The Huckel theory, with an extended basis set consisting of 2s and 2p carbon and 1s hydrogen orbitals, with inclusion of overlap and all interactions, yields a good qualitative solution of most hydrocarbon conformational problems. Calculations have been performed within the same parametrization for nearly all simple saturated and unsaturated compounds, testing a variety of geometries for each. Barriers to internal rotation, ring conformations, and geometrical isomerism are among the topics treated. Consistent σ and π charge distributions and overlap populations are obtained for aromatics and their relative roles discussed. For alkanes and alkenes charge distributions are also presented. Failures include overemphasis on steric factors, which leads to some incorrect isomerization energies; also the failure to predict strain energies. It is stressed that the geometry of a molecule appears to be its most predictable quality.

An Extended Hückel Theory. I. Hydrocarbons

Fluorescence quenching rate constants, kq, ranging from 106 to 2 × 1010 M−1 sec−1, of more than 60 typical electron donor-acceptor systems have been measured in de-oxygenated acetonitrile and are shown to be correlated with the free enthalpy change, ΔG23, involved in the actual electron transfer process

in the encounter complex and varying between + 5 and −60 kcal/mole. The correlation which is based on the mechanism of adiabatic outer-sphere electron transfer requires ΔG≠23, the activation free enthalpy of this process to be a monotonous function of ΔG23 and allows the calculation of rate constants of electron transfer quenching from spectroscopic and electrochemical data.

A detailed study of some systems where the calculated quenching constants differ from the experimental ones by several orders of magnitude revealed that the quenching mechanism operative in these cases was hydrogen-atom rather than electron transfer.

The conditions under which these different mechanisms apply and their consequences are discussed.

Kinetics of Fluorescence Quenching by Electron and H‐Atom Transfer

1. Advantages of Chemical Redox Agents 878 2. Disadvantages of Chemical Redox Agents 879 C. Potentials in Nonaqueous Solvents 879 D. Reversible vs Irreversible ET Reagents 879 E. Categorization of Reagent Strength 881 II. Oxidants 881 A. Inorganic 881 1. Metal and Metal Complex Oxidants 881 2. Main Group Oxidants 887 B. Organic 891 1. Radical Cations 891 2. Carbocations 893 3. Cyanocarbons and Related Electron-Rich Compounds 894

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Chemical Redox Agents for Organometallic Chemistry

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

Two 3d-4f heterometallic coordination polymers {[Ln(PDA)3Mn1.5(H2O)3].3.25H2O}infinity with 1D channels were synthesized under hydrothermal conditions (PDA = pyridine-2,6-dicarboxylic acid; Ln = Eu (1); Ln = Tb (2)). The emission intensities of 1 and 2 increased significantly upon addition of Zn2+, while the introduction of other metal ions caused the intensity to be either unchanged or weakened. The case implies that 1 and 2 may be used as luminescent probes of Zn2+.

Coordination Polymers Containing 1D Channels as Selective Luminescent Probes

A general theory of the linewidths in the electron spin resonance spectra of dilute solutions of free radicals has been developed in terms of the relaxation‐matrix theory of Bloch, Redfield, and Ayant. In contrast to previous theories, it is shown that a composite line arising from a set of degenerate nuclear‐spin states should, in general, consist of a sum of superimposed lines of Lorentzian shape with different widths rather than a single line with an over‐all Lorentzian shape. A single Lorentzian line is still obtained, however, as a limiting case when the variation of the widths of the different components of a composite line is small compared to the average width. Although the non‐Lorentzian shape of a composite line is often difficult to observe experimentally, a number of other observable properties are predicted by the present development that are outside the scope of the previous theories. For example, linewidth effects resulting from differences in the widths of the separate components of a composite line are predicted that explain the alternation in the linewidths from one hyperfine line to another recently observed in the ESR spectra in certain free radicals. The detailed form of the relaxation matrix is presented for intramolecular anisotropic and isotropic electron—nuclear dipolar interactions, quadrupole interactions, and g‐tensor relaxations. Modulations of the spin density and hyperfine splittings are included, as are internal motions, and a number of cross terms between the different relaxation mechanisms arise. In general the relaxation matrix of a composite line contains significant off‐diagonal elements, and the determination of the linewidths requires the evaluation of the eigenvalues of the matrix. Problems involving rapid chemical exchange, or modulation by jumps to a small number of sites, can be treated by the relaxation‐matrix theory and, under special restrictions, by either the modified Bloch equations or the Anderson theory of motional narrowing. When applicable, these latter procedures can be used over the entire range of exchange rates, while the relaxation‐matrix theory is limited to fast rates only.

