Theoretical and kinetic study of the H-atom abstraction reactions by Ḣ atom from alkyl cyclohexanes

Reaction kinetics of hydrogen atom abstraction from six alkyl cyclohexanes, methyl cyclohexane (MCH), ethyl cyclohexane (ECH), n-propyl cyclohexane (nPCH), iso-propyl cyclohexane (iPCH), sec-butyl cyclohexane (sBCH) and iso-butyl cyclohexane (iBCH), by the Ḣ atom are systematically studied in this work. The M06-2X method combined with the 6-311++G(d,p) basis set is used to perform geometry optimization, frequency analysis and zero-point energy calculations for all species. The intrinsic reaction coordinate (IRC) calculations are performed to confirm the transition states connecting the reactants and products correctly. One-dimensional hindered rotors are used to treat the low frequency torsional models with potentials scanned at the M06-2X/6-31G level of theory. Electronic single-point energy calculations for all reactants, transition states, and products are performed at the QCISD(T)/CBS level of theory. High-pressure limiting rate constants of 39 reaction channels are obtained using conventional transition state theory with asymmetric Eckart tunneling corrections in the temperature range 298.15-2000 K. Reaction rate rules for H-atom abstraction by the Ḣ atom from alkyl cyclohexanes on primary, secondary and tertiary carbon sites on both the side chain and ring are provided. The obtained rate constants are given by the Arrhenius expression in the temperature range 500-2000 K, which can be used for the combustion kinetics model development for alkyl cyclohexanes.

Atmospheric chemistry of ketones: Reaction of OH radicals with 2-methyl-3-pentanone, 3-methyl-2-pentanone and 4-methyl-2-pentanone

This work reports new kinetic and mechanistic information on the atmospheric chemistry of ketones. Both absolute and relative rate methods were used to determine the rate constants for OH reactions with 2-methyl-3-pentanone (2M3P), 3-methyl-2-pentanone (3M2P) and 4-methyl-2-pentanone (4M2P), three widely used compounds in the industry. This work constitutes the first temperature dependence study of the reactions of OH with 2M3P and 3M2P. The following rate constants values are recommended at 298 K (in 10^-12 cm³ molecule^-1 s^-1): k_OH+2M3P = 3.49 ± 0.5; k_OH+3M2P = 6.02 ± 0.14 and k_OH+4M2P = 11.02 ± 0.42. The following Arrhenius expressions (in units of cm³ molecule^-1 s^-1) adequately describe the measured rate constants for OH reactions with 2M3P and 3M2P in the temperature range 263-373 K: k_2M3P = (2.33 ± 0.06) × 10^-12 exp((127.4 ± 18.6)/T) and k_3M2P = (1.05 ± 0.14) × 10^-12 exp((537 ± 41)/T). Products studies from the reactions of OH with the investigated ketones were conducted in a 7.3 m³ simulation chamber using PTR-ToF-MS, UHPLC-MS and GC-MS. A series of short chain carbonyl compounds including formaldehyde, acetone, acetaldehyde, 2-butanone and 2-methypropanal were observed as products. Combining the yields of carbonyls measured with those estimated from the SAR method, we propose various mechanistic degradation schemes of the investigated ketones initiated by reaction with OH radicals.

Inhibition of hydrocarbon autoxidation by nitroxide-catalyzed cross-dismutation of hydroperoxyl and alkylperoxyl radicals

Nitroxides are putative intermediates in the accepted reaction mechanisms of the diarylamine and hindered amine antioxidants that are universally added to preserve synthetic and natural hydrocarbon-based materials. New methodology which enables monitoring of hydrocarbon autoxidations at low rates of radical generation has revealed that diarylnitroxides and hindered nitroxides are far better inhibitors of unsaturated hydrocarbon autoxidation than their precursor amines, implying intervention of a different mechanism. Experimental and computational investigations suggest that the nitroxides catalyze the cross-dismutation of hydroperoxyl and alkylperoxyl radicals to yield O2 and a hydroperoxide, thereby halting the autoxidation chain reaction. The hydroperoxyl radicals - key players in hydrocarbon combustion, but essentially unknown in autoxidation - are proposed to derive from a tunneling-enhanced intramolecular (1,4-) hydrogen-atom transfer/elimination sequence from oxygenated radical addition intermediates. These insights suggest that nitroxides are preferred additives for the protection of hydrocarbon-based materials from autoxidation since they exhibit catalytic activity under conditions where their precursor amines are less effective and/or inefficiently converted to nitroxides in situ.

