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The primary electron donor (D) of photosynthetic bacteria (Rhodobacter sphaeroides) is a dimer of bacteriochlorophyll a (BChl a) molecules. The molecular origin of the electronic asymmetry of the cation radical of D (D+•) is investigated by EPR/ENDOR on D+• in a heterodimer hydrogen bond double mutant HL (M202)/LH (L131). HL (M202) is the heterodimer mutant, in which DL is a BChl and DM a bacteriopheophytin. In D+• of this mutant the unpaired electron is localized on DL. The LH (L131) mutation introduces a hydrogen bond to DL, which shifts spin density from ring I into ring III. Molecular orbital calculations on BChl a +• are performed in order to predict the effect of acetyl group rotation and hydrogen bonding on the spin density distribution. Interaction of the newly introduced His L131 with DL seems weaker than that of the acetyl hydrogen bond formed by His L168 in the native structure. Additional EPR signals observed in HL (M202)/LH (L131) suggest further paramagnetic species, which might be a result of the high oxidation potential of D in this mutant.

Publisher Summary It has reported that NO molecule, which is a radical, is a very good probe to investigate Lewis acid sites in zeolites. This peculiarity comes from the fact that the ground state of the NO molecule is degenerated and therefore the magnetic moment is equal to zero. To observe an ESR signal from NO one has to quench the orbital momentum of the free electron. This can be done by the electric field originating from an ion on which NO molecule is sorbed, for example from the Lewis site in a zeolite. To adsorb the NO on this site and therefore to get an ESR spectra one has to cool the whole sample with the gas to the low temperature (77 K). At that temperature one can see very nice ESR spectrum, which shows the hyperfine splitting coming from the 14N nucleus and the superhyperfine splitting from 27AI nucleus.

Recent Electron-Nuclear-Double and -Triple Magnetic Resonance measurements on single crystals of reaction centers (RC’s) of the bacterium Rb. sphaeroides R-26 have revealed an asymmetric spin density distribution of the primary donor radical cation P 865 + 1. The observed ratio of spin densities, ρ L/ρ M, on the monomeric halves DL and DM of this dimer is approximately 2 : 1. A similar result had been found earlier for the primary donor radical cation P 960 +. in RC’s of Rps. viridis in liquid solution 2,3.

The rates of the electron transfer processes in reaction centers (RC’s) of photosynthetic bacteria are controlled both by the spatial and the electronic structure of the involved donor and acceptor molecules. The spatial structure of bacterial RC's has been determined by X-ray diffraction for Rhodopseudomonas (Rp.) viridis 1 and for Rhodobacter (Rb.) sphaeroides.2 The electronic structure of the transient radical species formed in the charge separation process can be elucidated by EPR and ENDOR techniques.3 The information is contained in the electron-nuclear hyperfine couplings (hfc’s) which, after assignment to specific nuclei, yield a detailed picture of the valence electron spin density distribution in the respective molecules.3,4

Porphycenes are novel structural isomers of porphyrins. The radical anions of several porphycenes were studied by ESR, ENDOR, and TRIPLE resonance in liquid solution yielding the isotropic hyperfine coupling constants including signs. For the unsubstituted free-base porphycene, the 2,7,12,17-tetra-n-propylporphycene, and the 9,10,19,20-tetra-n-propylporphycene, the experimental findings are compared with results of all-valence-electrons self-consistent field molecular orbital calculations (RHF-INDO/SP).

In addition to the spatial structure of the reaction center (RC) of Rhodopseudomonas (Rps.) viridis obtained from X-ray crystallography (1) a knowledge of the electronic structure is required for a basic understanding of the functional details of the RC. This can be obtained for the radical ions formed in the charge separation process by EPR and ENDOR techniques (2) which yield electron-nuclear hyperfine couplings (hfc’s). From the hfc’s a map of the valence electron spin distribution over the molecule is obtained. In this work we studied the intermediate electron acceptor radical anion I−•, a monomeric bacteriopheophytin (BPh) b (3), Fig. 1, that was trapped at low temperature (77 K) in the RC (4). EPR and ENDOR results on I−• had been reported earlier (5). Improved instrumental design enabled us to measure additional hfc’s with higher accuracy.

