Stephanie M. Villano

High-pressure rate rules for alkyl + O2 reactions. 1. The dissociation, concerted elimination, and isomerization channels of the alkyl peroxy radical.

The reactions of alkyl peroxy radicals (RO(2)) play a central role in the low-temperature oxidation of hydrocarbons. In this work, we present high-pressure rate estimation rules for the dissociation, concerted elimination, and isomerization reactions of RO(2). These rate rules are derived from a systematic investigation of sets of reactions within a given reaction class using electronic structure calculations performed at the CBS-QB3 level of theory. The rate constants for the dissociation reactions are obtained from calculated equilibrium constants and a literature review of experimental rate constants for the reverse association reactions. For the concerted elimination and isomerization channels, rate constants are calculated using canonical transition state theory. To determine if the high-pressure rate expressions from this work can directly be used in ignition models, we use the QRRK/MSC method to calculate apparent pressure and temperature dependent rate constants for representative reactions of small, medium, and large alkyl radicals with O(2). A comparison of concentration versus time profiles obtained using either the pressure dependent rate constants or the corresponding high-pressure values reveals that under most conditions relevant to combustion/ignition problems, the high-pressure rate rules can be used directly to describe the reactions of RO(2).

High-pressure rate rules for alkyl + O2 reactions. 2. The isomerization, cyclic ether formation, and β-scission reactions of hydroperoxy alkyl radicals.

The unimolecular reactions of hydroperoxy alkyl radicals (QOOH) play a central role in the low-temperature oxidation of hydrocarbons as they compete with the addition of a second O(2) molecule, which is known to provide chain-branching. In this work we present high-pressure rate estimation rules for the most important unimolecular reactions of the β-, γ-, and δ-QOOH radicals: isomerization to RO(2), cyclic ether formation, and selected β-scission reactions. These rate rules are derived from high-pressure rate constants for a series of reactions of a given reaction class. The individual rate expressions are determined from CBS-QB3 electronic structure calculations combined with canonical transition state theory calculations. Next we use the rate rules, along with previously published rate estimation rules for the reactions of alkyl peroxy radicals (RO(2)), to investigate the potential impact of falloff effects in combustion/ignition kinetic modeling. Pressure effects are examined for the reaction of n-butyl radical with O(2) by comparison of concentration versus time profiles that were obtained using two mechanisms at 10 atm: one that contains pressure-dependent rate constants that are obtained from a QRRK/MSC analysis and another that only contains high-pressure rate expressions. These simulations reveal that under most conditions relevant to combustion/ignition problems, the high-pressure rate rules can be used directly to describe the reactions of RO(2) and QOOH. For the same conditions, we also address whether the various isomers equilibrate during reaction. These results indicate that equilibrium is established between the alkyl, RO(2), and γ- and δ-QOOH radicals.

The lowest singlet and triplet states of the oxyallyl diradical

Small S-T splitting : The photoelectron spectrum of the oxyallyl radical anion (see picture) reveals that the electronic ground state of oxyallyl is singlet, and the lowest triplet state is separated from the singlet state by only (55 ± 2) meV in adiabatic energy.

Reactions of α-Nucleophiles with Alkyl Chlorides: Competition between SN2 and E2 Mechanisms and the Gas-Phase α-Effect

Reaction rate constants and deuterium kinetic isotope effects for the reactions of BrO(-) with RCl (R = methyl, ethyl, isopropyl, and tert-butyl) were measured using a tandem flowing afterglow-selected ion flow tube instrument. These results provide qualitative insight into the competition between two classical organic mechanisms, nucleophilic substitution (S(N)2) and base-induced elimination (E2). As the extent of substitution in the neutral reactants increases, the kinetic isotope effects become increasingly more normal, consistent with the gradual onset of the E2 channel. These results are in excellent agreement with previously reported trends for the analogous reactions of ClO(-) with RCl. [Villano et al. J. Am. Chem. Soc. 2006, 128, 728.] However, the reactions of BrO(-) and ClO(-) with methyl chloride, ethyl chloride, and isopropyl chloride were found to occur by an additional reaction pathway, which has not previously been reported. This reaction likely proceeds initially through a traditional S(N)2 transition state, followed by an elimination step in the S(N)2 product ion-dipole complex. Furthermore, the controversial alpha-nucleophilic character of these two anions and of the HO(2)(-) anion is examined. No enhanced reactivity is displayed. These results suggest that the alpha-effect is not due to an intrinsic property of the anion but instead due to a solvent effect.

