We present an analysis of the performances of a parameter free density functional model (PBE0) obtained combining the so called PBE generalized gradient functional with a predefined amount of exact exchange. The results obtained for structural, thermodynamic, kinetic and spectroscopic (magnetic, infrared and electronic) properties are satisfactory and not far from those delivered by the most reliable functionals including heavy parameterization. The way in which the functional is derived and the lack of empirical parameters fitted to specific properties make the PBE0 model a widely applicable method for both quantum chemistry and condensed matter physics.

Toward reliable density functional methods without adjustable parameters: The PBE0 model

Publisher Summary This chapter discusses the radical-scavenging efficiencies of flavonoids as antioxidants To test whether a certain substance acts as radical scavenger, the clearest evidence comes from the determination of reaction rate constants with a set of different specifically produced radicals In the tests discussed in the chapter, hydroxyl, azide, superoxide, linoleic acid peroxyl, tert-butoxyl, and sulfite radicals are used The spectral observation of radical reactions with flavonoids is assisted by the strong absorption characteristics of both the parent compounds and their respective aroxyl radicals Owing to the low solubility of flavonoids, conditions for pseudo-first order reactions with radical species can rarely be achieved for flavonoid aglycones Therefore, kinetic modeling calculations are employed The radical chemistry of flavonoids not only is of interest from a kinetic or mechanistic point of view but also offers considerable insight into structural relationships of highly evolved plant components First, the consistently high rate constants for attack by different types of radicals demonstrate the effective radicals scavenging capabilities of most flavonoids Second, owing to extensive electron delocalization as a prerequisite for radical stabilization, multiple mesomeric structures exist for aroxyl radical species of flavonoids

Flavonoids as antioxidants: determination of radical-scavenging efficiencies.

A new approximate self‐consistent‐field method for the determination of molecular orbitals for all valence electrons of a molecule is proposed. This method features neglect of differential overlap in all electron‐interaction integrals except those involving one center only. The parameters involved in the calculation are generally obtained semi‐empirically. The new method is known as the Intermediate Neglect of Differential Overlap (INDO) method, and may be regarded as an improvement over the CNDO method proposed in Part I, in that atomic term‐level splittings and unpaired spin distributions are better accommodated. Calculations on geometries of AB2 and AB3 molecules are reported to substantiate the proposed method, and calculated unpaired spin distributions for methyl and ethyl radicals are discussed.

Approximate Self‐Consistent Molecular‐Orbital Theory. V. Intermediate Neglect of Differential Overlap

This paper describes the implementation of a fully automated code for the building of anharmonic force constants and their use in a second-order perturbative evaluation of vibrorotational parameters. Next, a number of test applications are discussed, which show the strengths and limits of various computational levels.

Anharmonic vibrational properties by a fully automated second-order perturbative approach

Many, if not most, redox reactions are coupled to proton transfers. This includes most common sources of chemical potential energy, from the bioenergetic processes that power cells to the fossil fuel combustion that powers cars. These proton-coupled electron transfer or PCET processes may involve multiple electrons and multiple protons, as in the 4 e–, 4 H+ reduction of dioxygen (O2) to water (eq 1), or can involve one electron and one proton such as the formation of tyrosyl radicals from tyrosine residues (TyrOH) in enzymatic catalytic cycles (eq 2). In addition, many multi-electron, multi-proton processes proceed in one-electron and one-proton steps. Organic reactions that proceed in one-electron steps involve radical intermediates, which play critical roles in a wide range of chemical, biological, and industrial processes. This broad and diverse class of PCET reactions are central to a great many chemical and biochemical processes, from biological catalysis and energy transduction, to bulk industrial chemical processes, to new approaches to solar energy conversion. PCET is therefore of broad and increasing interest, as illustrated by this issue and a number of other recent reviews.

