Toluene Radical Cation Research Articles

Radiation-induced transformations of isolated toluene molecules in low-temperature matrices: Towards better understanding of molecular radiation chemistry in condensed phases

The transformations of isolated toluene molecules under X-ray radiolysis in solid noble gas matrices at 6 K were characterized by FTIR spectroscopy. It was found that the efficiency of the radiation-induced decomposition of toluene was generally comparable to that of benzene. Meanwhile, a remarkable matrix effect was observed: toluene is more stable than benzene in an argon matrix, whereas it becomes more sensitive in the case of xenon matrix. This result was ascribed to the increasing role of upper triplet excited states in a xenon matrix. In all the matrices two main radiolysis products observed at moderate absorbed doses were benzyl radical and 5-methylene-1,3-cyclohexadiene (MCHD). Benzyl radical predominates in all the cases, while the relative contribution of MCHD strongly decreases in the row Ar-Kr-Xe. On the basis of this comparison, the formation of MCHD was tentatively attributed to the rearrangement of excited primary toluene radical cations. Possible scheme of the radiation-induced processes is discussed.

Gas Phase Ionization of Toluene: Benzylium Versus Tropylium Pathway

Aim: In this investigation, we used accurate mass high-resolution gas chromatography mass spectrometry to study the gas phase carbocations rearrangements and fragmentation of toluene and halo-toluenes as well as their deuterium labeled compounds. Objective: Accurate mass of selected ions from ionization of toluene and related compounds revealed that the initially formed radical cation C7H8 +. does not rearrange to tropylium radical cation contradicting published literature. Methods: When the toluene radical cation was purely selected, it was found to lose a free radical (hydrogen atom) at collision energies greater than 5 eV and forming benzylium or tropylium cation C7H7 + (m/z = 91), with no other fragmentations. Results: The resulting cation at collision energy greater than 20 eV fragmented by losing acetylene or ethylene or allene molecule to form C5H5 + (m/z = 65), C5H3 + (m/z = 63) or C4H3 + (m/z = 51) respectively. Purely selected C5H5 + cation at collision energy greater than 30 eV lost acetylene molecule and formed C3H3 + (m/z =39). Conclusion: In this investigation toluene, halotoluene and their deuterated derivatives (structural isomers) were found to ionize in the gas phase with isomer retention. Historically, it has been suggested that the seven carbons and hydrogen atoms would become indistinguishable. However, this should be revised in the light of new technologies.

Unveiling New Isomers and Rearrangement Routes on the C7H8+ Potential Energy Surface

The unimolecular reactions of C7H8+ radical cations are among those most studied by mass spectrometry, especially the rearrangement of toluene and cycloheptatriene molecular ions, which are directly connected to the formation of benzylium and tropylium cations. This study reveals important new isomers and isomerization pathways on the C7H8+ potential energy surface, through the application of gas-phase electronic photodissociation spectroscopy in conjunction with ab initio calculations. Presented are the first gas-phase vibrationally resolved electronic spectra of the o-isotoluene, norcaradiene, bicyclo[3.2.0]hepta-2,6-diene radical cations, and ring-opened products from cyclic C7H8+ species. The isomerization route from the norbornadiene radical cation to the toluene radical cation, which competes with isomerization to the bicyclo[2.2.1]hepta-2-ene-5-yl-7-ylium radical cation, is identified. Further, this work expands understanding of the C7H8+ potential energy surface by connecting spiro[2.4]hepta-4,6-diene and acyclic 1,2,4,6-heptatetraene radical cations, and confirms the important role of the o-isotoluene radical cation in the interconversion pathways of C7H8+ species.

Visible-Light Selective Photooxidation of Aromatic Hydrocarbons via Ligand-to-Metal Charge Transfer Transition on Nb2O5

This study shows that a vacuum heat treatment of Nb2O5 permits the ligand-to-metal charge transfer (LMCT) transition between its surface and adsorbed aromatic hydrocarbons, which corresponds to visible light (>390 nm) absorption. Selective photooxidation of aromatic hydrocarbons to form the corresponding carbonyl compounds could effectively proceed under visible light irradiation by using this LMCT transition, and the catalytic activity is much superior to that of TiO2. Specifically, the LMCT bands of toluene-adsorbed Nb2O5 correlated well with the action spectrum for toluene photooxidation, suggesting that the reaction proceeded via the LMCT photoexcitation. We successfully detected toluene radical cation and its further oxidation product of benzylperoxy radical as intermediate species on toluene-adsorbed Nb2O5 through the LMCT transition. These findings suggest a novel reaction mechanism for the selective photooxidation of aromatic hydrocarbons via LMCT transition on Nb2O5.

