High-pressure Limit Research Articles

Time-Resolved Gas-Phase Kinetic, Quantum Chemical, and RRKM Studies of the Reaction of Silylene with 2,5-Dihydrofuran.

Time-resolved kinetics studies of silylene, SiH2, generated by laser flash photolysis of phenylsilane, were performed to obtain rate coefficients for its bimolecular reaction with 2,5-dihydrofuran (2,5-DHF). The reaction was studied in the gas phase over the pressure range of 1-100 Torr in SF6 bath gas, at five temperatures in the range of 296-598 K. The reaction showed pressure dependences characteristic of a third body assisted association. The second-order rate coefficients obtained by Rice-Ramsperger-Kassel-Marcus (RRKM)-assisted extrapolation to the high-pressure limit at each temperature fitted the following Arrhenius equation where the error limits are single standard deviations: log(k/cm(3) molecule(-1) s(-1)) = (-9.96 ± 0.08) + (3.38 ± 0.62 kJ mol(-1))/RT ln 10. End-product analysis revealed no GC-identifiable product. Quantum chemical (ab initio) calculations indicate that reaction of SiH2 with 2,5-DHF can occur at both the double bond (to form a silirane) and the O atom (to form a donor-acceptor, zwitterionic complex) via barrierless processes. Further possible reaction steps were explored, of which the only viable one appears to be decomposition of the O-complex to give 1,3-butadiene + silanone, although isomerization of the silirane cannot be completely ruled out. The potential energy surface for SiH2 + 2,5-DHF is consistent with that of SiH2 with Me2O, and with that of SiH2 with cis-but-2-ene, the simplest reference reactions. RRKM calculations incorporating reaction at both π- and O atom sites, can be made to fit the experimental rate coefficient pressure dependence curves at 296-476 K, giving values for k(∞)(π) and k(∞)(O) that indicate the latter is larger in magnitude at all temperatures, in contrast to values from individual model reactions. This unexpected result suggests that, in 2,5-DHF with its two different reaction sites, the O atom exerts the more pronounced electrophilic attraction on the approaching silylene. Arrhenius parameters for the individual pathways were obtained. The lack of a fit at 598 K is consistent with decomposition of the O-complex to give 1,3-butadiene + silanone.

Theoretical study of the oxidation mechanisms of thiophene initiated by hydroxyl radicals.

The mechanisms for the oxidation of thiophene by OH radicals under inert conditions (Ar) have been studied using density functional theory in conjunction with various exchange-correlation functionals. These results were compared with benchmark CBS-QB3 theoretical results. Kinetic rate constants were estimated by means of variational transition state theory (VTST) and the statistical Rice-Ramsperger-Kassel-Marcus (RRKM) theory. Effective rate constants were calculated via a steady-state analysis based upon a two-step model reaction mechanism. In line with experimental results, the computed branching ratios indicate that the most kinetically efficient process involves OH addition to a carbon atom adjacent to the sulfur atom. Due to the presence of negative activation energies, pressures larger than 10(4)bar are required to reach the high-pressure limit. Nucleus-independent chemical shift indices and natural bond orbital analysis show that the computed activation energies are dictated by changes in aromaticity and charge-transfer effects due to the delocalization of lone pairs from sulfur to empty π(*) orbitals. Graphical Abstract CBS-QB3 energy profiles for the reaction pathways 1-3 characterizing the oxidation of thiophene by hydroxyl radicals into the related products.

Ab Initio Reaction Kinetics of CH3OĊ(═O) and ĊH2OC(═O)H Radicals.

