Mechanism of oxidation of cyclohexane by the radical cation of pyridine N-Oxide

Oxidation of pyridine N-oxide in the presence of cyclohexane was studied on platinum and carbon glass rotating disk electrodes using the method of cyclic voltammetry in 0.2 N LiClO₄ in acetonitrile. It was hypothesized that the electrochemically generated N-oxide radical cation reacts with cyclohexane according to the mechanism of hydrogen atom cleavage.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1488–1491, July, 1990.     

The structure and decay dynamics of exciplexes derived from dibenzoyl-methanatoboron difluoride and alkylbenzenes in cyclohexane

Dibenzomethanatoboron difluoride (DBMBF2) interacted with alkylbenzenes from its singlet excited state to form exciplexes ranging from weak polarity up to contact radical ion pairs (CRIP); this exciplex series shows the characteristics in the Marcus "normal" region. In cyclohexane these exciplexes gave intense fluorescence spectra and high quantum yields (phi(infinity)ex). The dipole moment of these exciplexes calculated from the solvatochromic shift of the fluorescence maximums (vmax) was used to estimate the coefficient ("a" and "c") of the CT and LE terms in the exciplex wavefunction. On the basis of the measured lifetimes and phi(infinity)ex of these exciplexes, the radiative (k(ex)f) and nonradiative (k(ex)NR) rate constants were calculated. The former k(ex)f were also computed from a semi-empirical approach based on the assumption that the exciplex wavefunction could be adequately described by CT and LE states, and that *DBMBF2 primarily contributes to the probability of exciplex emission. Two results agree with each other with small systematic deviations for those less polar exciplexes. The plots of k(ex)r and k(ex)NR (or their logarithmic value) against the LE contribution (c2) and transition energy gaps (hvmax) afford better correlation than those against -deltaG(-et). This indicates the role played by the LE contribution in generating the stabilization energy (U(s)) in these exciplexes through the A-D+)<==> *AD) resonance interaction; U(s), in turn, modifies -deltaG(-et) to afford the decay driving force hvmax. Also, those plots from k(ex)f values (being determined directly from experiments) show better correlation than those from k(ex)NR. In contrast to the CRIP type exciplexes in the Marcus "inverted" region, these k(ex)f and k(ex)NR increase in the common trend with increasing transition energy gaps. The k(ex)NR plots show less steep slopes and attains more quickly a minimum toward the CRIP region; the latter is identified as the turning point from the "normal" to "inverted" region. Both the attenuation and reversal of the k(ex)NR value with increasing polarity are believed to be generated by the emerging contribution of the intersystem crossing process as an additional nonradiative process, which is induced by the increased spin-orbit coupling in highly polar exciplexes.     

The decay of 1-chloropropyne cation studied by photoelectron-photoion coincidence spectroscopy

The decay of internal energy selected 1-chloropropyne cations is investigated using the fixed wavelength (He-Iα) photoelectron-photoion coincidence technique. The breakdown curves of the molecular ion and the C₃H₂Cl⁺, C₃HCl⁺, CCl⁺, C₃H⁺₃, C₃H⁺₃, C₃H⁺ fragment ions are reported. For 1-chloropropyne cations initially formed in their $\text{A}\text{?}$²E state it is found that four fragmentation channels compete with a non-dissociative relaxation pathway. The average kinetic energies released on formation of C₃H⁺₃ and C₃H⁺₃ are deduced from the time-of-flight distributions of these fragment ions measured at different internal energies of the molecular ion. The coincidence data are supplemented by electron impact appearance energies. The obtained decay pattern of 1-chloropropyne cation is compared with the breakdown diagrams reported for the C₃H⁺₄ isomers, i.e. allene-, propyne- and cyclopropene cations.     

On the origin of the high density of liquid cyclohexane

Cyclohexane and 2,3-dimethylbutane molecules are the most compact among saturated C6-hydrocarbons. They have a similar size and are arranged in the liquid phase like atoms in simple liquids. However, the cyclohexane density is higher approximately by 20% than that of 2,3-dimethylbutane. The reasons of this distinction are discussed. It can be explained within the concepts of the physics of simple liquids. According to them, a small variation of the radius of a particle hard core or the pair interaction energy can lead to appreciable changes in the structure and density of a liquid. The obtained results give grounds to discard the explanations based exclusively on the features of the shape of the given molecules.     