Laser-based ion mobility spectrometer for the direct analysis of aromatic compounds in liquids

A laser-based ionization source for the direct analysis of liquid samples in ion mobility (IM) spectrometry is presented and characterized. Ionization of aromatic substances in liquids is achieved, analogous to atmospheric pressure laser ionization (APLI) in mass spectrometry, by vaporizing the liquid and subsequently ionizing the aromatic substances by resonance-enhanced multiphoton ionization (REMPI). The effects of parameters, such as composition and flow rate of the solvent as well as laser wavelength and pulse energy, are systematically investigated. The characterization of the IM spectrometer is carried out by means of selected substances from diverse fields of applications, e.g., polycyclic aromatic hydrocarbons (PAH), pesticides, wood preservatives and drug compounds. Limits of detection (LOD) down to 10 fmol and linear ranges up to three orders of magnitude are established. In addition to direct laser ionization, indirect laser ionization via dopants (toluene) for substances with low ionization efficiencies is investigated. Ionization occurs as a result of proton transfer from toluene radical cations to substances of sufficiently high proton affinities. As a result of indirect laser ionization, LOD could be decreased by up to two orders of magnitude. Ionization products are investigated by means of a combination of IM and mass spectrometer. Depending on the substance investigated primary ions (radical cations) and secondary ions (protonated molecules) resulting from ion molecule reactions are formed.

MECHANISM OF IONIZATION OF POLYCYCLIC AROMATIC HYDROCARBONS BY A TOLUENE/ANISOLE MIXTURE AS A DOPANT IN LIQUID CHROMATOGRAPHY/DOPANT-ASSISTED ATMOSPHERIC-PRESSURE PHOTOIONIZATION/MASS SPECTROMETRY

Ionization mechanisms of 16 polycyclic aromatic hydrocarbons (PAHs) by dopant-assisted atmospheric-pressure photoionization (DA-APPI) with a toluene/anisole mixture as a dopant were examined in the reversed-phase liquid chromatography mode with a methanol/water to acetonitrile linear gradient mobile phase. When toluene/anisole mixtures were used as dopants, their characteristics were governed by anisole. Since the IE of anisole (8.20 eV) is below that of toluene (8.83 eV), anisole is ionized not only by photons from the krypton lamp but also by charge transfer from toluene radical cations. In the latter half of chromatogram, when acetonitrile was dominant in the eluent, ions originating from both toluene and anisole decreased while acetonitrile-cluster ions began to increase. Because acetonitrile-cluster ions are not formed with anisole alone, but are formed with toluene alone, toluene radical cations would be consumed to form acetonitrile-cluster ions.

Theoretical investigation of the formation of the tropylium ion from the toluene radical cation

AbstractThe formation of the tropylium ion, C7H7+, in the mass spectrum of toluene is a chemical process that has been extensively studied. There is, however, still debate as to the structure of the moieties and the reaction pathways involved. This work presents the first computationally complete reaction schemes for the formation of tropylium from toluene to be reported. The calculations were performed at the HF/6‐31G(d, p) and the DFT/B3LYP/6‐311++G(2d) levels of theory using Gaussian 03W. The previously unreported optimized structures and energies for a transition state and an intermediate in one scheme and a transition state in the other have been determined. These results are consistent with the previously reported literature and the available experimental data. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Rearrangement and Hydrogen Scrambling Pathways of the Toluene Radical Cation: A Computational Study

A computational study is undertaken to provide a unified picture for various rearrangement reactions and hydrogen scrambling pathways of the toluene radical cation (1). The geometries are optimized with the BHandHLYP density functional, and the energies are computed with the ab initio CCSD(T) method, in conjunction with the 6-311+G(d,p) basis set. In particular, four channels have been located, which may account for hydrogen scrambling, as they are found to have overall barriers lower than the observed threshold for hydrogen dissociation. These are a stepwise norcaradiene walk involved in the Hoffman mechanism, a rearrangement of 1 to the methylenecyclohexadiene radical cation (5) by successive [1,2]-H shifts via isotoluene radical cations, a series of [1,2]-H shifts in the cycloheptatriene radical cation (4), and a concerted norcaradiene walk. In addition, we have also investigated other pathways such as the suggested Dewar-Landman mechanism, which proceeds through 5, via two consecutive [1,2]-H shifts. This pathway is, however, found to be inactive as it involves too high reaction barriers. Moreover, a novel rearrangement pathway that connects 5 to the norcaradiene radical cation (3) has also been located in this work.