The dissociation and isomerization kinetics of the methyl ester combustion intermediates methoxycarbonyl radical (CH3OĊ(═O)) and (formyloxy)methyl radical (ĊH2OC(═O)H) are investigated theoretically using high-level ab initio methods and Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation (ME) theory. Geometries obtained at the hybrid density functional theory (DFT) and coupled cluster singles and doubles with perturbative triples correction (CCSD(T)) levels of theory are found to be similar. We employ high-level ab initio wave function methods to refine the potential energy surface: CCSD(T), multireference singles and doubles configuration interaction (MRSDCI) with the Davidson-Silver (DS) correction, and multireference averaged coupled-pair functional (MRACPF2) theory. MRSDCI+DS and MRACPF2 capture the multiconfigurational character of transition states (TSs) and predict lower barrier heights than CCSD(T). The temperature- and pressure-dependent rate coefficients are computed using RRKM/ME theory in the temperature range 300-2500 K and a pressure range of 0.01 atm to the high-pressure limit, which are then fitted to modified Arrhenius expressions. Dissociation of CH3OĊ(═O) to ĊH3 and CO2 is predicted to be much faster than dissociating to CH3Ȯ and CO, consistent with its greater exothermicity. Isomerization between CH3OĊ(═O) and ĊH2OC(═O)H is predicted to be the slowest among the studied reactions and rarely happens even at high temperature and high pressure, suggesting the decomposition pathways of the two radicals are not strongly coupled. The predicted rate coefficients and branching fractions at finite pressures differ significantly from the corresponding high-pressure-limit results, especially at relatively high temperatures. Finally, because it is one of the most important CH3Ȯ removal mechanisms under atmospheric conditions, the reaction kinetics of CH3Ȯ + CO was also studied along the PES of CH3OĊ(═O); the resulting kinetics predictions are in remarkable agreement with experiments.

Ab Initio Unimolecular Reaction Kinetics of CH2C(═O)OCH3 and CH3C(═O)OCH2 Radicals

The unimolecular dissociation and isomerization kinetics of the methyl ester combustion intermediates methoxycarbonylmethyl (CH2C(═O)OCH3) and acetyloxylmethyl (CH3C(═O)OCH2) are theoretically investigated using high-level ab initio methods and the Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation (ME) theory. Potential energy surfaces (PESs) are obtained using coupled cluster singles and doubles with perturbative triples correction (CCSD(T)), multireference singles and doubles configuration interaction (MRSDCI) with the Davidson-Silver (DS) correction, and multireference averaged coupled pair functional (MRACPF2) theory. The transition states exhibit high T1 diagnostics in coupled cluster calculations, suggesting the need for a multireference correlated wave function treatment. MRSDCI+DS and MRACPF2 capture their multiconfigurational character well, yielding lower barrier heights than CCSD(T) for these reactions. The rate coefficients are computed using the RRKM/ME theory over a 500-2500 K temperature range and at a pressure range of 0.01 atm to the high-pressure limit. The temperature- and pressure-dependent rate coefficients are given in modified Arrhenius expressions. The β-scission of CH2C(═O)OCH3 is predicted to have a much higher barrier than the corresponding isomerization reaction and the β-scission of CH3C(═O)OCH2. Consequently, the rate coefficients for β-scission of CH2C(═O)OCH3 are the smallest among the three reactions and the isomerization followed by decomposition to CH3C(═O) and HCHO is the dominant reaction pathway for CH2C(═O)OCH3. Both radicals CH2C(═O)OCH3 and CH3C(═O)OCH2 are predicted to mainly decompose to CH3C(═O) + HCHO rather than to the bimolecular product CH2C(═O) + CH3O. A newly developed MA combustion mechanism, using our theoretical rate coefficients for the MA-related reactions, predicts combustion properties in good agreement with available experimental data.

Rate Constant for the OH + CO Reaction at Low Temperatures.

