Reaction of OH with Aliphatic and Aromatic Isocyanates

Isocyanates are highly relevant industrial intermediates for polyurethane production. In this work, we used quantum chemistry and transition state theory (TST) to investigate the gas-phase reaction of isocyanates with the OH radical, which is likely one of the most significant chemical sinks for these compounds in the troposphere. para-Tolyl-isocyanate (p-tolyl-NCO) was chosen as a proxy substance for the large-volume aromatic diisocyanate species toluene diisocyanate and methylene diphenyl diisocyanate. Besides p-tolyl-NCO + OH, the model reactions CH3NCO + OH, H2CCHNCO + OH, C6H5-NCO + OH, C6H5-CH3 + OH, and C6H6 + OH have been studied as well to analyze various substituent effects and to allow for comparison with literature. Quantum chemical computations at the CCSD(T)/cc-pV(T,Q → ∞)Z//M06-2X/def2-TZVP level were used as the basis for tunneling-corrected canonical TST calculations. For CH3NCO + OH, H abstraction by OH at the methyl group is the main reaction channel according to our calculations and predicted to be four orders of magnitude faster than OH addition at the NCO group. The calculated rate coefficient (8.8 × 10–14 cm3 molecule–1 s–1) at 298 K is in good agreement with experimental data from the literature. Likewise, for aromatic isocyanates, OH attack at the isocyanate group was found to be only a minor pathway compared to addition to the aromatic ring. In the OH + p-tolyl-NCO reaction, OH addition at the ortho-position relative to the NCO group has been identified as the main initial reaction channel (branching fraction: 53.2%), with smaller but significant branching fractions for the H abstraction at the methyl group (9.6%) as well as the other ring addition reactions (ipso: 2.3%, meta: 24.5%, para: 10.5%, all relative to the NCO group). By comparing OH addition to the aromatic ring in p-tolyl-NCO with the respective ring addition reactions of phenyl isocyanate and toluene, the site-selective reactivity trends observed for ring addition in the OH + p-tolyl-NCO could be rationalized by a dominating positive mesomeric effect of the NCO group and a positive electron-donating (inductive) effect of the CH3 group. Except for the OH ring adduct formed from OH addition in ipso-position to the NCO group, we estimate the first-generation radical intermediates in the OH + p-tolyl-NCO reaction to have sufficiently long lifetimes to react with O2 under atmospheric conditions and undergo typical oxidative reaction cascades like those known for benzene or toluene

Synchrotron Photoionization Measurements of OH-Initiated Cyclohexene Oxidation: Ring-Preserving Products in OH + Cyclohexene and Hydroxycyclohexyl + O₂ Reactions

Earlier synchrotron photoionization mass spectrometry experiments suggested a prominent ring-opening channel in the OH-initiated oxidation of cyclohexene, based on comparison of product photoionization spectra with calculated spectra of possible isomers. The present work re-examines the OH + cyclohexene reaction, measuring the isomeric products of OH-initiated oxidation of partially and fully deuterated cyclohexene. In particular, the directly measured photoionization spectrum of 2-cyclohexen-1-ol differs substantially from the previously calculated Franck–Condon envelope, and the product spectrum can be fit with no contribution from ring-opening. Measurements of H₂O₂ photolysis in the presence of C₆D₁₀ establish that the addition–elimination product incorporates the hydrogen atom from the hydroxyl radical reactant and loses a hydrogen (a D atom in this case) from the ring. Investigation of OH + cyclohexene-4,4,5,5-<i>d</i>₄ confirms this result and allows mass discrimination of different abstraction pathways. Products of 2-hydroxycyclohexyl-<i>d</i>₁₀ reaction with O₂ are observed upon adding a large excess of O₂ to the OH + C₆D₁₀ system

Facile Rearrangement of 3‑Oxoalkyl Radicals is Evident in Low-Temperature Gas-Phase Oxidation of Ketones

