Solvent effects on the thermochemistry of free-radical reactions

Spin-Spin Coupling Controls the Gas-Phase Reactivity of Aromatic σ-Type Triradicals

Examination of the reactions of σ-type quinolinium-based triradicals with cyclohexane in the gas phase demonstrated that the radical site that is the least strongly coupled to the other two radical sites reacts first, independent of the intrinsic reactivity of this radical site, in contrast to related biradicals that first react at the most electron-deficient radical site. Abstraction of one or two H atoms and formation of an ion that formally corresponds to a combination of the ion and cyclohexane accompanied by elimination of a H atom ("addition-H") were observed. In all cases except one, the most reactive radical site of the triradicals is intrinsically less reactive than the other two radical sites. The product complex of the first H atom abstraction either dissociates to give the H-atom-abstraction product and the cyclohexyl radical or the more reactive radical site in the produced biradical abstracts a H atom from the cyclohexyl radical. The monoradical product sometimes adds to cyclohexene followed by elimination of a H atom, generating the "addition-H" products. Similar reaction efficiencies were measured for three of the triradicals as for relevant monoradicals. Surprisingly, the remaining three triradicals (all containing a meta-pyridyne moiety) reacted substantially faster than the relevant monoradicals. This is likely due to the exothermic generation of a meta-pyridyne analog that has enough energy to attain the dehydrocarbon atom separation common for H-atom-abstraction transition states of protonated meta-pyridynes.

Mechanistic insights for the photoredox organocatalytic fluorination of aliphatic carbons by anthraquinone using time-resolved and DFT studies

Nanosecond TAS and DFT calculations reveal mechanistic insights for photoredox fluorination of aliphatic C–H bonds and evidence of an anthraquinone–Selectfluor® exciplex.

Comparison of the reactivity of the three distonic isomers of the pyridine radical cation toward tetrahydrofuran in solution and in the gas phase

The reactivity of the three distonic isomers of the pyridine radical cation toward tetrahydrofuran is compared in solution and in the gas phase. In solution, the distonic ions were generated by UV photolysis at 300 nm from iodo-precursors in acidic 50:50 tetrahydrofuran/water solutions. In the gas phase, the ions were generated by collisionally activated dissociation (CAD) of protonated iodo-precursors in an FT-ICR mass spectrometer, as described in the literature. The same major reaction, hydrogen atom abstraction, was observed in solution and in the gas phase. Attempts to cleave the iodine atom from the 2-iodopyridinium cation in the gas phase and in solution yielded the 2-pyridyl cation in addition to the desired 2-dehydropyridinium cation. In the gas phase, this ion was ejected prior to the examination of the desired ion's chemical properties. This was not possible in solution. This study suggests that solvation effects are not significant for radical reactions of charged radicals. On the other hand, the even-electron ion studied, the 2-pyridyl cation, shows substantial solvation effects. For example, in solution, the 2-pyridyl cation forms a stable adduct with tetrahydrofuran, whereas in the gas phase, only addition/elimination reactions were observed.

Photochemical generation and reversible cycloaromatization of a nine-membered ring cyclic enediyne

Irradiation of the nine-membered ring enediyne precursor, which has one of its triple bonds masked as cyclopropenone, efficiently (Phi = 0.34) generates the reactive 4,5-benzocyclonona-2,6-diynol. The latter rapidly equilibrates with the corresponding 1,4-didehydronaphthalene diradical and then undergoes rate-limiting hydrogen abstraction to produce the ultimate product of the Bergman cyclization, benz[f]indanol.

Bond-dissociation enthalpies in the gas phase and in organic solvents: Making ends meet

Solvent effects are responsible for the difference between gas- and solution-phase bond-dissociation enthalpies (BDEs), and are thus crucial for understanding reactivity in solution. While solvation effects can be negligible (e.g., in reactions involving carbon-centered radicals), they may be rather significant (e.g., when oxygen-centered radicals are formed). This paper reviews a number of models which have been proposed to deal with the difference between the solvation energetics of a radical and its parent molecule. It is concluded that the radical-solvent interaction may be larger than previously anticipated.

