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The title diradical was synthesized and investigated by ESR and UV-vis spectroscopy. It was found to have a lifetime of weeks even in the presence of oxygen, and even survives brief heating in toluene up to about 60 °C. In the UV-vis spectrum, the diradical showed reversible thermochromic behavior in the -10 to 50 °C range. In the ESR spectrum, hyperfine analysis showed nearly isolated behavior by the phenoxyl and nitroxide spin carrying units. Upon warming, additional, broad lines appeared at the expense of the lower temperature lines. This temperature behavior was reversible over the 0 to 50 °C range, so long as the raised temperatures were not maintained for long time periods. The spectral behavior is interpreted as being due to temperature-dependent conformational effects on the exchange coupling between the spin carrying units, i.e.

The photochemistry of azidopyridine 1-oxides was studied using an array of glass and matrix isolation techniques. As with room temperature, the photochemistry of 4-azidopyridine 1-oxide is dominated by triplet nitrene chemistry. However, in the case of the 3-azide, matrix photolysis indicates the formation of diazabicyclo[4.1.0]hepta-2,4,6-triene N-oxide and diazacycloheptatetraene N-oxide intermediates as well as triplet nitrene.

2-(4-Azidophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (14), 2-(4-azidophenyl)benzimidazole-1-oxide-3-oxyl (16), 2-(4-azidophenyl)-1,2,6-triphenylverdazyl (19), 2-(3-azidophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (21), and (3-azidophenyl)-N-tert-butyl-N-aminoxyl (25) were photolyzed in frozen solution to give S = 3/2 state ESR spectra of the corresponding nitrenophenyl radicals with the following zero-field splitting parameters: |D/hc| = 0.277 cm(-1), |E/hc| < or = 0.002 cm(-1) (7 from 14); |D/hc| = 0.256 cm(-1), |E/hc| < or = 0.002 cm(-1) (8 from 16); |D/hc| = 0.288 cm(-1), |E/hc| < or = 0.002 cm(-1) (9 from 19); |D/hc| = 0.352 cm(-1), |E/hc| = 0.006 cm(-1) (10 from 21); |D/hc| = 0.336 cm(-1), |E/hc| = 0.004 cm(-1) (11 from 25). UB3LYP/6-31G computations and ESR spectroscopic analyses suggest that these are nitreno radicals, even para-linked systems with possible quinonoidal resonance forms. Neat samples of azidophenyl radicals 14 and 21 showed bulk paramagnetic behavior, consistent with the lack of close contacts in their crystal structures. Efforts to make photolabile coordination complexes of 14 and 21 with paramagnetic transition metal ions were unsuccessful: Cu(ClO4)2 x 6H2O instead oxidized them to the corresponding diamagnetic nitrosonium perchlorate salts.

The title diradical was synthesized and investigated by ESR and UV−vis spectroscopy. It was found to have a lifetime of weeks even in the presence of oxygen, and even survives brief heating in tolu...

The electronic states of open-shell stilbene diradicals were studied. Previously used semiempirical MO-CI methods were applied to stilbenes having dimethylene, dioxyl, and dinitroxide centers in o,o'-, o,m'-, o,p'-, m,m'-, m,p'-, and p,p'-substitution patterns, and the vinous singlet-triplet energy gaps were computed to predict qualitative ground state spin multiplicities. Singlet and triplet spin states were essentially degenerate in the m,m'-isomers of disjoint connectivity, while nondisjoint o,m'- and m,p'-isomers had triplet computed ground states. Nitroxide-based diradicals showed considerably smaller computational exchange coupling energies than did other diradical models studied. As a test of these computations, stilbenes with two N-tert-butyl nitroxide groups at the o,m'-, m,m'-, and m,p'-positions were prepared, and their magnetic properties were studied by ESR and magnetic susceptibility measurements. The m,m'- and m,p'-isomers were found experimentally to have singlet and triplet ground states, respectively, in accord with the qualitative computed predictions

