Benzene Radical Cation Research Articles

Halogenated Benzene Cation Radicals

The halogenated benzenes C(6)HF(5), 2,4,6-C(6)H(3)F(3), 2,3,5,6-C(6)H(2)F(4), C(6)F(6), C(6)Cl(6), C(6)Br(6), and C(6)I(6) were converted into their corresponding cation radicals by using various strong oxidants. The cation-radical salts were isolated and characterized by electron paramagnetic resonance (EPR) spectroscopy and by single-crystal X-ray diffraction. The thermal stability of the cation radicals increased with decreasing hydrogen content. As expected, the cation radicals [C(6)HF(5)](+) and 2,3,5,6-[C(6)H(2)F(4)](+) had structures with the same geometry as C(6)HF(5) and 2,3,5,6-[C(6)H(2)F(4)]. In contrast, the cation radicals [C(6)F(6)](+), [C(6)Cl(6)](+), and possibly also [C(6)Br(6)](+) exhibited Jahn-Teller-distorted geometries in the crystalline state. In the case of C(6)F(6)(+)Sb(2)F(11)(-), two low-symmetry geometries were observed in the same crystal. Interestingly, the structures of the cation radicals 2,4,6-[C(6)H(3)F(3)](+) and C(6)I(6)(+) did not exhibit Jahn-Teller distortions. DFT calculations showed that the explanation for the lack of distortion of these cations from the D(3h) or D(6h) symmetry of the neutral benzene precursor was different for 2,4,6-[C(6)H(3)F(3)](+) than for [C(6)I(6)](+).

Photocatalytic alkoxylation of benzene with 3-cyano-1-methylquinolinium ion

One-pot alkoxylation of benzene with alcohols occurs under photoirradiation of 3-cyano-1-methylquinolinium ion in an oxygen-saturated acetonitrile solution containing benzene and alcohols. The photocatalytic reaction mechanism was clarified by nanosecond laser flash photolysis. The photocataytic alkoxylation of benzene is initiated by photoinduced electron transfer from benzene to the singlet excited state of 3-cyano-1-methylquinolinium ion, followed by the nucleophilic addition of alcohols to benzene radical cation. In the case of photoethoxylation, the optimal product and quantum yields of ethoxybenzene were 20% and 10%, respectively.

Regioselective Photochemical C–OMe Bond Formation Initiated by One‐Electron Transfer and N–OMe Bond Fragmentation in Electron Donor–Acceptor Systems

AbstractCompounds that integrate electron donor–acceptor subunits with N‐methoxyisoquinolinium as acceptors and substituted (methoxy)benzenes as donors were synthesized and their luminescent and photochemical properties studied. Photolysis yielded the corresponding photomethoxylation products in a two‐step process that involves N–OMe bond scission followed by C–OMe bond formation. Homolysis of the N–OMe bond restores the aromatic isoquinoline nucleus and produces a methoxy radical that can couple to the required ring carbon atom in the benzene cation radical to give the products in a regioselective process controlled by the spin density of the cation radical. This photoprocess involves two different pathways: methoxylation of the acceptor (intracomponent methoxylation) or the donor (intercomponent metoxylation). Both methoxy‐transfer pathways are controlled by the donoating ability (redox potential) of the donor subunit, consistent with the emission observed upon excitation of the charge‐transfer state in systems that undergo intermethoxylation.

