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Liquid solution ENDOR and TRIPLE resonance experiments have been performed on texaphyrin radical anions generated by sodium reduction in tetrahydrofuran. Six proton and two nitrogen hyperfine couplings could be determined, including their signs. The results were compared with theoretical isotropic hyperfine couplings. They were calculated for energy-minimized texaphyrin structures, including the sodium metallo-derivative, by state-of-the-art SCF-MO methods (RHF-INDO/SP). This comparison suggests that in the course of the alkali metal reduction of texaphyrin the acidic hydrogen in the macrocycle is replaced by sodium.

The mechanochemical activation of polycrystalline mixtures of porphyrins and quinones (as well as some other donors and quinones) was found to produce large concentrations of triplet radical pairs with unusual stability in the solid state matrix. High-field/high-frequency (2 mm band) EPR experiments allowed observation of low-temperature spin polarization and determination of the sign of the zero-field splitting parameters D. They appeared to be positive, in contrast to most in chemically or photochemically produced donor—acceptor radical pairs, for which the point dipole approximation (D

The EPR g -factor rotation pattern of the primary donor cation radical in the x  y plane of illuminated reaction center single crystals of Rb. sphaeroides R-26 near room temperature was measured at 3 mm wavelength (W-band, 95 GHz). In contrast to earlier X-band (9.5 GHz) EPR experiments, the two magnetically inequivalent sites of the four reaction centers per unit cell were well resolved. in frozen reaction center solution was also measured by 3 mm EPR. The analysis of both the frozen-solution spectrum and the single-crystal rotation pattern yields the principal values of the g -tensor and the phase angle between the two magnetically inequivalent sites in the x  y plane.

From electrolytically generated phcophytin a anion radicals in solution, 12 proton and 2 nitrogen hyperfine coupling constants, including signs, were determined by ENDOR and TRIPLE resonance and interpreted by MO calculations. The data are compared with those from the anion radicals of chlorophyll a and the bacterial analogues bacteriopheophytin a and bacteriochlorophyll a.

From the electrolytically generated chlorophyll a anion radical in solution, 12 proton and 4 nitrogen hyperfine coupling constants, including their signs and the number of contributing equivalent nuclei, were determined by ESR, ENDOR, and TRIPLE resonance. The data are compared with recent MO calculations, and possible assignments to molecular positions are discussed.

The radical cations of chlorophyll a (Chl a) and bis ( chlorophyll )cyclophane (BCP), a synthetic model dimer of chlorophyll, were studied by ENDOR and TRIPLE resonance in liquid solution. The measured hyperiine coupling constants (hfcs) of Chl a + were compared with the results of molecular orbital calculations. By comparing the hfcs of BCP + and Chl a + a localization of the unpaired electron on one macrocycle of BCP + is deduced.

A phosphorescence line-narrowing technique involving direct quartet ground state to doublet excited state pumping, is used to resolve completely optical—Zeeman components at field strengths compatible with Q-band EPR spectroscopy. By incorporating this technique, ODMR of the lowest doublet excited state of Cr3+ ions in an organometallic host crystal (tris(acetylacetonato)aluminum(III)) has been obtained for several magnetic field orientations. Selective, direct optical pumping shows that the three prominent, close-lying phosphorescence bands at 773, 776 and 778 nm correspond to three zero-phonon origins. ODMR performed on the 778 nm origins yields the values|g⊥|=1.921 ± 0.003, |{g∥|}|=1.60 ± 0.03 for the excited-state g-tensor and provides an order of magnitude estimate of ≈ 10 μs for the excited-state spin—lattice relaxation time.

Solution 25 Mg ENDOR and TRIPLE resonance experiments have been performed on the bacteriochlorophyll a (BChl a) anion and cation radicals enriched in 25 Mg. The 25 Mg isotropic hyperfine coupling constants for BChl a + · and BChl a − · have been determined. The values are in good agreement with those obtained from recent MO calculations.

From chemically generated anion radicals of anti[2.2](1,4)naphthalenophane and 1,4-dimethylnaphthalene, hyperfine coupling constants including relative signs are determined by ENDOR and TRIPLE resonance. The data are compared with spin density distributions obtained by MO calculations. The results are discussed with respect to the spin density distribution in the “special pair” primary donor of bacterial photosynthesis.

The ketone ω,ω-dimethoxy-ω-phenyl-acetophenone (DMPA) is an efficient initiator in UV-curing polymerization processes started by photochemically generated radicals. The short-lived radical intermediates created during the photolysis of DMPA have been investigated by time-resolved ESR and ENDOR spectroscopy taking advantage of a strong CIDEP effect.

