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Using isotope labeled water (D2O and H217O) and pulsed W-band (94 GHz) high- field multiresonance EPR spectroscopies, such as ELDOR-detected NMR and ENDOR, the biologically important question of detection and quantification of local water in proteins is addressed. A bacterial reaction center (bRC) from Rhodobacter sphaeroides R26 embedded into a trehalose glass matrix is used as a model system. The bRC hosts the two native radical cofactor ions Image ID:c7cp03942e-t1.gif (primary electron donor) and Image ID:c7cp03942e-t2.gif (primary electron acceptor) as well as an artificial nitroxide spin label site-specifically attached to the surface of the H-protein domain. The three paramagnetic reporter groups have distinctly different local environments. They serve as local probes to detect water molecules via magnetic interactions (electron–nuclear hyperfine and quadrupole) with either deuterons or 17O nuclei. bRCs were equilibrated in an atmosphere of different relative humidities allowing us to control precisely the hydration levels of the protein. We show that by using oxygen-17 labeled water quantitative conclusions can be made in contrast to using D2O which suffers from proton–deuterium exchange processes in the protein. From the experiments we also conclude that dry trehalose operates as an anhydrobiotic protein stabilizer in line with the “anchorage hypothesis” of bio-protection. It predicts selective changes in the first solvation shell of the protein upon trehalose–matrix dehydration with subsequent changes in the hydrogen-bonding network. Changes in hydrogen-bonding patterns usually have an impact on the global function of a biological system.

On the basis of experiments at 275 GHz, we reconsider the dependence of the continuous-wave EPR spectra of nitroxide spin-labeled protein sites in sensory- and bacteriorhodopsin on the micro-environment. The high magnetic field provides the resolution necessary to disentangle the effects of hydrogen bonding and polarity. In the gxx region of the 275 GHz EPR spectrum, bands are resolved that derive from spin-label populations carrying no, one or two hydrogen bonds. The gxx value of each population varies hardly from site to site, significantly less than deduced previously from studies at lower microwave frequencies. The fractions of the populations vary strongly, which provides a consistent description of the variation of the average gxx and the average nitrogen-hyperfine interaction Azz from site to site. These variations reflect the difference in the proticity of the micro-environment, and differences in polarity contribute marginally. Concomitant W-band ELDOR- detected NMR experiments on the corresponding nitroxide in perdeuterated water resolve population-specific nitrogen-hyperfine bands, which underlies the interpretation for the proteins.

The frozen solution electron paramagnetic resonance spectrum of the primary donor cation P + in reaction centers of site-directed mutants of Rhodobacter ( Rb. ) sphaeroides has been obtained at a microwave frequency ν =360 GHz and a magnetic field B 0 =12.8 T. Due to the high Zeeman resolution of the powder pattern, all three principal components of the rhombic g -tensors at T =160 K could be determined with high accuracy. We compare spectra of the site-directed mutants, in which the axial ligand histidine M202 of the primary donor is replaced by glutamic acid (HE(M202)) or leucine (HL(M202)), with those of the strain R26, whose primary donor is similar to that of the wild type and only lacks the carotenoid. For HE(M202), this is the first determination of its g -tensor with the principal components g xx =2.00335(3), g yy =2.00236(2) and g zz =2.00191(2). While in R26 the primary donor is a bacteriochlorophyll a dimer, the HL(M202) and HE(M202) mutants have previously been shown to be bacteriochlorophyll:bacteriopheophytin heterodimers. Their g -tensor anisotropy Δ g = g xx − g zz shows significant variations in opposite directions when compared with R26, with an increased anisotropy for HE(M202) and a decreased one for HL(M202). Calculations employing Density Functional Theory suggest that the observed shifts originate in different torsional angles of the acetyl group attached to the spin-carrying bacteriochlorophyll half L of the dimer.