A quantum-classical simulation of a multi-surface multi-mode nuclear dynamics on $ {\rm C_{6}H}_{6}^{+} $ incorporating degeneracy among electronic states

We have performed a nuclear dynamics simulation to calculate photoelectron spectra and population profiles of benzene radical cation (C6H\(_6^+\)) employing the parallelized Time Dependent Discrete Variable Representation (TDDVR) approach. For this purpose, we have considered two multi-state multi-mode model Hamiltonians of C6H\(_6^+\) with degeneracy among the eectronic states: (a) One consists of three states and eight modes, which in turn leads to a five state thirteen mode Hamiltonian (X 2E1g -B 2E2g -C 2A2u ) due to the degeneracy; and (b) The other is constituted of three states and thirteen modes which is basically a five state twenty mode Hamiltonian (B 2E2g -D 2E1u -E 2B2u ) for the same reason. Since these electronic states are interconnected by several conical intersections in the vicinity of the Franck Condon region, it will be challenging to pursue such large dynamical calculation in the presence of nonadiabaticity among the electronic states. The spectral as well as population profiles calculated with the advent of TDDVR approach show reasonably good agreement with the results obtained by the Multi Configuration Time Dependent Hartree (MCTDH) methodology. The photoelectron spectra of benzene is quantum - classically simulated using parallelized time dependent discrete variable representation (TDDVR) method, where the degeneracy among different states is considered in the realistic model Hamiltonian of corresponding system.

Structure of the C8H8•+ Radical Cation Formed by Electron Impact Ionization of Acetylene Clusters. Evidence for a (Benzene•+·Acetylene) Complex

Here, we report ion mobility experiments and theoretical studies aimed at elucidating the identity of the C8H8•+ ion formed by electron impact ionization of neutral acetylene clusters. The ion dissociates by a dominant low-energy channel involving the loss of an acetylene molecule, leaving behind a stable benzene radical cation. Using equilibrium thermochemical measurements, the enthalpy and entropy changes of association of acetylene to the benzene radical cation are measured as −4.0 ± 1 kcal/mol and −11.4 ± 2.5 cal/(mol K), respectively. Ion mobility measurement indicates the presence of mainly one isomer with an average collision cross section in helium of 61.0 ± 3 Å2, significantly larger than the calculated cross sections of all of the covalently bonded C8H8•+ ions and in excellent agreement with that of the benzene•+·acetylene complex. The results provide strong evidence that the C8H8•+ ion is predominantly present as an acetylene molecule associated with the benzene radical cation in ionized acetylene clusters.

Structures of protonated benzene dimer and intermolecular interaction decomposition via symmetry-adapted perturbation theory

Intermolecular interactions of the protonated benzene dimer are examined through computed interaction energies of three distinct dimer configurations and the decomposition of the interaction into electrostatic, induction, exchange-repulsion, and dispersion components. Protonated benzene dimer is an effective prototype for the study of positively charged, non-covalent interactions commonly encountered throughout aromatic regions of complex biological systems. Three distinct configurations of the dimer are identified: parallel-displaced, T-shaped, and canted. Equilibrium interaction energies computed with CCSD(T) range from −11.4 to −10.0 kcal mol −1 where the lowest energy structures involve direct interaction of the protonation site with the top face of benzene. All three conformers contain unorthodox, cationic, CH–π hydrogen bonds. Comparison of the parallel-displaced structures of C 6 H 6 – C 6 H 7 + and ( C 6 H 6 ) 2 + reveals that the interaction between benzene and protonated benzene is weaker than the interaction between benzene and benzene radical cation. Symmetry adapted perturbation theory provides evidence that the benzene–benzenium attractive interaction consists primarily of equal contributions from dispersion and electrostatics.

A “classical” trajectory driven nuclear dynamics by a parallelized quantum‐classical approach to a realistic model Hamiltonian of benzene radical cation

AbstractWe explore the workability of a parallelized algorithm of time‐dependent discrete variable representation (TDDVR) methodology formulated by involving “classical” trajectories on each DOF of a multi‐mode multi‐state Hamiltonian to reproduce the population dynamics, photoabsorption spectra and nuclear dynamics of the benzene radical cation. To perform such dynamics, we have used a realistic model Hamiltonian consists of five lowest electronic states (X2E1g, B2E2g, C2A2u, D2E1u, and E2B2u) which are interconnected through several conical intersections with nine vibrational modes. The calculated nuclear dynamics and photoabsorption spectra with the advent of our parallelized TDDVR approach show excellent agreement with the results obtained by multiconfiguration time‐dependent Hartree method and experimental findings, respectively. The major focus of this article is to demonstrate how the “classical” trajectories for the different modes and the “classical” energy functional for those modes on each surface can enlight the time‐dependent feature of nuclear density and its' nodal structure. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Competitive Adsorption and Interaction of Benzene and Oxygen on Cu/HZSM5 Zeolites