Rate constants for the reaction of OH + CO → products (1) have been measured using laser photolysis/laser-induced fluorescence (LP/LIF) over the temperature range 193–296 K and at pressures of 50–700 Torr of Ar and N2. The reaction was studied under pseudo-first-order conditions, monitoring the decay of OH in the presence of a large excess of CO. The rate constants can be expressed as a combination of bimolecular and termolecular components. The bimolecular component was found to be temperature-independent with an expression given by kbi(T) = (1.54 ± 0.14) × 10(–13)[e(–(13±17)/T)] cm(3) molecule(–1) s(–1), with an error of one standard deviation. The termolecular component was fitted to the expression, kter = k0(T)[M]/[1 + (k0(T)[M]/k∞(T)] × 0.6({1+[log10(k0(T)[M]/k∞(T))]2}−1) where k0(T) = k0(300)(T/300)(−n) and k∞(T) = k∞(300)(T/300)(−m). The parameters for k0(T) were determined to be k0(300) = (6.0±0.5) × 10(−33) cm(6) molecule(–2) s(–1) in N2 and k0(300) = (3.4 ± 0.3) × 10(–33) cm(6) molecule(–2) s(–1) in Ar, with n = 1.9±0.5 and 2.0±0.4 in N2 and Ar, respectively. These parameters were determined using k0(T) and m from the NASA kinetics data evaluation (JPL Publication No. 10-6) since the experimental pressure range was far from the high-pressure limit. Addition of low concentrations of O2 had no discernible effect on the mechanism of the OH + CO reaction but resulted in secondary reactions which regenerated OH.

Low- and intermediate-temperature oxidation of ethylcyclohexane: A theoretical study

To get a better understanding of low temperature oxidation chemistry of long-side-chain cycloalkanes, the low- and intermediate-temperature oxidation mechanisms and kinetics of ethylcyclohexane (ECH) have been investigated by using a combination of electronic-structure theory, transition state theory (TST) and Rice-Ramsberger-Kassel-Marcus/Master-Equation (RRKM/ME) theory in this work. The high pressure limit rate constants for reactions with tight transition states are obtained by the TST, while the high pressure limit rate constants for barrierless reactions are obtained by the variational transition state theory (VTST). The rate constants in the fall-off range for pressure-dependent reactions are obtained by RRKM/ME theory. Depending on the position of the extracted H atom on the ring or on the side chain, the oxidation of ECH is mainly initiated by H-abstraction with OH radical to form cyclic C8H15 radicals having six isomers (R1–R6) at low (500−900 K) and intermediate (900−1100 K) temperature and in this study we focus on the kinetics of the reactions starting from these isomers. At low temperature, it is shown that, for addition reactions of O2 to cyclic C8H15 radicals, the calculated rate constants when the unpaired electron of the radicals is in the side-chain are smaller than when in the ring. The intramolecular isomerization reactions of six cycloalkylperoxy radicals (R1OO–R6OO) by H-shift can lead to a large number of hydroperoxycycloalkyl radicals (QOOH) and it is shown that the 1,5 H-shift is more competitive than the 1,6 H-shift in the R1OO–R6OO. Furthermore, unimolecular elimination reactions of QOOH can form OH, HO2, cyclic ethers, ketones, aldehydes and conjugated olefins. The calculated results indicate that the formation cyclic ethers of ECH oxidation are inclined to 1-oxaspiro[3.5]octane, 8-methyl-7-oxa-bicyclo[4.2.0]octane, 7-methyl-6-oxa-bicyclo[3.2.1]octane and 2-ethyl-6-oxa-bicyclo[3.1.1]heptane. At intermediate temperature, the most important reaction type of R1–R6 consumption is the C–C bond fission to form smaller products. The reaction pathways and potential energy surfaces to form the main products (ethylene, butadiene, 1-butene, and so on) of R1–R6 decomposition are also performed and the rate constants of the corresponding reactions are calculated by using the TST. Rate constants of these reactions are fitted by a nonlinear least-squares method to the form of a modified Arrhenius rate expression, which can be directly used for the modeling study of low temperature oxidation of cycloalkanes.

Study of the Thermodynamics (Thermal and Cl Catalyzed) and Kinetics of the Cis and Trans Isomerizations of CF3CF═CHF, CF3CH═CHCF3, and CH3CH═CHCH3 in 100-950 Torr of N2 Diluent at 296-875 K: Effect of F and CF3 Substitution on the Isomerization Process Including the Fluorine "Cis Effect".