Theory of Linewidths in Electron Spin Resonance Spectra

A quantitative theory of the isotropic electron‐nuclear spin interactions of carbon 13 in pi‐electron radicals is presented and applied to the hyperfine splittings observed in the electron spin resonance spectra of these substances. The splittings arise from sigma‐pi interactions which polarize both the 1s and 2s electrons. The 1s‐orbital spin polarization is shown to contribute a term of negative sign with a magnitude comparable to that from the 2s electrons. For an sp2 hybridized carbon atom that is bonded to three atoms, Xi (i=1, 2, 3), the hyperfine constant aC has the form aC=(SC+ ∑ i=13QCXiC)ρπ+ ∑ i=13QXiCCρiπ, where ρπ and ρiπ(i=1,2,3) are the pi‐electron spin densities on atoms C and Xi, respectively. The contribution of the 1s electrons is determined by SC and that of the 2s electrons by the Q's, where QBCA is the sigma‐pi parameter for the nucleus of atom A resulting from the interaction between the bond BC and the pi‐electron spin density on atom B. Calculations for a planar CHC2 fragment model...

Theoretical Interpretation of Carbon‐13 Hyperfine Interactions in Electron Spin Resonance Spectra

Electron Spin Resonance of Electrolytically Generated Nitrile Radicals

Measurements have been made of the g values in the electron spin resonance spectra of 20 aromatic free radicals with an accuracy which has not previously been attainable. The largest source of error for both absolute and relative measurements resulted from variations in the difference between the magnetic field at the ESR sample and at the proton NMR probe, located just outside the center of the microwave cavity, which was used to measure the field. Except for the p‐benzosemiquinone radical, g values were not found to depend on temperature over the range from room temperature to −100°C. The dependence of the g value on solvent and counterion was also investigated to some extent, and the g value of the perinaphthenyl radical was found to vary markedly with solvent. Agreement with Stone's theory of the g values of aromatic radicals is excellent, except for those radicals in which molecular distortions are expected according to the Jahn—Teller theorem and for radicals in which the unpaired electron is in a nonbonding molecular orbital.

Measurement of g Values in the Electron Spin Resonance Spectra of Free Radicals

Molecular orbital calculations using the Huckel LCAO theory and the approximate configuration interaction treatment suggested by McLachlan have been performed on a large series of nitrosubstituted aromatic anion radicals By properly adjusting the Coulomb and resonance integrals for the nitro group, a good quantitative correlation is obtained between the calculated spin densities and the experimental proton hyperfine splittings for most of the compounds studied Procedures are developed to account for the variations in hyperfine splittings with different solvents, and an equation is formulated which gives excellent predictions of the splitting from N14 nuclei in the nitro groups In addition, the calculations provide a basis for the qualitative discussion of the effects of sterically hindered nitro groups on the hyperfine splittings, and of the apparently anomalous spin densities and linewidth variations in some of the meta‐dinitrobenzene anions Polarographic half‐wave potentials are also correlated with

George K. Fraenkel

Papers

Theory of Linewidths in Electron Spin Resonance Spectra

Theoretical Interpretation of Carbon‐13 Hyperfine Interactions in Electron Spin Resonance Spectra

Electron Spin Resonance of Electrolytically Generated Nitrile Radicals

Measurement of g Values in the Electron Spin Resonance Spectra of Free Radicals

Analysis of the Electron Spin Resonance Spectra of Aromatic Nitrosubstituted Anion Radicals