Cyclopentyl Radicals Research Articles

Product Identification in the Low-Temperature Oxidation of Cyclohexane Using a Jet-Stirred Reactor in Combination with SVUV-PEPICO Analysis and Theoretical Quantum Calculations.

Cyclohexane oxidation chemistry was investigated using a near-atmospheric pressure jet-stirred reactor at T = 570 K and equivalence ratio ϕ = 0.8. Numerous intermediates including hydroperoxides and highly oxygenated molecules were detected using synchrotron vacuum ultraviolet photoelectron photoion coincidence spectroscopy. Supported by high-level quantum calculations, the analysis of photoelectron spectra allowed the firm identification of molecular species formed during the oxidation of cyclohexane. Besides, this work validates recently published gas chromatography and synchrotron vacuum ultraviolet photoionization mass spectrometry data. Unambiguous detection of characteristic hydroperoxides (e.g., γ-ketohydroperoxides) and their respective decomposition products provides support for the conventional O2 addition channels up to the third addition and their relative contribution to the cyclohexane oxidation. The results were also compared with the predictions of a recently proposed new detailed kinetic model of cyclohexane oxidation. Most of the predictions are in line with the current experimental findings, highlighting the robustness of the kinetic model. However, the analysis of the recorded slow photoelectron spectra indicating the possible presence of C5 species in the kinetic model provides hints that the substituted cyclopentyl radicals from cyclohexyl ring opening might play a minor role in cyclohexane oxidation. Potentially important missing reactions are also discussed.

High-resolution CH stretch spectroscopy of jet-cooled cyclopentyl radical: First insights into equilibrium structure, out-of-plane puckering, and IVR dynamics.

First, high-resolution sub-Doppler infrared spectroscopic results for cyclopentyl radical (C5H9) are reported on the α-CH stretch fundamental with suppression of spectral congestion achieved by adiabatic cooling to Trot ≈ 19(4) K in a slit jet expansion. Surprisingly, cyclopentyl radical exhibits a rotationally assignable infrared spectrum, despite 3N - 6 = 36 vibrational modes and an upper vibrational state density (ρ ≈ 40-90 #/cm-1) in the critical regime (ρ ≈ 100 #/cm-1) necessary for onset of intramolecular vibrational relaxation (IVR) dynamics. Such high-resolution data for cyclopentyl radical permit detailed fits to a rigid-rotor asymmetric top Hamiltonian, initial structural information for ground and vibrationally excited states, and opportunities for detailed comparison with theoretical predictions. Specifically, high level ab initio calculations at the coupled-cluster singles, doubles, and perturbative triples (CCSD(T))/ANO0, 1 level are used to calculate an out-of-plane bending potential, which reveals a C2 symmetry double minimum 1D energy surface over a C2v transition state. The inversion barrier [Vbarrier ≈ 3.7(1) kcal/mol] is much larger than the effective moment of inertia for out-of-plane bending, resulting in localization of the cyclopentyl wavefunction near its C2 symmetry equilibrium geometry and tunneling splittings for the ground state too small (<1MHz) to be resolved under sub-Doppler slit jet conditions. The persistence of fully resolved high-resolution infrared spectroscopy for such large cyclic polyatomic radicals at high vibrational state densities suggests a "deceleration" of IVR for a cycloalkane ring topology, much as low frequency torsion/methyl rotation degrees of freedom have demonstrated a corresponding "acceleration" of IVR processes in linear hydrocarbons.

Developing the Low-Temperature Oxidation Mechanism of Cyclopentane: An Experimental and Theoretical Study.