The equilibrium constants for the Z to E isomerizations of CF3CF═CHF (K1) and CF3CH═CHCF3 (K2) have been measured using GC/FID analysis over the temperature ranges 360–850 and 297–850 K, respectively. At lower temperature, K was determined using Cl atom catalysis. At higher temperature, K was measured without a catalyst. The temperature-dependent expressions are K1 (Z to E) = 1.45(±0.15)e(–[2845(±100)/RT]) and K2 (Z to E) = 1.9(±0.22)e(+[4330(±120)/RT]) (where the gas constant R ≡ 1.986 cal mol(–1) K(–1)). For isomerization 1, the Z (fluorine cis) isomer is 2.85 kcal mol(–1) lower in energy than the E (fluorine trans) isomer, providing another example of the fluorine "cis effect" in olefins. For isomerization 2, the E(trans) isomer is 4.3 kcal mol(–1) lower in energy than the corresponding Z(cis) isomer as is normal for olefins. The isomerization rate constant in a single direction was also measured for each fluorinated compound: k–1(E to Z) = 10(13.87±0.24)e(–59530(±887)/RT) s(–1); and k2(Z to E) = 10(13.89±0.23)e(–58845(±675)/RT) s(–1). To verify the experimental method, cis to trans (k3) and trans to cis (k–3) isomerization rate constants were also measured for cis- and trans-2-butene for comparison to several previous studies. The rate constants determined herein are k3 (cis to trans) = 10(13.95±0.23)e(–63245(±815)/RT) s(–1); and k–3 (trans to cis) = 10(14.32±0.28)e(–64993(±1132)/RT) s(–1). k3 agrees well with four previous measurements and represents the best available rate constant for 2-butene. All errors quoted here are 2σ. The typical total pressure for these experiments was 760 ± 100 Torr. Limited experiments performed at 100 Torr showed no pressure dependence for any of the compounds above 100 Torr. Thus, all isomerization rate constants represent high-pressure limits. The rates of the addition reactions of Cl to the double bonds of CF3CF═CHF (k4) and CF3CH═CHCF3 (k7) (used in Cl catalysis) were also measured in pure N2 and in pure O2. In O2, the rate constants expressions are k4 = 1.56 (±0.22) × 10(–11) e(+(643/RT)) cm3 molecule(–1) s(–1); and k7 = 1.05 × 10(–12) e(+(1874/RT)) cm(3) molecule(–1) s(–1). In N2, k4 and k7 decrease several orders of magnitude as the temperature increases because of the increasing reversibility of the Cl addition reaction, which produces the catalytic effect.

The Reaction Kinetics of Amino Radicals with Sulfur Dioxide

Abstract Application of the laser photolysis–laser-induced fluorescence method to the reaction NH2 + SO2 in argon bath gas yields pressure-dependent, third-order kinetics which may be summarized as k = (1.49 ± 0.15) × 10−31 (T/298 K)−0.83 cm6 molecule –2 s –1 over 292–555 K, where the uncertainty is the 95% confidence interval and includes possible systematic errors. The quenching of vibrationally excited NH2 is consistent with a high-pressure limit for NH2 + SO2 of (1.62 ± 0.25) × 10−11 cm3 molecule –1 s –1 over the temperature range 295–505 K, where again the 95% confidence interval is shown. Ab initio analysis yields a H2N–SO2 dissociation enthalpy of 73.5 kJ mol –1, and comparison with RRKM theory and the exponential down model for energy transfer yields 〈ΔE〉down = 350 cm –1 for Ar at room temperature.

Group Additive Kinetics for Hydrogen Transfer Between Oxygenates.