A semiempirical AM1 molecular orbital plus configuration interaction model is applied to π-conjugated organic open-shell molecules in order to understand qualitatively the manner and degree to which they are spin-coupled by various electronic mechanisms, either in simpler open-shell systems or, in some examples, in layer polyradicals. A «library» of spin-coupling spacer groups (X in radical-X-radical or nitrene-X nitrene) is delineated with relative exchange interaction strengths and natures described, including alkenes and polyalkenes, alkynes, phenyl rings, heteroatoms, and nonalternant pseudoaromatic systems. Overall, the semiempirical MO-Cl method is shown to be both conceptually useful and highly cost effective for semiquantitative study and classification of broad classes of electron exchange coupling effects in non-Kekule molecules

Hydrogen-bonding triarylamines, 4-(N,N-bis(4-methoxyphenyl)amino)benzoic acid (TPA1), 5-(N,N-bis(4-methoxyphenyl)amino)isophthalic acid (TPA2), and N-(4-(1H-benzimidazol-2-yl)phenyl)-N,N-bis(4-methoxyphenyl)amine (BImTPA), were synthesized as radical cation precursors. TPA1 and TPA2 are readily p-doped by AgSbF(6) to give highly persistent radical cations. Poor solid-state spin yields of the radical cation from BImTPA may be due to spin delocalization.

Stable radicals 1-(4-(N-tert-butyl-N-aminoxyl)phenyl)pyrrole (BNPP) and 1-(4-(N-[para-methoxyphenyl]-N-aminoxyl)phenyl)pyrrole (MNPP) were synthesized and characterized by crystallography and magnetism. BNPP crystals exhibit 1-D chains of intermolecular nitroxide NO to nitroxide CH3 contacts, but polycrystalline magnetic susceptibility measurements show quite small antiferromagnetic (AFM) exchange interactions. MNPP shows stronger AFM exchange interactions that appear to be associated with a 2-D planar mesh of crystallographic nitroxide to nitroxide (N)O...N(O) contacts of 4.0-4.2 A. The AFM behavior of MNPP can be fitted to a 2-D square planar Heisenberg antiferromagnetic exchange model with J/k = (-)0.78 +/- 0.04 K and mean field constant theta = (-)0.77 +/- 0.12 K.

Triplet 2-(dicyclopropylmethylidene)cyclopentane-1,3-diyl, CP2TMM, has a Deltam(s) = 2 ESR transition revealing hyperfine splitting consistent with coupling to two equivalent alpha hydrogens and five equivalent beta hydrogens and one other hydrogen with a small hyperfine coupling constant. This is consistent only with a conformation in which one cyclopropane ring conjugates with the pi triplet so that the other cyclopropane is twisted allowing the tertiary cyclopropyl hydrogen to hyperconjugate. Geometry optimization of the triplet state of CP2TMM was carried out using the 3-21G basis set with a UHF wave function and while the singlet state was optimized using a GVB perfect pairing wave function. The most stable conformation of each spin state is a C(2)-symmetric structure providing a 10 kcal/mol singlet-triplet energy gap favoring the triplet. This gap is reproduced at the 6-31G level using the 3-21G geometries. However, this structure cannot reproduce the hyperfine splitting observed; 1 kcal/mol higher in energy is an asymmetrical conformation of triplet CP2TMM which can account for the hyperfine interactions.

The use of aryloxy oxalyl chlorides (AOCs), aryloxy oxalyl tert-butyl peroxides (AOBs), and diaryl oxalates (DAOs) for unimolecular generation of phenoxyl-based radicals under solution and rigid matrix conditions is described. AOCs are usable for photochemical generation of phenoxyl radicals, but are only conveniently stable as precursors when 2,6-di-tert-butylated derivatives are used. AOBs may be used as thermal precursors to aryloxyl radicals, since they typically decompose within 2-3 h at 60-85 degrees C to give phenols. (1)H-NMR solution kinetic studies find that DeltaH() = 31 kcal/mol, and DeltaS() = +3.4 cal/mol-K for decomposition of phenoxyoxalyl tert-butyl peroxide, consistent with substantial concertedness in peroxide bond cleavage. AOBs and the more stable DAOs are also convenient photochemical phenoxyl radical precursors. AOBs yield phenoxyl radicals more readily by photolysis than do corresponding DAOs, but the DAOs have fewer side reactions that can quench the product phenoxyl radicals.