Effect of a porous medium on the phase transitions and mobility of cyclohexane molecules

The effect of a porous medium on the phase transitions and molecular mobility of cyclohexane at a liquid content corresponding to a monolayer is studied by pulsed NMR. The times of longitudinal T ₁ and transverse T ₂ magnetic relaxation of protons of cyclohexane introduced into granulated porous glasses of the Vycor type with average pore diameters of 4, 11, and 32 nm are measured in the temperature range of 128–293 K. In spite of a relatively low liquid content, two phase transitions are observed for all porous glass samples at temperatures lower than those inherent in pure cyclohexane. At low temperatures, nonfreezing cyclohexane volumes with characteristic times of T ₂ ～ 100–200 μs and relative populations of 5–10% remain preserved due to the presence of a small number of micropores commensurable with molecular sizes. The appearance of an additional component with T ₂ ～ 200 μs upon temperature elevation to 148 K attests to thawing out of some cyclohexane volumes, which begins long before the crystal-plastic crystal phase transition. The nonexponential character of the transverse magnetization decay of cyclohexane above the temperature of the plastic crystal-liquid phase transition in the porous glass with a pore diameter of 4 nm suggests the existence of barriers for rapid molecular exchange. The obtained experimental results are indicative of the cluster mechanism of cyclohexane adsorption in the studied porous glasses.     

Electrogenerated cation radicals of heteroaromatic N-oxides and oxidation of cyclohexane induced by them

The primary products of electrooxidation of heteroaromatic N-oxides are the corresponding cation-radicals. The redox properties of N-oxides, and also the stability of the cation-radicals generated from them, are determined by the number of N atoms and N-oxide groups in the heterocyclic ring and also by the nature of the substituents. Using phenazine di-N-oxide as an example, it was shown that the generation of the cation radical induces the oxidation of cyclohexane into cyclohexanol and cyclohexanone and is accompanied by deoxygenation of the di-N-oxide into a mono-N-oxide. The oxidation of cyclohexane proceeds by two paths, in one of which molecular oxygen participates, while in the second path an oxygen transfer probably takes place from the di-N-oxide cation radical to cyclohexane.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 994–999, May, 1990.The authors wish to express their gratitude to A. E. Shilov for his participation in the discussion of the results, and also to R. E. Lokhov and A. V. Ryzhakov for providing samples of a series of N-oxides.     

Charge effect of the cation on the structure and the molecular mobility of polymer electrolytes

A study of the thermal and mechanical characteristics of polyethylene oxide-Fe(SCN)₃ complexes shows the existence of structures which can be described by considering a crystalline phase of pure polyethelene oxide (PEO) and an amorphous phase of PEO with dissolved salt. Increasing salt contents decrease the content of crystals, favoring the formation of complexes with a high degree of noncrystallinity as a result of strong polymersalt interactions which tend to enhance the distortion degree of the polymeric skeleton. The microscopic homogeneity of the complexes is also confirmed by the presence in the thermal and mechanical spectra of single glass transitions, which shift to higher temperatures with increasing salt content. Two molecular relaxations are present in the mechanical behavior of these systems, the γ-and the α_a-processes at low and high temperatures, respectively, and show a relaxation strength which increases with decreasing degree of crystallinity of the polymer up to a X ～ 0.10 molar fraction. Both relaxations exhibit a marked nonexponentiality which has been well accounted for in terms of a gaussian distribution of relaxation times for the γ-process and of the Kolrausch-Williams-Watt exponential function for the α_a-process. In the glassy region, the elastic modulus E' reveals a linear temperature dependence which has been interpreted as arising from the anharmonicity of vibrational modes. Increasing noncrystallinity of the system gives rise to an increase of the anharmonicity parameter, which has been ascribed to the influence of the “free volume” in determining the thermal expansivity. ©1995 John Wiley & Sons, Inc.     