Hydrogen abstraction reactions involving oxygenates in gaseous phase play an important role in many biomass-related conversion processes. In this work, group additivity is used to provide Arrhenius parameters in a temperature range of 300-2500 K for hydrogen abstractions between oxygenate compounds such as alcohols, ethers, esters, acids, ketones, diketones, aldehydes, hydroxyperoxides, alkyl peroxides, and unsaturated ethers and ketones. The group additive values for Arrhenius parameters of hydrogen transfer reactions of the type O--H--C and O--H--O are derived from CBS-QB3 calculations in the high-pressure limit. From a total set of 118 reactions, 43 group additivity values are determined. Inclusion of an additional 37 corrections accounting for cross-resonance effects in the transition state further improves the accuracy of the model. For a set of 25 ab initio calculated and 60 experimental rate coefficients, group additive modeling reproduces rate coefficients within a mean factor of deviation of ∼3. Hence, the developed group additive models can be reliably used for an accurate and fast prediction of the kinetics of hydrogen abstractions involving oxygenates.

Thermal Decomposition of 2(3H) and 2(5H) Furanones: Theoretical Aspects.

The thermal decomposition reactions of 2(3H) and 2(5H) furanones and their methyl derivatives are explored. Theoretical calculations of the barriers, reaction enthalpies, and the properties of these and intermediate species are reported using the composite model chemistry CBS-QB3 and also the functional M06-2X allied to the 6-311++G(d,p) basis set. Thus, the bond dissociation enthalpies, ionization energies, and unimolecular chemical kinetic rate constants in the high-pressure limit were computed. We show that flow reactor experiments that intimated that heating the 2(3H) furanone converts it to the isomeric 2(5H) furanone occurs via a 1 → 2 H-transfer reaction to an open ring ketenoic aldehyde. The latter can then ring close to the other isomeric structure. The final products acrolein and carbon monoxide are only formed from 2(3H), and acrolein will further decompose to ethylene and CO. Comparable channels explain the interconversion of 5-methyl-2(3H) furanone to its 2(5H) isomer and to the formation of methyl vinyl ketone and CO. The influence of the methyl group at other positions on the ring is hardly of significance except in the case of 5-methyl-2(5H) furanone where a hydrogen atom transfer from the methyl group leads to the formation of a doubly unsaturated carboxylic compound, 2,4-pentadienoic acid. Studies of the UV photolysis of the parent compounds in both low-temperature inert argon matrices and in solution are broadly in accord with the thermal findings insofar as product formation is concerned and with our theoretical calculations. The dominant features of the early decomposition chemistry of these compounds are simple hydrogen transfer and simultaneous ring opening reactions, which do however result in some quite unusual species.

The role of ontogeny in physiological tolerance: decreasing hydrostatic pressure tolerance with development in the northern stone crabLithodes maja

Extant deep-sea invertebrate fauna represent both ancient and recent invasions from shallow-water habitats. Hydrostatic pressure may present a significant physiological challenge to organisms seeking to colonize deeper waters or migrate ontogenetically. Pressure may be a key factor contributing to bottlenecks in the radiation of taxa and potentially drive speciation. Here, we assess shifts in the tolerance of hydrostatic pressure through early ontogeny of the northern stone crab Lithodes maja, which occupies a depth range of 4-790 m in the North Atlantic. The zoea I, megalopa and crab I stages were exposed to hydrostatic pressures up to 30.0 MPa (equivalent of 3000 m depth), and the relative fold change of genes putatively coding for the N-methyl-D-aspartate receptor-regulated protein 1 (narg gene), two heat-shock protein 70 kDa (HSP70) isoforms and mitochondrial Citrate Synthase (CS gene) were measured. This study finds a significant increase in the relative expression of the CS and hsp70a genes with increased hydrostatic pressure in the zoea I stage, and an increase in the relative expression of all genes with increased hydrostatic pressure in the megalopa and crab I stages. Transcriptional responses are corroborated by patterns in respiratory rates in response to hydrostatic pressure in all stages. These results suggest a decrease in the acute high-pressure tolerance limit as ontogeny advances, as reflected by a shift in the hydrostatic pressure at which significant differences are observed.