Atmospheric Pressure Photoionization Mass Spectrometry of Polyisobutylene Derivatives

Atmospheric pressure photoionization mass spectrometric (APPI-MS) study on three types of polyisobutylene derivatives is reported. Two of the polyisobutylenes investigated were polyisobutylene with dihydroxy and diolefinic end-groups derived from aromatic moieties [dicumyl chloride, 1,4-bis(2-chloro-2-propyl)benzene], and the third contained no aromatic moieties with a monohydroxy end-group. All three polyisobutylene derivatives (PIBs) had an average molecular weight (M(n)) of approximately 2000 g/mol, with a polydispersity lower than 1.2. In the positive ion APPI mode, protonated PIB molecules were formed, but the molecular weights obtained were considerably lower than those expected, indicating fragmentation of the PIB chains. In the negative APPI mode, using solvents such as tetrahydrofuran and toluene as dopants, no signal was obtained. However, in chlorinated solvents, such as CCl(4), CHCl(3), and CH(2)Cl(2), in the presence of toluene dopant, PIB adducts with chloride ions were formed with relatively high signal intensity. In the case of CH(2)Cl(2), no dopant (toluene) was necessary to generate chlorinated adduct ions, albeit increasing the toluene concentration in the flow increased the PIB signal intensity. The effect of the toluene concentration on PIB signal intensity was studied and models that include (1) photoionization of toluene, (2) formation of chloride ions from the chlorinated solvents by dissociative electron capture, (3) formation of chlorinated adduct ions and charge recombination reactions between the toluene radical cation, (4) chloride ions, and (5) chlorinated adduct ions are proposed based on the experimental results.

A ring walk of methylene groups in toluene radical cations. An extension of the toluene-cycloheptatriene rearrangement of aromatic radical cations. Theory and experiment.

The minimum energy reaction pathway (MERP) of the toluene-cycloheptatriene radical cation rearrangement (TOL/CHT-rearrangement) has been calculated by the UHF and DFT model at the level UHF/6-311+G(3df,2p)//UHF/6-31G(d) and B3LYP/6-311+G(3df,2p)//B3LYp/6-31G(d), respectively, including the ring walk of the substituent by a 1,2-shift around the aromatic ring. This ring walk corresponds to interconversion of distonic ions and norcaradiene radical cations (the two intermediates of the TOL/CHT-rearrangement) by making and breaking of the external C-C bonds of the cyclopropane moiety of the intermediate norcaradiene structure. For toluene radical cation 1, UHF calculations adequately reproduce earlier results(4) and show, that the ring walk of the CH(3)-substituents requires slightly more energy than formation of the cycloheptatriene radical cation. By the DFT model, the distonic ion, which is formed initially by a 1,2-H shift from CH(3) to the benzene ring, is not stable but the transition state of an interconversion of norcaradiene radical cations along a ring walk of the CH(3) substituent. The activation energy for this ring walk exceeds that for formation of the cycloheptatriene radical cation by c. 30 kJ mol(-1). Thus, isomerization of 1 by a ring walk of the CH(3)-substituent competes with the TOL/CHT-rearrangement likely only for excited 1. The calculation was repeated for the MERPs of a TOL/CHT-rearrangement of para-xylene radical cation 5 and ethylbenzene radical cation 2, yielding basically the same results as for 1. According to the calculation, polar substituents alter significantly the relative energies of the competing routes of isomerization. For benzylcyanide 3 (X = CN), the activation energy for a ring walk of the NC-CH(2)-substituent is distinctly below that of a ring enlargement. For benzyl methyl ether 4 (X = OCH(3)), the distonic intermediate along the UHF-MERP is unusually stable. Further, the 7-methoxy-norcaradiene radical ion is unstable and corresponds to a transition state between isomeric distonic intermediates differing by a 1,2-shift of the side chain. In contrast, the 7-methoxy-norcaradiene radical ion is the only intermediate of the DFT-MERP, and the distonic ion is the transition state for a 1,2-shift of the cyclopropane ring. A ring walk of the CH(3)OCH(2)-substituent is much more favorable than formation of a 7-methoxy-cycloheptatriene radical cation in both MERPs. The findings of the theoretical calculation are substantiated by the mass spectrometric fragmentations of meta- and para-methoxymethylated 1-phenylethanols 8 and 9 and of para-methoxymethyl substituted benzyl ethyl ether 10 and benzyl n-propyl ether 11. Important fragmentation routes of metastable molecular ions of these compounds correspond to elimination of alcohols. Use of deuterated derivatives shows that the elimination occurs by a "false" ortho-effect which requires migration of a ROCH(2)-substituent around the benzene ring. Results of particular interest are obtained for the asymmetric bis-ethers 10 and 11. Here, the MIKE spectra of the molecular ions of deuterated analogs reveal a selective ring walk of the C(2)H(5)OCH(2)- and n-C(3)H(7)OCH(2)-side chain, respectively.