The formation of byproducts in the autoxidation of cyclohexane

In this work, a complementary experimental and theoretical approach is used to unravel the formation of byproducts in the autoxidation of cyclohexane. The widely accepted vision that cyclohexanone would be the most important precursor of undesired products was found inconsistent with several experimental observations. However, the propagation reaction of cyclohexyl hydroperoxide, which we recently put forward as the missing source of cyclohexanol and cyclohexanone, is now unambiguously identified also as the dominant path leading to byproducts. Indeed, this overlooked reaction produces large amounts of cyclohexoxy radicals, able to ring-open via a beta-C--C cleavage to omega-formyl radicals. The pathway by which these radicals are converted into the observed and quantified byproducts is derived in this work. In this liquid-phase reaction, solvent cages were found very important, steering the fate of nascent species.     

Comparative analysis of the gas-phase reactions of cylindrospermopsin and the difference in the alkali metal cation mobility

Cylindrospermopsin (CYN) belongs to a group of toxins produced by several strains of freshwater cyanobacteria. It is a compact zwitterionic molecule composed of a uracil section and a tricyclic guanidinium portion with a primarily hepatotoxic effect. Using low multi-stage and high-resolution mass spectrometry, the gas-phase reactions of this toxin have been investigated. Our data show that collision-induced dissociation (CID) spectra of CYN are dominated by neutral losses, and three major initial fragmentation pathways are clearly distinguishable. Interestingly, comparative analysis of protonated and cationizated molecules showed a significant difference in the balance of the SO3 and terminal ring elimination. These data indicate that the differential ion mobility of H+, Li+, Na+ and K+ leads to different fragmentation pathways, giving rise to mass spectra with different profiles.     

Quantitative information in decay curves obtained with a pulsed ion mobility spectrometer

Ion mobility spectrometry (IMS) is well known for its very high sensitivity, and thus IMS spectra are commonly used in the identification of trace gases. Extracting quantitative information from IMS spectra is, in contrast, difficult, especially regarding the reproducibility due to the nature of the processes involved in the measurement of the spectra. Here we present data extracted from signal decay curves obtained with a pulsed IMS, which can support the determination of substance concentrations in the lower ppb range with good stability.     

Aromatization of methylcyclopentane and cyclohexane over zinc-containing high silica zeolites

High catalytic activity and selectivity of high silica zeolites containing no cations of noble metals, towards aromatization of methylcyclopentane (MCPA) and cyclohexane (CHA) has been revealed.

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Effect of positive ion scavengers on recombination times: An attempt to use secondary electron transfer reactions as a probe for the existence of a high mobility hole in cyclohexane

In order to probe for the existence of a high mobility hole in cyclohexane, secondary electron transfer reactions between CH₃Cl^? and other electron scavenging solutes have been studied in the absence and presence of positive ion scavengers. There is no evidence in these experiments for a change in the time scale for neutralization of the negative ions which can be attributed to trapping of the positive ions.     

Variable high-pressure and concentration study of cyclohexane pyrolysis at high temperatures

A high-pressure and temperature cyclohexane pyrolysis shock tube study was completed with the goal of extending the experimental cyclohexane pyrolysis data to pressures relevant to current and future combustors and to investigate whether ring contraction products observed in high-pressure, supercritical phase cyclohexane and cycloalkane pyrolysis experiments can form at matching, and higher, pressures in the gas phase. The experiments in the current work were completed over a range of 950–1650 K and at nominal pressures of 40, 100, and 200 bar. No alkylcyclopentanes, possible ring contraction products, were observed to form under the conditions of the current study. The production of methylenecyclopentane and 1,3-cyclopentadiene, and the other three cyclic species quantified: cyclohexene, benzene, and toluene, increased significantly with a substantial increase in the initial fuel concentration. Two sets of experimental data obtained at 200 bar were compared with a literature and laboratory-generated model. Both models had difficulty capturing the propadiene and propyne profiles, and the literature model significantly overestimated the benzene observed in the set of experiments completed with the more dilute fuel mixture. The literature model was able to better predict propadiene, propyne, and benzene product profiles in the 200 bar set of experiments, which used a higher concentration of fuel in the test gas. These results suggest that despite both cyclohexane and benzene being well-studied and important species in combustion chemistry, their reaction pathways and reaction rates would benefit from further refinement.     