Global uncertainty analysis for RRKM/master equation based kinetic predictions: A case study of ethanol decomposition

A precise understanding of the accuracy of reaction rate constants, whether determined experimentally or theoretically, is of considerable importance to kinetic modelers. While the uncertainties of experimentally measured rate constants are commonly provided, the “error bars” of computed (temperature- and pressure-dependent) rate constants are rarely evaluated rigorously. In this work, global uncertainty and sensitivity analysis is applied to the propagation of the uncertainties in the input parameters (e.g. barrier heights, frequencies and collisional energy transfer parameters et al.) to those in the rate constants computed by the RRKM/master equation method for the decomposition of ethanol. This case study provides a systematic exploration of the effect of temperature and pressure on the parametric uncertainties in RRKM/master equation calculations for a prototypical single-well multiple-channel dissociation. In the high pressure limit, the uncertainties in the theoretical predictions are controlled by the uncertainties in the input parameters involved in the transition state theory calculations, with the most important ones being those describing the energetics of the decomposition. At lower pressures, where fall-off is important, the uncertainties in the collisional energy transfer parameters play a significant role, particularly for the higher energy of the two channels. Remarkably, the competition between dissociation and collisional excitation leads to uncertainties of more than a factor of 100 in the predictions for the higher energy channel. These large uncertainties are related to the need for large-scale single-collision-induced transitions in energy in order to produce the higher energy products in the low pressure limit. The present study illustrates the value of detailed qualitative and quantitative studies of the uncertainties in theoretical kinetics predictions.

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.

Reaction CH3 + CH3 → C2H6 Studied over the 292-714 K Temperature and 1-100 bar Pressure Ranges.

Reaction of recombination of methyl radicals, CH3 + CH3 → C2H6 (1) was studied using pulsed laser photolysis coupled to transient UV-vis absorption spectroscopy over the 292-714 K temperature and 1-100 bar pressure ranges (bath gas He), very close to the high-pressure limit. Methyl radicals were produced by photolysis of acetone at 193.3 nm or in the reaction of electronically excited oxygen atoms O((1)D), produced in the photolysis of N2O at 193.3 nm, with CH4, and subsequent reaction of OH with CH4. Temporal profiles of CH3 were recorded via absorption at 216.36 and 216.56 nm using a xenon arc lamp and a spectrograph. The absolute intensity of the photolysis light inside the reactor was determined by an accurate in situ actinometry based on the ozone formation in photolysis of N2O/O2/N2 mixtures. The rate constant of reaction 1 in the high-pressure limit has a negative temperature dependence: k1,inf = (5.66 ± 0.43) × 10(-11)(T/298 K)(-0.37) cm(3) molecule(-1) s(-1) (292-714 K).

Kinetics and Products of Vinyl + 1,3-Butadiene, a Potential Route to Benzene.

The reaction between vinyl radical, C2H3, and 1,3-butadiene, 1,3-C4H6, has long been recognized as a potential route to benzene, particularly in 1,3-butadiene flames, but the lack of reliable rate coefficients has hindered assessments of its true contribution. Using laser flash photolysis and visible laser absorbance (λ = 423.2 nm), we measured the overall rate coefficient for C2H3 + 1,3-C4H6, k1, at 297 K ≤ T ≤ 494 K and 4 ≤ P ≤ 100 Torr. k1 was in the high-pressure limit in this range and could be fit by the simple Arrhenius expression k1 = (1.1 ± 0.2) × 10(-12) cm(3) molecule(-1) s(-1) exp(-9.9 ± 0.6 kJ mol(-1)/RT). Using photoionization time-of-flight mass spectrometry, we also investigated the products formed. At T ≤ 494 K and P = 25 Torr, we found only C6H9 adduct species, while at 494 K ≤ T ≤ 700 K and P = 4 Torr, we observed ≤∼10% branching to cyclohexadiene in addition to C6H9. Quantum chemistry master-equation calculations using the modified strong collision model indicate that n-C6H9 is the dominant product at low temperature, consistent with our experimental results, and predict the rate coefficient and branching ratios at higher T where chemically activated channels become important. Predictions of k1 are in close agreement with our experimental results, allowing us to recommend the following modified Arrhenius expression in the high-pressure limit from 300 to 2000 K: k1 = 6.5 × 10(-20) cm(3) molecule(-1) s(-1) T(2.40) exp(-1.76 kJ mol(-1)/RT).