Spin density calculations were performed on the oxidized primary electron donor in reaction centers (RC’s) from Rhodobacter sphaeroides R-26. The calculations were based on the all-valence-electron MO method RHF-INDO/SP and on the three-dimensional structure determined by X-ray diffraction. Results are compared with experimental isotropic hyperfine couplings (hf c’s) obtained from ENDOR/TRIPLE magnetic resonance spectra and with previous studies on in RC’s from Rhodopseudomonas viridis. Both bacteriochlorophyll dimer species and show marked deviations of their spin distributions from C2-symmetry. However, is more symmetric than with respect to sums of spin densities on the A and B halves of the dimers. The effects of various environmental (inclusion of surrounding amino acid residues) and structural refinements (e.g. rotation of acetyl groups) on hfc’s and excited state properties were studied. The latter were obtained by additional INDO/S calculations. In both species, P865 and P960, the excited electron in the lowest unoccupied molecular orbital (LUMO) is predicted to be preferentially on the B half of the dimer, which is close to the accessory bacteriochlorophyll monomer on the photoactive A branch. Thus the preferential electron transfer along the A branch originates in the excited state of the dimer.

Light-induced triplet electron transfer (ET) of covalently linked porphyrin-quinone molecules (P-Q) have been studied with time-resolved EPR in glass forming solvents at temperatures between 130 K and 160 K. Both, the porphyrin excited triplet state TP-Q and the radical pair triplet state (+P-Q-) T formed after ET from TP-Q, are observed.

Anisotropic hyperfine interactions of P 865 +· are obtained from ENDO experiments by rotation about the C2 axes of reaction center (RC) single crystals of Rb. Sphaeroids R-26. An assignment to individual molecular positions in P 865 +· is aqchieved by comparison with calculated dipolar HFS tensors using atomic coordinates from x-ray diffraction.

The primary charge separation step in bacterial photosynthesis from an excited bacteriochlorophyll (BChl) dimer to a bacteriopheophytin with a possible intermediate BChl monomer is controlled both by the three dimensional arrangement and by the electronic structure of the reacting pigments [1,2]. The three dimensional structure of bacterial photosynthetic reaction centers (RC’s) has been determined for R. viridis [3,4] and for Rb. sphaeroides R-26 [5–7] by X-ray diffraction.

EPR spectra of NO adsorbed on both Na-ZSM-5 and H-ZSM-5 zeolites have been recorded at 10 and 78 K. Using the experimental parameters, it was possible to simulate the spectra on the assumption that in the studied zeolites, an NO molecule is tightly bound to an Na+ ion, already at 78 K. In the case of a ‘true’ Lewis site in these zeolites, a tight binding of NO at 10 K, but a rotation of the molecule around its z axis at 78 K, was assumed. It was possible to estimate the Al–N distance and to propose a structure of the ‘true’ Lewis site.

Deuterium quadrupole coupling constants have been determined by distant ENDOR spectroscopy for a series of carboxylic acids in which deuterium is substituted at the H-bond position. The measured coupling constants cover a range of 40 to 175 kHz corresponding to a variation of the bridge (O-H … O) length of 2·45 to 2·70 A. Features of the electronic and geometrical structure which principally determine e 2 qQ and η of deuterium in these H-bond situations have been assessed. To aid this assessment a simple model is proposed on which theoretical calculations of the INDO MO type are based. Good agreement between experiment and theory is obtained. It is concluded that for calculation of the electric-field gradient tensor two and three-centre integrals in the vicinity of the H-bond cannot be neglected, and further, in the cases of short and symmetrical H-bonds, e 2 qQ can change sign. Our studies also indicate that in non-ionic systems the asymmetry parameter η is primarily determined by the molecule itself, ra...

ENDOR measurements have been performed on 13 C labelled biradicals, namely Yang's biradical and p -phenylenebisgalvinoxyl, in frozen solutions. Besides intense ENDOR lines at the free nuclear Zeeman frequencies, hyperfine shifted 1 H and 13 ENDOR lines could be detected with excellent signal-to-noise ratios. From these spectra not only the magnitudes, but also the signs of the hyperfine splitting components relative to the zero field splitting parameter D could be derived. It is demonstrated that the sign of D in Yang's biradical must be negative. This may be due to a spin-orbit coupling contribution to the zero field splitting parameter.