The symmetry of the arrangement of objects has fascinated philosophers, artists and scientists for a long time, and still does. Symmetries often exist in nature, but are also created artificially, for instance by chemical synthesis of novel molecules and materials. The one-sided, non-orientable Möbius band topology is a paradigm of such a symmetry-based fascination. In the early 1960s, in synthetic organic chemistry the interest in molecules with Möbius symmetry was greatly stimulated by a short paper by Edgar Heilbronner. He predicted that sufficiently large [n]annulenes with a closed-shell electron configuration of 4n π-electrons should allow for sufficient π-overlap stabilization to be synthesizable by twisting them with a 180° phase change into the Möbius symmetry of their hydrocarbon skeleton. In 2007, the group of Lechosław Latos-Grażyński succeeded in synthesizing the compound di-p-benzi[28 ]hexa-phyrin(1.1.1.1.1.1), compound 1, which can dynamically switch between Hückel and Möbius conjugation depending, in a complex manner, on the polarity and temperature of the surrounding solvent. This discovery of “topology switching” between the two-sided (Hückel) and one-sided (Möbius) molecular state with closed-shell electronic configuration was based primarily on the results of NMR spectroscopy and DFT calculations. The present EPR and ENDOR work on the radical cation state of compound 1 is the first study of a ground- state open-shell system which exhibits a Hückel–Möbius topology switch that is controlled by temperature, like in the case of the closed-shell precursor. The unpaired electron interacting with magnetic nuclei in the molecule is used as a sensitive probe for the electronic structure and its symmetry properties. For a Hückel conformer with its higher symmetry, we expect – and observe – fewer ENDOR lines than for a Möbius conformer. The ENDOR results are supplemented by and in accordance with theoretical calculations based on density functional theory at the ORCA level.

The photoexcited triplet state of free-base porphycene randomly oriented in a glassy toluene matrix has been investigated by time-resolved electron paramagnetic resonance and electron spin echo spectroscopy over the temperature range 4–100 K. Spectral broadening and anisotropic relaxation are both observed as the temperature is raised, demonstrating the complementary use of these techniques. It is concluded that two triplet states are present in the EPR spectra: only one at 4 K, and the other becoming populated at higher temperatures. This behaviour is explained in terms of intramolecular N–H tautomerism in the porphycene macrocycle. The N–H tautomerism is estimated to have a maximum rate of 7×108 s-1 at 100 K.

The results of calculations of doublet state G tensors of organic radicals often show unsatisfactory agreement with the respective experimental data. This led us to reconsider the results of the Rayleigh–Schrodinger perturbation theory approach found in the literature. We came to the conclusion that neither the expressions given for restricted nor for unrestricted Hartree–Fock functions are fully correct. In this paper new expressions for the G tensor to be used with restricted Hartree–Fock functions will be introduced and results of calculations employing these new formulas in conjunction with molecular states from semiempirical Hartree–Fock-type calculations are discussed.

Some organisms can survive complete dehydration and high temperatures by adopting an anhydrobiotic state in which the intracellular medium contains large amounts of disaccharides, particularly trehalose and sucrose. Trehalose is most effective also in protecting isolated in vitro biostructures. In an attempt to clarify the molecular mechanisms of disaccharide bioprotection, we compared the structure and dynamics of sucrose and trehalose matrices at different hydration levels by means of high-field W-band EPR and FTIR spectroscopy. The hydration state of the samples was characterized by FTIR spectroscopy and the structural organization was probed by EPR using a nitroxide radical dissolved in the respective matrices. Analysis of the EPR spectra showed that the structure and dynamics of the dehydrated matrices as well as their evolution upon re-hydration differ substantially between trehalose and sucrose. The dehydrated trehalose matrix is homogeneous in terms of distribution of the residual water and spin-probe molecules. In contrast, dehydrated sucrose forms a heterogeneous matrix. It is comprised of sucrose polycrystalline clusters and several bulk water domains. The amorphous form was found only in 30% (volume) of the sucrose matrix. Re-hydration leads to a structural homogenization of the sucrose matrix, whilst in the trehalose matrix several domains develop differing in the local water/radical content and radical mobility. The molecular model of the matrices provides an explanation for the different protein–matrix dynamical coupling observed in dried ternary sucrose and trehalose matrices, and accounts for the superior efficacy of trehalose as a bioprotectant. Furthermore, for bacterial photosynthetic reaction centers it is shown that at low water content the protein–matrix coupling is modulated by the sugar/protein molar ratio in sucrose matrices only. This effect is suggested to be related to the preference for sucrose, rather than trehalose, as a bioprotective disaccharide in some anhydrobiotic organisms.