Pressure-dependent kinetics of initial reactions in iso-octane pyrolysis.

This study focuses on the studies of the main pressure-dependent reaction types of iso-octane (iso-C8H18) pyrolysis, including initial C-C bond fission of iso-octane, isomerization, and β-scission reactions of the alkyl radicals produced by the C-C bond fission of iso-octane. For the C-C bond fission of iso-octane, the minimum energy potentials are calculated at the CASPT2(2e,2o)/6-31+G(d,p)//CAS(2e,2o)/6-31+G(d,p) level of theory. For the isomerization and the β-scission reactions of the alkyl radicals, the optimization of the geometries and the vibrational frequencies of the reactants, transition states, and products are performed at the B3LYP/CBSB7 level, and their single point energies are calculated by using the composite CBS-QB3 method. Variable reaction coordinate transition state theory (VRC-TST) is used for the high-pressure limit rate constant calculation and Rice-Ramsperger-Kassel-Marcus/master equation (RRKM/ME) is used to calculate the pressure-dependent rate constants of these channels with pressure varying from 0.01-100 atm. The rate constants obtained in this work are in good agreement with those available from literatures. We have updated the rate constants and thermodynamic parameters for species involved in these reactions into a current chemical kinetic mechanism and also have improved the concentration profiles of main products such as C3H6 and C4H6 in the shock tube pyrolysis of iso-octane. The results of this study provide insight into the pyrolysis of iso-octane and will be helpful in the future development of branched paraffin kinetic mechanisms.

Quantum Chemical Study of Autoignition of Methyl Butanoate.

Methyl butanoate is a widely studied surrogate for fatty acid esters used in biodiesel fuel. Here we report a detailed analysis of the thermodynamics and kinetics of the autoignition chemistry of methyl butanoate. We employ composite CBS-QB3 calculations to construct the potential energy profiles of radicals derived from methyl butanoate. We compare our results with recently published G3MP2B3 results for reactions of peroxy (ROO(•)) and hydroperoxy alkyl ((•)QOOH) radicals and comment on differences in barrier heights and reaction enthalpies. Our emphasis, however, is on hydroperoxy alkylperoxy ((•)OOQOOH) radicals that are critical for autoignition of diesel fuel. We examined four classes of reactions: peroxy radical interconversion of (•)OOQOOH ((•)OOQOOH→ HOOQOO(•)), H-migration reactions (from carbon to oxygen), HO2 elimination, and cyclic ether formation with elimination of OH radical. We evaluate the significance of reaction pathways by comparing rate coefficients in the high-pressure limit. Unexpectedly, we find a low activation barriers for 1,8 H-migration of RC(═O)OCH2OO(•). We also find peroxy radical interconversion of (•)OOQOOH radicals from methyl butanoate commonly possess the lowest barriers of any unimolecular reaction of these radicals, despite that they proceed via 8-, 10- and 11-member ring transition states. At temperatures relevant to autoignition, these peroxy radical interconversions are dominant or significant reaction pathways. This means that some (•)OOQOOH radicals that were expected to be produced in negligible yields are, instead, major products in the autoignition of methylbutanoate (MB). These reactions have not previously been considered for MB, and will require revision of models of autoignition of methyl butanoate and other esters.

