Putting aniline radical cations in a bottle

Salts containing aniline radical cations have been isolated and characterized by electron paramagnetic resonance(EPR)spectroscopy, UV-Vis absorption spectroscopy and single crystal X-ray diffraction. The EPR spectra and theoretical calculations indicate the unpaired electron is delocalized on phenyl rings and nitrogen atoms. Both radical cations feature a quinoidal geometry with a partially double C–N bond, but are distinct in that the C–N bond is coplanar to the phenyl plane in one cation while deviates from the plane in the other due to steric crowding. The work provides the first unequivocal examples of stable aniline radical cations.     

Reactions of radical cations of acetals: Evidence for unimolecular decomposition

The thermal and photochemical reactions of the methylal radical cation /I/ in freon matrices were studied using selective deuteration for elucidating the structure of the resulting species. /I/ has been shown to decay by unimolecular reaction upon heating to 140 K as well as upon photolysis in CFCl₃ matrix and the product of decay has been assumed to be the complex of formaldehyde radical cation with CFCl₃. Such decay reaction has been demonstrated for 1,3-dioxolan radical cation as well.     

Photoinduced deprotonation of cyclopentene oxide radical cations in low-temperature Freon matrices

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Computation of pKa values of substituted aniline radical cations in dimethylsulfoxide solution

A newly developed computation strategy was used to calculate the absolute pKa values of 18 substituted aniline radical cations in dimethylsulfoxide (DMSO) solution with the error origin elucidated and deviation minimized. The B3LYP/6-311++G(2df,2p) method was applied and was found to be capable of reproducing the gas-phase proton-transfer free energies of substituted anilines with a precision of 0.83 kcal/mol. The IEF-PCM solvation model with gas-phase optimized structures was adopted in calculating the pKa values of the substituted neutral anilines in DMSO, regenerating the experimental results within a standard deviation of 0.4 pKa unit. When the IEF-PCM solvation model was applied to calculate the standard redox potentials of anilide anions, it showed that the computed values agreed well with experiment, but the redox potentials of substituted anilines were systematically overestimated by 0.304 eV. The cause of this deviation was found to be related to the inaccuracy of the calculated solvation free energies of aniline radical cations. By adjusting the size of the cavity in the IEF-PCM method, we derived a reliable procedure that can reproduce the experimental pKa values of aniline radical cations within 1.2 pKa units to those from experiment.     

Mobile protons versus mobile radicals: Gas-phase unimolecular chemistry of radical cations of cysteine-containing peptides

A combination of electrospray ionization (ESI), multistage, and high-resolution mass spectrometry experiments are used to examine the gas-phase fragmentation reactions of radical cations of cysteine containing di- and tripeptides. Two different chemical methods were used to form initial populations of radical cations in which the radical sites were located at different positions: (1) sulfur-centered cysteinyl radicals via bond homolysis of protonated S-nitrosocysteine containing peptides; and (2) α-carbon backbone-centered radicals via Siu’s sequence of reactions (J. Am. Chem. Soc. 2008, 130, 7862). Comparison of the fragmentation reactions of these regiospecifically generated radicals suggests that hydrogen atom transfer (HAT) between the α C-H of adjacent residues and the cysteinyl radical can occur. In addition, using accurate mass measurements, deuterium labeling, and comparison with an authentic sample, a novel loss of part of the N-terminal cysteine residue was shown to give rise to the protonated, truncated N-formyl peptide (an even-electron x_n ion). DFT calculations were performed on the radical cation [GCG]^.+ to examine: the relative stabilities of isomers with different radical and protonation sites; the barriers associated with radical migration between four possible radical sites, [G.CG]⁺, [GC.G]⁺, [GCG.]⁺, and [GC(S.)G]⁺; and for dissociation from these sites to yield b₂-type ions.     

Contrastive photoreduction pathways of benzophenones governed by regiospecific deprotonation of imidazoline radical cations and additive effects

[Reaction: see text]. In the photoreaction of benzophenones with 1,3-dimethyl-2-phenylbenzimidazoline (DMPBI), benzhydrols were major products. Addition of H2O accelerated the reaction with no change in the product distribution, while AcOH, PhOH, and metal salts such as LiClO4 and Mg(ClO4)2 were effective additives to produce benzpinacols. In contrast, benzpinacols were exclusively formed regardless of the solvent and the additive in the reactions with 2-(o-hydroxyphenyl)-1,3-dimethylbenzimidazoline (o-HPDMBI). These observations are consistent with the hypothesis that DMPBI*+ donates a proton at the C2 position to the benzophenone ketyl radicals while o-HPDMBI*+ donates a phenol proton.     

