Isotope effects on gas phase reaction processes. 2. Equilibrium isotope effects on the proton transfer reactions of methylbenzenes

Wolf, J. F.; Devlin, John L.; Defrees, D. J.; Taft, Robert W.; Hehre, Warren J.

doi:10.1021/ja00433a008

Cited by 10 publications

(5 citation statements)

References 2 publications

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“…In addition, solution studies of rearrangements of toluenium, xylenium, and heptamethylbenzenium ions have been reported, but detailed considerations of the transition states involved were not made. Complementary theoretical results for the structures and energies of σ complexes, C 6 H 6 CH 3 + and C 6 H 6 C(CH 3 ) 3 + , have been reported at semiempirical − and ab initio levels. , …”

Section: Introductionmentioning

confidence: 67%

Carbocation−π Interaction: Computational Study of Complexation of Methyl Cation with Benzene and Comparisons with Related Systems

1998

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An investigation of the interaction of carbocations with aromatic rings has been initiated by a computational study of complexation of methyl cation with benzene to determine if this is appropriately included as an example of η6 cation−π interaction. Specifically, electronic structure calculations for three types of π complex of methyl cation with benzene, 1(η6), 2(η2), 3(η1), and the Wheland σ complex 4 have been obtained at several theoretical levels. The results indicate that inclusion of electron correlation is required for accurate calculation of intermolecular distances and binding energies and that the B3LYP/6-31G* level of theory provides a practical, reliable approach for the study of carbocation−π interactions. Although none of the π complexes is an energy minimum, the maximum binding energy of CH3 + above the periphery of the ring is more than twice as large as that of optimum binding above the ring centroid. In addition, the association energies in η2 and η1 complexes are ca. 80% of the binding energy calculated for the equilibrium σ complex. The comparison of results for C6H6- -CH3 +, C6H6- -SiH3 +, and C6H6- -Na+ with earlier theoretical work and with experiment confirms the reliability of the B3LYP/6-31G* method. An examination of the dependence of binding in complexes 1−4 on intermolecular separation was also conducted. The results indicate that at distances >2 Å, a “π approach” toward 2 or 3 has a binding energy which is competitive with the approach to σ complex (4) formation. This work also shows clearly that, in contrast to complexes of coordinatively saturated cations with benzene, at intermolecular distances <3.5 Å an η6 geometry is not the most favorable for π complexation of carbocations. Significantly greater binding energy is obtained anywhere over the periphery of the ring (as in 2 or 3) at any binding distance less than ca. 3.5 Å. Even at distances >3.5 Å, binding at the periphery of the aromatic system is comparable in energy to η6 binding. With respect to the postulated stabilization of carbocation intermediates in biochemical reactions via π interactions with aromatic residues, the results show that very substantial stabilization can be afforded to carbocations positioned appropriately over any portion of a benzene ring and at distances considerably greater than typical covalent bonding distances. Thus, an enforced separation between carbocation and aromatic amino acid side chain residue to avoid unwanted covalent bond formation between protein and substrate would still allow substantial stabilization via carbocation−π interaction.

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Section: Introductionmentioning

confidence: 67%

Carbocation−π Interaction: Computational Study of Complexation of Methyl Cation with Benzene and Comparisons with Related Systems

1998

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“…Furthermore, studies of rearrangement of the resulting carbonium ion such as toluenium or xylenium in gas as well as the solution phase have been reported. − However, detailed theoretical studies including characterization of TSs are very limited . Semiempirical and ab initial studies of structures and energetic relations of some σ-complexes may be found in the literature. − …”

Section: Resultsmentioning

confidence: 99%

Alkylation of Phenol: A Mechanistic View

Chakraborty²,

Faglioni³

et al. 2006

J. Phys. Chem. A

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The current work utilizes the ab initio density functional theory (DFT) to develop a molecular level of the mechanistic understanding on the phenol alkylation in the presence of a cation-exchange resin catalyst, Amberlyst-15. The catalyst is modeled with the benzene sulfonic acid, and the effect of this acid on olefins such as isopropene (i-Pr) and tributene (t-Bu) in a phenol solution mimics the experimental condition. A neutral-pathway mechanism is established to account for early-stage high concentration of the phenolic ether observed in experiments. The mechanism involves an exothermic reaction between olefin and the benzene sulfonic acid to form ester followed by three reaction pathways leading to direct O-alkylation, o-C-alkylation, and p-C-alkylation. Our calculations conclude that O-alkylation to form the phenolic ether is the most energetically favorable in the neutral condition. An ionic rearrangement mechanism describes intramolecular migrations of the alkyl group from the phenolic ether to form C-alkylphenols, while the positively charged protonation significantly lowers transition barriers for these migrations. The ionic rearrangement mechanism accounts for high yields of o-C-alkylphenol and p-C-alkylphenol. Competition between the H atom and the alkyl R group at the substitutive site of the protonated ortho configuration is found to be the determining factor to the ortho/para ratio of C-alkylation products.

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“…Another possibility is the presence of a secondary isotope effect operating on the simple silyl transfers between water and ethanol. Since the whole trimethylsilyl group is transferred, the isotope effect is unlikely to have a large enough magnitude to account for the observed results (19)(20)(21)(22). Competition for exchange and reversion to reactants could occur; however, as discussed below, reversion to reactants is unlikely.…”

Section: Resultsmentioning

confidence: 95%

Measurement of hydrogen/deuterium ratios in ethanol/ethanol-O-d mixtures by chemical ionization mass spectrometry with tetramethylsilane as reagent gas

Ellenberger¹,

Hendewerk²,

Weil³

et al. 1982

Anal. Chem.

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Isotope effects on gas phase reaction processes. 2. Equilibrium isotope effects on the proton transfer reactions of methylbenzenes

Cited by 10 publications

References 2 publications

Carbocation−π Interaction: Computational Study of Complexation of Methyl Cation with Benzene and Comparisons with Related Systems

Carbocation−π Interaction: Computational Study of Complexation of Methyl Cation with Benzene and Comparisons with Related Systems

Alkylation of Phenol: A Mechanistic View

Measurement of hydrogen/deuterium ratios in ethanol/ethanol-O-d mixtures by chemical ionization mass spectrometry with tetramethylsilane as reagent gas

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