Rate Rules, Branching Ratios, and Pressure Dependence of the HO2 + Olefin Addition Channels

In this work, we present high-pressure rate rules and branching ratios for the addition of HO2 to olefins through the concerted addition channel to form an alkyl peroxy radical (HO2 + olefin → RO2) and through the radical addition channel to form a β-hydroperoxy alkyl radical (HO2 + olefin → β-QOOH). These rate rules were developed by calculating rate constants for a series of addition reactions involving olefins with varying degrees of branching. The individual rate expressions were determined from electronic structure calculations performed at the CBS-QB3 level of theory combined with TST calculations. The calculated rate constants were found to be in good agreement with those reported in the literature. Next, we calculated apparent pressure- and temperature-dependent rate constants for HO2 addition to the terminal site of 1-butene using an energy-grained master equation (ME) approach and QRRK calculations with a modified strong collision (MSC) approximation. The two methods gave similar results for both reaction classes. We found that, for the radical addition reaction, the high-pressure limit for the stabilization channel is not reached until unusually high pressures (>1000 atm). Instead, this reaction leads to the direct formation of an oxirane + OH. In general, the results for the major channels are in reasonable agreement with prior theoretical and experimental data. Finally, to explicitly examine the effect of pressure, we compared concentration-time profiles for the reactions of HO2 plus butene in air that were obtained using both high-pressure and pressure-dependent mechanisms at 10 and 100 atm. These simulations showed that, contrary to general expectations, the manifestation of pressure falloff effects in kinetic modeling studies might be more prevalent at increasing pressures. This behavior is attributed to the reaction of β-QOOH with O2, the rate of which increases with increasing pressure of air. This bimolecular reaction competes with the unimolecular reactions of β-QOOH under conditions where falloff effects are important for that channel.

Reactivity–Structure-Based Rate Estimation Rules for Alkyl Radical H Atom Shift and Alkenyl Radical Cycloaddition Reactions

Intramolecular H atom shift reactions of alkyl radicals and cycloaddition reactions of alkenyl radicals are two important reaction classes in hydrocarbon combustion and pyrolysis. In this work, we derive high-pressure rate estimation rules that are based on the results of electronic structure calculations at the CBS-QB3 level of theory combined with transition state theory calculations. The rules for the H atom shift reactions of alkyl radicals cover the 1,2- up to the 1,7-H shifts. The rules for the cycloaddition reactions of alkenyl radicals are for both the endo- and exo-cycloaddition and include the formation of three- to seven-member ring products. The results are in good agreement with available experiment measurements and other theoretical studies. Both types of reactions proceed via cyclic transition state structures. The impact of ring size and substituent groups on pre-exponential factors and activation energies are discussed in the context of a Benson-type structure-reactivity relationship. Similar relationships between the pre-exponential factors and the number of internal rotors lost in formation of the transition state are derived for both H-shift and cycloaddition reactions. The activation energies are found to be more complicated. The ring strain contribution to the barrier is much lower for the exo-cycloaddition reactions than it is for the other two investigated reaction systems. The ring strains for the H-shift and endo-cycloaddition are similar to one another and are comparable to that of cycloalkanes for three- to six-member rings, but are significantly lower for the larger rings. The results suggest that the 1,6-H shift and 1,7-endo-cycloaddition reactions might be faster than previous estimates.

Gas-Phase Carbene Radical Anions: New Mechanistic Insights

The gas-phase reactivity of the CHCl*- anion has been investigated with a series of halomethanes (CCl4, CHCl3, CH2Cl2, and CH3Cl) using a FA-SIFT instrument. Results show that this anion primarily reacts via substitution and by proton transfer. In addition, the reactions of CHCl*- with CHCl3 and CH2Cl2 form minor amounts of Cl2*- and Cl-. The isotopic distribution of these two products is consistent with an insertion-elimination mechanism, where the anion inserts into a C-Cl bond to form an unstable intermediate, which eliminates either Cl2*- or Cl- and Cl*. Neutral and cationic carbenes are known to insert into single bonds; however, this is the first observation of such reactivity for carbene anions.

