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Stephen J Klippenstein

Publications and source records attributed to Stephen J Klippenstein.

At least 19 recordsLinked to original sources
PubMed2006

Energy-resolved photoionization of alkylperoxy radicals and the stability of their cations.

The photoionization of alkylperoxy radicals has been investigated using a newly developed experimental apparatus that combines the tunability of the vacuum ultraviolet radiation of the Advanced Light Source at Lawrence Berkeley National Laboratory with time-resolved mass spectrometry. Methylperoxy (CH(3)OO) and ethylperoxy (C(2)H(5)OO) radicals are produced by the reaction of pulsed, photolytically produced alkyl radicals with molecular oxygen, and the mass spectrum of the reacting mixture is monitored in time by using synchrotron-photoionization with a double-focusing mass spectrometer. The kinetics of product formation is used to confirm the origins and assignments of ionized species. The photoionization efficiency curve for CH(3)OO has been measured, and an adiabatic ionization energy of (10.33 +/- 0.05) eV was determined with the aid of Franck-Condon spectral simulations, including ionization to the lowest triplet and singlet cation states. Using the appearance energy of CH(3)(+) from CH(3)OO, an enthalpy of formation for CH(3)OO of Delta(f) (CH(3)OO) = (22.4 +/- 5) kJ mol(-1) is derived. The enthalpy of formation of CH(3)OO(+) is derived as Delta(f) = (1019 +/- 7) kJ mol(-1) and the CH(3)(+)-OO bond energy as (CH(3)(+) - O(2)) = (80 +/- 7) kJ mol(-1). The C(2)H(5)OO(+) signal is not detectable; however, the time profile of the ethyl cation signal suggests its formation from dissociative ionization of C(2)H(5)OO. Electronic structure calculations suggest that hyperconjugation reduces the stability of the ethylperoxy cation, making the C(2)H(5)OO(+) ground state only slightly bound with respect to the ground-state products, C(2)H(5)(+) and O(2). The value of the measured appearance energy of C(2)H(5)(+) is consistent with dissociative ionization of C(2)H(5)OO via the Franck-Condon favored ionization to the ã (1)A' state of C(2)H(5)OO(+).

Journal Article↗
PubMed2006

Master equation methods in gas phase chemical kinetics.

In this article, we discuss the application of master equation methods to problems in gas phase chemical kinetics. The focus is on reactions that take place over multiple, interconnected potential wells and on the dissociation of weakly bound free radicals. These problems are of paramount importance in combustion chemistry. To illustrate specific points, we draw on our experience with reactions we have studied previously.

Journal Article↗
PubMed2006

Reaction of ethylene with hydroxyl radicals: a theoretical study.

Ab initio calculations of portions of the C2H5O potential energy surface critical to the title reaction are presented. These calculations are based on QCISD geometries and frequencies and RQCISD(T) energies extrapolated to the complete-basis-set limit. Rate coefficients for the reaction of C2H4 with OH are calculated using this surface and the two transition-state model of Greenwald and co-workers [J. Phys. Chem. A 2005, 109, 6031] for the association of OH with C2H4. The present calculations reproduce most of the experimental data, including the temperature and pressure dependence of the rate coefficients, with only a small (0.4 kcal/mol) adjustment to the energy barrier for direct hydrogen abstraction. We confirm the importance of this channel above 800 K and find that a significant fraction of the total rate coefficient (approximately 10%) is due to the formation of vinyl alcohol above this temperature. Calculations of the vinyl alcohol channel are consistent with the recent observation of this molecule in low-pressure flames [Taatjes, C. A.; Hansen, N.; McIlroy, A.; Miller, J. A.; Senosiain, J. P.; Klippenstein, S. J.; Qi, F.; Sheng, L.; Zhang, Y.; Cool, T. A.; Wang, J.; Westmoreland, P. R.; Law, M. E.; Kasper, T.; Kohse-Höinghaus, K. Science 2005, 308, 1887] and suggest that this reaction should be included in hydrocarbon oxidation mechanisms.