The pulsed photolytic chlorine-initiated oxidation of methyl-<i>tert</i>-butyl ketone (MTbuK), di-<i>tert</i>-butyl ketone (DTbuK), and a series of partially deuterated diethyl ketones (DEK) is studied in the gas phase at 8 Torr and 550–650 K. Products are monitored as a function of reaction time, mass, and photoionization energy using multiplexed photoionization mass spectrometry with tunable synchrotron ionizing radiation. The results establish that the primary 3-oxoalkyl radicals of those ketones, formed by abstraction of a hydrogen atom from the carbon atom in γ-position relative to the carbonyl oxygen, undergo a rapid rearrangement resulting in an effective 1,2-acyl group migration, similar to that in a Dowd–Beckwith ring expansion. Without this rearrangement, peroxy radicals derived from MTbuK and DTbuK cannot undergo HO₂ elimination to yield a closed-shell unsaturated hydrocarbon coproduct. However, not only are these coproducts observed, but they represent the dominant oxidation channels of these ketones under the conditions of this study. For MTbuK and DTbuK, the rearrangement yields a more stable tertiary radical, which provides the thermodynamic driving force for this reaction. Even in the absence of such a driving force in the oxidation of partially deuterated DEK, the 1,2-acyl group migration is observed. Quantum chemical (CBS-QB3) calculations show the barrier for gas-phase rearrangement to be on the order of 10 kcal mol^–1. The MTbuK oxidation experiments also show several minor channels, including β-scission of the initial radicals and cyclic ether formation

Synchrotron Photoionization Mass Spectrometry Measurements of Product Formation in Low-Temperature <i>n</i>‑Butane Oxidation: Toward a Fundamental Understanding of Autoignition Chemistry and <i>n</i>‑C₄H₉ + O₂/<i>s</i>‑C₄H₉ + O₂ Reactions

Product formation in the laser-initiated low-temperature (575–700 K) oxidation of <i>n</i>-butane was investigated by using tunable synchrotron photoionization time-of-flight mass spectrometry at low pressure (∼4 Torr). Oxidation was triggered either by 351 nm photolysis of Cl₂ and subsequent fast Cl + <i>n</i>-butane reaction or by 248 nm photolysis of 1-iodobutane or 2-iodobutane. Iodobutane photolysis allowed isomer-specific preparation of either <i>n</i>-C₄H₉ or <i>s</i>-C₄H₉ radicals. Experiments probed the time-resolved formation of products and identified isomeric species by their photoionization spectra. For stable primary products of butyl + O₂ reactions (e.g., butene or oxygen heterocycles) bimodal time behavior is observed; the initial prompt formation, primarily due to chemical activation, is followed by a slower component arising from the dissociation of thermalized butylperoxy or hydroperoxybutyl radicals. In addition, time-resolved formation of C₄-ketohydroperoxides (C₄H₈O₃, <i>m</i>/<i>z</i> = 104) was observed in the <i>n</i>-C₄H₉ + O₂ and Cl-initiated oxidation experiments but not in the <i>s</i>-C₄H₉ + O₂ measurements, suggesting isomeric selectivity in the combined process of the “second” oxygen addition to hydroperoxybutyl radicals and subsequent internal H-abstraction/dissociation leading to ketohydroperoxide + OH. To further constrain product isomer identification, Cl-initiated oxidation experiments were also performed with partially deuterated <i>n</i>-butanes (CD₃CH₂CH₂CD₃ and CH₃CD₂CD₂CH₃). From these experiments, the relative yields of butene product isomers (<i>cis</i>-2-butene, <i>trans</i>-2-butene, and 1-butene) from C₄H₈ + HO₂ reaction channels and oxygenated product isomers (2,3-dimethyloxirane, 2-methyloxetane, tetrahydrofuran, ethyloxirane, butanal, and butanone) associated with OH formation were determined. The current measurements show substantially different isomeric selectivity for oxygenated products than do recent jet-stirred reactor studies but are in reasonable agreement with measurements from butane addition to reacting H₂/O₂ mixtures at 753 K

Unconventional Peroxy Chemistry in Alcohol Oxidation: The Water Elimination Pathway

