98-06-6Relevant academic research and scientific papers
Accurate oxidation potentials of benzene and biphenyl derivatives via electron-transfer equilibria and transient kinetics
Merkel, Paul B.,Luo, Pu,Dinnocenzo, Joseph P.,Farid, Samir
experimental part, p. 5163 - 5173 (2009/12/06)
(Graph Presented) Nanosecond transient absorption methods were used to determine accurate oxidation potentials (Eox) in acetonitrile for benzene and a number of its alkyl-substituted derivatives. Eox values were obtained from a combination of equilibrium electron-transfer measurements and electron-transfer kinetics of radical cations produced from pairs of benzene and biphenyl derivatives, with one member of the pair acting as a reference. Using a redox-ladder approach, thermodynamic oxidation potentials were determined for 21 benzene and biphenyl derivatives. Of particular interest, Eox values of 2.48 ± 0.03 and 2.26 ± 0.02 V vs SCE were obtained for benzene and toluene, respectively. Because of a significant increase in solvent stabilization of the radical cations with decreasing alkyl substitution, the difference between ionization and oxidation potentials of benzene is ～0.5 eV larger than that of hexamethylbenzene. Oxidation potentials of the biphenyl derivatives show an excellent correlation with substituent σ+ values, which allows Eox predictions for other biphenyl derivatives. Significant dimer radical cation formation was observed in several cases and equilibrium constants for dimerization were determined. Methodologies are described for determining accurate electrontransfer equilibrium constants even when dimer radical cations are formed. Additional equilibrium measurements in trifluoroacetic acid, methylene chloride, and ethyl acetate demonstrated that solvation differences can substantially alter and even reverse relative Eox values.
Coupled reactions of condensation and charge transfer. 1. Formation of olefin dimer ions in reactions with ionized aromatics. Gas-phase studies
Meot-Ner, Michael,Pithawalla, Yezdi B.,Gao, Junling,El-Shall, M. Samy
, p. 8332 - 8341 (2007/10/03)
The toluene radical ion C6H5CH3(·)+, generated by resonance two-photon ionization, does not react with a single isobutene molecule (i-C4H8) which has a significantly higher ionization potential (ΔIP = 0.42 eV). However, a reaction is observed involving two i-C4H8 molecules, to form the dimer ion C8H16(·)+. A coupled reaction of dimer formation and charge transfer to the dimer is exothermic if the product is an ionized hexene with a low IF. Correspondingly, the observed nominal second-order rate coefficients, (5-25) x 10-12 cm3 s-1, are enhanced by a factor of > 105 over the expected value for direct endothermic charge transfer. Pressure and concentration effects suggest a sequential mechanism that proceeds through a C6H5CH3·+(i-C4H8) reactive π complex. The complex can isomerize to a nonreactive CH3C6H4-t-C4H9(·)+ adduct,or react with a second i-C4H8 molecule to form a C6H5CH3·+-(i-C4H8)2 complex, in which the olefin molecules are activated by the aromatic ion. Similar reactions are observed in the benzene/propene system with a somewhat larger ΔIP of 0.48 eV, suggesting that the charge density on the olefin in the complex is still sufficient to activate it for nucleophilic attack. However, aromatic/olefin systems with ΔIP > 0.87 eV show no olefin dimer formation. At low [i-C4H8] and [Ar] number densities, the rate of formation of C8H16(·)+ is proportional to [i-C4H8]2[Ar]. The corresponding fourth-order rate coefficient shows a strong negative temperature coefficient with k = 11 x 10-42 cm9 s-1 at 300 K and 2 x 10-42 cm9 s-1 at 346 K, suggesting that the mechanism can be efficient in low-temperature industrial and interstellar environments. The direct formation of the dimer bypasses the monomer olefin cation and its consequent side-reactions, and directs the products selectively into radical ion polymerization. The products and energy relationships that apply in the gas phase are observed also in clusters.
Alkyl and Silyl Derivatives of Benzene Radical-cations formed by Radiolysis : an Electron Spin Resonance Study
Ramakrishna Rao, D. N.,Chandra, Harish,Symons, Martyn C. R.