At present, the reactivity of cyclic alkanes is estimated by comparison with acyclic hydrocarbons. Due to the difference in the structure of cycloalkanes and acycloalkanes, the thermodynamic data obtained by analogy are not applicable. In this study, a molecular beam sampling vacuum ultraviolet photoionization time-of-flight mass spectrometer (MB-VUV-PI-TOFMS) was applied to study the low-temperature oxidation of cyclopentane (CPT) at a total pressure range from 1-3 atm and low-temperature range between 500 and 800 K. Low-temperature reaction products including cyclic olefins, cyclic ethers, and highly oxygenated intermediates (e. g., ketohydroperoxide KHP, keto-dihydroperoxide KDHP, olefinic hydroperoxides OHP and ketone structure products) were observed. Further investigation of the oxidation of CPT - electronic structure calculations - were carried out at the UCCSD(T)-F12a/aug-cc-pVDZ//B3LYP/6-31+ G(d,p) level to explore the reactivity of O2 molecules adding sequentially to cyclopentyl radicals. Experimental and theoretical observations showed that the dominant product channel in the reaction of CPT radicals with O2 is HO2 elimination yielding cyclopentene. The pathways of second and third O2 addition - the dissociation of hydroperoxide - were further confirmed. The results of this study will develop the low-temperature oxidation mechanism of CPT, which can be used for future research on accurately simulating the combustion process of CPT.

Reaction Pathways, Thermodynamics, and Kinetics of Cyclopentanone Oxidation Intermediates: A Theoretical Approach.

Despite the promising role of cyclopentanone as a bioderived fuel, thermodynamic and kinetic data are lacking for low temperature oxidation regimes. In this study, ab initio calculations at the CBS-QB3 level explore the subsequent reactivity that results from O2-addition to 2- and 3-oxo cyclopentyl radicals, including expected reaction classes such as intra-H migration, HO2-elimination, cyclic ether formation, and β-scission along with their thermodynamic parameters. Some of the rates are similar to the analogous reactions of cyclopentane, but some other reactions of cyclopentanone are very different. The carbonyl group hinders H-migration from the α' position but promotes HO2-elimination. Enol peroxy formation from some hydroperoxy alkyl radicals of cyclopentanone is unexpectedly important, and so is HO2-elimination by β-scission. Our calculations also indicated that at engine relevant conditions the α-RO2 prefers to go back to the reactants 2-oxo cyclopentyl radical and O2. Therefore, the reactions resulting from HO2-addition to 2-oxo cyclopentyl are also provided. The lowest barrier channel identified on the singlet surface corresponds to an unexpected intra OH-migration path concerted with ring opening. This valuable information will advance the construction of improved kinetic models for the oxidation of cylopentanone.

A Shock Tube Study of H Atom Addition to Cyclopentene

ABSTRACTWe report shock tube studies of the kinetics of H atom addition to cyclopentene and modeling of the subsequent decomposition of cyclopentyl. Hydrogen atoms were generated with thermal precursors in dilute mixtures of cyclopentene and a reference compound in argon. Addition of H to the double bond leads to a cyclopentyl radical that rapidly ring opens and decomposes to ethene and allyl radical. The process was monitored by postshock gas chromatographic analysis of ethene and rate constants determined relative to H atom displacement of methyl from 1,3,5‐trimethylbenzene (135TMB). At 863–1167 K and 160–370 kPa, we find urn:x-wiley:05388066:kin21150:equation:kin21150-math-0001 and, with , we obtain urn:x-wiley:05388066:kin21150:equation:kin21150-math-0003Using experimental values of about 3:1 for the ratio of C─C to C─H beta scission in cyclopentyl radicals and a corresponding transition‐state‐theory/Rice‐Ramsberger‐Kassel‐Marcus (TST/RRKM) model, the high‐pressure rate expression for addition of H to cyclopentene at 863–1167 K is derived as urn:x-wiley:05388066:kin21150:equation:kin21150-math-0004Combined with literature results from lower temperatures and a fitted TST model, the rate expression between 298 and 2000 K is determined as urn:x-wiley:05388066:kin21150:equation:kin21150-math-0005Results are compared with related systems. Near 1000 K, our data require a minimum value of 1.5 for branching between beta C─C and C─H scission in cyclopentyl radicals to maintain established trends in H addition rates. This conflicts with current computed values and those used in existing kinetics models of cyclopentane combustion. We additionally report and discuss minor observed channels in the decomposition of cyclopentene, including formation of 1,4‐pentadiene, (E/Z)‐1,3‐pentadiene, 1,3‐butadiene, and the direct elimination of H2 from cyclopentene to give cyclopentadiene.