Predictive simulation for designing efficient engines requires detailed modeling of combustion chemistry, for which the possibility of unknown pathways is a continual concern. Here, we characterize a low-lying water elimination pathway from key hydroperoxyalkyl (QOOH) radicals derived from alcohols. The corresponding saddle-point structure involves the interaction of radical and zwitterionic electronic states. This interaction presents extreme difficulties for electronic structure characterizations, but we demonstrate that these properties of this saddle point can be well captured by M06-2X and CCSD­(T) methods. Experimental evidence for the existence and relevance of this pathway is shown in recently reported data on the low-temperature oxidation of isopentanol and isobutanol. In these systems, water elimination is a major pathway, and is likely ubiquitous in low-temperature alcohol oxidation. These findings will substantially alter current alcohol oxidation mechanisms. Moreover, the methods described will be useful for the more general phenomenon of interacting radical and zwitterionic states

Time- and Isomer-Resolved Measurements of Sequential Addition of Acetylene to the Propargyl Radical

Soot formation in combustion is a complex process in which polycyclic aromatic hydrocarbons (PAHs) are believed to play a critical role. Recent works concluded that three consecutive additions of acetylene (C₂H₂) to propargyl (C₃H₃) create a facile route to the PAH indene (C₉H₈). However, the isomeric forms of C₅H₅ and C₇H₇ intermediates in this reaction sequence are not known. We directly investigate these intermediates using time- and isomer-resolved experiments. Both the resonance stabilized vinylpropargyl (<i>vp</i>-C₅H₅) and 2,4-cyclopentadienyl (<i>c</i>-C₅H₅) radical isomers of C₅H₅ are produced, with substantially different intensities at 800 K vs 1000 K. In agreement with literature master equation calculations, we find that <i>c</i>-C₅H₅ + C₂H₂ produces only the tropyl isomer of C₇H₇ (<i>tp</i>-C₇H₇) below 1000 K, and that <i>tp</i>-C₇H₇ + C₂H₂ terminates the reaction sequence yielding C₉H₈ (indene) + H. This work demonstrates a pathway for PAH formation that does not proceed through benzene

Facile Rearrangement of 3‑Oxoalkyl Radicals is Evident in Low-Temperature Gas-Phase Oxidation of Ketones

The pulsed photolytic chlorine-initiated oxidation of methyl-<i>tert</i>-butyl ketone (MTbuK), di-<i>tert</i>-butyl ketone (DTbuK), and a series of partially deuterated diethyl ketones (DEK) is studied in the gas phase at 8 Torr and 550–650 K. Products are monitored as a function of reaction time, mass, and photoionization energy using multiplexed photoionization mass spectrometry with tunable synchrotron ionizing radiation. The results establish that the primary 3-oxoalkyl radicals of those ketones, formed by abstraction of a hydrogen atom from the carbon atom in γ-position relative to the carbonyl oxygen, undergo a rapid rearrangement resulting in an effective 1,2-acyl group migration, similar to that in a Dowd–Beckwith ring expansion. Without this rearrangement, peroxy radicals derived from MTbuK and DTbuK cannot undergo HO₂ elimination to yield a closed-shell unsaturated hydrocarbon coproduct. However, not only are these coproducts observed, but they represent the dominant oxidation channels of these ketones under the conditions of this study. For MTbuK and DTbuK, the rearrangement yields a more stable tertiary radical, which provides the thermodynamic driving force for this reaction. Even in the absence of such a driving force in the oxidation of partially deuterated DEK, the 1,2-acyl group migration is observed. Quantum chemical (CBS-QB3) calculations show the barrier for gas-phase rearrangement to be on the order of 10 kcal mol^–1. The MTbuK oxidation experiments also show several minor channels, including β-scission of the initial radicals and cyclic ether formation

Low-Temperature Combustion Chemistry of <i>n-</i>Butanol: Principal Oxidation Pathways of Hydroxybutyl Radicals

