Role of hyperconjugation in determining carbon-oxygen bond dissociation enthalpies in alkylperoxyl radicals

[reaction: see text] Theoretical calculations of carbon-oxygen bond dissociation enthalpies in substituted methylperoxyl radicals (YCH(2)OO*) reveal that bond strengths are not governed solely by the stability of YCH2* radicals but are largely affected by hyperconjugation when Y is electron-donating or conjugating. In many cases, this hyperconjugative effect is greater than stabilization of the methyl radical by Y. All electron-withdrawing Y exert small destabilizing effects via inductive withdrawal of electrons from the polarized C-OO* bond.     

Anthracene derivatives and the corresponding dimers with TEMPO radicals

Anthracene derivatives with several TEMPO radicals (2-4, 10) were prepared, and each photodimerization reaction was investigated. Although the photodimerization was unsuccessful in obtaining the dimers of anthracenes 2 and 3, which could be alternatively prepared in a stepwise manner, the photodimers of anthracenes 4 and 10 were available by the direct photoreaction. The dissociation reaction of the dimers proceeded well by heating them in solution to give the corresponding monomers in each case, and thus the reversible system could be constructed in the latter two systems. While no large difference was observed in their magnetic behaviors between the monomer/dimer pair of 4 and 8, an intriguing difference was found in the magnetic behaviors for the pair of 10 and 11 from ferromagnetic interactions in 10 to the variable magnetic interactions in 11 depending on the solvent molecules incorporated in the crystals.     

Substituent effects on the bond dissociation enthalpies of aromatic amines

Bond dissociation enthalpy differences, Z-X DeltaBDE = BDE(4-YC(6)H(4)Z-X) - BDE(C(6)H(5)Z-X), for Z = CH(2) and O are largely independent of X and are determined mainly by the stabilization/destabilization effect of Y on the 4-YC(6)H(4)Z(*) radicals. The effects of Y are small (< or =2 kcal/mol for all Y) for Z = CH(2), but they are large for Z = O, where good correlations with sigma(p)(+)(Y) yield rho(+) = 6.5 kcal/mol. For Z = NH, two sets of electrochemically measured N-H DeltaBDEs correlate with sigma(p)(+)(Y), yielding rho(+) = 3.9 and 3.0 kcal/mol. However, in contrast to the situation with phenols, these data indicate that the strengthening effect on N-H BDEs of electron-withdrawing (EW) Y's is greater than the weakening effect of electron-donating (ED) Y's. Attempts to measure N-H DeltaBDEs in anilines using two nonelectrochemical techniques were unsuccessful; therefore, we turned to density functional theory. Calculations on 15 4-YC(6)H(4)NH(2) gave N-H DeltaBDEs correlating with sigma(p)(+) (rho(+) = 4.6 kcal/mol) and indicated that EW and ED Y's had comparable strengthening and weakening effects, respectively, on the N-H bonds. To validate theory by connecting it to experiment, the N-H DeltaBDEs of four 4,4'-disubstituted diphenylamines and five 3,7-disubstituted phenothiazines were both calculated and measured by the radical equilibration EPR technique. For all compounds, theory and experiment agreed to better than 1 kcal/mol. Dissection of N-H DeltaBDEs in 4-substituted anilines and O-H DeltaBDEs in 4-substituted phenols into interaction enthalpies between Y and NH(2)/OH (molecule stabilization/destabilization enthalpy, MSE) and NH*/O* (radical stabilization/destabilization enthalpy, RSE) reveals that for both groups of compounds, ED Y's destabilize the molecule and stabilize the radical, while the opposite holds true for EW Y's. However, in the phenols the effects of substituents on the radical are roughly 3 times as great as those in the molecule, whereas in the anilines the two effects are of comparable magnitudes. These differences arise from the stronger ED character of NH(2) vs OH and the weaker EW character of NH* vs O*. The relatively large contributions to N-H BDEs in anilines arising from interactions in the molecules suggested that N-X DeltaBDEs in 4-YC(6)H(4)NH-X would depend on X, in contrast to the lack of effect of X on O-X and CH(2)-X DeltaBDEs in 4-YC(6)H(4)O-X and 4-YC(6)H(4)CH(2)-X. This suggestion was confirmed for X = CH(3), H, OH, and F, for which the calculated NH-X DeltaBDEs yielded rho(+) = 5.0, 4.6, 4.0, and 3.0 kcal/mol, respectively.     

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.     

Solvent effects on solution enthalpies of adamantyl derivatives

Solution enthalpies of 1-bromoadamantane, 1-adamantanol, and 2-adamantanone in a large set of protic and aprotic solvents are reported at 298.15 K. Solvent effects on the solution processes of these solutes are analyzed in terms of a modified TAKA equation, involving δ_cav h ^s as the cavity term. The nature and magnitude of the major interactions which influence these processes are assessed and discussed in terms of the solutes’ characteristics. New insights on the solution processes under scrutiny are presented.     