Semiempirical molecular orbital calculations of the electronic g-tensor have been performed on the primary donor cation radical P865+· of Rb. sphaeroides R-26 reaction centers (RCs). These calculations are based on Stone's theory for free radical doublet states and RHF-INDO wave functions and orbital energies. The result is in qualitative agreement with recent W-band (95 GHz) EPR measurements of the g-tensor in RC single crystals which show a breaking of C2 symmetry in the primary donor electronic structure.

Transient EPR spectra of protonated and deuterated Zn-substituted reaction centres of Rhodobacter sphaeroides R-26, measured at a microwave frequency of 95 GHz and an external magnetic field of 3.5 T, are presented. The high-field/high-frequency spin-polarized spectrum of the correlated coupled radical pair P 865 +. Q A −. after laser flash excitation is a very sensitive monitor of the relative orientation of the two g -matrices and the dipolar tensor with respect to each other. Therefore, detailed structural information of the RC concerning the relative orientation of the primary donor P 865 +. and the acceptor Q A −. with respect to the axis connecting the two molecules in the RC can be derived. Together with the information obtained by high-field cw-EPR on single crystals, the enhanced resolution of the polarized high-field spectra allows an unambiguous assignment of the g -matrix of the donor P 865 +. to the molecular axis system. The experimental results are compared with earlier X-band (9 GHz) and K-band (24 GHz) EPR experiments.

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

Intramolecular electron transfer (ET) in cis and trans isomers of a covalently linked porphyrin–cyclohexylene–quinone, oriented in liquid crystals (LCs), is monitored by time‐domain electron paramagnetic resonance (EPR) spectroscopy over an extended range of temperature, i.e., 210

The last decade witnessed a tremendous growth in combined efforts of biologists, chemists and physicists to understand the dominant factors determining the specificity and directionality of transmembrane transfer processes in proteins. A large variety of experimental techniques is being used including X-ray and neutron diffraction, but also time-resolved optical, infrared and magnetic resonance spectroscopy. This is done in conjunction with genetic engineering strategies to construct site-specific mutants for controlled modification of the proteins. As a general perception of these efforts, the substantial influence of weak interactions within the protein and its membrane interfaces is recognized. The weak interactions are subject to subtle changes during the reaction cycle owing to the inherent flexibility of the protein-membrane complex. Specific conformational changes accomplish molecular-switch functions for the transfer process to proceed with optimum efficiency. Characteristic examples of time varying non-bonded interactions are specific H-patterns and/or polarity effects of the microenvironment. The present perception has emerged from the coupling of newly developed spectroscopic techniques – and advanced EPR certainly deserves credit in this respect – with newly developed computational strategies to interpret the experimental data in terms of protein structure and dynamics. By now, the partners of this coupling, particularly high-field EPR spectroscopy and DFT-based quantum theory, have reached a level of sophistication that applications to large biocomplexes are within reach. In this review, a few large paradigm biosystems are surveyed which were explored lately in our laboratory. Taking advantage of the improved spectral and temporal resolution of high-frequency/high-field EPR at 95 GHz/3.4 T and 360 GHz/12.9 T, as compared to conventional X-band EPR (9.5 GHz/0.34 T), three biosystems are characterized with respect to structure and dynamics: (1) Light-induced electron-transfer intermediates in wild-type and mutant reaction-centre proteins from the photosynthetic bacterium Rhodobacter sphaeroides, (2) light-driven proton-transfer intermediates of site-specifically nitroxide spin-labelled mutants of bacteriorhodopsin proteins from Halobacterium salinarium, (3) refolding intermediates of site-specifically nitroxide spin-labelled mutants of the channel-forming protein domain of Colicin A bacterial toxin produced in Escherichia coli. The detailed information obtained is complementary to that of protein crystallography, solid-state NMR, infrared and optical spectroscopy techniques. A unique strength of high-field EPR is particularly noteworthy: it can provide highly desired detailed information on transient intermediates of proteins in biological action. They can be observed and characterized while staying in their working states on biologically relevant time scales. The review introduces the audience to origins and basic experiments of EPR in relation to NMR, describes the underlying strategies for extending conventional EPR to high-field/high-frequency EPR, and highlights those details of molecular information that are obtained from high-field EPR in conjunction with genetic engineering and that are not accessible by “classical” spectroscopy. The importance of quantum-chemical interpretation of the experimental data by DFT and advanced semiempirical molecular-orbital theory is emphasized. A short description of the laboratory-built 95 GHz and 360 GHz EPR/ENDOR spectrometers at FU Berlin is also presented. The review concludes with an outlook to future opportunities and challenges of advanced bio-EPR in interdisciplinary research.