Reactions of hydroxybutyl radicals with O₂ were investigated by a combination of quantum-chemical calculations and experimental measurements of product formation. In pulsed-photolytic Cl-initiated oxidation of <i>n</i>-butanol, the time-resolved and isomer-specific product concentrations were probed using multiplexed tunable synchrotron photoionization mass spectrometry (MPIMS). The interpretation of the experimental data is underpinned by potential energy surfaces for the reactions of O₂ with the four hydroxybutyl isomers (1-hydroxybut-1-yl, 1-hydroxybut-2-yl, 4-hydroxybut-2-yl, and 4-hydroxybut-1-yl) calculated at the CBS-QB3 and RQCISD(T)/cc-pV∞Z//B3LYP/6-311++G­(d,p) levels of theory. The observed product yields display substantial temperature dependence, arising from a competition among three fundamental pathways: (1) stabilization of hydroxybutylperoxy radicals, (2) bimolecular product formation in the hydroxybutyl + O₂ reactions, and (3) decomposition of hydroxybutyl radicals. The 1-hydroxybut-1-yl + O₂ reaction is dominated by direct HO₂ elimination from the corresponding peroxy radical forming butanal as the stable coproduct. The chemistry of the other three hydroxybutylperoxy radical isomers mainly proceeds via alcohol-specific internal H-atom abstractions involving the H atom from either the −OH group or from the carbon attached to the −OH group. We observe evidence of the recently reported water elimination pathway (Welz et al. <i>J. Phys. Chem. Lett.</i> <b>2013</b>, <i>4</i> (3), 350–354) from the 4-hydroxybut-2-yl + O₂ reaction, supporting its importance in γ-hydroxyalkyl + O₂ reactions. Experiments using the 1,1-<i>d</i>₂ and 4,4,4-<i>d</i>₃ isotopologues of <i>n</i>-butanol suggest the presence of yet unexplored pathways to acetaldehyde

Isomer Specific Product Detection in the Reaction of CH with Acrolein

The products formed in the reaction between the methylidene radical (CH) and acrolein (CH₂CHCHO) are probed at 4 Torr and 298 K employing tunable vacuum-ultraviolet synchrotron light and multiplexed photoionization mass-spectrometry. The data suggest a principal exit channel of H loss from the adduct to yield C₄H₄O, accounting for (78 ± 10)% of the products. Examination of the photoionization spectra measured upon reaction of both CH and CD with acrolein reveals that the isomeric composition of the C₄H₄O product is (60 ± 12)% 1,3-butadienal and (17 ± 10)% furan. The remaining 23% of the possible C₄H₄O products cannot be accurately distinguished without more reliable photoionization spectra of the possible product isomers but most likely involves oxygenated butyne species. In addition, C₂H₂O and C₃H₄ are detected, which account for (14 ± 10)% and (8 +10, −8)% of the products, respectively. The C₂H₂O photoionization spectrum matches that of ketene and the C₃H₄ signal is composed of (24 ± 14)% allene and (76 ± 22)% propyne, with an upper limit of 8% placed on the cyclopropene contribution. The reactive potential energy surface is also investigated computationally, and specific rate coefficients are calculated with RRKM theory. These calculations predict overall branching fractions for 1,3-butadienal and furan of 27% and 12%, respectively, in agreement with the experimental results. In contrast, the calculations predict a prominent CO + 2-methylvinyl product channel that is at most a minor channel according to the experimental results. Studies with the CD radical strongly suggest that the title reaction proceeds predominantly via cycloaddition of the radical onto the CO bond of acrolein, with cycloaddition to the CC bond being the second most probable reactive mechanism

VUV Photoionization Cross Sections of HO₂, H₂O₂, and H₂CO

The absolute vacuum ultraviolet (VUV) photoionization spectra of the hydroperoxyl radical (HO₂), hydrogen peroxide (H₂O₂), and formaldehyde (H₂CO) have been measured from their first ionization thresholds to 12.008 eV. HO₂, H₂O₂, and H₂CO were generated from the oxidation of methanol initiated by pulsed-laser-photolysis of Cl₂ in a low-pressure slow flow reactor. Reactants, intermediates, and products were detected by time-resolved multiplexed synchrotron photoionization mass spectrometry. Absolute concentrations were obtained from the time-dependent photoion signals by modeling the kinetics of the methanol oxidation chemistry. Photoionization cross sections were determined at several photon energies relative to the cross section of methanol, which was in turn determined relative to that of propene. These measurements were used to place relative photoionization spectra of HO₂, H₂O₂, and H₂CO on an absolute scale, resulting in absolute photoionization spectra