Group additive modeling of cyclopentane pyrolysis

The pyrolysis of cyclopentane is not well established although it is an abundant compound in typical naphtha feedstocks and can be considered a model compound for cyclic fuels. The studies in literature so far have focused primarily on the initial decomposition of cyclopentane in shock tubes. This article therefore explores the pyrolysis of cyclopentane in a continuous flow tubular reactor with pure cyclopentane feed at reactor conditions 0.17MPa, 973–1073K, and a residence time of 0.5s. Conversions of 5% to 75% were realized while the product concentrations were quantified using two dimensional gas chromatography. A mechanism composed of elementary high pressure limit reactions has been generated using the automatic network generation tool “Genesys”. Kinetics of the reactions originate from high level ab-initio calculations and new group additive values derived from ab-initio kinetic data in literature. Overall the Genesys model outperforms the models available in literature and there is a good agreement between model calculated mass fraction profiles and experimental data for 22 products ranging from hydrogen to naphthalene without any adjustments of the kinetic parameters. Reaction path analysis reveals that cyclopentane consumption is initiated by the unimolecular isomerization to 1-pentene, but overall dominated by hydrogen abstraction reactions by allyl radicals and hydrogen atoms to give cyclopentyl radicals, whose ring opening and further scissions lead to smaller molecules. Dominant routes for the major products are discussed.

Elucidating reactivity regimes in cyclopentane oxidation: Jet stirred reactor experiments, computational chemistry, and kinetic modeling

This study is concerned with the identification and quantification of species generated during the combustion of cyclopentane in a jet stirred reactor (JSR). Experiments were carried out for temperatures between 740 and 1250K, equivalence ratios from 0.5 to 3.0, and at an operating pressure of 10atm. The fuel concentration was kept at 0.1% and the residence time of the fuel/O2/N2 mixture was maintained at 0.7s. The reactant, product, and intermediate species concentration profiles were measured using gas chromatography and Fourier transform infrared spectroscopy. The concentration profiles of cyclopentane indicate inhibition of reactivity between 850–1000K for φ = 2.0 and φ = 3.0. This behavior is interesting, as it has not been observed previously for other fuel molecules, cyclic or non-cyclic. A kinetic model including both low- and high-temperature reaction pathways was developed and used to simulate the JSR experiments. The pressure-dependent rate coefficients of all relevant reactions lying on the PES of cyclopentyl+O2, as well as the C–C and C–H scission reactions of the cyclopentyl radical were calculated at the UCCSD(T)-F12b/cc-pVTZ-F12//M06-2X/6-311++G(d,p) level of theory. The simulations reproduced the unique reactivity trend of cyclopentane and the measured concentration profiles of intermediate and product species. Sensitivity and reaction path analyses indicate that this reactivity trend may be attributed to differences in the reactivity of allyl radical at different conditions, and it is highly sensitive to the C–C/C–H scission branching ratio of the cyclopentyl radical decomposition.

Shock tube study of the decomposition of cyclopentyl radicals

The kinetics of the thermal decomposition of cyclopentyl radicals has been studied in a single-pulse shock tube over a temperature range of 950–1116K and pressures of 2.3–3.4bar. Cyclopentyl radicals were generated by abstraction of H from cyclopentane by H atoms, which were produced by the thermal decomposition of hexamethylethane. Products were analyzed using dual-column gas chromatography under cryogenic conditions with simultaneous flame-ionization and mass-spectrometric detection. The main stable products resulting from the decomposition of cyclopentyl radicals are ethene and cyclopentene. These results are consistent with the interconversion of cyclopentyl and pent-4-en-1-yl radicals. The product branching ratio of ethene to cyclopentene is determined as [ethene]/[cyclopentene]=(8.2±0.7)exp[−(1067±88)/T), where the listed uncertainty (1σ) is from the statistical analysis. The overall accuracy in our measurement of the branching ratio is estimated at about 8% (1σ). In contrast to previous estimates, ring opening of cyclopentyl is found to be faster than ejection of a hydrogen atom. The former reaction is favored by about a factor of three. An RRKM/Master Equation treatment was adopted to assign the rate parameters to elementary reactions. Details of the model and its compatibility with the present and literature results are presented. Kinetic results for the pyrolysis of cyclopentyl and pent-4-en-1-yl radicals have been derived over relevant combustion conditions [0.1–1000bar, 700–1900K].

