbenzyl→ o -CH 2 C 6 H 4 CH 2 +H (R2 o ) 3-methylbenzyl→ m -CH 2 C 6 H 4 CH 2 +H (R2 m ) 4-methylbenzyl→ p -CH 2 C 6 H 4 CH 2 +H (R2 P ) We followed the rate of H-atom formation by using atom resonance absorption spectroscopy (H-ARAS) at 121.6 nm. We report the first directly measured rate coefficients for the decomposition of methylbenzyl radicals: k 2o =5×10 15 exp {−(310±4)kJ mol −1 / RT } s −1 , k 2m =5×10 15 exp{−(340±4) kJ mol −1 / RT } s −1 , and k 2p =5×10 15 exp{−(295±4) kJ mol −1 / RT } s −1 These rate coefficients are about a factor of 4 below the high-pressure limit, and the accuracy is estimated to be 30%. The complete H-atom concentration-time profiles were successfully modeled using a simple mechanism.

Experimental and modeling study of the temperature and pressure dependence of the reaction C2H5 + O2 (+ M) → C2H5O2 (+ M).

The reaction C2H5 + O2 (+ M) → C2H5O2 (+ M) was studied at 298 K at pressures of the bath gas M = Ar between 100 and 1000 bar. The transition from the falloff curve of an energy transfer mechanism to a high pressure range with contributions from the radical complex mechanism was observed. Further experiments were done between 188 and 298 K in the bath gas M = He at pressures in the range 0.7-2.0 Torr. The available data are analyzed in terms of unimolecular rate theory. An improved analytical representation of the temperature and pressure dependence of the rate constant is given for conditions where the chemical activation process C2H5 + O2 (+ M) → C2H4 + HO2 (+ M) is only of minor importance.

Contribution of the radical-complex mechanism to the rate of the reaction CH3 + O-2 (+ M) -> CH3O2 (+ M) at high pressures.

Earlier experimental studies of the falloff curves of the reaction CH(3) + O(2) (+ M) → CH(3)O(2) (+ M) in the bath gases M = Ar and N(2) (Fernandes et al., J. Phys. Chem. A 2006, 110, 4442), in addition to the usual behavior of the energy-transfer (ET) mechanism, showed first evidence for a participation of the radical-complex (RC) mechanism in the reaction at pressures above about 300 bar and at temperatures below 400 K. By extending these measurements to the bath gas M = CO(2), more pronounced deviations from the ET mechanism were now observed. This unambiguously confirms the presence of the RC mechanism at high pressures in a medium-sized molecular system, analogous to earlier observations for larger systems such as the dimerization of benzyl radicals (Luther et al., Phys. Chem. Chem. Phys. 2004, 6, 4133).

Advanced fuel chemistry for advanced engines.

Autoignition chemistry is central to predictive modeling of many advanced engine designs that combine high efficiency and low inherent pollutant emissions. This chemistry, and especially its pressure dependence, is poorly known for fuels derived from heavy petroleum and for biofuels, both of which are becoming increasingly prominent in the nations fuel stream. We have investigated the pressure dependence of key ignition reactions for a series of molecules representative of non-traditional and alternative fuels. These investigations combined experimental characterization of hydroxyl radical production in well-controlled photolytically initiated oxidation and a hybrid modeling strategy that linked detailed quantum chemistry and computational kinetics of critical reactions with rate-equation models of the global chemical system. Comprehensive mechanisms for autoignition generally ignore the pressure dependence of branching fractions in the important alkyl + O{sub 2} reaction systems; however we have demonstrated that pressure-dependent formally direct pathways persist at in-cylinder pressures.