Journal Article↗
PubMed2006

Pathways and rate coefficients for the decomposition of vinoxy and acetyl radicals.

The potential in the vicinity of the stationary points on the surface for the decomposition of ground-state vinoxy and acetyl radicals has been calculated using the RQCISD(T) method extrapolated to the infinite-basis set limit. Rate coefficients for the decomposition pathways of these two radicals were computed using the master equation and variational transition state theory. Agreement between our calculated rate coefficients for H + CH(2)CO <--> CH(3) + CO and experimental data is very good, without the need for empirical adjustments to the ab initio energy barriers. Multireference configuration-interaction calculations indicate two competitive channels for vinoxy decomposition, with the channel leading to H + CH(2)CO being preferred at photodissociation energies. However, at typical combustion conditions, vinoxy decomposes primarily to CO and methyl. In contrast, decomposition of acetyl shows only one decomposition channel, leading to CO and methyl. The implications of a low-lying exit channel for the calculation of theoretical rate coefficients are discussed briefly.

Journal Article↗
PubMed2006

Identification of C5Hx isomers in fuel-rich flames by photoionization mass spectrometry and electronic structure calculations.

The isomeric composition of C(5)H(x) (x = 2-6, 8) flame species is analyzed for rich flames fueled by allene, propyne, cyclopentene, or benzene. Different isomers are identified by their known ionization energies and/or by comparison of the observed photoionization efficiencies with theoretical simulations based on calculated ionization energies and Franck-Condon factors. The experiments combine flame-sampling molecular-beam mass spectrometry with photoionization by tunable vacuum-UV synchrotron radiation. The theoretical simulations employ the rovibrational properties obtained with B3LYP/6-311++G(d,p) density functional theory and electronic energies obtained from QCISD(T) electronic structure calculations extrapolated to the complete basis set limit. For C(5)H(3), the comparison reveals the presence of both the H(2)CCCCCH (i-C(5)H(3)) and the HCCCHCCH (n-C(5)H(3)) isomer. The simulations also suggest a modest amount of cyclo-CCHCHCCH-, which is consistent with a minor signal for C(5)H(2) that is apparently due to cyclo-CCHCCCH-. For C(5)H(4), contributions from the CH(2)CCCCH(2) (1,2,3,4-pentatetraene), CH(2)CCHCCH, and CH(3)CCCCH (1,3-pentadiyne) isomers are evident, as is some contribution from CHCCH(2)CCH (1,4-pentadiyne) in the cyclopentene and benzene flames. Signal at m/z = 65 originates mainly from the cyclopentadienyl radical. For C(5)H(6), contributions from cyclopentadiene, CH(3)CCCHCH(2), CH(3)CHCHCCH, and CH(2)CHCH(2)CCH are observed. No signal is observed for C(5)H(7) species. Cyclopentene, CH(2)CHCHCHCH(3) (1,3-pentadiene), CH(3)CCCH(2)CH(3) (2-pentyne), and CH(2)CHCH(2)CHCH(2) (1,4-pentadiene) contribute to the signal at m/z = 68. Newly derived ionization energies for i- and n-C(5)H(3) (8.20 +/- 0.05 and 8.31 +/- 0.05 eV, respectively), CH(2)CCHCCH (9.22 +/- 0.05 eV), and CH(2)CHCH(2)CCH (9.95 +/- 0.05 eV) are within the error bars of the QCISD(T) calculations. The combustion chemistry of the observed C(5)H(x) intermediates and the impact on flame chemistry models are discussed.

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PubMed2006

Identification and chemistry of C4H3 and C4H5 isomers in fuel-rich flames.