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Thermochemistry of Proton-Coupled Electron Transfer Reagents and its Implications

Electron spin resonance methods have been used to observe alkyl radicals in liquid hydrocarbon systems during irradiation with 2.8‐MeV electrons. These investigations provide detailed structural, radiation chemical, and kinetic information about a large number of radicals.In general, in these studies the ESR lines are found to be narrow; considerable fine structure is observable, permitting positive assignment of the radical species. Accurate hyperfine constants are reported for 21 alkyl and cycloalkyl radicals (including several deuterated species), vinyl, 1‐methylvinyl, 3‐butenyl, allyl, and cyclohexadienyl radicals, and hydrogen and deuterium atoms. Except for cyclopropyl radical, all the alkyl and cycloalkyl radicals have α coupling constants in the range 21–23 G. The β coupling constants in cases where they have been rotationally averaged isotropically are found to decrease with increasing substitution of alkyl groups on the α carbon atom. In general the values for primary, secondary, and tertiary ra...

Electron spin resonance studies of transient alkyl radicals

The ESR spectrum of the CF3 radical has been observed during the irradiation of liquid C2F6. The isotropic fluorine hyperfine splitting in this radical has a value of 144.75 G which is considerably larger than other known values for α fluorine atoms. Well‐resolved spectra of CF3·, CHF2·, and CH2F·, as well as certain of the lines of the 13C containing radicals, have also been observed in krypton and xenon matrices. The 13CF3 and 13CHF2 radicals have carbon hyperfine constants of 271.6 and 148.8 G showing that the unpaired electron is in an orbital with considerable s character and that these radicals are therefore non‐planar. The carbon and fluorine splittings in CF3· are sufficiently large that third‐order correction terms are necessary in considering the line positions. As a result it is demonstrable that the carbon and fluorine hyperfine constants are of the same sign.

ESR Spectra and Structure of the Fluorinated Methyl Radicals

The rate constant for reaction of hydrated electrons with N/sub 2/O in aqueous solutions has been determined to be (9.1 +- 0.2) x 10/sup 9/ M/sup -1/ s/sup -1/ from pulse radiolysis observations on the growth and the decay of the absorption of e/sub aq//sup -/. Improvements in instrumental response have allowed observations to be carried out directly on solutions saturated with N/sub 2/O at atmospheric pressure where the reaction period is only 2.9 ns. An approach to numerical simulation of the observed time dependence is described which allows one to take into account in detail the track processes which occur on this time scale as well as the pulse profile and instrumental characteristics. Critical comparisons of the observed growth and decay with the results of these simulations establish well-defined limits for the reaction period. By using the simulated responses to provide reference information we show that it is possible to use the integrated absorbance to evaluate the reaction period with an accuracy of a few percent. The approaches used here allow rate information to be obtained for reaction periods as low as approx. 0.3 ns even though the pulse duration and recording response periods are 1 order of magnitudemore » greater.« less

Rate constant for scavenging eaq- in nitrous oxide-saturated solutions

A study was undertaken to calibrate the thiocyanate dosimeter most commonly used for determination of OH concentration in pulse radiolysis experiments. The dosimeter which involves 10/sup -2/ M SCN/sup -/ in N/sub 2/O-saturated solutions was calibrated against the Fricke system. The calibration led to a value of 46,400 +- 600 (molecules/100 eV) M/sup -1/ cm/sup -1/ for the product of yield and extinction coefficient of (SCN)/sub 2//sup -/ at 10/sup -2/ M SCN/sup -/. This yield is 10-15% greater than most previously quoted values. It is emphasized that yields of OH scavenging processes are significantly dependent on solute concentration and that reports of radiolysis dosimetry should specify the reference system being used in order that results may be correlated. (BLM)

Robert H. Schuler

Papers

Electron spin resonance studies of transient alkyl radicals

ESR Spectra and Structure of the Fluorinated Methyl Radicals

Rate constant for scavenging eaq- in nitrous oxide-saturated solutions

Yield for the scavenging of hydroxyl radicals in the radiolysis of nitrous oxide-saturated aqueous solutions

Electron spin resonance spectra of radical intermediates in the oxidation of ascorbic acid and related substances