The competitive adsorption of benzene and oxygen on Cu/HZSM5 zeolites has been studied by FTIR and EPR to understand the gas-phase oxidation of benzene to phenol using molecular oxygen. Benzene proved to be the stronger adsorbate on copper, leading to a poisoning of the catalyst. Phenol, water, and deep combustion products could be detected. The evolution of the benzene radical cation was observed in situ, while benzene and oxygen were adsorbed on the sample either simultaneously or alternatingly. At the same time, the total spin density of the sample and thus the oxidation state of copper were monitored. A new reaction path based on the interplay of the zeolitic hydroxyl groups and the exchanged copper ions was proposed. The benzene resides inside the zeolites channel intersections, but due to benzene molecules blocking the channels, the products are hardly able to diffuse out and thus have to be extracted.

ChemInform Abstract: Reactions of Dihalobenzene Radical Cations with Ammonia in the Gas Phase. Reactivity Pattern for Nucleophilic Aromatic Substitution.

The reactions of homo- and heterodisubstituted dihalobenzene radical cations with NH3 were investigated by FT-ICR spectrometry. A halogen atom X (X = Cl, Br, I) is substituted in a gas-phase nucleophilic ipso substitution, yielding haloanilinium ions. The reaction efficiency, i.e., the percentage of reactive ion-molecule collisions, ranges from < 0.006% for 1-chloro-4-iodobenzene radical cations to 18% for 1-bromo-2-chlorobenzene radical cations. The reactivity of the halogenated benzene radical cations was found to be governed by two structural parameters. First, the radical cations with a low ionization energy, i.e., iodobenzene and its derivatives, react especially slowly regardless of the reaction exothermicity. Second, the reactivity of all dihalobenzene radical cations is strongly influenced by the substitution pattern. The reactivity of isomeric radical cations is always highest for the 1,2-isomer and lowest for the 1,4-isomer. These results show that the nucleophilic substitution of the halo- and dihalobenzene radical cations by NH3 proceeds by a multistep reaction mechanism with a double-well potential energy surface. The rate-determining step is the addition of NH3 to the aromatic radical cation in the collision complex. The structural parameters influencing this reaction step can be analyzed by the reactivity model of polar reactions of Shaik and Pross. From this the most important feature of the substitution reaction of mono- and dihalobenzene radical cations with NH3 is the different charge localization in reactants and products. This results in a strong influence of the difference of the ionization energies of the halogenated benzene and NH3 and of the dipole moment of the halogenated benzene, i.e., the precursor of the ionic reactant, on the activation energy of the addition step.

Spectroscopic Study of the Adsorption of Benzene on Cu/HZSM5 Zeolites

By interpretation of the sequence in which perturbations or new FTIR bands appear as a function of benzene loading it is concluded that the adsorption path follows the route through the straight channels into the channel intersections and at last into the sinusoidal channels of HZSM5. FTIR-derived site-specific adsorption isotherms revealed the adsorption and saturation behavior of benzene on the hydroxyl groups of the zeolite and on the copper ions. The overall adsorption order for ZSM5 zeolites is Cu > terminal Si(OH) > internal Si(OH)Al. The benzene molecules adsorb atop of the cation or are slightly tilted. While some of the Cu(II) ions are reduced, others suffer a change of environment, and benzene cation radicals are formed. There are two copper peroxide dimers. One of them is sensitive to benzene adsorption but is not cleaved by it, as in the case of the adsorption of oxygen. The other one seems to be secluded and unable to interact either with benzene or with oxygen. The presence of Na+ ions does ...