Quantitative identification of isomers of hydrocarbon radicals in flames is critical to understanding soot formation. Isomers of C4H3 and C4H5 in flames fueled by allene, propyne, cyclopentene, or benzene are identified by comparison of the observed photoionization efficiencies with theoretical simulations based on calculated ionization energies and Franck-Condon factors. The experiments combine molecular-beam mass spectrometry (MBMS) with photoionization by tunable vacuum-ultraviolet synchrotron radiation. The theoretical simulations employ the rovibrational properties obtained with B3LYP/6-311++G(d,p) density functional theory and electronic energies obtained from QCISD(T) ab initio calculations extrapolated to the complete basis set limit. For C4H3, the comparisons reveal the presence of the resonantly stabilized CH2CCCH isomer (i-C4H3). For C4H5, contributions from the CH2CHCCH2 (i-C4H5) and some combination of the CH3CCCH2 and CH3CHCCH isomers are evident. Quantitative concentration estimates for these species are made for allene, cyclopentene, and benzene flames. Because of low Franck-Condon factors, sensitivity to n-isomers of both C4H3 and C4H5 is limited. Adiabatic ionization energies, as obtained from fits of the theoretical predictions to the experimental photoionization efficiency curves, are within the error bars of the QCISD(T) calculations. For i-C4H3 and i-C4H5, these fitted adiabatic ionization energies are (8.06 +/- 0.05) eV and (7.60 +/- 0.05) eV, respectively. The good agreement between the fitted and theoretical ionization thresholds suggests that the corresponding theoretically predicted radical heats of formation (119.1, 76.3, 78.7, and 79.1 kcal/mol at 0 K for i-C4H3, i-C4H5, CH3CCCH2, and CH3CHCCH, respectively) are also quite accurate.

Carbon Compounds, Inorganic↗
PubMed2006

Predictive theory for the combination kinetics of two alkyl radicals.

An ab initio transition state theory based procedure for accurately predicting the combination kinetics of two alkyl radicals is described. This procedure employs direct evaluations of the orientation dependent interaction energies at the CASPT2/cc-pvdz level within variable reaction coordinate transition state theory (VRC-TST). One-dimensional corrections to these energies are obtained from CAS+1+2/aug-cc-pvtz calculations for CH3 + CH3 along its combination reaction path. Direct CAS+1+2/aug-cc-pvtz calculations demonstrate that, at least for the purpose of predicting the kinetics, the corrected CASPT2/cc-pvdz potential energy surface is an accurate approximation to the CAS+1+2/aug-cc-pvtz surface. Furthermore, direct trajectory simulations, performed at the B3LYP/6-31G* level, indicate that there is little local recrossing of the optimal VRC transition state dividing surface. The corrected CASPT2/cc-pvdz potential is employed in obtaining direct VRC-TST kinetic predictions for the self and cross combinations of methyl, ethyl, iso-propyl, and tert-butyl radicals. Comparisons with experiment suggest that the present dynamically corrected VRC-TST approach provides quantitatively accurate predictions for the capture rate. Each additional methyl substituent adjacent to a radical site is found to reduce the rate coefficient by about a factor of two. In each instance, the rate coefficients are predicted to decrease quite substantially with increasing temperature, with the more sterically hindered reactants having a more rapid decrease. The simple geometric mean rule, relating the capture rate for the cross reaction to those for the self-reactions, is in remarkably good agreement with the more detailed predictions. With suitable generalizations the present approach should be applicable to a wide array of radical-radical combination reactions.

Journal Article↗
PubMed2005

The reaction of acetylene with hydroxyl radicals.

The potential energy surface for the reaction between OH and acetylene has been calculated using the RQCISD(T) method and extrapolated to the complete basis-set limit. Rate coefficients were determined for a wide range of temperatures and pressures, based on this surface and the solution of the one-dimensional and two-dimensional master equations. With a small adjustment to the association energy barrier (1.1 kcal/mol), agreement with experiments is good, considering the discrepancies in such data. The rate coefficient for direct hydrogen abstraction is significantly smaller than that commonly used in combustion models. Also in contrast to previous models, ketene + H is found to be the main product at normal combustion conditions. At low temperatures and high pressures, stabilization of the C2H2OH adduct is the dominant process. Rate coefficient expressions for use in modeling are provided.