The kinetics of competing multiple-barrier unimolecular dissociations of o-, m-, and p-chlorotoluene radical cations

The kinetics of competing multiple-barrier unimolecular dissociations of o-, m-, and p-chlorotoluene radical cations to C7H7(+) (benzyl and tropylium) are studied by ab initio/Rice-Ramsperger-Kassel-Marcus (RRKM) calculations. This system presents a very intriguing kinetic example in which the conventional approach assuming a single-barrier or a double-well potential surface with one transition state cannot predict or explain the outcome. The molecular parameters obtained at the SCF level of theory with the DZP basis set are utilized for the evaluation of microcanonical RRKM rate constants with no adjustable parameters. First-principles calculations provide the microscopic details of the reaction kinetics along the two competing multiple-barrier reaction pathways: the rate-energy curves for all elementary steps; temporal variations of the reactants, the reaction intermediates, and the products; and the product yield as a function of energy. The rate constant for each channel is calculated as a function of the internal energy at 0 K. After the thermal correction, the calculated rate-energy curves for the benzyl channel agree well with the photoelectron photoion coincidence data obtained at room temperature for all three isomers. Close agreement between experiments and theory suggests that first-principles calculations taking the full sequence of kinetic steps into account offer a useful kinetic model capable of correctly predicting the outcome of competing multiple-barrier reactions. The slowest process is identified as [1,2] and [1,3] alpha-H migration at the entrance to the tropylium and benzyl channel, respectively. However, the overall rate is determined not by the slowest process, but by the combination of the slowest rate and the net flux toward the product, which is multiplicatively reduced with an increasing number of reaction intermediates. The product yield calculation confirms the benzyl cation as the predominant product. For all isomers, the thermodynamically most stable tropylium ion is produced much less than expected because a large fraction of flux coming into the tropylium channel goes back to the benzyl channel. The benzyl channel is kinetically favored because it involves a lower entrance barrier with fewer rearrangements than the tropylium channel.     

Collisional and unimolecular ionisation of monatomic halogen cations

Charge-stripping processes of monatomic halogen cations F⁺, Cl⁺, Br⁺ and I⁺ having keV translational energies were investigated, and found to closely resemble those of the isoelectronic dications of the noble gases. In the absence of collision gas, an ionisation process is still observed which cannot be attributed to charge-stripping collisions of very high cross-section with background gas molecules, nor to surface interactions, nor to field ionisation. The observed time-dependence of the yield from this collision-free process is, however, also difficult to reconcile with an interpretation in terms of autoionisation of some highly excited metastable state. In addition, the appearance energy of the states involved was found to lie within 1– eV of the ionisation onset. A mechanism was proposed which can account qualitatively for these observations. The crucial step in this proposal involves excitation of Rydberg states by ambient (300 K) black-body radiation, a phenomenon well established for neutral atoms.     

Relative reactivities of isopropyl,ethyl and methyl groups in the gas-phase side-chain deprotonation of alkylaromatic radical cations

The relative reactivity of isopropyl, ethyl and methyl groups in the gas-phase side-chain deprotonation of alkylaromatic radical cations by some pyridines has been determined by using Fourier transform mass spectrometry.     

The unimolecular dissociation of electronic state-selected methyl iodide cations

Within the context of a recently proposed model, the angle-bending Schrödinger equation for linear molecules is cast in the form of the Legendre equation with an added potential. This is solved by a complete set expansion in terms of associated Legendre functions. The resulting solutions are incorporated into the existing model. Computation of the lower vibrational excitations of HCN is carried out.     

The deprotonation of benzyl alcohol radical cations: a mechanistic dichotomy in the gas phase as in solution

The gas-phase acidity of ionized benzyl alcohol and of some of its derivatives with selected reference bases has been studied by Fourier Transform Ion Cyclotron Resonance (FT-ICR) mass spectrometry. The aim was to relate the gas-phase reactivity to the behavior in aqueous solution of the radical cations of benzyl alcohols bearing methoxy substituent(s) on the phenyl ring which are known to undergo deprotonation at both the CH2 and OH groups. The dual reactivity behavior is confirmed in the gas phase, in which the prototypical ion, C6H5CH2OH*+, is deprotonated at both the CH2 and OH groups, whereas the ring hydrogens are not involved. An increasing extent of O-deprotonation is shown as the strength of the base increases. Appropriate methyl substitution, as in the radical cations of C6H5C(Me)2OH and C6H5CH2OMe, allows only O- or C-acidity. The two processes are characterized by comparable thermodynamic features with a Gas-phase Basicity (GB) value of 852 kJ mol(-1) for the cumyloxyl radical and 850 kJ mol(-1) for the alpha-methoxybenzyl radical. The possible origin of the observed mechanistic dichotomy is discussed.     

Enthalpy considerations applied to the relatively slow unimolecular reactions of a series of homologous CnH2n radical cations

The concept that the slow, unimolecular reactions of the homologous series of radical cations are largely determined by the relative energetics of the possible product combinations is discussed. The concept is shown to provide a reasonably consistent rationalization of the known metastable dissociations of and correctly predicts the reactions undergone by . The concept may be useful in discussing the unimolecular chemistry of isomers.     