Substituent Effects on the Rearrangements of Cyclohexyl to Cyclopentyl Radicals Involving Avermectin-Related Radicals

The rearrangement of a substituted cyclohexyl radical to a cyclopentylmethyl radical on the skeleton of avermectin B1 has been investigated using density functional (UB3LYP/6-31G(d)) and G3MP2B3 computational methods. The rearrangement is preferred when highly radical stabilizing groups are present at the 2- and 3-positions of the cyclohexyl radical. A substituent on the 3-position of the cyclohexyl radical enables ring-cleavage of the cyclohexyl radical, while a radical stabilizing substituent on the 2-position of the cyclohexyl radical stabilizes the final cyclopentylmethyl radical, enabling the overall rearrangement and reversing the normal thermodynamic preference for the hexenyl radical ring closure.

A Study of Vinyl Radical Cyclization Using N-Alkenyl-7-bromo-Substituted Hexahydroindolinones

A new method for the synthesis of the octahydropyrrolo[3,2,1-ij]quinoline ring system that possesses the characteristic skeleton of the aspidosperma family of alkaloids has been developed. The method utilizes an intramolecular Diels-Alder reaction of an amido-substituted furan across a tethered indole pi-bond. To apply this strategy to the synthesis of the indole alkaloid spegazzinidine, it was necessary to address the problem of assembling the final D-ring of the pentacyclic skeleton. Radical cyclization of a model N-allyl-7-bromo-3a-methylhexahydroindolinone system was found to preferentially lead to the 6-endo-trig cyclization product, with the best yield being obtained under high dilution conditions. The six-membered cyclized product is generated through two reaction pathways: (a) 6-endo-trig ring closure and (b) rearrangement of an intermediate methylene-cyclopentyl radical obtained by 5-exo-trig cyclization. A number of related 7-bromo-substituted hexahydroindolinones containing tethered olefinic groups were prepared and found to undergo efficient cyclization under both radical and palladium-mediated reaction conditions. Vinyl radical cyclization with several N-butenyl-substituted systems afforded a mixture of 6-exo and 7-endo cyclization products. A protocol to introduce an ethyl substituent into the C20-position of the aspidospermidine skeleton was also developed.

Infrared Frequency-Modulation Probing of Product Formation in Alkyl + O2 Reactions: III. The Reaction of Cyclopentyl Radical (c-C5H9) with O2 between 296 and 723 K

The production of HO2 from the reaction of c-C5H9 + O2 has been investigated as a function of temperature (296−723 K) by using laser photolysis/CW infrared frequency modulation spectroscopy. The HO2 yield is derived by comparison with the Cl2/CH3OH/O2 system and is corrected to account for HO2 signal loss due to competing reactions involving HO2 radical and the adduct c-C5H9O2. The time behavior of the HO2 signal following cyclopentyl radical formation displays two separate components. The first component is a prompt production of HO2, which increases with temperature and is the only component observed between 296 and 500 K. The yield from the prompt production rises from less than 1% at 296 K to ∼23% at 693 K. At temperatures above 500 K a second slower rise in the HO2 signal is also observed. The production of HO2 on a slower time scale is attributable to cyclopentylperoxy radical decomposition. The total HO2 yield, including the contribution from the slower rise, increases dramatically with temperature from ∼2% at 500 K to ∼100% at 683 K. From 683 to 723 K the total HO2 yield remains constant. The second slower rise accounts for a majority of the product formation at these higher temperatures. The biexponential time behavior of the HO2 production from c-C5H9 + O2 is similar to that previously observed in studies of C2H5 + O2 and C3H7 + O2 reactions. The rate of formation for delayed HO2 production from c-C5H9 + O2 is larger than the rate of formation from either C2H5 + O2 or C3H7 + O2 at each temperature. However, apparent activation energies, obtained by an Arrhenius plot of the rates of formation for delayed HO2 formation, are very similar for the three systems (C2H5 + O2, C3H7 + O2, and c-C5H9 + O2). The results suggest a similar coupled mechanism for HO2 production in the C2H5 + O2, C3H7 + O2, and c-C5H9 + O2 reactions, with concerted elimination of HO2 from the RO2 radical responsible for HO2 + alkene production.