Journal Article↗
PubMed2005

A two transition state model for radical-molecule reactions: a case study of the addition of OH to C2H4.

A two transition state model is applied to the study of the addition of hydroxyl radical to ethylene. This reaction serves as a prototypical example of a radical-molecule reaction with a negative activation energy in the high-pressure limit. The model incorporates variational treatments of both inner and outer transition states. The outer transition state is treated with a recently derived long-range transition state theory approach focusing on the longest-ranged term in the potential. High-level quantum chemical estimates are incorporated in a variational transition state theory treatment of the inner transition state. Anharmonic effects in the inner transition state region are explored with direct phase space integration. A two-dimensional master equation is employed in treating the pressure dependence of the addition process. An accurate treatment of the two separate transition state regions at the energy and angular momentum resolved level is essential to the prediction of the temperature dependence of the addition rate. The transition from a dominant outer transition state to a dominant inner transition state is predicted to occur at about 130 K, with significant effects from both transition states over the 10 to 400 K temperature range. Modest adjustment in the ab initio predicted inner saddle point energy yields theoretical predictions which are in quantitative agreement with the available experimental observations. The theoretically predicted capture rate is reproduced to within 10% by the expression [4.93 x 10(-12) (T/298)(-2.488) exp(-107.9/RT) + 3.33 x 10(-12) (T/298)(0.451) exp(117.6/RT); with R = 1.987 and T in K] cm3 molecules(-1) s(-1) over the 10-600 K range.

Journal Article↗
PubMed2005

Channel specific rate constants relevant to the thermal decomposition of disilane.

Rate constants for the thermal dissociation of Si2H6 are predicted with a novel transition state model. The saddle points for dissociation on the Si2H6 potential energy surface are lower in energy than the corresponding separated products, as confirmed by high level ab initio quantum mechanical calculations. Thus, the dissociations of Si2H6 to produce SiH2 + SiH4 (R1) and H3SiSiH + H2 (R2) both proceed through tight inner transition states followed by loose outer transition states. The present "dual" transition state model couples variational phase space theory treatments of the outer transition states with ab initio based fixed harmonic vibrator treatments of the inner transition states to obtain effective numbers of states for the two transition states acting in series. It is found that, at least near room temperature, such a dual transition state model is generally required for the proper description of each of the dissociations. Only at quite high temperatures, i.e., above 2000 K for (R1) and 600 K for (R2), does a single fixed inner transition state provide an adequate description. Similarly, only at quite low temperatures (below 100 and 10 K for (R1) and (R2), respectively) does a single outer transition state provide an adequate description. Pressure dependent rate constants are obtained from solutions to the multichannel master equation. These calculations confirm that dissociation channel (R2) is negligible under conditions relevant to the thermal chemical vapor deposition (CVD) processes. Rate constants for the chemical activation reactions, SiH2 + SiH4 --> Si2H6 (R-1) and SiH2 + SiH4 --> H3SiSiH + H2 (R3), are also evaluated within the dual transition state model. It is found that reaction R3 is the dominant channel for low pressures and high temperatures, i.e., below 100 Torr for temperatures above 1100 K.

Journal Article↗
PubMed2005

The vinyl + NO reaction: determining the products with time-resolved Fourier transform spectroscopy.