Symmetry considerations for the targeted assembly of entropically stabilized colloidal crystals via Voronoi particles.

The relationship between colloidal building blocks and their assemblies is an active field of research. As a strategy for targeting novel crystal structures, we examine the use of Voronoi particles, which are hard, space-filling particles in the shape of Voronoi cells of a target structure. Although Voronoi particles stabilize their target structure in the limit of high pressure by construction, the thermodynamic assembly of the same structure at moderate pressure, close to the onset of crystallization, is not guaranteed. Indeed, we find that a more symmetric crystal is often preferred due to additional entropic contributions arising from configurational or occupational degeneracy. We characterize the assembly behavior of the Voronoi particles in terms of the symmetries of the building blocks as well as the symmetries of crystal structures and demonstrate how controlling the degeneracies through a modification of particle shape and field-directed assembly can significantly improve the assembly propensity.

Muonium Addition Reactions and Kinetic Isotope Effects in the Gas Phase: k∞ Rate Constants for Mu + C2H2.

The kinetics of the addition reaction of muonium (Mu) to acetylene have been studied in the gas phase at N2 moderator pressures mainly from ∼800 to 1000 Torr and over the temperature range from 168 to 446 K, but also down to 200 Torr at 168 K and over a much higher range of pressures, from 10 to 44 bar at 295 K, demonstrating pressure-independent rate constants, kMu(T). Even at 200 Torr moderator pressure, the kinetics for Mu + C2H2 addition behave as if effectively in the high-pressure limit, giving k∞ = kMu due to depolarization of the muon spin in the MuC2H2 radical formed in the addition step. The rate constants kMu(T) exhibit modest Arrhenius curvature over the range of measured temperatures. Comparisons with data and with calculations for the corresponding H(D) + C2H2 addition reactions reveal a much faster rate for the Mu reaction at the lowest temperatures, by 2 orders of magnitude, in accord with the propensity of Mu to undergo quantum tunneling. Moreover, isotopic atom exchange, which contributes in a major way to the analogous D atom reaction, forming C2HD + H, is expected to be unimportant in the case of Mu addition, a consequence of the much higher zero-point energy and hence weaker C-Mu bond that would form, meaning that the present report of the Mu + C2H2 reaction is effectively the only experimental study of kinetic isotope effects in the high-pressure limit for H-atom addition to acetylene.

Ab initio Variational Transition State Theory and Master Equation Study of the Reaction (OH)3SiOCH2 + CH3 ⇌ (OH)3SiOC2H5

Abstract In this paper we use variable reaction coordinate variational transition state theory (VRC-TST) to calculate the reaction rate constants for the two reactions, R1: (OH)3SiOCH2 + CH3 ⇌ (OH)3SiOC2H5, and R2: CH2OH + CH3 ⇌ C2H5OH. The first reaction is an important channel during the thermal decomposition of tetraethoxysilane (TEOS), and its rate coefficient is the main focus of this work. The second reaction is analogous to the first and is used as a basis for comparison. The interaction energies are obtained on-the-fly at the CASPT2(2e,2o)/cc-pVDZ level of theory. A one-dimensional correction to the sampled energies was introduced to account for the energetic effects of geometry relaxation along the reaction path. The computed, high-pressure rate coefficients were calculated to be, R1: k 1 = 2.406 × 10−10 T −0.301 exp (− 271.4/T) cm3 molecule –1 s –1 and R2: k 2 = 1.316 × 10−10 T −0.189 exp (− 256.5/T) cm3 molecule –1 s –1. These rates differ from each other by only 10% – 30% over the temperature range 300–2000 K. A comparison of the computed rates with experimental data shows good agreement and an improvement over previous results. The pressure dependency of the reaction R1 is explored by solving a master equation using helium as a bath gas. The results obtained show that the reaction is only weakly pressure dependent over the temperature range 300–1700 K, with the predicted rate constant being within 50% of its high-pressure limit at atmospheric pressure.