A quantum-classical simulation of the nuclear dynamics in NO 2 − and C6H 6 + with realistic model Hamiltonian

We perform the nuclear dynamics simulation to calculate the broad band as well as better resolved (a) photodetachment spectra of nitrogen dioxide anion (NO −2 ), and (b) photoelectron spectra of benzene radical cation (C6H +6 ) with degeneracy among the electronic states using our parallelized time dependent discrete variable representation approach. For this purpose, we first consider a theoretical model that describes the photodetachment spectrum originating due to the Franck-Condon (FC) transition from the anionic ground state to the conically intersecting \(\tilde X^2 A_1 - \tilde A^2 B_2\) electronic manifold of the neutral NO2, whereas two multi-state, multi-mode model Hamiltonians of C6H +6 with degeneracy among the electronic states are explored for its’ photoelectron spectra. The dynamics of C6H +6 has been carried out with two Hamiltonians: (a) one consists with four states and nine modes, and (b) the other is constituted with three states and seven modes. Since the electronic states of both the systems are interconnected by several conical intersections in the vicinity of the FC region, it would be challenging to persue the dynamical calculation due to the impact of non-adiabaticity among the electronic states. The spectral profiles obtained with the advent of TDDVR approach show reasonably good agreement with the results obtained by other theoretical methodologies and experimental measurements.

Monomer and Dimer Radical Cations of Benzene, Toluene, and Naphthalene

Pulse radiolytic generation of monomeric and dimeric cations of benzene, toluene, and naphthalene in aqueous acid media at room temperature and their spectrophotometric characterization is discussed. Results presented include measurements of each aromatic's solubility in H(2)O-H(2)SO(4) and H(2)O-HClO(4) media over the acidity range pH 1 to H(0) -7.0, facile oxidative generation, and real-time identification of appropriate cationic transients with respective lambda(max) (nm) and epsilon (M(-1) cm(-1)) values measured as follows: C(6)H(6)(*+) (443, 1145 +/- 75), C(6)H(5)CH(3)(*+) (428, 1230 +/- 90), C(10)H(8)(*+) (381, 3650 +/- 225, and 687, 2210 +/- 160), (C(6)H(6))(2)(*+) (860, 2835 +/- 235), (C(6)H(5)CH(3))(2)(*+) (950, 1685 +/- 155), and (C(10)H(8))(2)(*+) (1040, 4170 +/- 320). Kinetic measurements reveal the respective formation rates of monomeric cations to be near-diffusion controlled, while the forward rate values for the dimeric species generation are marginally slower. The proton activity corrected pK(a) values are found to remain between -2.6 and -1.3 for the ArH(*+) species (C(6)H(6)(*+) most acidic, C(10)H(8)(*+) least acidic), while the pK(a) values of (ArH)(2)(*+) species vary from -5.0 to -3.0 ((C(6)H(6))(2)(*+) most acidic, (C(10)H(8))(2)(*+) least acidic). In H(0) -5 in aqueous H(2)SO(4), the respective stabilization energy of (C(6)H(6))(2)(*+), (C(6)H(5)CH(3))(2)(*+), and (C(10)H(8))(2)(*+) is estimated to be 16.6, 15.0, and 13.7 kcal mol(-1). Thus, the aqueous acid solution emerges as an alternative medium for typical radical-cationic studies, while offering compatibility for the deprotonated radical characterization near neutral pH.