We have studied the vinyl + NO reaction using time-resolved Fourier transform emission spectroscopy, complemented by electronic structure and microcanonical RRKM rate coefficient calculations. To unambiguously determine the reaction products, three precursors are used to produce the vinyl radical by laser photolysis: vinyl bromide, methyl vinyl ketone, and vinyl iodide. The emission spectra and theoretical calculations indicate that HCN + CH2O is the only significant product channel for the C2H3 + NO reaction near room temperature, in contradiction to several reports in the literature. Although CO emission is observed when vinyl bromide is used as the precursor, it arises from the reaction of NO with photofragments other than vinyl. This conclusion is supported by the absence of CO emission when vinyl iodide or methyl vinyl ketone is used. Prompt emission from vibrationally excited NO is evidence of the competition between back dissociation and isomerization of the initially formed nitrosoethylene adduct, consistent with previous work on the pressure dependence of this reaction. Our calculations indicate that production of products is dominated by the low energy portion of the energy distribution. The calculation also predicts an upper bound of 0.19% for the branching ratio of the H2CNH + CO channel, which is consistent with our experimental results.

Journal Article↗
PubMed2005

Predictive theory for hydrogen atom-hydrocarbon radical association kinetics.

Procedures for accurately predicting the kinetics of hydrogen atom associations with hydrocarbon radicals are described and applied to a series of reactions. The approach is based on CASPT2/cc-pvdz evaluations of the orientation-dependent interaction energies within variable reaction coordinate transition state theory. One-dimensional corrections to the interaction energies are estimated from CAS+1+2/aug-cc-pvtz evaluations for the H + CH3 reaction, and a dynamical correction factor of 0.9 is applied. This corrected CASPT2 approach yields results that are within 10% of those obtained with the full CAS+1+2/aug-cc-pvtz potential for the H + CH3, H + C2H5, H + C2H3, and H + C2H reactions. New predictions are made for the H + iso-C3H7, H + tert-C4H9, H + C6H5, and H + C10H7 reactions. For the H + CH3 and H + C2H3 reactions, where the experimental values appear to be the most well-determined, theory and experiment essentially agree to within their error bars. For the other reactions, the agreement is reasonably satisfactory given the often large dispersion in the experimental results. For the reactions with saturated alkyl radicals, the theory predicts that each additional CH3 group increases the steric factor by approximately a factor of 2. In contrast, for the unsaturated radicals, the H + C6H5 and H + C10H7 high-pressure association rate coefficients are nearly identical to that for H + C2H3.

Journal Article↗
PubMed2005

The addition of hydrogen atoms to diacetylene and the heats of formation of i-C4H3 and n-C4H3.

In this article, we discuss in detail the addition of hydrogen atoms to diacetylene and the reverse dissociation reactions, H + C(4)H(2)<==>i-C(4)H(3) (R1) and H + C(4)H(2)<==>n-C(4)H(3) (R2). The theory utilizes high-level electronic structure methodology to characterize the potential energy surface, Rice-Ramsperger-Kassel-Marcus (RRKM) theory to calculate microcanonical/J-resolved rate coefficients, and a two-dimensional master-equation approach to extract phenomenological (thermal) rate coefficients. Comparison is made with experimental results where they are available. The rate coefficients k1(T, p) and k2(T, p) are cast in forms that can be used in chemical kinetic modeling. In addition, we predict values of the heats of formation of i-C(4)H(3) and n-C(4)H(3) and discuss their importance in flame chemistry. Our basis-set extrapolated, quadratic-configuration-interaction with single and double excitations (and triple excitations added perturbatively), QCISD(T), predictions of these heats of formation at 298 K are 130.8 kcal/mol for n-C(4)H(3) and 119.3 kcal/mol for the i-isomer; multireference CI calculations with a nine-electron, nine-orbital, complete-active-space (CAS) reference wavefunction give just slightly larger values for these parameters. Our results are in good agreement with the recent focal-point analysis of Wheeler et al. (J. Chem. Phys. 2004, 121, 8800-8813), but they differ substantially for DeltaH0(f 298)(n-C(4)H(3)) with the earlier diffusion Monte Carlo predictions of Krokidis et al. (Int. J. Chem. Kinet.2001, 33, 808-820).

Journal Article↗
PubMed2005

Long-range transition state theory.