Influence of amine buffers on carbon tetrachloride reductive dechlorination by the iron oxide magnetite

The influence of amine buffers on carbon tetrachloride (CCl4) reductive dechlorination by the iron oxide magnetite (FeIIFeIII2O4) was examined in batch reactors. A baseline was provided by monitoring the reaction in a magnetite suspension containing NaCl as a background electrolyte at pH 8.9. The baseline reaction rate constant was measured at 7.1 x 10(-5)+/-6.3 x 10(-6) L m(-2) h(-1). Carbon monoxide (CO) was the dominant reaction product at 82% followed by chloroform (CHCl3) at 5.2%. In the presence of 0.01 M tris-(deuteroxymethyl)aminomethane (TRISd), the reaction rate constant nearly tripled to 2.1 x 10(-4)+/-6.5 x 10(-6) L m(-2) h(-1) but only increased the CHCl3 yield to 11% and did not cause any statistically significant changes to the CO yield. Reactions in the presence of triethylammonium (TEAd) (0.01 M) increased the rate constant by 17% to 8.6 x 10(-5)+/-8.1 x 10(-6) L m(-2) h(-1) but only increased the CHCl3 yield to 8.8% while leaving the CO yield unchanged. The same concentration of N,N,N',N'-tetraethylethylenediamine (TEEN) increased the reaction rate constant by 18% to 8.7 x 10(-5)+/-4.8 x 10(-6) L m(-2) h(-1) but enhanced the CHCl3 yield to 34% at the expense of the CO yield that dropped to 35%. Previous work has shown that CHCl3 can be generated either through hydrogen abstraction by a trichloromethyl radical (radical CCl3), or through proton abstraction by the trichlorocarbanion (-:CCl3). These two possible hydrogenolysis pathways were examined in the presence of deuterated buffers. Deuterium tracking experiments revealed that proton abstraction by the trichlorocarbanion was the dominant hydrogenolysis mechanism in the magnetite-buffered TRISd and TEAd systems. The only buffer that had minimal influence on both the reaction rate and product distribution was TEAd. These results indicate that buffers should be prescreened and demonstrated to have minimal impact on reaction rates and product distributions prior to use. Alternatively, it may be preferable, to utilize the buffer capacity of the solids to avoid organic buffer interactions entirely.

Thermochemistry of Aqueous Hydroxyl Radical from Advances in Photoacoustic Calorimetry and ab Initio Continuum Solvation Theory

Photoacoustic signals from dilute ( approximately 30 mM) solutions of H2O2 were measured over the temperature range from 10 to 45 degrees C to obtain the reaction enthalpy and volume change for H2O2(aq) --> 2 OH(aq) from which we ultimately determined DeltafG degrees , DeltafH degrees and partial molal volume, v degrees , of OH (aq). We find DeltarH = 46.8 +/- 1.4 kcal/mol, which is 4 kcal/mol smaller than the gas-phase bond energy, and DeltaVr = 6.5 +/- 0.4 mL/mol. The v degrees for OH in water is 14.4 +/- 0.4 mL/ml: smaller than the v degrees of water. Using ab intio continuum theory, the hydration free energy is calculated to be -3.9 +/- 0.3 kcal/mol (for standard states in number density concentration units) by a novel approach devised to capture in the definition of the solute cavity the strength and specific interactions of the solute with a water solvent molecule. The shape of the cavity is defined by "rolling" a three-dimensional electron density isocontour of water on the ab initio water-OH minimum interaction surface. The value of the contour is selected to reproduce the volume of OH in water. We obtain for OH(aq): DeltafH degrees = -0.2 +/- 1.4 and DeltafG degrees = 5.8 +/- 0.4 kcal/mol that are in agreement with literature values. The results provide confidence in the pulsed PAC technique for measuring aqueous thermochemistry of radicals and open the way to obtaining thermochemistry for most radicals that can be formed by reaction of OH with aqueous substrates while advancing the field of continuum solvation theory toward ab initio-defined solute cavities.

Equilibrium Thermodynamic Studies in Water: Reactions of Dihydrogen with Rhodium(III) Porphyrins Relevant to Rh−Rh, Rh−H, and Rh−OH Bond Energetics

Aqueous solutions of rhodium(III) tetra p-sulfonatophenyl porphyrin ((TSPP)Rh(III)) complexes react with dihydrogen to produce equilibrium distributions between six rhodium species including rhodium hydride, rhodium(I), and rhodium(II) dimer complexes. Equilibrium thermodynamic studies (298 K) for this system establish the quantitative relationships that define the distribution of species in aqueous solution as a function of the dihydrogen and hydrogen ion concentrations through direct measurement of five equilibrium constants along with dissociation energies of D(2)O and dihydrogen in water. The hydride complex ([(TSPP)Rh-D(D(2)O)](-4)) is a weak acid (K(a)(298 K) = (8.0 +/- 0.5) x 10(-8)). Equilibrium constants and free energy changes for a series of reactions that could not be directly determined including homolysis reactions of the Rh(II)-Rh(II) dimer with water (D(2)O) and dihydrogen (D(2)) are derived from the directly measured equilibria. The rhodium hydride (Rh-D)(aq) and rhodium hydroxide (Rh-OD)(aq) bond dissociation free energies for [(TSPP)Rh-D(D(2)O)](-4) and [(TSPP)Rh-OD(D(2)O)](-4) in water are nearly equal (Rh-D = 60 +/- 3 kcal mol(-1), Rh-OD = 62 +/- 3 kcal mol(-1)). Free energy changes in aqueous media are reported for reactions that substitute hydroxide (OD(-)) (-11.9 +/- 0.1 kcal mol(-1)), hydride (D(-)) (-54.9 kcal mol(-1)), and (TSPP)Rh(I): (-7.3 +/- 0.1 kcal mol(-1)) for a water in [(TSPP)Rh(III)(D(2)O)(2)](-3) and for the rhodium hydride [(TSPP)Rh-D(D(2)O)](-4) to dissociate to produce a proton (9.7 +/- 0.1 kcal mol(-1)), a hydrogen atom (approximately 60 +/- 3 kcal mol(-1)), and a hydride (D(-)) (54.9 kcal mol(-1)) in water.