The multistate multimode vibronic dynamics of benzene radical cation with a realistic model Hamiltonian using a parallelized algorithm of the quantumclassical approach

We demonstrate the workability of a parallelized algorithm of the time-dependent discrete variable representation (TDDVR) method to explore the detailed dynamical aspects of vibronic interaction in two three-state model Hamiltonians (X (2)E(1g), B (2)E(2g), C (2)A(2u) and B (2)E(2g), D (2)E(1u), E (2)B(2u)) of benzene radical cation along with a preliminary investigation on its five electronic states (X (2)E(1g), B (2)E(2g), C (2)A(2u), D (2)E(1u), and E(2)B(2u)). Since those electronic states are interconnected through a series of conical intersections, we have used six and nine vibronically important modes for the three- and five-state Hamiltonians, respectively, in order to perform the quantum dynamics on such system. The population profiles calculated by using our TDDVR approach show reasonably good agreement with the results obtained by exact quantum mechanical (multiconfiguration time-dependent Hartree) method, whereas the corresponding (calculated) photoabsorption spectra originating from various electronic states agree well with the experimental ones. It is important to note that the parallelized algorithm of our TDDVR approach reduces the computation cost by more than an order of magnitude compared to its serial analog. The TDDVR approach appears to be a good compromise between accuracy and speed for such large molecular system, where quantum mechanical description is needed in a restricted region.

Radical cations of aromatic molecules with high ionization potentials on the surfaces of oxide catalysts: Formation, properties, and reactivity

The results obtained by the authors on the formation and properties of the radical cations of benzene and other molecules with high ionization potentials on the surfaces of ZSM-5 zeolites and sulfated zirconium dioxide are analyzed in this paper. It was found that radical cations of aromatic compounds can be obtained both by a thermal process and under illumination with visible light. Radical cations were found to be key intermediates in the low-temperature polycondensation of aromatic compounds on these catalysts. A possible mechanism of the formation of radical cations and the nature of the sites responsible for these processes are discussed.

The Jahn–Teller and pseudo-Jahn–Teller effects in the low-lying electronic states of 1,3,5-trifluorobenzene radical cation

Static and dynamic aspects of the multimode Jahn-Teller (JT) and pseudo-Jahn-Teller (PJT) interactions in the low-lying electronic states of 1,3,5-trifluorobenzene radical cation (TFBZ(+)) are investigated by an ab initio quantum dynamical approach. The electronic ground state (tilde X(2)E'') of TFBZ(+) is energetically well separated from its excited states. The first three excited electronic states (tilde A(2)A(2)", tilde B(2)E' and tilde C(2)A(2)') are however, energetically close. A sequence of low-energy conical intersections underlying the JT and PJT interactions among these electronic states is established. Nonadiabatic effects due to these intersections on the vibronic dynamics are examined in detail. The theoretical results are compared with the low resolution and also better resolved experimental data. The impact of increasing fluorination on the structure and dynamics of the excited states is discussed in relation to the parent benzene radical cation and its mono- and di-fluoro derivatives.

Radical cation/radical reactions: A Fourier transform ion cyclotron resonance study of allyl radical reacting with aromatic radical cations

A method for the study of reactions of open-shell neutrals (radicals) and radical cations is described. Pyrolysis (25–1500 °C) of thermally labile compounds, such as, 1,5-hexadiene via a Chen nozzle yields a seeded beam of reactive species in helium. The pyrolysis products are then analyzed by electron ionization (EI) or reacted with stored ions. Electron ionization of the pyrolysis products of 1,5-hexadiene shows that both the allyl radical and allene are generated. Reactions of benzene radical cations and the pyrolysis products of 1,5-hexadiene result in carbon–carbon bond formation. Those reactions of allyl radical with the benzene radical cation yield the C 7H 7 + ion of m/ z 91, permitting an unusual entry into arenium ions. The reaction of allene with benzene radical cation in contrast yields C 9H 10 + and C 9H 9 +.

