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 method is developed for the assignment to nuclear positions of the hyperfine splittings in the electron spin resonance spectra of free radicals in solution. The procedure is applicable to the splittings from the nuclei of atoms with a nonvanishing pi‐electron spin density, and it is thus useful for 13C and 14N splittings, but cannot be employed for proton splittings. The technique used depends on linewidth effects. It is shown that one contribution to the linewidths is approximately proportional to the square of the spin density on the atom containing the nucleus of interest (the local spin density), and that, under appropriate conditions, splittings from nuclei at positions with high spin density show greater linewidth effects than those from positions with low spin density. The technique is applied to two of the 13C splittings from nuclei present in natural abundance in the spectrum of the anthracene negative ion radical.

Assignment of Hyperfine Splittings in Electron Spin Resonance Spectra by Linewidth Analyses

Measurements have been made of the g values of the naphthalene radical anion associated with several alkali‐metal cations at a series of temperatures and in different solvents It has been suggested previously that the temperature dependences of the g factors of spectra exhibiting alkali‐metal splittings can be explained either in terms of equilibrium changes between two paired species (Hirota model) or in terms of changes in the populations of the vibrational energy levels of a paired species of fixed electronic structure (Atherton–Weissman model) Evidence is presented to show that the Hirota model adequately accounts for the g‐factor variations of the sodium naphthalenide ion pair in tetrahydrofuran, but not for the cesium naphthalenide ion pair in 1,2‐dimethoxyethane It is argued that the latter ion pair is probably a tight ion pair which does not incorporate solvent molecules, and that the variations of the spectral parameters of such complexes are best described by the Atherton–Weissman model

Effect of Ion Pairing on the g Value of the Naphthalene Anion Radical

Paramagnetic resonance of ultramarine1

Paramagnetism of the system s8-so3-h2o

The Giacometti–Nordio–Pavan modification of McConnell's relation between proton hyperfine splittings and spin densities has been shown to reduce to a simpler form than that originally proposed. It has been demonstrated further that the Giacometti–Nordio–Pavan modification should not be regarded as arising from a nearest‐neighbor effect. A statistical analysis of the available experimental proton hyperfine splittings from even‐alternant hydrocarbon ions indicates, however, that the Colpa–Bolton relation is superior to the Giacometti–Nordio–Pavan relation in explaining the differences between proton splittings in corresponding cation and anionx radicals. These results are valid within the framework of Huckel molecular‐orbital theory.

Proton Hyperfine Splittings in the Electron Spin Resonance Spectra of Hydrocarbon Ions: A Comparison of the Colpa‐Bolton and Giacometti–Nordio–Pavan Relations

George K. Fraenkel

Papers

Assignment of Hyperfine Splittings in Electron Spin Resonance Spectra by Linewidth Analyses

Effect of Ion Pairing on the g Value of the Naphthalene Anion Radical

Paramagnetic resonance of ultramarine1

Paramagnetism of the system s8-so3-h2o

Proton Hyperfine Splittings in the Electron Spin Resonance Spectra of Hydrocarbon Ions: A Comparison of the Colpa‐Bolton and Giacometti–Nordio–Pavan Relations