Effect of oxygen on the formation and decay of stilbene radical cation during the resonant two-photon ionization

Formation and decay of radical cations of trans-stilbene and p-substituted trans-stilbenes (S.+) during the resonant two-photon ionization (TPI) of S in acetonitrile in the presence and absence of O(2) have been studied with laser flash photolysis using a XeCl excimer laser (308 nm, fwhm 25 ns). The transient absorption spectra of S.+ were observed with a peak around 470-490 nm. The formation quantum yield of S.+ (0.06-0.29) increased with decreasing oxidation potential (E(ox)) and increasing fluorescence lifetime (tau(f)) of S, except for trans-4-methoxystilbene which has the lowest E(ox) and longer tau(f) among S. The considerable low yield and fast decay in a few tens of nanoseconds time scale were observed for trans-4-methoxystilbene.+ in the presence of O(2), but not for other S.+ . It is suggested that formation of the ground-state complex between trans-4-methoxystilbene and O(2) and the distonic character of trans-4-methoxystilbene.+ with separation and localization of the positive charge on the oxygen of the p-methoxyl group and an unpaired electron on the beta-olefinic carbon are responsible for the fast reaction of trans-4-methoxystilbene.+ with O(2) or superoxide anion, leading to the considerable low yield and fast decay of trans-4-methoxystilbene.+ . The mechanism based on the transient absorption measurement of S.+ during the TPI is consistent with the relatively high oxidation efficiency of trans-4-methoxystilbene among S based on the product analysis during the photoinduced electron transfer in the presence of a photosensitizer such as 9,10-dicyanoanthracene and O(2) in acetonitrile.     

Formation and decay of the ABTS derived radical cation: A comparison of different preparation procedures

Bleaching of a preformed solution of the blue‐green radical cation 2,2′‐azinobis (3‐ethylbenzothizoline‐6‐sulfonic acid) (ABTS^+·) has been extensively used to evaluate the antioxidant capacity of complex mixtures and individual compounds. The reaction of the preformed radical with free‐radical scavengers can be easily monitored by following the decay of the sample absorbance at 734 nm. The ABTS radical cation can be prepared employing different oxidants. Results obtained using MnO₂ as oxidant show that the presence of manganese ions increases the rate of [ABTS]^+· autobleaching in a concentration‐dependent manner. The radicals can also be obtained by oxidizing ABTS with 2,2^′‐azobis(2‐amidinopropane)hydrochloride (AAPH) or peroxodisulfate (PDS). The oxidation by AAPH takes place with a large activation energy and a low reaction order in ABTS. The data support a mechanism in which the homolysis of AAPH is the rate‐limiting step, followed by the reaction of ABTS with the peroxyl radicals produced after the azocompound thermolysis. On the other hand, the low activation energy measured employing PDS, as well as the kinetic law, are compatible with the occurrence of a bimolecular reaction between the oxidant and ABTS. Regarding the use of ABTS‐based methodologies for the evaluation of free radical scavengers, radical cations obtained employing AAPH as oxidant can be used only at low temperatures, conditions where further decomposition of the remaining AAPH is minimized. The best results are obtained with ABTS derived radicals generated in the reaction of PDS with an ABTS/PDS concentration ratio equal (or higher) to two. However, even with radicals prepared by this procedure, stoichiometric coefficients considerably larger than two are obtained for the consumption of the radical cation employing tryptophane or p‐terbutylphenol as reductants. This casts doubts on the use of ABTS‐based procedures for the estimation of antioxidant capacities. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 659–665, 2002     

Unexpected high oxidation of cyclohexane by Fe salts and dihydrogen peroxide in acetonitrile