Multifrequency continuous wave (cw) and echo-detected (ED) electron paramagnetic resonance (EPR) was employed to study the mobility of nitroxides confined in nanocapsules. The complexes p-hexanoyl calix[4]arene with 4-methoxy-2,2,6,6-tetramethylpiperidine-N-oxyl (MT) and N-(2-methylpropyl)-N-(1-diethylphosphono-2,2-dimethylpropyl)-aminoxyl (DEPN) were studied by X-, W-band and 360 GHz cw EPR at temperatures between 90 and 370 K. Thereby we were able to extract the canonical values of the hyperfine and g-tensors of the encapsulated radicals as well as information on restricted orientational dynamics of the caged spin probes. Comparing cw and ED-EPR data obtained on MT@C6OH we found that between 90 and 200 K the caged nitroxide undergoes isotropic small-angle fluctuations (librations), whereas at higher temperatures restricted rotations of the radical with correlation times of 0.75 x 10(-10) s and 1.2 x 10(-10) s dominate at 325 and 300 K, respectively. The activation energy of the rotational motion of encapsulated MT radicals was evaluated as E(a) = 1.0 kcal mol(-1), which is comparable to the magnitude of van der Waals interactions. Compared to MT, the reorientational motion of DEPN was found to be slower and more isotropic.

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.

Pulsed high-field/high-frequency EPR (3.4 T magnetic field, 95 GHz microwave frequency) is performed on the primary electron acceptor ubisemiquinone-10 anion radical (UQ-10 −. ) in Zn-substituted photosynthetic reaction centers (RCs) of Rhodobacter sphaeroides R26. In addition, in vitro measurements of UQ-10 −. in a frozen solution of deuterated isopropanol are presented. The method is two-dimensional EPR, with the swept magnetic field as the first and the pulse separation time as the second variable of the echo-detected spectra. The high Zeeman field leads to a spectral resolution of the anisotropic g -tensor. This allows a sensitive detection of the orientation-dependent echo-decay functions. The T 2 relaxation anisotropy at high field is dominated by librations. Comparison of in vivo and in vitro measurements shows different anisotropies of the echo-decay functions. The results are interpreted by calculations considering angular motions around the principal axes of the g -tensor. Hydrogen bonds between UQ-10 −. and its protic environment are considered.

We present an X-ray diffraction (XRD) and multi frequency electron spin resonance (ESR) study of the structure and dynamics of an inclusion complex of p-hexanoyl calix[4]arene (C6OH) with 4-methoxy-2,2,6,6-tetramethylpiperidine-N-oxyl (MT). The single crystal XRD experiments reveal that MT along with ethanol (solvent) molecules are entrapped in a capsular type crystalline lattice of the host C6OH material. ESR measurements were performed at 9.2 GHz/0.33 T (X-band) and at 360 GHz/14 T. In order to avoid ESR line broadening resulting from electron dipole-dipole interaction between nitroxides occupying neighbouring capsules in the crystal lattice, the capsules containing nitroxides were separated from each other by capsules containing diamagnetic dibenzylketone (DBK). Due to the extremely high g-tensor resolution of ESR at 360 GHz, we were able to distinguish, by shifts of their gxx component, between encapsulated nitroxide molecules forming a hydrogen bond between their O-(N) group and the OH group of an ethanol molecule occupying the same capsule and nitroxides missing this interaction. Temperature dependent ESR measurements revealed an orientational anisotropy in the motion of MT encapsulated in C6OH. Solid lipid nanoparticles (SLN) prepared from C6OH and loaded with the nitroxide retained the microcrystalline capsular structure of the pertinent inclusion complex. We found that encapsulated MT in SLNs becomes inaccessible to reducing agents such as sodium ascorbate.