The radical cation of bacterioehlorophyll b (BChl b) is investigated by ENDOR and TRIPLE resonance in liquid solution. The experimental hyperfine coupling constants, ten proton and three nitrogen couplings, are compared with the predictions from advanced molecular-orbital calculations (RHF INDO/SP). The detailed picture obtained of the spin density distribution is a prerequisite for the investigation of the primary electron donor radical cation in BChl b containing photosynthetic bacteria.

The first 14 N nuclear quadrupole coupling constants for a representative porphyrin, zinc(octaethylporphyrin) (ZnOEP), in its diamagnetic ground state are reported. Well-resolved ODNQR spectra show that the four 14 N nuclei are nearly equivalent with average constants ¦ e 2 qQ/h ¦=2.16 MHz and η = 0.56.

From electrolytically generated bacteriochlorophyll a anion radicals in solution, eight proton and four nitrogen hfs constants including signs were measured by ENDOR and TRIPLE resonance. Optimum experimental conditions for ENDOR of different nuclei were examined. The hfs data are compared with results from recent MO calculations, yielding satisfactory agreement.

ESR, 1 H and 13 C NMR, ENDOR and TRIPLE resonance have been employed to study photochemical reactions of 2,2,5,5-tetramethyl-4-phenyl-3-imidazoline-3-oxide-1-oxyl and to determine the electronic structures of the initial and final radicals. The light-induced isomerization of a phenyl nitron group to an oxaziridine group has been shown to proceed without involving the radical centre.

The photoexcited triplet state PT of Rhodopseudomonas sphaeroides R-26 has been investigated by ENDOR measurements performed on frozen photosynthetic reaction centre solutions. For the first time hyperfine data could be obtained for PT. These data indicate a delocalisation of the triplet state over two bacteriochlorophyll a molecules.

On two photolytically generated transient radicals showing CIDEP effects. ENDOR experiments have been performed by integrating the ESR signal between 4 and 5 μs after the irradiating laser flash. Different ENDOR effects are observed for a net effect electron polarization in p -benzosemiquinone and for a multiplet polarization in benzyl radicals.

From electrolytically generated bacteriopheophytin a anion radical in solution, 10 proton and 4 nitrogen hyperfine coupling constants, including signs, have been measured by ESR, ENDOR and TRIPLE resonance techniques. The experimental results are compared to those from MO calculations and possible assignments to molecular positions discussed.

The light-induced radical cation of the primary electron donor P960+• in photosynthetic reaction centers from Rhodopseudomonas viridis has been investigated by ESR, ENDOR and TRIPLE techniques. Both the comparison with the cation radical of monomeric bacteriochlorophyll b (BChl b) and with molecular-orbital calculations performed on P960+• using the results of an X-ray structure analysis, consistently show an asymmetric distribution of the unpaired electron over the two BChl b molecules which constitute P960+•. The possible relevance of this result for the primary electron transfer step in the reaction center is briefly discussed.

Two representative porphyrin dimers, each containing meso-tetraphenylporphyrin (TPP) monomers as constituents, have been studied by resonance Raman (RR) spectroscopy. In both cases the monomer units are linked through one common phenyl ring. The first incorporates a para-phenylene bridge thereby retaining a centre of inversion; the second incorporates a metaphenylene bridge and has no centre of inversion. Modes giving rise to distinguishing features (monomer versus dimer and the p -dimer versus the m -dimer) in the 300–420 cm −1 region are qualitatively assigned. It is concluded that in each dimer the componentmonomers are weakly interacting in their Soret excited states. The excitation may be considered monomer-localised on the RR time scale of ≈ 1 ps.

Quadrupole splittings of organic nitroxide radicals in solution have been observed by ENDOR in a liquid crystal. The analysis of the line shifts yields e 2 q ′ 33 Q/h = −2.5 MHz. This result could be rationalized by a population analysis of the nitrogen bonds. The theory predicts that the main contribution to the observed splitting comes from the unpaired π-electron.

From the ENDOR spectra of the mononegative ions of biphenyl and ortho-, meta- and para-terphenyl in solution complete sets of hyperfine splitting constants could be determined. The splittings are interpreted in terms of SCF MO spin densities obtained from McLachlan type calculations. It turns out that π-σ spin delocalization effects do not play a significant role in the spin density distribution in these radicals although some of them deviate considerably from coplanarity.

In the liquid crystal “Phase IV” which exhibits its nematic range between 16 and 76 °C ENDOR spectra of perinaphthenyl and tri- t -butylphenoxyl radicals could be recorded. It is demonstrated that the combined use of the ENDOR and liquid crystal methods yeilds valuable structural information.