Why Do Enolate Anions Favor O-Alkylation over C-Alkylation in the Gas Phase? The Roles of Resonance and Inductive Effects in the Gas-Phase SN2 Reaction between the Acetaldehyde Enolate Anion and Methyl Fluoride.

Contributions by resonance and inductive effects toward the net activation barrier were determined computationally for the gas-phase SN2 reaction between the acetaldehyde enolate anion and methyl fluoride, for both O-methylation and C-methylation, in order to understand why this reaction favors O-methylation. With the use of the vinylogue extrapolation methodology, resonance effects were determined to contribute toward increasing the size of the barrier by about 9.5 kcal/mol for O-methylation and by about 21.

Why is sulfuric acid a much stronger acid than ethanol? Determination of the contributions by inductive/field effects and electron-delocalization effects.

Two different and complementary computational methods were used to determine the contributions by inductive/field effects and by electron-delocalization effects toward the enhancement of the gas-phase deprotonation enthalpy of sulfuric acid over ethanol. Our alkylogue extrapolation method employed density functional theory calculations to determine the deprotonation enthalpy of the alkylogues of sulfuric acid, HOSO2-(CH2CH2)n-OH, and of ethanol, CH3CH2-(CH2CH2)n-OH. The inductive/field effect imparted by the HOSO2 group for a given alkylogue of sulfuric acid was taken to be the difference in deprotonation enthalpy between corresponding (i.

Why are esters and amides weaker carbon acids than ketones and Acid fluorides? Contributions by resonance and inductive effects.

Two computational methodologies-a vinylogue extrapolation methodology and a block localized wave function (BLW) methodology-were employed to determine the contributions by resonance and inductive effects toward the gas-phase deprotonation enthalpies at the alpha carbons of acetone, acetamide, acetic acid, and acetyl fluoride, which were taken to be model compounds for ketones, amides, esters, and acid fluorides, respectively. Results from the vinylogue methodology suggest that resonance serves to enhance the gas-phase deprotonation enthalpy of a ketone by 34.3 kcal/mol, an amide by 26.

Molecular rotations and dipole-bound state lifetimes.

A computational model based on classical molecular rotation provides insight into the observability of dipole-bound states. The observability is related to the lifetime of the state prior to rotational autodetachment of the electron. The model tracks an ensemble of dipole-bound states.

Enolate structure and electron affinity.

Photodetachment cross sections for a series of cyclic enolates were measured using a continuous wave (CW) ion cyclotron resonance instrument to generate and detect the ions. We report electron affinities for the radicals corresponding to the removal of the extra electron from the following anions: 2-methylcyclopent-1-enolate, 3-methylcyclopent-1-enolate, 4-methylcyclopent-1-enolate, 5-methylcyclopent-1-enolate, 2-methylcyclohex-1-enolate, 3-methylcyclohex-1-enolate, 4-methylcyclohex-1-enolate, 4-ethylcyclohex-1-enolate, 5-methylcyclohex-1-enolate, and 6-methylcyclohex-1-enolate. Some of these anions are mixed with their tautomers, derived from deprotonation of the parent ketone; the consequences of this are analyzed.

Y-aromaticity: why is the trimethylenemethane dication more stable than the butadienyl dication?

[reactions: see text] Resonance energies of the trimethylenemethane dication (1) and the butadienyl dication (4) were evaluated using two independent computational methodologies to provide insight into the validity of Y-aromaticity. One methodology employed density functional theory calculations and examined the resonance contribution of the C=C double bond toward the double hydride abstraction enthalpies of methylpropene (6) and 2-butene (8), yielding 1 and 4, respectively. These resonance contributions by the double bond were determined by calculating the double hydride abstraction enthalpies of both the parallel and perpendicular conformations of vinylogues of 6 and 8, in which n = 1-4 vinyl units were inserted between the central carbon-carbon double bond and each of the reaction centers.

Resonance energies of the allyl cation and allyl anion: contribution by resonance and inductive effects toward the acidity and hydride abstraction enthalpy of propene.

Density functional theory was employed to calculate the acidities and hydride abstraction enthalpies of propene (3) and propane (4), along with their vinylogues (5 and 6, respectively). The same reaction enthalpies were calculated for the propene vinylogues in which the terminal vinyl group was rotated perpendicular to the rest of the conjugated system (7). The contribution by resonance and inductive effects toward the acidity and hydride abstraction enthalpy of each vinylogue of 5 (n = 1-3) was computed and extrapolated to n = 0 (the parent propene system).

Origin of the acidity enhancement of formic acid over methanol: resonance versus inductive effects.

Density functional theory calculations were employed to study the relative contribution of resonance versus inductive effects toward the 37 kcal/mol enhanced gas-phase acidity (DeltaH degrees (acid)) of formic acid (1) over methanol (2). The gas-phase acidities of formic acid, methanol, vinyl alcohol (5), and their vinylogues (6, 8, and 9) were calculated at the B3LYP/6-31+G level of theory. Additionally, acidities were calculated for the formic acid and vinyl alcohol vinylogues in which the formyl group and the vinyl group, respectively, were perpendicular to the rest of the conjugated system.

Conformation-dependent reaction thermochemistry: study of lactones and lactone enolates in the gas phase.

Gas-phase acidities (Delta H degrees (acid)) of lactones with ring sizes from four to seven have been measured on a Fourier transform ion cyclotron resonance mass spectrometer. Electron affinities (EAs) of the corresponding lactone enolate radicals were measured on a continuous-wave ion cyclotron resonance mass spectrometer, and the bond dissociation energies (BDEs) of the alpha C-H bonds were derived. In order of increasing ring size, Delta H degrees (acid) = 368.