, p. 1201 - 1206 (2007/10/02)
Exposure of dilute solutions of variuos alkyl- and silyl-benzenes in trichlorofluoromethane to 60CoSg-rays at 77 K gave species whose e.s.r. spectra are characteristic of substituted benzene cations.For the ethyl derivative large hyperfine coupling to the methyl protons establishes a preferred conformation in which the methyl group lies in the plane of the benzene ring.For the isopropyl derivative, and particularly for p-cymene cations, several conformes were detected, the sterically most favourable being the least stable.This is interpreted in terms of strong electron-donation from the C-H ?-orbitals into the ring ?-orbital with is greater than that from C-Me ?-orbitals so that ?-overlap with the C-H bonds is maximised.The energy difference is slightly greater than the steric energy differences.The SOMO for the silyl derivatives (SiH3, SiHMe2, and SiMe3) is also the a1 orbital (ΠSa) which places maximum spin-density on the position of the substituent.However, the degree of hyperconjugation involving Si-H ?-orbitals is reduced by a factor of ca. 2, as judged by the 1H hyperfine coupling constants.Nevertheless, it was not found necessary to invoke a mixture of the a1 and a2 orbitals (ΠSS and ΠSA) to explain the results.These results are compared wiyh those for the corresponding radical-anions and for the neutral cyclopentadienyl radicals.
Absorption Spectra and Photochemical Rearrangements of Alkyl- and Dialkylbenzene Cations in Solid Argon
Kelsall, Benuel J.,Andrews, Lester
, p. 5893 - 5898 (2007/10/02)
Matrix photoionization of alkyl- and dialkylbenzenes produced and trapped the parent radical cations.Irradiation in the visible parent cation absorption induced α-H transfer to the cation ring to give substituted methylenecyclohexadiene cations.The ease of 1,3-hydrogen transfer in these experiments suggests that this may be an important rearrangement in gaseous alkylbenzene cations.Subsequent ultraviolet photolysis of these samples produced substituted styrene cations.
Carbon-Hydrogen Bond Dissociation Energies in Alkylbenzenes. Proton Affinities of the Radicals and the Absolute Proton Affinity Scale
Meot-Ner (Mautner), Michael
, p. 5 - 10 (2007/10/02)
Rate constants (k) were measured for proton-transfer reactions from alkylbenzene ions RH+ to a series of reference bases B, i.e., RH+ + B -> BH+ + R*.For exothermic reactions (ΔH -1.For example, the reaction C6H5CH3+ + B -> BH+ + C6H5CH2* is fast (reaction efficiency = k/kcol >/= 0.5) when B = MeO-t-Bu or stronger bases, but k/kcol is significantly smaller when B is n-Pr2O or weaker bases.From the falloff curve of reaction efficiency vs.PA(B), we find PA(n-Pr2O) = PA(C6H5CH2*) + 0.8 kcal mol-1 = 200.0 kcal mol-1.Since PA(C6H5CH2*) is obtained from known thermochemical data, this relation defines the absolute PA of n-Pr2O.Through a ladder of known PA, we then obtain PA(i-C4H8) = 186.8 kcal mol-1; we also obtain the absolute PAs of other oxygen bases.Falloff curves of reaction efficiencies of 3-FC6H4CH3+, C6H5C2H5+, C6H5-n-C3H7+, and C6H5-i-C3H7+ with these reference bases give then the following PAs of R* and R-H bond dissociation energies (Do) (all in kcal mol-1) as R*, PA(R*), Do(R-H): 3-FC6H4CH2*, 197.2, 89.4; , 197.9, 86.2; , 199.1, 86.1; , 199.6, 86.1.In similar manner, rate constants for H+ transfer from C6H5NH2+ to reference pyridines and amines yield PA(C6H5NH*) = 221.5 and Do(C6H5NH-H) = 85.1 kcal mol-1 (1 kcal mol-1 = 4.18 kJ mol-1).