Determination of enthalpies of formation of organic free radicals based on bond dissociation energies. 4. Alkyl-substituted derivatives of phenyl and benzyl

The enthalpies of formation (H _f°) of 16 alkyl-substituted phenyl and benzyl radicals (R^·) were determined for the first time by the published values of energies of R—X bond dissociation. For the initial molecules of RX, alkyl-substituted benzenes, the additive-group procedure was developed for the calculation of H _f°. In the framework of the additive-group model for considered R^·, we studied the structure-property interrelation, analyzed the obtained H _f°(R^·) values, and confirmed their reliability. The influence of nonvalent interactions on H _f°(R^·) was systematized and detailed. The parameters, from which it is possible to calculate H _f° of the 51 radicals, were proposed.     

Theoretical calculations of carbon-oxygen bond dissociation enthalpies of peroxyl radicals formed in the autoxidation of lipids

Theoretical calculations were carried out to provide a framework for understanding the free radical oxidation of unsaturated lipids. The carbon[bond]hydrogen bond dissociation enthalpies (BDEs) of organic model compounds and oxidizable lipids (R[bond]H) and the carbon[bond]oxygen bond dissociation enthalpies of peroxyl radical intermediates (R[bond]OO*) have been calculated. The carbon[bond]hydrogen BDEs correlate with the rate constant for propagation of free radical autoxidation, and the carbon[bond]oxygen BDEs of peroxyl radicals correlate with rate constants for beta-fragmentation of these intermediates. Oxygen addition to intermediate carbon radicals apparently occurs preferentially at centers having the highest spin density. The calculated spin distribution therefore provides guidance about the partitioning of oxygen to delocalized carbon radicals. Where the C[bond]H BDEs are a function of the extent of conjugation in the parent lipid and the stability of the carbon radical derived therefrom, C[bond]OO* BDEs are also affected by hyperconjugation. This gives way to different rates of beta-fragmentation of peroxyl radicals formed from oxygen addition at different sites along the same delocalized radical. We have also studied by both theory and experiment the propensity for benzylic radicals to undergo oxygen addition at their ortho and para carbons which, combined, possess an equivalent unpaired electron spin density as the benzylic position itself. We find that the intermediate peroxyl radicals in these cases have negative C[bond]OO* BDEs and, thus, have rate constants for beta-fragmentation that exceed the diffusion-controlled limit for the reaction of a carbon-centered radical with oxygen.     

Computational study on the bond dissociation enthalpies in the enolic and ketonic forms of beta-diketones: their influence on metal-ligand bond enthalpies

A computational study on the thermodynamic properties of 13 beta-diketones is presented. The B3LYP//6-311+G(2d,2p)//B3LYP/6-31G(d) theoretical approach was employed to compute the O-H and C-H bond dissociation enthalpies and enthalpy of tautomerization and to estimate standard gas-phase enthalpies of formation for the radicals and for the parent molecules. The gas-phase enthalpies of formation for the neutral molecules are in excellent agreement with available experimental data, supporting the estimates made for the radicals. The latter are very important for the clarification of the thermochemistry of many beta-diketonato metal complexes previously reported in the literature. Importantly, when substituents R = -CHR' are attached to the beta-diketone's scaffold, C-H homolytic bond cleavage is always favored with respect to O-H bond scission.     

A computational study of bond dissociation enthalpies in haloethenes

Carbon–hydrogen bond dissociation enthalpies (BDEs) were computed for all haloethenes, C₂H_4−nX_n (n=0–3, X=F, Cl, Br, I), at the B3LYP/6-311+G(3df,2p) level using isodesmic reactions. It was found that C–H bond strengths in the monohaloethenes varied substantially, by as much as 18 kJ mol⁻¹, dependent upon the bond's stereochemical position relative to the halogen. BDEs in the dihaloethanes varied in the order CX₂CH–H>(E)-CHXCX–H>(Z)-CHXCX–H. Trends in the computed bond enthalpies were discussed and explained on the basis of relative steric repulsions and hyperconjugative delocalization interactions, as determined from Natural Bond Orbital analysis.     

Involvement of proton transfer in the reductive repair of DNA guanyl radicals by aniline derivatives

The most easily oxidized sites in DNA are the guanine bases, and major intermediates produced by the direct effect of ionizing radiation (ionization of the DNA itself) are electron deficient guanine species. By means of a radiation chemical method (gamma-irradiation of aqueous thiocyanate), we are able to produce these guanyl radicals in dilute aqueous solutions of plasmid DNA where the direct effect would otherwise be negligible. Stable modified guanine products are formed from these radicals. They can be detected in the plasmid conversion to strand breaks after a post-irradiation incubation with a DNA base excision endonuclease enzyme. If aniline compounds are also present, the yield of modified guanines is strongly attenuated. The mechanism responsible for this effect is electron donation from the aniline compound to the guanyl radical, and it is possible to derive rate constants for this reaction. Aniline compounds bearing electron withdrawing groups (e.g., 4-CF3) were found to be less reactive than those bearing electron donating groups (e.g., 4-CH3). At physiological pH values, the reduction of a guanyl radical involves the transfer of a proton as well as of an electron. The mild dependence of the rate constant on the driving force suggests that the electron is not transferred before the proton. Although the source of the proton is unclear, our observations emphasize the importance of an accompanying proton transfer in the reductive repair of oxidative damage to guanine bases which are located in a biologically active double stranded plasmid DNA substrate.     