Formation of furan along with HO₂ during the OH-initiated oxidation of 2,5-DHF and 2,3-DHF: an experimental and computational study

Experimental characterization of products during OH-initiated oxidation of dihydrofurans (DHF) confirms the formation of furan accompanied by the formation of HO2 to be a significant channel in 2,5-DHF (21 ± 3%), whereas it is absent in 2,3-DHF. Theoretical investigations on the reaction of OH with these molecules are carried out to understand this difference. All possible channels of reaction are studied at M06-2X level with 6-311G* basis set, and the stationary points on the potential energy surface are optimized. The overall rate coefficients calculated using conventional TST with Wigner tunneling correction for 2,5-DHF and 2,3-DHF are 2.25 × 10(-11) and 4.13 × 10(-10) cm(3) molecule(-1) s(-1), respectively, in the same range as the previously determined experimental values. The branching ratios of different channels were estimated using the computed rate coefficients. The abstraction of H atom, leading to dihydrofuranyl radical, is found to be a significant probability, equally important as the addition of OH to the double bond in the case of 2,5-DHF. However, this probability is very small in the case of 2,3-DHF because the rate coefficient of the addition reaction is more than 10 times that of the abstraction reaction. This explains the conspicuous absence of furan among the products of the reaction of OH with 2,3-DHF. The calculations also indicate that the abstraction reaction, and hence furan formation, may become significant for OH-initiated oxidation of 2,3-DHF at temperatures relevant to combustion.

Atmospheric chemistry of C3-C6 cycloalkanecarbaldehydes

The rate coefficients for the gas phase reaction of NO3 and OH radicals with a series of cycloalkanecarbaldehydes have been measured in purified air at 298 +/- 2 K and 760 +/- 10 Torr by the relative rate method using a static reactor equipped with long-path Fourier transform infrared (FT-IR) detection. The values obtained for the OH radical reactions (in units of 10(-11) cm3 molecule(-1) s(-1)) were the following: cyclopropanecarbaldehyde, 2.13 +/- 0.05; cyclobutanecarbaldehyde, 2.66 +/- 0.06; cyclopentanecarbaldehyde, 3.27 +/- 0.07; cyclohexanecarbaldehyde, 3.75 +/- 0.05. The values obtained for the NO3 radical reactions (in units of 10(-14) cm3 molecule(-1) s(-1)) were the following: cyclopropanecarbaldehyde, 0.61 +/- 0.04; cyclobutanecarbaldehyde, 1.99 +/- 0.06; cyclopentanecarbaldehyde, 2.55 +/- 0.10; cyclohexanecarbaldehyde, 3.19 +/- 0.12. Furthermore, the reaction products with OH radicals have been investigated using long-path FT-IR spectroscopy and proton-transfer-reaction mass spectrometry (PTR-MS). The measured carbon balances were in the range 89-97%, and the identified products cover a wide spectrum of compounds including nitroperoxycarbonyl cycloalkanes, cycloketones, cycloalkyl nitrates, multifunctional compounds containing carbonyl, hydroxy, and nitrooxy functional groups, HCOOH, HCHO, CO, and CO2.

Structure−Activity Relationship for the Addition of OH to (Poly)alkenes: Site-Specific and Total Rate Constants

A novel site-specific structure-activity relationship was developed for the site-specific addition of OH radicals to (poly)alkenes at 298 K. From a detailed structure-activity analysis of some 65 known OH + alkene and diene reactions, it appears that the total rate constant for this reaction class can be closely approximated by a sum of independent partial rate constants, ki, for addition to the specific (double-bonded) C atoms that depend only on the stability type of the ensuing radical (primary, secondary, etc.), that is, on the number of substituents on the neighboring C atom in the double bond. The (nine) independent partial rate constants, ki, were derived, and the predicted rate constants (kOH,pred = Sigmak(i)) were compared with experimental k(OH,exp) values. For noncyclic (poly)alkenes, including conjugated structures, the agreement is excellent (Delta < 10%). The SAR-predicted rate constants for cyclic (poly)alkenes are in general also within <15% of the experimental value. On the basis of this SAR, it is possible to predict the site-specific rate constants for (poly)alkene + OH reactions accurately, including larger biogenic compounds such as isoprene and terpenes. An important section is devoted to the rigorous experimental validation of the SAR predictions against direct measurements of the site-specific addition contributions within the alkene, for monoalkenes as well as conjugated alkenes. The measured site specificities are within 10-15% of the SAR predictions.

Can topological indices be used to predict gas-phase rate coefficients of importance to tropospheric chemistry? Reactions of alkenes with OH, NO3 and O3

Rate coefficients for the gas-phase reactions of unsaturated hydrocarbons with OH, NO(3) and O(3) are of vital importance to atmospheric modelling. Many of these rate coefficients are unknown--possibly resulting from the variety of these compounds and the inherent expense of deriving these data experimentally--and a reliable method for their prediction would therefore be of value to such models. This study presents a method for estimating rate coefficients for C(2)-C(10) olefins. Measured rate coefficients for the reaction of unsaturated hydrocarbons with OH and NO(3) radicals and O(3) are correlated with the Randić topological descriptor and an established correlation parameter, i.e. ionization potentials calculated using the frontier molecular orbital (FMO) approach. Although the ionization potential method produces better correlations in general, OH correlations of aliphatic species, subdivided into groups of varying total number of primary (1 degree), secondary (2 degrees) and tertiary (3 degrees) carbon atoms (Sigma x degrees (x)) produced several new trends with the Randić index that were not present in the ionization potential correlations. Strong correlation was observed with a Randić-type index optimized to include a term for side chain length (Sigma x degrees (x)). The presence of strong relationships, made predictable by molecular constitution, makes topological descriptors a useful and accessible tool for estimating rate coefficients for the reactions of OH with aliphatic species. This study indicates that alkene reactivity is affected strongly by constitution and that abstraction as well as addition may be important for some classes of compound.