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.

Electron nuclear double resonance (ENDOR) experiments were performed on fluorinated benzophenone and semi-quinone anion radicals. By detecting both fluorine and proton ENDOR signals it was possible to unravel complex hyperfine patterns. The hyperfine coupling constants obtained are discussed in the frame of MO calculations of the McLachlan and INDO type. By comparing the ENDOR enhancements for the fluorine and proton nucluei information could be obtained about the dominant relaxation mechanisms. The observed increase of ENDOR linewidths in fluorinated radicals could be explained on the basis of Freed's relaxation theory. In the fluoranil anion pronounced cross-relaxation effects were observed which are discussed to some extent.

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.

High-resolution ODMR- and ODNQR spectroscopy have been performed on acridine- d 9 in its photoexcited triplet state. The nuclear quadrupole constants e 2 qQ/h = −4.36(3) MHz and η = 0.22(1) have been measured for the 14 N nucleus, where the sign of e 2 qQ/h is determined relative to the signs of the fine structure parameters, D and E . The 14 N ODNQR spectrum, with linewidths as low as 60 kHz, exhibits a satellite structure attributable to the 2 D nucleus located at the meso-position in the molecule. For this nucleus | e 2 qQ/h | = 180(30) kHz may be estimated. The intensity behaviour of the ODNQR transitions as a function of the NMR rf power level leads to a coarse determination of the in-plane components of the 14 N hyperfine tensor with | A yy N | ⩽ 0.2 MHz and | A yy N | ⩽ 4.7 MHz. Owing to accidental overlap of two transitions in the 2 E -ODMR spectrum of acridine, the ODNQR technique had to be extended to a five-pulse sequence.

Three stable dimer configurations of bacteriochlorophyll a are predicted by a self-consistent-field molecular orbital calculation (all valence electrons, restricted Hartree-Fock, INDO-parametrization, perturbational treatment of spin polarization) including geometry optimization by energy minimization. Spin-density distributions of the cation radicals are also calculated and compared with results from magnetic resonance experiments. The largest binding energy is obtained from a strong overlap of the π-systems combined with the formation of two symmetrical MgO bonds between the monomeric components. This structure is compatible with the experimental spin density distribution and very similar to the recently determined X-ray structure of the bacteriochlorophyll b dimer in the photosynthetic reaction center of Rps. viridis. Some possible perturbations by the protein environment 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.

Electron nuclear triple resonance (TRIPLE) experiments on free radicals in solution not only give considerable signal‐to‐noise improvement as compared to ENDOR, but also decrease the observed linewidth and give additional information about the number of protons contributing to a particular hfs constant. The experimental setup is described and the results are discussed in terms of a phenomenological theory.

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.

The three principal phosphorescent bands of Cr3+ (1%)/tris(acetylacetonato)aluminium(III) single crystals at 1.8 K, have been examined at field strengths of 0–5.7 T in various orientations. The principal g values of the corresponding doublet excited states are determined to lie in the ranges ‖g∥ ‖:1.4–1.7, ‖g⊥ ‖:1.8–2.0, in marked contrast to those of the inorganic analogue, ruby.

For the radical anions of low-symmetry molecules (1-phenyl-naphthalene and 2-phenyl-naphthalene) in solution ENDOR spectra were recorded. By careful studies of the temperature dependence of the ENDOR lines different dynamic processes like hindered rotation and ion pair formation could be distinguished. The thermodynamic constants for both processes could be determined. Simple McLachlan type MO calculations were used together with approximate consideration of electrostatic repulsion to interpret the data. These model calculations are found adequate to explain the observed shifts in porton hfs couplings due to ion pairing, which are much larger for the low-symmetry phenyl-naphthalene anions than for the naphthalene anion.

ODMR and ODENDOR spectra were obtained for quinoxaline and p-dichlorobenzene in their excited triplet states. These molecules were studied in mixed crystals using heavy atom containing hosts. For protons the complete hyperfine tensors could be evaluated, yielding information about the symmetry and geometry of these molecules in their triplet states. For quinoxaline no deviations from planarity have been observed whereas triplet p-dichlorobenzene turned out to be non-planar. The measured π-spin densities in p-dichlorobenzene confirm the 3B1u symmetry of this molecule in its first excited triplet state.