Selective removal of ethylene, a deposit precursor, from a "dirty" synthesis gas stream via gas-phase partial oxidation.

A fundamental issue in the gasification of biomass is that in addition to the desired synthesis gas product (a mixture of H(2) and CO), the gasifier effluent contains other undesirable products that need to be removed before any further downstream processing can occur. This work assesses the potential to selectively remove hydrocarbons from a synthesis gas stream via gas-phase partial oxidation. Specifically, the partial oxidation of methane-doped, ethylene-doped, and methane/ethylene-doped model synthesis gas mixtures has been investigated at ambient pressures over a temperature range of 760-910 degrees C and at residence times ranging from 0.4 to 2.4 s using a tubular flow reactor. For the synthesis gas mixtures that contain either methane or ethylene, the addition of oxygen substantially reduces the hydrocarbon concentration while only a small reduction in the hydrogen concentration is observed. For the synthesis gas mixtures doped with both methane and ethylene, the addition of oxygen preferentially removes ethylene while the concentrations of methane and hydrogen remain relatively unaffected. These results are compared to the predictions of a plug flow model using a reaction mechanism that is designed to describe the pyrolysis and partial oxidation of small hydrocarbon species. The agreement between the experimental observations and the model predictions is quite good, allowing us to explore the underlying chemistry that leads to the hydrocarbon selective oxidation. The implications of these results are briefly discussed in terms of using synthesis gas to produce liquid fuels and electrical power via a solid oxide fuel cell.

Gas Phase Study of C+ Reactions of Interstellar Relevance

The current uncertainty in many reaction rate constants causes difficulties in providing satisfactory models of interstellar chemistry. Here we present new measurements of the rate constants and product branching ratios for the gas phase reactions of C+ with NH3, CH4, O2, H2O, and C2H2, using the flowing afterglow-selected ion flow tube (FASIFT) technique. Results were obtained using two instruments that were separately calibrated and optimized; in addition, low ionization energies were used to ensure formation of ground-state C+, the purities of the neutral reactants were verified, and mass discrimination was minimized.

Photoelectron spectroscopy and thermochemistry of the peroxyacetate anion.

The 351.1 nm photoelectron spectrum of the peroxyacetate anion, (CH3C(O)OO−) was measured. Analysis of the spectrum shows that the observed spectral features arise almost exclusively from transitions between the trans-conformer of the anion and the X˜22A″ and Ã2A′ states of the corresponding radical. The electron affinity of trans-CH3C(O)OO is 2.381 ± 0.007 eV and the term energy splitting of the Ã2A′ state is 0.691 ± 0.009 eV, in excellent agreement with two prior values [Zalyubovsky et al. J. Phys. Chem. A 107, 7704 (2003); Hu et al. J. Phys. Chem. 124, 114305/1 (2006); Hu et al. J. Phys. Chem. 110, 2629 (2006)]. The gas-phase acidity of trans-peroxyacetic acid was bracketed between the acidity of acetic acid and tert-butylthiol at ΔaG298(trans-CH3C(O)OOH) = 1439 ± 14 kJ mol−1 and ΔaH298(trans-CH3C(O) OOH) = 1467 ± 14 kJ mol−1. The acidity of cis-CH3C(O)OOH was found by adding a calculated energy correction to the acidity of the trans-conformer; ΔaG298[cis-CH3C(O)OOH] = 1461 ± 14 kJ mol−1 and ΔaH298[cis-CH3C(O)OOH] = 1490 ± 14 kJ mol−1. The O–H bond dissociation energies for both conformers were determined using a negative ion thermodynamic cycle to be D0[trans-CH3C(O)OOH] = 381 ± 14 kJ mol−1 and D0[cis-CH3C(O)OOH] = 403±14 kJ mol−1. The atmospheric implications of these results and relations to the thermochemistry of peroxyacetyl nitrate are discussed briefly.