No abstract

No abstract

Using the pulse radiolysis technique, studies on reactions of 3,4,5-trihydroxybenzoic acid [gallic acid (GA)] with radical species generated in water are reported. At pH 6.8 and 9.7, OH radicals react with GA to give an adduct initially with rate constants of the order of 1 ] 1010 dm3 mol~1 s~1. This adduct then reacts with parent GA molecules with rate constants of the order 5 ] 108 dm3 mol~1 s~1 to give phenoxyl type radical species having absorption maxima in the 350 nm region. At pH 12 and 13.6, OH/O~radicals directly bring about oxidation of GA. SpeciÐc oxidants like azidyl radical bring about one electron oxidation at neutral and alkaline pHs with almost di †usion controlled rate constants. Rate constants for the reaction of radicals Br 2 ãre found to be lower than those for OH radicals by an order of magnitude. At pH 0, both OH and Cl 2 radicals react with GA to give phenoxyl type radicals. The phenoxyl radicals formed are quite stable at higher pHs, which make GA a good antioxidant. Rate constants for the reactions of with di †erent protolytic e aq forms of GA are determined.

No abstract

No abstract

Publication costs assisted by Natick LaboratoriesA detailed examination of the one-electron reduction of aromatic nitrogen heterocyclic compounds in water was carried out using the technique of pulse radiolysis and kinetic absorption spectrophotometry. The compounds studied include pyrazine (Pz), pyrimidine (Pm), pyridazine (Pd), quinoxaline (Qx), phthalazine, and acridine. The reducing agents used were eaq~, (CH3)2COH and (CH3)2CQ~r adicals. The efficiency and the rate of electron transfer from the latter two radicals were correlated with the redox potentials of the aza-aromatic compounds. The radical anions of these compounds are very weak acids and undergo fast protonation by water or by other proton donors to form the neutral monohydro radicals and the dihydroradical cations. Based on the determined transient absorption spectra of the intermediates, and their change over the pH range 0-14, it was possible to assign and derive the ionization constants of the radical observed. For • PzH2+, • PmH2+, PdH2+, and QxH2+ dihydroradical cations the pJfa values are 10.5 ± 0.2, 7.6 ± 0.1, 7.6 ± 0.3, and 8.8 ± 0.1, respectively. No ionization of the corresponding neutral radicals -PzH, •PmH, •PdH, and *QxH to the radical anions was observed up to pH 14, and presumably occurs only in very alkaline solution. An interesting feature of the spectral characteristics of these intermediates is the blue shifting of the absorption maxima of the neutral monohydro radicals compared to the dihydro radical cations. Furthermorethe radical cations are relatively inert to oxygen while the neutral radicals react with 02 with k > 108 M-1 sec-1.

No abstract

3345culated dipole moments of hydrocarbons can be predicted in from fair to good agreement with observations. From the work presented here, we see that variations in the C-H bond strength of hydrocarbons can also be explained in terms of pure electrostatic effects. It has also been possible to demonstrate2= that the barrier to rotation in ethane barrier and the instability of the gauche conformation relative to trans conformation in n-butane could not be explained by electrostatic interactions. These phenomena require other explanations, and the simplest is a nonbonded H --H repulsion of the form originally proposed by hug gin^.^ According to Huggins, a pair of H atoms attached to two different C atoms will repel each other if the distance between them is smaller than 2.7 A. The energy associated with this repulsion can be as high as 1 .O kcal mol-' for every interaction a t a distance of -2.3 A. Once this repulsive potential is added to the electrostatic potential, an excellent agreement is obtained between the model and the experimental value for the barrier to rotation along C-C axis and for the relative instability of the gauche conformation. In some hydrocarbon molecules, the electrostatic model had predicted a heat of formation more negative than the experimental value. In all these cases, we found that a t least one pair of nonbonded H atoms is separated by less than 2.5 A. Adding the repulsion energy associated with this interaction, a better agreement with the experimental observation is obta ined.The various formal charges that we have selected in this series to explain the electrostatic stability of hydrocarbons are only an approximation, but they were consistent in all cases within the experimental uncertainty. The actual formal charges can be slightly different. This could be deter-mined only when one will make the exact geometrical model and will take into account the exact energetic values for the H . H nonbonded interaction, the electrostatic interactions, and the potential function for the structure.A preliminary calculation of AHfO fo,r compounds containing heteroatoms has already shown that this simple model by itself will not give as good an agreement with the observed results. Once an atom with a lone pair of electrons is introduced into the molecule, it appears that it is necessary to take into account the interaction between the dipole moment associated with the lone pair and the other formal charges in the molecule. Polarization eFfects also became significant energetically and, a t the moment, we have not sorted them out. Appendix IThe heats of formation of various free radicals, in kcal mol-', as a function of the formal charges y , and 6, (Iyl = 0.28 X References and Notes(1) Postdoctoral Research Associate. (2) (a) Part I of this series: S. W. Benson and M. Luria, J. Am. Chem. SOC.. 97, 704 (1975); (b) part II: S. W. Benson and M. Luria, preceding paper in this issue.( 3) H. E. O Abstract:The one-electron reduction of purine (PH), 9-methylpurine (MP), adenosine (A) and 1 -methylguanosi...