Calculations of the effects of substituents on bond localization in annelated cyclopentadienyl radicals

UB3LYP/6-31G(d) calculations have been performed in order to predict the ground states, 2A2 or 2B1, of cyclopentadienyl radicals that are mono- and bis-annelated with a wide variety of substituents. Unlike the case in the annelated cyclooctatetraenes, studied by Baldridge and Siegel, our calculations find that the sizes of the coefficients of the degenerate MOs at the annelated carbons are more important than the symmetries of the substituent's frontier orbitals in determining the mode of bond localization in the annelated cyclopentadienyl radicals.     

Synthesis and study of electrochemical properties of the sterically hindered phenol derivatives and the corresponding radicals

1-(3,5-Di-tert-butyl-4-hydroxyphenyl)-2-arylbenzimidazoles were synthesized by the condensation of 2,6-di-tert-butyl-p-benzoquinone imine with aromatic aldehydes. 2-(3,5-Di-tert-butyl-4-hydroxybenzylidene) benzimidazoles were synthesized by the reaction of 2-aminobenzimidazole with 2,6-di-tert-butyl-4-hydroxybenzaldehyde. The substances were characterized by elemental analysis, IR and NMR spectra. The electrochemical reduction and oxidation of these compounds and phenoxy radicals derived from them was studied by cyclic voltammetry. The stability of the studied phenoxy radicals was confirmed by the electron spin resonance method.     

The gas phase enthalpies of formation of hydrazine, its methylated derivatives, and the corresponding values for ammonia and its methylated derivatives

Hydrazine and some of its methylated derivatives are highly important species. Despite their molecular simplicity and those of the other methylated derivatives, ammonia, and the corresponding amines, there are significant problems with the literature values of their gas phase enthalpies of formation. We present our high level quantum chemical calculations at the G4 level for all of these species.     

Theoretical reinvestigation of opposite electronic effects on bond lengths in thiophenols and thiophenolic radicals

Our previous study has revealed that para-substituents have opposite electronic effects on the C-S bond lengths of thiophenols and thiophenolic radicals. Although a theoretical elucidation has been given, it has not been supported by theoretically calculated atomic charges. To give an alternative explanation, we calculated the C-S bond lengths, C-S bond electron densities, and Mulliken charges on the carbon and sulfur atoms for thiophenols, thiophenolic radicals, and thiophenolic radical cations by means of the B3LYP density functional theory method using the 6-31G(d, p) basis set. It was revealed that the C-S bond length is adequately defined in terms of C-S bond electron density. The distinct electronic effects on the C-S bond lengths of thiophenols, thiophenolic radicals and thiophenolic radical cations are well elucidated by the different electronic states (electron-deficient or-rich) of the phenyl ring and SH group.     

Thermochemical studies on 3-methyl-quinoxaline-2-carboxamide-1,4-dioxide derivatives: enthalpies of formation and of N-O bond dissociation

The standard molar enthalpies of formation of the 3-methyl-N-R-2-quinoxalinecarboxamide-1,4-dioxides (R = H, phenyl, 2-tolyl) in the gas phase were derived using the values for the enthalpies of combustion of the crystalline compounds, measured by static bomb combustion calorimetry, and for the enthalpies of sublimation, measured by Knudsen effusion, at T = 298.15 K. These values have also been used to calibrate a computational procedure that has been employed to estimate the gas-phase enthalpies of formation of the corresponding 3-methyl-N-R-2-quinoxalinecarboxamides and also to compute the first, second, and mean N-O bond dissociation enthalpies in the gas phase. It is found that the size of the substituent almost does not influence the computed N-O bond dissociation enthalpies; the maximum enthalpic difference is approximately 5 kJ.mol-1.     

A passive chemical dosimeter for assessing long-term exposure in air containing aniline and aniline derivatives

A passive chemical dosimeter for assessing long-term exposure to toxic substances (aniline and its derivatives) in the atmosphere of inhabitable premises was described. The compounds to be determined were accumulated on an absorption filter due to a chemical reaction with a selective agent upon the many-hour exposure of the device. Analytical performance of chemical dosimeters (diffusion coefficients, chemisorption pre-concentration rate, and detection limits) was evaluated. The concentrations of aniline, p-chloroaniline, 3,4-dichloroaniline, and o-toluidine in the air of chemical laboratories and smoking rooms were determined.     

Substituent effects on ultrafast excited-state proton transfer of protonated aniline derivatives in aqueous solution

Substituent effects on the proton dissociation of protonated aniline derivatives upon femtosecond laser pulse excitation are investigated for m- and p-cyanoanilines and m- and p-methoxyanilines in aqueous solution. The cyano substitution at the meta-position increases the rate significantly (k_dis=3.7 × 10¹¹ s⁻¹) compared to aniline (k_dis=1.3 × 10¹⁰ s⁻¹), while the methoxy substitution reduces the rate remarkably. Either substituent at the para-position shows only slight influences on the rate. The strong dependences of the k_dis value on the substituent and the substituted position are reasonably explained on the basis of the free energy change.