Reactivity and reaction pathways of alkylalkoxybenzene radical cations. Part 3. Effects of 2-alkyl substituents on the relative importance of ring-substitution over deprotonation of 2-alkyl-1,4-dimethoxybenzene radical cations

One-electron oxidation of 2-alkyl-1,4-dimethoxybenzenes 1a-f (2-alkyl=Me, Et, i-Pr, cy-C₃H₅CH₂, PhCH₂ and t-Bu) by 4-nitrobenzoyl peroxide 2 and pentaflurobenzoyl peroxide 3 was proved by the observation of great acceleration of decomposition of the peroxides at room temperature, the detection of the corresponding radical cations 1 ^+? a-f and product analysis. The product studies have disclosed that under the conditions employed (in acetonitrile at 40°C), the reaction pathways of the radical cations are greatly dependent on the nature of 2-alkyl substituents: Ring-4-nitrobenzoloxylation product at C ₅ and C ₆ were obtained exclusively in the reactions of the donors with aliphatic 2-alkyl substituents bearing at least one α-hydrogen atom, such as 1a, 1b, 1c and 1d; whereas in the case of 1e (with 2-benzyl group), both ring-substitution at C ₅ (4e) and C ₆ (5e) and deprotonation/4-nitrobenzoloxylation products 8e were isolated; from the donor without α-hydrogen atom, 1f, de-t-butylation products 12 and t-butyl 4-nitrobenzoate 13 were incorporated with ring-substitution at C ₅ (4f) and C ₆ (5f). Furthermore, the product distribution (4 over 5) is also affected by the bulkiness of 2-alkyl group. For all the electron-transfer reactions, large amounts of the benzoic acid (4-NO₂-C₆H₄COOH or C₆F₅COOH) were generated and trace amounts of de-methylation product (2-alkyl-1,4-benzoqinones 6) were also detected by ¹H NMR.     

Efficient production of enol ether radical cations by heterolytic cleavage of beta-mesylate radicals

[reaction: see text] alpha-Methoxy-beta-mesyloxy radicals were produced in laser flash photolysis reactions, and yields of enol ether radical cations formed by heterolytic fragmentation of the mesylate group were determined. The mesylate heterolysis reaction is faster than heterolyses of phosphate and bromide groups in analogous radicals and highly efficient in medium-polarity solvents.     

Polyfluorinated radical cations

The synthesis of the first trifluoromethanesulfonate esters of the type CF₃SO₃(CH₂)_nO₃SCF₃ (n=1,2,3) are reported. The new compounds are prepared from Cl(CH₂)_nCl by substitutive electrophilic dehalogenation reactions with CF₃SO₂OX (x=Cl,Br). The extension of this reaction to HCCl₃ results in HC(O₃SCF₃)₃ but the compound is unstable at 22°.     

Oxidation of aminoindole radical cations

The varying stabilities of certain aminoindole radical cations toward oxidation with molecular oxygen have been studied. Oxidation leads to different products depending upon the environment around the N-amino nitrogen. A plausible reaction mechanism is proposed based on electronic and magnetic resonance spectroscopy.     

The unimolecular chemistry of [1,2-propanediol]+˙: a rationale in terms of hydrogen-bridged radical cations

By combining results from a variety of mass spectrometric techniques (metastatle ion, collisional activation, collision-induced dissociative ionization, neutralization–reionization spectrometry and appearance energy measurements) and the classical method of isotopic labelling, a unified mechanism is proposed for the complex unimolecular chemistry of ionized 1,2-propanediol. The key intermediates involved are the stable hydrogen-bridged radical cations [CH₂?C(H)? H…?O…?O(H)CH₃]⁺˙, which were generated independently from [4-methoxy, 1-butanol]⁺˙ (loss of C₂H₄) and [1-methoxyglycerol]⁺˙ (loss of CH₂O), [CH₃? C?O…?H…?O(H)CH₃]⁺˙ and the related ion-dipole complex [CH₂?C(OH)CH₃/H₂O]⁺˙. The latter species serves as the precursor for the loss of CH³˙ and in this reaction the same non-ergodic behaviour is observed as in the loss of CH³˙ from the ionized enol of acetone.     

Aminium cation radical of glycylglycine and its deprotonation to aminyl radical in aqueous solution

The photochemical reaction between glycylglycine and triplet 4-carboxybenzophenone has been investigated using time-resolved chemically induced dynamic nuclear polarization (CIDNP). It is shown that the mechanism of the peptide reaction with triplet excited carboxybenzophenone is electron transfer from the amino group of the peptide, leading to the formation of an aminium cation radical that deprotonates to a neutral aminyl radical. Simulation of the CIDNP kinetics leads to an estimation of the paramagnetic relaxation time for the alpha-protons at the N-terminus at 20 to 40 mus with the best-fit value of 25 mus.