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. Technical Note Report No. 9

Persistent and stable radicals of the heavier main group elements and related species.

Structure, Reactivity, and Chemistry of Fluoroalkyl Radicals.

Reaction conditions are described for the oxidation of anilines furnishing nitrosoarenes and the synthesis of unsymmetrically substituted azobenzenes. In a comparative study, the catalytic oxidation of methyl 4-aminobenzoate by hydrogen peroxide was investigated, and SeO2 proved to be superior or equal to methyl trioxorhenium (MTO) and Na2WO4, respectively. Nevertheless, the application of the inexpensive, environmentally friendly, Oxone in a biphasic system proved to be more efficient, and a variety of useful nitrosoarenes for the synthesis of azo compounds were prepared in high yield and purity on a large scale.

Efficient Preparation of Nitrosoarenes for the Synthesis of Azobenzenes

This review describes the synthesis of 1,3- and 1,2-tetrachalcogenafulvalenes, the chemical transformation of substituents, reactions of the ring systems and the synthesis of polymers with 1,3-tetrathiafulvalene units in the backbone and in the side chain. For selected charge-transfer complexes of tetrachalcogenafulvalenes the relation between molecular structure and solid-state structure, respectively, and electric conductivity are discussed. Superconducting properties of some charge-transfer complexes with tetrachalcogenafulvalenes as donor are reported. Preparations of important starting compounds for the synthesis of tetrachalcogenafulvalenes (like 1,2- and 1,3-dithiole, 1,3-thiaselenole, and 1,3-diselenole derivatives) have been considered in this review. Selected properties (melting point, UV-VIS absorption data and oxidation potentials) of 1,2- and 1,3-tetrachalcogenafulvalenes are summarized in appropriate tables.

Synthesis, Reactions, and Selected Physico-Chemical Properties of 1,3-and 1,2-Tetrachalcogenafulvalenes

The peri-bridged naphthalenes C10H6XY with XY = SS, SSe, STe, SeSe, SeTe, TeTe all exhibit low first vertical ionization potentials at about 7.1 eV and, therefore, can be oxidized using AlCl3/H2CCl2. The radical cations generated (XY = SS to SeTe) are stable at room temperature. The chalcogen bridges dominate the species M⋅⊕ regarding spin population as well as charge distribution and, correspondingly, the ESR spectra reflect the effect of increasing spin/orbit interaction S < Se < Te: line widths grow and g values raise linearly from 2.0086 (XY = SS) to 2.0409 (XY = SeTe). The PES ionization energies and the ESR coupling constants of sulfur-bridged derivatives are satisfactorily reproduced by a molecular state parametrized HMO model.



Radikalionen, 46. Die Einelektronen-Oxidation 1,8-Chalkogen-uberbruckter Naphthaline



Die peri-uberbruckten Naphthaline C10H6XY mit XY = SS, SSe, STe, SeSe, SeTe, TeTe weisen alle niedrige erste vertikale Ionisierungspotentiale von etwa 7.1 eV auf und lassen sich daher mit AlCl3/H2CCl2 oxidieren. Die erzeugten Radikalkationen (XY = SS bis SeTe) sind bei Raumtemperatur stabil. Die Chalcogen-Brucken pragen die Species M⋅⊕ sowohl bezuglich der Spinpopulation wie der Ladungsverteilung und dementsprechend spiegeln die ESR-Spektren den Effekt zunehmender Spin/Bahn-Wechselwirkung S < Se < Te wider: Die Linienbreiten wachsen und die g-Faktoren erhohen sich linear von 2.0086 (XY = SS) bis auf 2.0409 (XY = SeTe). Die PES-Ionisierungsenergien und die ESR-Kopplungskonstanten der Schwefel-Derivate werden von einem HMO-Modell mit Molekulzustands-Parametern zufriedenstellend wiedergegeben.