Structure-reactivity relationships in the photoreduction of n,pi*-excited ketones and azoalkanes: the effect of reaction thermodynamics, excited-state electrophilicity, and antibonding character in the transition state

The reaction enthalpies for the photoreduction of n,pi*-excited states (acetone, benzophenone, 2.3-diazabicyclo[2.2.2]oct-2-ene, and a 2,3-diazabicyclo[2.2.1]hept-2-ene derivative) by model hydrogen donors (methanol and dimethylamine) were calculated on the basis of a critically evaluated data set of bond dissociation energies for donors and reduced acceptors. These were compared with the observed photoreactivity, which can be assessed through quenching rate constants of the excited states by hydrogen donors. The intriguing observation of a preferential attack at electrophilic hydrogen atoms, i.e., N-H or O-H, by n,pi*-excited azoalkanes is rationalised on the basis of the calculated thermochemical data, differences in electrophilicity, and varying contributions of antibonding character in the transition state. Singlet-excited azoalkanes act as nucleophilic species, while excited ketones display an electrophilic reactivity. This is in line with the pictorial description of the electron distribution in these excited states.

Solvation enthalpies of free radicals: O-O bond strength in di-tert-butylperoxide

The photolysis reaction of di-tert-butylperoxide was studied in various solvents by photoacoustic calorimetry (PAC). This technique allows the determination of the enthalpy of this homolysis reaction, which by definition corresponds to the O-O bond dissociation enthalpy of the peroxide in solution, DHsin(degrees)(O-O). The derived value from these experiments in benzene, 156.7 +/- 9.9 kJ mol(-1), is very similar to a widely accepted value for the gas-phase bond dissociation enthalpy, DH(degrees)(O-O) = 159.0 +/- 2.1 kJ mol(-1). However, when the PAC-based value is used together with auxiliary experimental data and Drago's ECW model to estimate the required solvation terms, it leads to 172.3 +/- 10.2 kJ mol(-1) for the gas-phase bond dissociation enthalpy. This result, significantly higher than the early literature value, is however in excellent agreement with a recent gas-phase determination of 172.5 +/- 6.6 kJ mol(-1). The procedure to derive the gas-phase DH(degrees)(O-O) was tested by repeating the PAC experiments in carbon tetrachloride and acetonitrile. The average of the values thus obtained was DH(degrees)(O-O) = 179.6 +/- 4.5 kJ mol(-1), confirming that the early gas-phase result is a lower limit. More importantly, the present study questions the usual assumption that the solvation terms of homolysis reactions producing free radicals in solution should cancel, and suggests a methodology to estimate solvation enthalpies of free radicals.

Gas-Phase and Solution-Phase Homolytic Bond Dissociation Energies of H-N(+) Bonds in the Conjugate Acids of Nitrogen Bases

The oxidation potentials of 19 nitrogen bases (abbreviated as B: six primary amines, five secondary amines, two tertiary amines, three anilines, pyridine, quinuclidine, and 1,4-diazabicyclo[2,2,2]octane), i.e., E(ox)(B) values in dimethyl sulfoxide (DMSO) and/or acetonitrile (AN), have been measured. Combination of these E(ox)(B) values with the acidity values of the corresponding acids (pK(HB)(+)) in DMSO and/or AN using the equation: BDE(HB)(+) = 1.37pK(HB)(+) + 23.1 E(ox)(B) + C (C equals 59.5 kcal/mol in AN and 73.3 kcal/mol in DMSO) gave estimates of solution phase homolytic bond dissociation energies of H-B(+) bonds. Gas-phase BDE values of H-B(+) bonds were estimated from updated proton affinities (PA) and adiabatic ionization potentials (aIP) using the equation, BDE(HB(+))(g) = PA + aIP - 314 kcal/mol. The BDE(HB)(+) values estimated in AN were found to be 5-11 kcal/mol higher than the corresponding gas phase BDE(HB(+))(g) values. These bond-strengthening effects in solution are interpreted as being due to the greater solvation energy of the HB(+) cation than that of the B(+*) radical cation.