Search

A thermodynamic cycle is used to carry out several proton affinity, as well as electron-binding measurements, which are then used to effectively and accurately measure the cycloalkane and cycloalkene carbon-hydrogen (C-H) bond dissociation energies in the gas phase. The results show that the normally cited bond energies for such cyclic hydrocarbons are much smaller than their actual values, while a vast difference is observed between the actual and calculated values of their bond strengths.

The heat of hydrogenation of phenylcyclobutadiene is determined via a thermodynamic cycle by carrying out gas-phase measurements on 1-phenylcyclobuten-3-yl cation. The results are compared to G3, G3(MP2), and B3LYP computations and represent the first experimental measurements of the energy of a monocyclic cyclobutadiene.

The alpha,2-, alpha,3- and alpha,4-dehydrophenoxide anions are regiospecifically generated by collisional activation of o-, m-, and p-nitrobenzoate. The alpha,2 and alpha,4 isomers are synthesized by reacting o-benzyne radical anion with carbon dioxide and electron ionization of p-diazophenol.

Mechanistic and energetic aspects for the conversion of camphor to 5-exo-hydroxycamphor by the compound I iron-oxo species of cytochrome P450 are discussed from B3LYP DFT calculations. The reaction occurs in a two-step manner along the lines that the oxygen rebound mechanism suggests.

Hydrogen bonds are the dominant motif for organizing the three-dimensional structures of biomolecules like carbohydrates, nucleic acids and proteins and have served as templates for proton transfer reactions. The computations in dimethyl sulfoxide on a series of polyols have shown that multiple hydrogen bonds to a single charged center have led to enhanced acidities and a new class of Bronsted acids is described.

A study is conducted, in which the enthalpies of formation of o-, m- and p-benzoquinone are determined by carrying out gas-phase ion energetic measurements, combustion calorimetry and phase-change determinations, and high-level quantum chemical computations. By combining the results obtained in thermodynamic cycles, the heats of hydrogenation and the heats of formation of o-, m- and p-benzoquinone are derived.

Bracketing and equilibrium techniques were used for examining the gas-phase acidity of 3,3-dimethylcyclopropene. The C-H bond energies of 3,3-dimethylcyclopropene differs by 26.1 plusmn 3.8 kcal mol(super -1).

Hydrogen bonds are the dominant motif for organizing the three-dimensional structures of biomolecules such as carbohydrates, nucleic acids, and proteins, and serve as templates for proton transfer reactions. Computations, gas-phase acidity measurements, and pKa determinations in dimethyl sulfoxide on a series of polyols indicate that multiple hydrogen bonds to a single charged center lead to greatly enhanced acidities. A new class of Brønsted acids, consequently, is proposed. [ABSTRACT FROM AUTHOR]

We have regiospecifically generated the α,2-, α,3-, and α,4-dehydrophenoxide anions by collisional activation of o-, m-, and p-nitrobenzoate. The α,2 and α,4 isomers also were synthesized by reacting o-benzyne radical anion with carbon dioxide and electron ionization of p-diazophenol. All three dehydrophenol radical anions were differentiated from each other and identified by probing their chemical reactivity with several reagents. Each isomer was converted to phenoxide and its corresponding quinone as well. Thermochemical measurements were carried out on all three radical anions and their hydrogenatom affinities, proton affinities, and electron binding energies are reported. These measured quantities are combined in thermodynamic cycles to derive the heats of formation of each of the radical anions and their corresponding carbenes (i.e., α,2-, α,3-, and α,4-dehydrophenol). These results are compared to MCQDPT2, G3, G2+(MP2), and B3LYP calculations and experimental data for appropriate reference compounds. [ABSTRACT FROM AUTHOR]

The pK(a) of an acyclic aliphatic heptaol ((HOCH(2)CH(2)CH(OH)CH(2))(3)COH) was measured in DMSO, and its gas-phase acidity is reported as well. This tertiary alcohol was found to be 10(21) times more acidic than tert-butyl alcohol in DMSO and an order of magnitude more acidic than acetic acid (i.e., pK(a) = 11.4 vs 12.3). This can be attributed to a 21.9 kcal mol(-1) stabilization of the charged oxygen center in the conjugate base by three hydrogen bonds and another 6.3 kcal mol(-1) stabilization resulting from an additional three hydrogen bonds between the uncharged primary and secondary hydroxyl groups. Charge delocalization by both the first and second solvation shells may be used to facilitate enzymatic reactions. Acidity constants of a series of polyols were also computed, and the combination of hydrogen-bonding and electron-withdrawing substituents was found to afford acids that are predicted to be extremely acidic in DMSO (i.e., pK(a) < 0). These hydrogen bond enhanced acids represent an attractive class of Brønsted acid catalysts.

Both C-H bond dissociation energies for cyclobutene were measured in the gas phase (BDE = 91.2 +/- 2.3 (allyl) and 112.5 +/- 2.5 (vinyl) kcal mol-1) via a thermodynamic cycle by carrying out proton affinity and electron-binding energy measurements on 1- and 3-cyclobutenyl anions. The results were compared to those for an acyclic model compound, cis-2-butene, and provide the needed information to experimentally establish the heat of formation of cyclobutadiene. Chemically accurate G3 and W1 calculations also were carried out on cycloalkanes, cycloalkenes, and selected reference compounds. It appears that commonly cited bond energies for cyclopropane, cyclobutane, and cyclohexane are 3 to 4 kcal mol-1 too small and their pi bond strengths, as given by BDE1 - BDE2, are in error by up to 8 kcal mol-1.