622 research outputs found
Internal Solvation Effects on the Reactivity of alpha,omega-Diphenylalkanes toward ME(3)C(+) Ions
No full textCrestoni ME, Fornarini S, Kuck D. Internal Solvation Effects on the Reactivity of alpha,omega-Diphenylalkanes toward ME(3)C(+) Ions. Journal of Physical Chemistry. 1995;99(10):3144-3149.The reactivity of alpha,omega-diphenylalkanes (1 less than or equal to n less than or equal to 4) toward Me(3)C(+) ions has been investigated with the radiolytic technique at 720 Torr, at temperatures of 47 and 120 degrees C. The intramolecular isotopic discrimination of Me(3)C(+), favoring attack at the unlabeled ring of C6H5(CH2)(2)C6D5 by a factor of 1.5 at 120 degrees C, contrasts with the lack of intermolecular isotopic discrimination, reflected by the same reactivity toward (C6H5CH2)(2) and (C6D5CH2)(2). Competition experiments show an appreciably higher reactivity of Ph(CH2)(n)Ph (n = 2-4) relative to toluene and diphenylmethane (DPM), but, when Ph(CH2)(n)Ph (n = 3, 4) or (3-CH3C6H4)(CH2)(2)C6H5 compete with Ph(CH2)(2)Ph, relative reactivities level off. The lack of substrate selectivity, in contrast to an intramolecular discrimination in the tert-butylation of (3-CH3C6H4)(CH2)(2)C6H5 of a factor of 2, together with the related variation of kinetic isotope effects, points out the kinetic role of the collision complex 1 from Me(3)C(+) and diphenylalkanes. The additional (''spectator'') ring of the higher homologues Ph(CH2)(n)Ph (n = 2-4) prevents dissociation of 1, making its formation irreversible and causing tert-butylation to occur at the encounter rate, in contrast to DPM, whose second ring appears essentially inert. Experiments involving substitution by Me(3)Si(+) at 120 degrees C show that this electrophile attacks preferably the unlabeled ring of C6H5(CH2)(2)C6D5 by a factor of 1.5, consistent with the 1.4 times higher reactivity toward (C6H5CH2)(2) With respect to (C6D5CH2)(2). In this case, the isotopic discrimination is traced to the competition between desilylation and deprotonation of intermediate ipso-silylated arenium ions
Gas-Phase Protonation of Diphenylalkanes
No full textCrestoni ME, Fornarini S, Kuck D. Gas-Phase Protonation of Diphenylalkanes. Journal of Physical Chemistry. 1995;99(10):3150-3155.The gas-phase protonation of alpha,omega-diphenylalkanes ranging from diphenylmethane to 1,6-diphenylhexane and of 1-phenyl-2-(m-tolyl)ethane has been examined by FT-ICR mass spectrometry. Their gas-phase basicities (GB) at 300 K are consistently higher than that of toluene, as expected from their higher polarizability, and depend on the length of the methylene chain joining the two phenyl groups. This noticeable structure dependence is ascribed to an intramolecular solvation effect exerted by the second phenyl ring on the protonated one, allowed by a proper conformation of the linking chain. This effect is confirmed by a ca. 2 kcal mol(-1) decrease in basicity when the protonated and the neutral phenyl rings are separated by the same number of C-C bonds in a rigid trans-cyclohexane-1,4-diyl unit versus a flexible chain
Biomimetic Oxidation Reactions of a Naked Manganese(V)–Oxo Porphyrin Complex
No full textThe intrinsic reactivity of a manganese(V)-oxo porphyrin complex, a typically fleeting intermediate in catalytic oxidation reactions in solution, has been elucidated in a study focused on its gas-phase ion-chemistry. The naked high-valent Mn(V)-oxo porphyrin intermediate 1 ([(tpfpp)Mn(V)O](+); tpfpp=meso-tetrakis(pentafluorophenyl) porphinato dianion), has been obtained by controlled treatment of [(tpfpp)Mn(III)]Cl (2-Cl) with iodosylbenzene in methanol, delivered in the gas phase by electrospray ionization and assayed by FT-ICR mass spectrometry. A direct kinetic study of the reaction with selected substrates, each containing a heteroatom X (X = S, N, P) including amines, sulfides, and phosphites, was thus performed. Ionic products arising from electron transfer (ET), hydride transfer (HT), oxygen-atom transfer (OAT), and formal addition (Add) may be observed, with a predominance of two-electron processes, whereas the product of hydrogen-atom transfer (HAT), [(tpfpp)Mn(IV)OH](+), is never detected. A thermochemical threshold for the formation of the product radical cation allows an evaluation of the electrontransfer ability of a Mn(V)-oxo complex, yielding a lower limit of 7.85 eV for the ionization energy of gaseous [(tpfpp)Mn(IV)O]. Linear free-energy analyses of the reactions of para-substituted N, N-dimethylanilines and thioanisoles indicate that a considerable amount of positive charge is developed on the heteroatom in the oxidation transition state. Substrates endowed with different heteroatoms, but similar ionization energy display a comparable reaction efficiency, consistent with a mechanism initiated by ET. For the first time, the kinetic acidity of putative hydroxo intermediates playing a role in catalytic oxidations, [(tpfpp)Fe(IV)OH](+) and [(tpfpp)Mn(IV)OH](+), has been investigated with selected reference bases, revealing a comparatively higher basicity for the ferryl, [(tpfpp)Fe(IV)O], with respect to the manganyl, [(tpfpp)Mn(IV)O], unit. Finally, the neat association reaction of 2 has been studied with various ligands showing that harder ligands are more strongly bound
Role of the spectator ring in the gas-phase alkylation of 1,2-diphenylethane by (CH3)2X+ (X = F, Cl) and CF3+ ions.