Iodine as a radical scavenger in the radiolysis of cyclopentane

Scavenging techniques with iodine in the concentration range of 0.1 to 30 mM were used to elucidate the radical yields in the γ-radiolysis of cyclopentane. In solutions below about 1 mM iodine reacts solely with alkyl radicals in the bulk of the medium. However, above this concentration the scavenging of thermal hydrogen atoms becomes significant and virtually all of the hydrogen atoms are scavenged at an iodine concentration of 30 mM. High concentrations of iodine are also found to reduce the cyclopentene yield, probably by reaction with ionic species produced as intermediates in cyclopentane radiolysis. At low iodine concentrations the yield of the main scavenging product, cyclopentyl iodide, and thereby the yield of the scavenged cyclopentyl radicals that escape the spur into the bulk medium was found to be 4.0 radicals/100 eV. The total cyclopentyl radical yield derived from the observed k d/ k c ratio and the bicyclopentyl yield is estimated to be 4.9 radicals/100 eV.

Synthesis and Reactivity of Cross-Conjugated Polyenones with a Planar Chirality

The complexes 6 are the first examples of a novel class of cross conjugated polyenones bearing a planar chirality introduced by an organometallic moiety. Their synthesis is described using, as a key intermediate, the easily accessible new phosphorane 9. The free double bond of the selectively protected polyenes 6 reacts efficiently, although with low diastereoselectivities, in Diels−Alder or 1,3 dipolar cycloaddition reactions or with nucleophiles. Cyclopentyl radical also adds to 6, and this is the first example of a radical reaction in the presence of carbonyliron complexes. Decomplexation of the various adducts leads, in good yields, to the corresponding polyfunctionalized free dienes, which can be of further synthetic use.

Spectral and kinetic characteristics of cycloalkyl radicals in the liquid phase

The C5−C10 cyclolakyl radicals have a weak light absorption in the 240–300 nm wavelength range that is due to Rydberg transition to the 3s orbital. The extinction coefficients at 250 nm are in the range of 350–900 mol−1 dm3 cm−1. At this wavelength for the C6−C10 radicals a local maximum appears. The radical decay obeys second order kinetics. The kinetic characteristics of the cyclic and linear radicals are generally similar, indicating that the rings are flexible and can easily overcome steric constraints in the termination process. Both the light absorption and decay characteristics of the cyclopentyl radical are somewhat different from those of the other radicals that are attributed to the special co-planar arrangement.

Heavy-Ion Radiolysis of Cyclooctane

The radiolysis of neat and iodine solutions of cyclopentane has been performed with 1-15 MeV protons, 5-20 MeV helium ions, 12-31 MeV carbon ions, and 22-32 MeV oxygen ions. The yields of the major products cyclopentene, bicyclopentyl, 1-pentene, and cyclopentyl iodide as well as the yields of a few select minor products have been determined. All products show a decrease in yield with increasing linear energy transfer (LET) of the irradiating particle. Some of these decreases are very large: for instance, the observed drop in the yield of bicyclopentyl from {gamma}-rays (LET = 0.14 eV/nm) to oxygen ions (LET = 900 eV/nm) amounts to a net loss of about 4 cyclopentyl radicals/100 eV absorbed energy. This value is 80% of the total cyclopentyl radicals produced in {gamma}-radiolysis. There are some indications that the yields of radicals due to decomposition of excited cyclopentane molecules are dependent on the energy and type of irradiating particle. 24 refs., 6 figs., 4 tabs.

Radical Reactions in the Radiolysis of Cyclopentane

The end products produced in the [gamma]-radiolysis of cyclopentane have been measured at very low total doses (25-50 krad). Iodine scavenging techniques in solutions of 0.1-30 mM were used to elucidate radical yields and reaction mechanisms. The yields of the main radical species were found to be as follows: cyclopentyl, 4.9; 1-pentyl, 0.2; 3-cyclopentenyl, 0.07; H atom, 1.3 radical/100 eV. The change in yields from neat cyclopentane to 0.1 mM iodine solution suggests that about 79% of the cyclopentyl radicals escape the spur and react in the bulk medium with a disproportionation to combination ratio of 0.97. Radical precursors account for about 50% of the total end product yield, which is much smaller than found in the radiolysis of cyclohexane or cyclooctane. The radiolysis mechanism for cyclopentane is discussed and compared to those for cyclohexane and cyclooctane. 41 refs., 5 figs., 2 tabs.