Effect of matrix and substituent on the electronic structure of trapped benzene radical cations

The structure and dynamics of the radical cations produced from benzene, monodeuterated benzene and toluene in various low-temperature matrices were characterized by EPR and ENDOR spectroscopy. It was found that the nature of the matrix had a dramatic effect on the EPR spectra of benzene cation. Rigid structures corresponding to the 2B1g and 2B2g states are revealed in solid argon and halocarbon (CFCl3) matrices, respectively, whereas only dynamically averaged patterns are observed in other hosts used (krypton, xenon, sulfur hexafluoride). Deuterium monosubstitution has no appreciable effect on the cation structure observed in argon and halocarbon matrices, which implies matrix control of the preferred electronic state. In contrast, the toluene radical cation exhibits only a 2B2g-like structure both in argon and in CFCl3 matrices, that is, the internal structural effect strongly predominates over environment effects in this case. The results are discussed in qualitative terms taking into consideration the matrix and substituent effects on the charge distribution in benzene cation.

Benzyl Radicals from Toluene by Photosensitization with Naphthalene-1,4-dicarbonitrile – Benzylation and Hydroxymethylation of Unsaturated Compounds

In aprotic media, photoinduced electron transfer from toluene to 1,4-naphthalenedicarbonitrile (DCN) is followed by in-cage proton transfer and radical coupling. However, in the presence of a protic co-solvent, the radical ions diffuse out of the cage and deprotonation of the toluene radical cation takes place from the free solvated species. With tBuOH, this results in reductive benzylation of DCN through coupling of benzyl radicals with DCN–·. With MeOH, secondary hydrogen abstraction by PhCH2· yields hydroxymethyl radicals, which couple with DCN–·. Both benzyl and hydroxymethyl radicals are efficiently trapped by electrophilic alkenes. Thus, in the presence of dimethyl maleate, DCN-photosensitized benzylation – or hydroxymethylation – of the substrate occurs through direct activation of the C–H bond. The same reaction has been accomplished with acrylonitrile, albeit with a lower yield.

Toluene and isotoluene radical cations: alkoxymethyl substituents as a probe to study the formation, fragmentation and isomerization

The unimolecular reactions of toluene radical cations and tautomeric isotoluene radical cations (methylenecyclohexadiene radical cations) substituted by an ROCH2 group (R=CH3 , C2H5 , n -C3H7 , i-C3 H7) were studied by mass-analyzed ion kinetic energy (MIKE) spectrometry and collisional activation mass spectrometry. The alkoxymethyl-substituted toluene radical cations were obtained directly by 70 eV electron ionization, while the substituted isotoluene radical cations were prepared by elimination of an aldehyde or ketone molecule from ionized bis(alkoxymethyl)benzenes. It is shown that the fragmentation reactions induced by the alkoxymethyl side-chain in the toluene radical cations and isotoluene radical cations can be used conveniently to distinguish unambiguously between the tautomers. The substituted toluene radical cations eliminate abundantly the alkoxy group of the side-chain as aldehyde or ketone by a well known fragmentation mechanism. Typical of the substituted isotoluene radical cations is an abundant elimination of alcohol from the side-chain of metastable ions, which with the exception of some para-substituted ions surpasses the abundance of the loss of aldehyde/ketone. Further, the substituted toluene radical cations typically fragment by loss of the methyl group of the aromatic ring. The analysis of the MIKE spectra reveals that <10% of the isotoluene racial cations are isomerized into the toluene isomers if these ions are generated by energetically favorable processes from the bis(alkoxymethyl)benzenes. Otherwise, extensive tautomerization between toluene and isotoluene-like ions takes place. Thus, (substituted) toluene and isotoluene radical cations are stable species which can be detected by tandem mass spectrometry. However, the activation barrier between the tautomeric ions cannot be very large, and energetically excited isotoluene radical cations are converted into the more stable toluene isomer. Copyright © 1999 John Wiley & Sons, Ltd.