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 proton ENDOR spectra of various phenyl substituted hydrocarbon radicals in solution show a marked temperature dependence of the positions, widths, and amplitudes of particular lines. These effects are analyzed in terms of three relaxation mechanisms: (i) internal hindered rotation and (ii) torsional oscillations of the phenyl rings, and (iii) molecular tumbling. From the tempera-ture dependence of relaxation effects caused by mechanism (i) hindrance potentials for internal rotation of phenyl rings could be determined. The results are in good agreement with the pre-dictions of simple π-MO theory. Relaxation effects caused by (ii) turned out to be negligible. Mechanism (iii) produces strong variations in linewidths at low temperatures due to the different hyperfine anisotropics depending on nuclear position.

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.

High-resolution optically detected magnetic resonance (ODMR) and optically detected electron—electron double resonance (ODEEDOR) have been performed on the lowest excited triplet states of pyrazine- h 4 and pyrazine- d 4 , as dilute guests in polycrystalline benzene and cyclohexane host matrices. Computer simulated spectra have been fitted to those experimentally observed, using a spin hamiltonian which takes into account the 14 N and 1 H (or 2 D) nuclei of the pyrazine guest molecule. Good agreement is obtained when one 14 N nuclear quadrupole energy difference, |e y N - e z N | ⩽ 1.3 MHz, and the remaining component, |e x N | = 0.6 ± 0.2 MHz for pyrazine- d 4 in benzene- d 6 .

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.

First measurements of deuterium quadrupole coupling constants of a polyatomic doublet state radical are reported. The quadrupole couplings were determined by ENDOR and electron–nuclear–nuclear TRIPLE resonance in liquid crystals. For the partially deuterated perinaphthenyl radical the out‐of‐plane component of the deuterium quadrupole coupling was measured to be e2q33Q/h =−(94±3) kHz for both molecular sites. Within experimental error these values are equal to those measured earlier for deuterated alternant hydrocarbons in the diamagnetic singlet ground state.

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.

Electrolytically generated semiquinone radical ions have been studied in the nematic mesophase of p-azoxyanisole by EPR methods. The degree of molecular ordering of benzo-, naphtho-, and anthrasemiquinones varied from 0.085, 0.18 to 0.20, respectively. The observed shifts of the hfs splitting constants and the g factors are in reasonable agreement with theoretical predictions. The negative sign of the C1 13 splitting constant of benzosemiquinone has been confirmed.

In the nematic and isotropic phases of the liquid crystal p-azoxyanisole (PAA) the neutral radicals perinaphthenyl (PNT), triphenylmethyl (TPM), and pentaphenylcyclopentadienyl (PPCPD) have been studied by electron spin resonance techniques. A method is discussed for measuring the degree of ordering of the radicals by means of the observed g factors in the different phases. For PNT and TPM the components of the axially symmetric g tensor are determined and compared with the values predicted by STONE’S g factor theory. The shift of the hyperfine splitting constants (C13 and H1) of PNT and TPM agree rather well with the theoretical shifts calculated by the dipol-dipol interaction theory of Mc CONNELL and STRATHDEE. For TPM, however, the agreement is only satisfactory when all the long range contributions are considered. For PNT and TPM also the signs of the spin densities could be determined. PPCPD shows drastically that the validity of both g factor theory and hfs theory in nematic mesophases breaks down for unplanar molecules with large twist angles.

In the course of an investigation of electrolytically generated radical ions in the nematic mesophase of liquid crystals, we studied the influence of supporting electrolyte on the degree of molecular ordering. In the case of the radical anion of tetracyanoethylene (TCNE-) electrolytic generation results in a lower degree of ordering than chemical generation with sodium iodide. By using the neutral radical perinaphthenyl as a probe it was assured that the degree of ordering of the solvent is not disturbed by adding 0.1-m. tetrapropylammoniumperchlorate. A pronounced linewidth variation is observed for the chemically generated TCNE- which is attributed to spin density modulation by dynamic interaction with the Na+ counter ion.

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.