Addition of cyclopentane to slowly reacting mixtures of H2+ O2 between 673 and 783 K: reactions of H and OH with cyclopentane and of cyclopentyl radicals

A number of elementary reactions involved in the oxidation of cyclopentane (CP) have been studied by adding CP to H2–O2 mixtures over the temperature range 673–783 K. Kinetic studies of the relative rates of consumption of CP and H2 have given values of k21=(1.11 ± 0.25)× 1010 and k22=(1.84 ± 0.32)× 109 dm3 mol–1 s–1 at 753 K. Combination with independent data and critical analysis gives k21= 3.65 × 102T2.5 exp(519/T) dm3 mol–1 s–1, which is recommended between 250 and 1500 K, with error limits of 50 Torr, very little ring rupture has been observed in the initial products. The formation of C2H4 has been discussed in detail, and a value of k32= 1013.14 ± 0.55 exp(– 17260/T) s–1 has been obtained over the temperature range 580–783 K. c-C5H9→ CH2= CHCH2CH2CH2(32) The cyclisation of C3, C4, C5 and C6 alken-1-yl radicals has been discussed and it has been concluded that the predominant reaction of hex-5-en-1-yl radicals between 600 and 1300 K and at 1 atm O2 pressure is cyclisation giving c-hexyl radicals. As the latter rapidly undergo successive oxidation to cyclohexene, cyclohexadiene and then benzene, it has been further concluded that such C6 cyclisations could be major sources of benzene and substituted benzenes in the oxidation of conventional fuels.

Internal motion of the cyclopentyl radical in CF 2ClCFCl 2: An ESR investigation

The internal motion of the cyclopentyl radical, trapped in a halocarbon matrix, CF 2ClCFCl 2, has been studied utilizing electron spin resonance (ESR) spectroscopy. The lineshapes of the ESR spectra corresponding to the radical exhibit an alternating line-width effect in the temperature range 110–135 K. The experimental ESR spectra were simulated using a dynamical model based on interconversion between two twisted C 2 structures and the activation energy was estimated to 2.8 kcal/mol.

Hyperfine and Structural Isotope Effects in Muonated Cyclohexadienyl and Cyclopentyl Radicals

AbstractIncorporation of muonium, the ultra‐light isotope of hydrogen with a mass of only one‐ninth the mass of H, leads to pronounced isotope effects in hyperfine coupling constants of radicals. They reflect the mass dependence of the internal dynamics. A detailed analysis is presented for the cyclohexadienyl and the cyclopentyl radicals. It is concluded that the isotope dependent bond length in an anharmonic potential accounts quantitatively for the observed effects in rigid species.

Comparison of radiolysis of cyclopentane, 2,3-dimethylbutane, and neopentane at 4.2 K with that at 77 K as studied by analysis of dimer products

The γ-radiolysis of cyclopentane, 2,3-dimethylbutane, and neopentane at 4.2 K was compared with that at 77 K by analysis of dimer products with capillary gas chromatography. In the radiolysis of cyclopentane, the yield of bicyclopentyl dimer at 4.2 K is lower than that at 77 K. The difference of the yields at two temperatures was explained in terms of disproportionation reaction of cyclopentyl radicals at very low temperature. In the radiolysis of 2,3-dimethylbutane, the distribution of dimer products at 4.2 K is different from that at 77 K. The results is due to the effect of phase change between two temperatures. In the radiolysis of 2,3-dimethylbutane, the fraction of unsaturated dimers in all dimer products at 77 K is much larger than that at 4.2 K. The olefinic dimer-products at 77 K is related to favorable formation of olefinic cations at 77 K. In the radiolysis of solid neopentane, addition of helium gas promotes the formation of 2,2,3,3-tetramethylbutane by a non-radical process.