The reaction of cyclohexane (CyH) with 1.5 equivalent of dihydrogen peroxide (30% aqueous solution) in the presence of 1 mol% of an iron(II) or iron(III) salt (without added ligand), at 50 °C in acetonitrile, produces cyclohexanol (CyOH) and cyclohexanone (CyO) in high yields (up to 87% of CyH is converted to CyOH + CyO). Remarkably, CyH is totally converted within 2 h in the presence of Fe(ClO₄)₂ as catalyst under argon, producing 45% of CyOH and 42% of CyO. The addition of a tridentate Schiff-base ligand, namely 2,6-bis-[1-(benzylimino)ethyl]pyridine (dapb), leads to a clear increase of the selectivity towards CyOH + CyO, which are obtained with an overall yield of 93% (and even 100% selectivity to CyOH and CyO) in the case of the [Fe(ClO₄)₃/dapb] catalytic system.     

The adsorption of polyoxyethylated nonylphenol in cyclohexane solution

Summary The adsorption of polyoxyethylated nonylphenol on calcium carbonate and on carbon black in cyclohexane was studied and compared with that in water. The adsorption of this nonionic surfactant in cyclohexane on calcium carbonate showed a monolayer adsorption isotherm and that on carbon black showed a multilayer adsorption isotherm. The adsorption cross sectional area,A, of the surfactant molecule increased with the number of the ethylene oxide units,n, in both cases, and expressed by the equationsA=49.1n ^0.34 for the former case andA=270.n ^0.29 for the later case.When either one of the adsorbent or the solvent is polar and the other is nonpolar, the adsorbed molecules form simple oriented monolayer. When both of the adsorbent and the solvent are polar or nonpolar, however, the adsorption takes place in a complicated form and is delicately affected by the impurity and the balance of the polarity of the surfactant molecule.

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Zusammenfassung Die Adsorption von Polyoxyäthylen-nonylphenol an Calciumcarbonat und Ruß in Zyklohexan wurde untersucht und das Adsorptionsverhalten mit den Ergebnissen von wäßrigen Lösungen verglichen, die im letzten Bericht mitgeteilt sind. Die Adsorption dieser nichtionischen oberflächenaktiven Substanz in Zyklohexan an Calciumcarbonat zeigte eine monomolekulare und an Ruß eine höhermolekulare Adsorptionsisotherme. In beiden Fällen nahm der Adsorptionsquerschnitt,A, des oberflächenaktiven Moleküls mit der Anzahl der Äthylenoxid-Einheiten,n, zu, und es galten folgende Werte:A=49.1 n^0.29 im ersten Fall undA=270.n ^0.34 im letzten Fall.Die adsorbierten Moleküle bilden eine einfach orientierte monomolekulare Schicht, wenn von den beiden Komponenten Adsorbens oder Lösungsmittel eines polar und das andere unpolar ist. Wenn jedoch beide polar oder nichtpolar sind, verläuft die Adsorption komplizierter und wird von den Verunreinigungen und der Bilanz der Polarität empfindlich beeinflußt.

     

The properties of supported rhenium catalysts in the dehydrogenation of cyclohexane

The 2 % Re/sibunite catalyst is more active than 2 % Re/-Al₂O₃ and 2 % Re/-Al₂O₃ catalysts in the dehydrogenation of cyclohexane into benzene (T = 350 °C,w = 0.5 h–1). The substitution of NH₄ReO₄ by HReO₄ in the preparation of the catalyst enhances its activity by a factor of 1.3. Treatment with HNO₃ or oxalic acid increases the selectivity by a factor of 1.2 and 1.35, respectively, the overall conversion of cyclohexane being 32–40 %.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 2119–2121, August, 1996.     

The radical cation of cyclohexane in halocarbon matrices. evidence for matrix interaction as studied with electron spin echo modulation (ESEM) spectroscopy

ESEM measurements on the cation of cyclohexane stabilised in halocarbon matrices have been performed at 4.2 K. The results show that hyperfine interaction with fluorine matrix nuclei can be quite large or very small depending on the chosen matrix. No modulations from chlorine nuclei have been detected, probably due to the large quadrupole constant of chlorine. Dipolar tensors have been calculated and compared with the experimental results.