Toluene and isotoluene radical cations: alkoxymethyl substituents as a probe to study the formation, fragmentation and isomerization Dedicated to the memory of Professor Dr Wilhelm J. Richter

Toluene and isotoluene radical cations: alkoxymethyl substituents as a probe to study the formation, fragmentation and isomerization Dedicated to the memory of Professor Dr Wilhelm J. Richter

A Kinetic Evaluation of Carbon−Hydrogen, Carbon−Carbon, and Carbon−Silicon Bond Activation in Benzylic Radical Cations

A detailed study of the competition between C−C, C−H, and C−Si bond fragmentation in a series of 4-methoxy-α-substituted toluene radical cations (1•+), involving both product studies and kinetic analysis, is presented. C−C bond fragmentation occurs with several radical cations in acetonitrile. The rate constants for such processes, determined by laser flash photolysis, varied from 2.8 × 104 (1c•+) to 1.53 × 106 (1f•+) s-1. The activation parameters for C−C bond fragmentation are characterized by low activation enthalpies on the order of 30 kJ mol-1 and negative activation entropies in the range −34 to −55 J mol-1 K-1. Deprotonation of the radical cations is always a second-order process induced by nucleophiles [cerium(IV) ammonium nitrate (CAN) or nitrate anion], with second-order rate constants from 7.7 × 107 (1h•+) to 8.8 × 108 (1i•+) M-1 s-1 in neat acetonitrile (CAN assisted) and from 0.4 × 108 (1j•+) to 7.1 × 108 (1i•+) M-1 s-1 in the presence of nitrate anion. The rate constant for nitrate-induced d...

Is the tropylium ion (Tr +) formed from toluene at its thermochemical threshold?

The role of tropylium ion (Tr +) versus benzylium ion (Bz +) formation by H loss from toluene radical cation (TOL +) was studied by time-resolved photoionization mass spectrometry, by ab initio calculations and by RRKM-QET. Tr + is not formed at its thermochemical threshold owing to the presence of a reverse activation energy for the reaction leading from the cycloheptatrien radical cation (CHT +) to Tr + + H. The critical energies required to form Bz + and Tr + from TOL + are very close, being 2.18 and 2.11 eV, respectively. The computational results are in excellent agreement with experiments regarding appearance energies, heats of formation and Tr +/Bz + abundance ratios as a function of energy.

Anodic synthesis of benzaldehydes—I. Voltammetry of the anodic oxidation of toluenes in non-aqueous solutions

The electrode kinetic investigation of the three consecutive steps of anodic toluene oxidation in methanol, namely the two-electron oxidation of toluene to benzylalcohol methylether, the two-electron oxidation of benzylmethylether to benzaldehyde dimethylacetal and the multielectron oxidation of benzaldehyde dimethylacetal to a mixture of unknown products was performed, using voltammetry at rotating and stationary disc electrodes and the rotating ring-disc electrodes in methanol, ethanol and acetonitrile as solvents. Investigated were toluene, benzylalcohol and benzaldehyde as references and para-substituted toluenes, benzylalcohols and benzaldehydes. Substituents with (+)I-effect activate the toleune towards anodic attack, substituents with (−)I-effect deactivate the aromatic system. A detailed electrode kinetic investigation is possible for p-methoxytoluene, the respective benzylalcohol and benzaldehyde. The current-voltage curve is almost exclusively mass transfer controlled. The chemical reaction following the first charge transfer is a first order reaction and is likely to be the proton dissociation of the toluene radical cation. It is heterogeneously catalysed as the choice of the anode material influences the half wave potential. In acetonitrile, as a more acidic solvent, dimerization seems to become important. A full description of the current-voltage curves by appropriate modelling is impossible as experimental inaccuracies do not allow for proper determination of the kinetic parameters. The proton dissociation rate of p-methoxytoluene radical cations can be estimated to be faster than 10 5s −1.

Thermochemical parameters for organic radicals and radical ions. Part 1. The estimation of the pKa of radical cations based on thermochemical calculations

Three methods for estimating the pKa of a radical cation using thermochemical cycles are discussed. These methods enable the calculations of the pKa of radical cations which have not been determined experimentally. Radical cation acids are classified according to the nature of their respective conjugate bases. The application of these methods is illustrated for a σ-acid (the benzene radical cation) and π-acid (the toluene radical cation). The potential of these methods to estimate relative and absolute solvation energies is also discussed. The use of thermochemical cycles in estimating standard oxidation potentials of organic compounds is outlined.Calculations show that the toluene radical cation is an extremely strong acid and the benzene radical cation a moderately strong acid in acetonitrile solution. Difficulties in directly determining the pKa value of aromatic hydrocarbon radical cations are also discussed.