Radical Ions, 46. One-Electron Oxidation of 1,8-Chalcogen-Bridged Naphthalenes

Radikalkation und Radikalanion eines tetrasilylsubstituierten Ethylens

Die Oxidation von Hexakis(alkylthio)ethanen (RS)3C C(SR)3 mit AlCl3/H2CCl2, NOBF4 oder I2 fuhrt zu Tetrakis(alkylthio)ethen-Radikalionen (RS)2Ċ⊕C(SR)2. Das schwarzviolette Tetrahydrotetrathiafulvalenium-triiodid 1 · I3 (n = 2) konnte in Substanz isoliert werden. Weitere Derivate wie das Dibenzotetrathiafulvalen- (2) und das Chinodimethan-(4)-Radikalion werden in Losung ESR-spektroskopisch charakterisiert: Aus den Kopplungskonstanten und g-Faktoren folgt, das sich die Spindichte uberwiegend an den S-Zentren befindet. Die Alkylprotonen-Kopplungen lassen sich vorteilhaft mit winkelunabhangiger Spinpolarisation beschreiben. Fur Tetrathiafulvalen ergeben geometrieoptimierte open-shell-MNDO-Berechnungen, das sich die zentrale C C-Bindung bei Oxidation verlangern, und das die positive Ladung uberwiegend am Schwefel zentriert sein sollte.



The Oxidation of Hexathioethanes to Tetrathioethene Radical Cations



The oxidation of hexakis(alkylthio)ethanes (RS)3C C(SR)3 by AlCl3/H2CCl2, NOBF4, or I2 yields tetrakis(alkylthio)ethene radical cations (RS)2Ċ⊕C(SR)2. The dark violet tetrahydrotetrathiafulvalenium triiodide 1 · I3 (n = 2) has been obtained as a crystalline solid. Other derivatives such as the dibenzotetrathiafulvalene (2) and the quinodimethane (4) radical cations have been characterized by ESR spectroscopy in solution: The coupling constants and the g values reveal that the spin density accumulates preferentially at the S centers. The alkyl proton coupling is rationalized advantageously by angle-independent spin polarization. For tetrathiafulvalene, geometry-optimized MNDO calculations predict a lengthening of the central C C bond on oxidation and the positive charge centered predominantly at sulfur.

Die Oxidation von Hexathioethanen zu Tetrathioethen‐Radikalkationen

Radical Cation and Radical Anion of a Tetrasilyl‐Substituted Ethylene

Bis(methylthio)naphthaline gehoren zu den wenigen Molekulen M. die sich sowohl zu Radikalkationen M•⊕ oxidieren als auch zu Radikalanionen M•⊖ reduzieren lassen. Fur die untersuchten 1.4-, 1.5-, 2.6- und 1,8-disubstituierten Derivate ergeben Photoelektronenspektren, das deren erste π-Ionisierungsenergien relativ zu der des Kohlenwasserstoffes C10H8 um mehr als 0.5eV absinken. Demgegenuber wird fur das unterste unbesetzte Naphthalin-π-Orbital eine vernachlassigbar geringe Storung durch die H3CS-Substituenten berechnet. Erwartungsgemas gleichen die aus den ESR-Spektren der Radikalanionen Entnehmbaren Spinverteilungen der von C10H8•⊖ wahrend in den Radikalkationen (H3CS)2C10H6•⊕ eine erhebliche Schwefelbeteiligung gemessen wird.



Radical Ions, XXXIII. Oxidation and Reduction of Methylthio-substituted Naphthalenes – A comparison of Molecular States



Bis(methylthio)naphthalenes belong to the few molecules M which may be oxidized to their radical cations M•⊕ as well as reduced to their radical anions M•⊖. For the 1.4-, 1.5-,2.6-, and 1,8- disubstituted derivatives investigated, photoelectron spectra demonstrate that the first π-ionization energies are lowered by at least 0.5eV relative to the parent hydrocarbon C10H8. On the contrary, only a negligible perturbation of the lowest unoccupied naphthalene π-orbital by the H3CS substituents is calculated. Correspondingly, the spin distributions inferred from the ESR spectra of the radical anions resemble that of C10H8•⊖, whereas in the radical cations(H3CS)2C10H6•⊕ sulfur participates considerably.

Georg Brähler

Papers

Radical Ions, 46. One-Electron Oxidation of 1,8-Chalcogen-Bridged Naphthalenes

Radikalkation und Radikalanion eines tetrasilylsubstituierten Ethylens

Die Oxidation von Hexathioethanen zu Tetrathioethen‐Radikalkationen

Radical Cation and Radical Anion of a Tetrasilyl‐Substituted Ethylene

Radikalionen, XXXIII. Oxidation und Reduktion methylthio‐substituierter Naphthaline — ein Vergleich von Molekülzuständen