Reijo Mäkelä

Electron spin resonance, ENDOR and TRIPLE resonance of some 9,10-anthraquinone and 9,10-anthraquinol radicals in solution

E.s.r., ENDOR and TRIPLE resonance spectra have been recorded for 2-methylanthraquinone, anthraquinone-2-sulphonate and anthraquinone-2,6-disulphonate radical anions, and in a strongly acidic medium for 2-methylanthraquinol, anthraquinol-2-sulphonate and anthraquinol-2,6-disulphonate radical cations.The hyperfine coupling constants (h.f.s.) and g factors are given. The ENDOR spectra show there to be more h.f.s. than were detected earlier by e.s.r. spectroscopy. The spectra of the di-deutero radicals of the anthraquinols and the assignment of h.f.s. are discussed.

The additivity relationship and substituent effects studied by EPR and multiresonance spectroscopy

Abstract The additivity relationship is studied for anion radicals of substituted benzenes using EPR spectroscopy. Additivity parameters were calculated separately for the symmetrical and antisymmetrical states of the degenerate parent molecule. It is shown that the additivity relationship gives better results when the signs of the coupling constants are taken into account. Hyperfine coupling constants, with their signs, were measured by ENDOR and TRIPLE resonance spectroscopy for two anion radicals of substituted benzene: cyanobenzene and m -dinitrobenzene. In addition a theory is developed for the electron donor and acceptor property of a substituent.

Electron paramagnetic resonance, ENDOR and TRIPLE resonance of some 9,10-anthraquinone radicals in solution. Part 3.—Hydroxyanthraquinones

EPR, ENDOR and TRIPLE resonance spectra have been recorded for 1,4-dihydroxy-9,10-anthraquinone, 1,5-dihydroxy-9,10-anthraquinone, 1,8-dihydroxy-9,10-anthraquinone, 2,6-dihydroxy-9,10-anthraquinone, 1,2-dihydroxy-9,10-anthraquinone, 1,2-dihydroxy-9,10-anthraquinone-3-sodium sulphonate and 1,2,5,8-tetrahydroxy-9,10-anthraquinone anion radicals. The hyperfine coupling constants and g factors are given. Use of a modified additivity relationship allowed assignment of the constants. The spectra of deuterated radicals are discussed.

Electron spin resonance, ENDOR and TRIPLE resonance of some 9,10-anthraquinone radicals in solution. Part 2.—Anthraquinonesulphonates

We report studies on radical anions of 9,10-anthraquinone-1-sulphonate, 9,10-anthraquinone-1,5-disulphonate, 9,10-anthraquinone-2-sulphonate, 9,10-anthraquinone-2,6-disulphonate and 9,10-anthraquinone-1,2-dihydroxy-3-sulphonate in alkaline ethanol and water mixtures carried out by multiple resonance spectroscopy. The hyperfine coupling constants and relative signs were determined in ENDOR and TRIPLE resonance experiments. The temperature dependence of the coupling constants was measured and the g values were also estimated. Use of a modified additivity relationship suggested a new assignment of the coupling constants.

Comparison of Spin Density Calculation Methods for Various Alkyl‐Substituted 9,10‐Anthraquinone Anion Radicals in the Solution Phase

EPR and ENDOR spectra were recorded for 2‐methyl‐9,10‐anthraquinone (2‐methylAQ), 2‐ethylAQ, 2‐tert‐butylAQ and 2,3‐dimethylAQ anion radicals in the solution phase. The EPR spectra were simulated with the help of ENDOR data. The experimental isotropic hyperfine coupling constants (IHFCs) were compared with calculated values from semi‐empirical INDO, spin‐restricted AM1/CI and B3PW91 density‐functional methods. The best computational methods for the IHFCs were the semi‐empirical AM1/CI method and the B3PW91 density‐functional method with a large basis set.

Electron paramagnetic resonance, ENDOR and TRIPLE resonance studies on substitution reactions of 2-methyl-1,4-benzoquinone and 2,3-dimethoxy-5-methyl-1,4-benzoquinone in alkaline methanol

Substitution of 2-methyl-1,4-benzoquinone with methanol takes place in alkaline methanol, giving rise to methoxy-substituted methyl-1,4-benzoquinone radical anions. The relative signs and absolute values of the hyperfine coupling constants (hfcs) of the various mono-, di- and tri-substituted methoxymethyl-1,4-benzoquinones have been measured by EPR, ENDOR and TRIPLE resonance spectroscopy. The hfcs of OCH3 and CH3 protons had the same positive sign, except in the 3,5-dimethoxy-2-methyl-1,4-benzoquinone radical anion. The ENDOR signals of the methoxy protons could be distinguished from the methyl proton signals by using perdeuterated methanol as solvent. Partial optimizations of the geometry of the radical were performed by using the unrestricted MNDO method. In the most stable conformers the methoxyl groups had twisted out of the aromatic plane. INDO calculations were performed by using optimized structures, and the hfcs were compared with those obtained using standard bond lengths and angles. The hfcs of the ring protons were assigned by McLachlan calculations and support of the assignment was provided by the additivity relationship method. The general TRIPLE technique allowed identification of the methyl, methoxy and ring protons of the different methoxy-substituted methyl-1,4-benzoquinone radical anions.

Electron paramagnetic resonance, ENDOR and TRIPLE resonance study of methyl-substituted benzoquinol radical cations

EPR and ENDOR spectra have been recorded for hydroquinone, methylhydroquinone, 2,3-dimethylhydroquinone, 2,5-dimethylhydroquinone, 2,6-dimethylhydroquinone, 2,3,5-trimethylhydroquinone and tetramethylhydroquinone radical cations. The radicals were generated by Ce4+ or Zr4+ oxidation in FSO3H or FSO3H and CH3NO2 solution. The radical cations were stable in strongly acidic media, where they existed in double-protonated form. The splittings of hydroxyl protons were assigned by deuteriation and other coupling constants by the additivity relationship. The cis–trans isomerism of the double-protonated cation radicals showed up clearly in EPR and ENDOR spectra of four of the radicals.

EPR, ENDOR and TRIPLE Resonance of Amino-Substituted 9,10-Anthraquinone Radicals and the Rotation of the Amino Groups in the Solution Phase

EPR, ENDOR and TRIPLE resonance spectra were recorded for 1‐amino‐9,10‐anthraquinone (1‐aminoAQ), 2‐aminoAQ, 1,2‐diaminoAQ, 1,4‐diaminoAQ, 1,5‐diaminoAQ and 2,6‐diaminoAQ anion radicals in the solution phase. The rotation of the amino groups is discussed with reference to the experimental data. Rotation barrier calculations were carried out using B3PW91/3–21G* density functional method. The amino groups at positions 1, 4, 5, 8 were found not to be in rotational movement up to the EPR time‐scale.

Electron paramagnetic resonance, ENDOR and TRIPLE resonance of some 9, 10-anthraquinol radicals in solution. Part 4.—Anthraquinolsulphonates

EPR, ENDOR and TRIPLE resonance spectra are recorded for 9, 10-anthraquinol-1-sodium sulphonate, 9,10-anthraquinol-2-sodium sulphonate, 9,10-anthraquinol-1,5-disodium sulphonate and 9,10-anthraquinol-2,6-disodium sulphonate cation radicals. The splittings of the 9,10-anthraquinol cation radical have been assigned by deuteration. The spectra of the deuterated anthraquinol cation radicals and the assignment of hyperfine coupling constants are discussed.