Multistate vibronic interactions in difluorobenzene radical cations. II. Quantum dynamical simulations

The multistate vibronic dynamics in the X-D electronic states of all three difluorobenzene radical cations are investigated theoretically by an ab initio quantum dynamical approach. The vibronic coupling scheme and the ab initio values of the system parameters are adopted from Paper I [S. Faraji and H. Koppel, J. Chem. Phys. 129, 074310 (2008)]. Extensive calculations by wave-packet propagation have been performed with the aid of the multiconfiguration time-dependent Hartree method. Five coupled electronic potential energy surfaces and 10 (11 in the case of the orthoisomer) vibrational degrees of freedom have been included in these calculations. The nonadiabatic interactions lead to the restructuring of the photoelectron spectral envelopes. Ultrafast internal conversion processes within the electronic manifolds in question demonstrate the strength of the nonadiabatic coupling effects and complement the analogous findings for the electronic spectra. The internal conversion dynamics is characterized by a stepwise transfer of the electronic population to the lowest electronic state on a time scale of femtoseconds to picoseconds. A difference between the three isomers is found to be related to the weaker interaction between the sets of X-A and B-C-D states (with high-energy conical intersections) in the meta isomer, as compared to the other isomers. The implications of these findings for the qualitative understanding of the fluorescence dynamics of fluorinated benzene radical cations are discussed.

HYPERFINE COUPLING CONSTANTS OF FLUORINATED BENZENE RADICAL CATIONS: A DFT B3LYP AND MP2 STUDY

The EPR spectra of a series of fluorinated benzene radical cations were first-time assigned by MP2//B3LYP quantum chemical calculations. Most of the calculated values are in agreement with the experimentally observed ones. The hyperfine structures of some cations with near-degenerated states were well assigned with the help of extra QCISD calculations.

Evidences for charge‐transfer complex formation in the benzene adsorption on sulfated TiO2–a resonance Raman spectroscopy investigation

AbstractBenzene adsorbed on highly acidic sulfated TiO2 (S‐TiO2) shows an intriguing resonance Raman (RR) effect, with excitation in the blue‐violet region. There are very interesting spectral features: the preferential enhancement of the e2g mode (1595 cm−1) in relation to the a1g mode (ring‐breathing mode at 995 cm−1) and the appearance of bands at 1565 and 1514 cm−1. The band at 1565 cm−1 is probably one of the components of the e2g split band, originally a doubly degenerate mode (8a, 8b) in neat benzene, and the band at 1514 cm−1 is assigned to the 19a mode, an inactive mode in neat benzene. These facts indicate a lowering of symmetry in adsorbed benzene, which may be caused by a strong interaction between S‐TiO2 and the benzene molecule with formation of a benzene to Ti (IV) charge transfer (CT) complex or by the formation of a benzene radical cation species. However, the RR spectra of the adsorbed benzene cannot be assigned to the benzene radical cation because the observed wavenumber of the ring‐breathing mode does not have the value expected for this species. Moreover, it was found by ESR measurements that the amount of radicals was very low, and so it was concluded that a CT complex is the species that originates the RR spectra. The most favorable intensification of the band at 1595 cm−1 in the RR spectra of benzene/S‐TiO2 at higher excitation energy corroborates this hypothesis, as an absorption band in this energy range, assigned to a CT transition, is observed. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Structural Studies on the Radical Cations of Benzene, Naphthalene, Biphenylene, and Anthracene Fully Annelated with Bicyclo[2.2.2]octene Frameworks [J. Am. Chem. Soc. 2000, 122, 10007−10016

ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionNEXTORIGINAL ARTICLEThis notice is a correctionStructural Studies on the Radical Cations of Benzene, Naphthalene, Biphenylene, and Anthracene Fully Annelated with Bicyclo[2.2.2]octene Frameworks [J. Am. Chem. Soc. 2000, 122, 10007−10016].Akira Matsuura, Tohru Nishinaga, and Koichi KomatsuCite this: J. Am. Chem. Soc. 2007, 129, 47, 14830Publication Date (Web):November 6, 2007Publication History Published online6 November 2007Published inissue 28 November 2007https://doi.org/10.1021/ja077519gCopyright © 2007 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views554Altmetric-Citations1LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (45 KB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information Get e-Alerts