No full textThe gas-phase alkylation of C6D5CH2CH2C6H5 by Me2F+, Me2Cl+, and CF3+ ions has been studied at 720 Torr, in the temperature range 47-120-degrees-C with the radiolytic technique. An interannular hydron-exchange process between the two aromatic rings of (beta-phenylethyl)arenium ions has been observed to occur on the nanosecond time scale and to obey thermal kinetics, with the normal positive dependence on temperature. When the efficiency of collisional deactivation is decreased (at 200 torr) or a highly exothermic reaction (alkylation by CF3+ ion) is carried out, an ''anomalously'' large extent of interannular hydron exchange takes place. The higher methylation rate (6.0:1) of 1,2-diphenylethane than of toluene and the marked positional selectivity (2 para/ortho ratio of 5.0) shown by Me2X+ ions have pointed out the fundamental role of persistent ion-neutral complexes between reactant pairs, as already suggested in previous radiolytic and mass spectrometric studies
Ionic Lewis superacids in the gas phase. Part 5. Competing ligand displacements in the adducts between SiF3-x(OH)x+ (x = 0-3) ions and H218O.
No full textThe actual nature of the ionic intermediates involved in the gas-phase reaction of SiF3+ with water has been ascertained using a Fourier-transform ion cyclotron resonance (FT-ICR) instrument equipped with an external ion source. The complete kinetic pattern following attack of SiF3-x(OH)x+ (x = 0-3) on H-2 O-18 has been investigated, revealing the occurrence of competing H-2 O-18-to-HF and H-2 O-18-to-H-2 O-16 displacement processes, when x = 1 or 2. The factors governing the relative extent of such competing reactions and the overall reactivity of SiF3-x(OH)x+ (x = 0-3) toward H-2 O-18 are discussed. Occurrence of a relatively fast displacement in the tetracoordinated adducts of SiF3-x(OH)x+ (x = 1-3) with H-2 O-18 prevents their FT-ICR structural characterization
Long-Lived ipso-Silylated p-Toluenium Ions: Evidence from a Kinetic Isotope Effect
No full textA sizeable hydrogen isotope effect arises from the competing abstractions of Me3Si+ and X (X = H, D) from the σ complex, which is formed by addition of gaseous Me3Si+ to toluene. This species may adequately describe the Et3Si+ -toluene pair recently reported in the solid state
Probing the cytochrome P450-like reactivity of high-valent oxo iron intermediates in the gas phase
No full textElectrospray ionization in combination with Fourier transform ion cyclotron resonance spectrometry is used to prepare and characterize at a molecular level high-valent oxoiron intermediates formed in the reaction of [(TPFPP)Fe-III]Cl (TPFPP= meso-tetrakis(pentafluorophenyl)porphinato dianion) (1-Cl) with H2O2 in methanol. The intrinsic reactivity in the gas phase of the iron(IV) oxo porphyrin cation radical complex, [(TPFPP)Fe-center dot+(IV)=O](+), has been probed toward selected substrates (S), chosen among naturally occurring and model compounds. Whereas CO and cyclohexane proved to be unreactive, olefins, sulfides, amines, and phosphites all undergo oxygen atom transfer in the gas phase yielding the reduction product 1 and/or an adduct ion ([1-S](+)). The reaction efficiencies show a qualitative correlation with the oxophilic character of the active site of S. A notable exception is nitric oxide, which displays a remarkably high reactivity, in line with the important role of NO reactions with iron porphyrin complexes. Furthermore, subsidiary information on the neat association reaction of 1 with selected ligands (L) has been obtained by a kinetic study showing that both the efficiency and the extent of ligation toward the naked ion 1 depend on the electron-donating ability of L
FT-ICR study of the gas phase basicity of N-nitroso-dimethylamine
No full textFourier transform ion cyclotron resonance study of the gas-phase
basicity of N-nitrosodimethylamin
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