The implementation of variational transition state theory (VTST) for long-range asymptotic potential forms is considered, with particular emphasis on the energy and total angular momentum resolved (microJ-VTST) implementation. A long-range transition state approximation yields a remarkably simple and universal description of the kinetics of reactions governed by long-range interactions. The resulting (microJ-VTST) implementation is shown to yield capture-rate coefficients that compare favorably with those from trajectory simulations (deviating by less than 10%) for a wide variety of neutral and ionic long-range potential forms. Simple analytic results are derived for many of these cases. A brief comparison with a variety of low-temperature experimental studies illustrates the power of this approach as an analysis tool. The present VTST approach allows for a simple analysis of the applicability conditions for some related theoretical approaches. It also provides an estimate of the temperature or energy at which the "long-range transition state" moves to such short separations that short-range effects, such as chemical bonding, steric repulsion, and electronic state selectivity, must be considered.

Journal Article↗
PubMed2005

Enols are common intermediates in hydrocarbon oxidation.

Models for chemical mechanisms of hydrocarbon oxidation rely on spectrometric identification of molecular structures in flames. Carbonyl (keto) compounds are well-established combustion intermediates. However, their less-stable enol tautomers, bearing OH groups adjacent to carbon-carbon double bonds, are not included in standard models. We observed substantial quantities of two-, three-, and four-carbon enols by photoionization mass spectrometry of flames burning representative compounds from modern fuel blends. Concentration profiles demonstrate that enol flame chemistry cannot be accounted for purely by keto-enol tautomerization. Currently accepted hydrocarbon oxidation mechanisms will likely require revision to explain the formation and reactivity of these unexpected compounds.

Journal Article↗
PubMed2005

Measurements and modeling of HO2 formation in the reactions of n-C3H7 and i-C3H7 radicals with O2.

The formation of HO(2) in the reactions of C(2)H(5), n-C(3)H(7), and i-C(3)H(7) radicals with O(2) is investigated using the technique of laser photolysis/long-path frequency-modulation spectroscopy. The alkyl radicals are formed by 266 nm photolysis of alkyl iodides. The formation of HO(2) from the subsequent reaction of the alkyl radicals with O(2) is followed by infrared frequency-modulation spectroscopy. The concentration of I atoms is simultaneously monitored by direct absorption of a second laser probe on the spin-orbit transition. The measured profiles are compared to a kinetic model taken from time-resolved master-equation results based on previously published ab initio characterizations of the relevant stationary points on the potential-energy surface. The ab initio energies are adjusted to produce agreement with the present experimental data and with available literature studies. The isomer specificity of the present results enables refinement of the model for i-C(3)H(7) + O(2) and improved agreement with experimental measurements of HO(2) production in propane oxidation.

Journal Article↗
PubMed2004

Vibrational spectroscopy and density functional theory of transition-metal ion-benzene and dibenzene complexes in the gas phase.

Metal-benzene complexes of the form M(benzene)(n) (M = Ti, V, Fe, Co, Ni) are produced in the gas-phase environment of a molecular beam by laser vaporization in a pulsed nozzle cluster source. These complexes are photoionized with an ArF excimer laser, producing the corresponding cations. The respective mono- and dibenzene complex ions are isolated in an ion-trap mass spectrometer and studied with infrared resonance enhanced multiple-photon dissociation (IR-REMPD) spectroscopy using a tunable free electron laser. Photodissociation of all complexes occurs by the elimination of intact neutral benzene molecules, and this process is enhanced on resonances in the vibrational spectrum, making it possible to measure vibrational spectroscopy for size-selected complexes. Vibrational bands in the 600-1700 cm(-1) region are characteristic of the benzene molecular moiety with systematic shifts caused by the metal bonding. The spectra in this solvent-free environment exhibit periodic trends in band shifts and intensities relative to the free benzene molecule that varies with the metal. Density functional theory calculations are employed to investigate the structures, energetics, and vibrational frequencies of these complexes. The comparison between experiment and theory provides fascinating new insight into the bonding in these prototypical organometallic complexes.

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