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 physical principles underlying steady-state electron-nuclear double resonance (ENDOR) and electron-nuclear-nuclear triple resonance (TRIPLE) experiments on radicals in solution are discussed. The gain in spectral resolution achievable by these techniques, as compared with standard ESR, allows hyperfine coupling constants to be measured even for large and low-symmetry radicals. ENDOR/TRIPLE is applied to cation radicals related to bacterial photosynthesis. The measured hyperfine couplings are interpreted by means of the all-valence electron MO method RHF-INDO/SP. Information about the spatial structure of chromophores in the reaction centers of photosynthetic bacteria, Rb. sphaeroides and Rps. viridis, is obtained from an analysis of the spin density distribution and from minimization of the total energy. The results are compared with recently available structural data of the primary donor obtained from X-ray diffraction experiments. The spin density distribution in the primary donor cation radical P+960, which consists of two bacteriochlorophyll b molecules, is discussed in terms of symmetry and perturbations from the protein environment.

Time-resolved spectroscopy in conjunction with X-ray structural data for reaction centers of Rhodopseudomonas viridis and Rhodobacter sphaeroides reveal a branching ratio a > 5 for the primary electron-transfer rates, favouring one of the two, almost symmetrical pigment/protein branches, L and M. In this paper we explore the origins of this unidirectionality of electron transfer between the excited singlet state of the bacteriochlorophyll dimer ( 1 P∗) and the bacteriopheophytin (H) along the L protein subunit. Nonadiabatic electron-transfer theory is applied to analyze the asymmetry of the electron-transfer rates, k L and k M across the L and M branches. The asymmetry originates from the cumulative contributions of the nuclear Franck-Condon factor and the electronic coupling, both of which enhance the electron transfer rate across the L branch. The nuclear Frank-Condon factors are modified by the energy difference ΔE LM between the states P + H − L and P + H − M , which is induced by the electrostatic interactions of these ion-pair states with the protein polar groups, as well as by asymmetric Coulomb and medium polarization interactions. The computation results in ΔE LM = −(0.09 ± 0.04) eV, which yields a nuclear enhancement contribution at 300 K of 1.5 (+0.8, −0.3) to k L k M and therefore is insufficient to explain alone the observed asymmetry in reaction centers of Rps. viridis . Another contribution to the unidirectionality originates from electronic superexchange coupling for 1 P∗-B-H via the virtual states of the accessory bacteriochlorophyll (B). The ratio of the intermolecular 1 P∗-B L and 1 P∗-B M electronic interaction terms was evaluated utilizing the tight-binding approximation with SCF-MO wavefunctions, together with the structural data for the prosthetic groups and for the polar amino acid side chains of the protein in reaction centers of Rps. viridis . The contribution to the enhancement of k L k M by the electronic superexchange is approx. 8 ± 4. This asymmetry was traced to the combination of an excess negative charge density on the M-dimer component P M , together with structural asymmetry, which enhances the P M -B L electronic overlap. Consequently, the 1 P∗-B L -H L superexchange is favoured over the 1 P∗-B M -H M interaction. The combined effects of asymmetric nuclear Franck-Condon factors and electronic couplings yield a branching ratio of the electron-transfer rates along the two pigment branches in reaction centers of Rps. viridis of an approx. 12 (−7, +15). This is sufficiently large to explain the experimentally observed unidirectionality.

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.

In the ENDOR spectra of some silacyclopentadiene anion radicals in solution, temperature-dependent line widths and line positions were observed. This dynamic behaviour could be interpreted as being due to hindered rotation of substituted phenyl rings. It was possible to determine activation energies and entropies, from which it is concluded that the main part of the potential barrier stems from π conjugation effects. The observed dynamic effects were also exploited for assignment of ENDOR lines to molecular positions.

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 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.

Zusammenfassung Die Synthese biradikalischer Kohlenwasserstoffe mit unterschiedlich verbrukten Tetraarylcyclopentadienyl-Halften (‘Bis-ZIEGLER-Radikalen”) wird beschrieben. Aus den EPR-Pulverspektren lassen sich bei den regellos orientierten Zweispinsystemen intramolekulare Elektronen-Spin-Spin-Kopplungen zu Triplettzustanden erkennen. Aus der Grosse der Nullfeldaufspaltung wird zunachst anhand einfacher Molekulmodellbetrachtungen auf die Molekulkonformation geschlossen. Insbesondere werden jedoch mit Hilfe quantenmechanischer Modellrechnungen und durch Vergleich der Rechenergebnisse mit den experimentellen Daten detailliertere Strukturuntersuchungen durchgefuhrt. Dabei wird der Einfluss der Verdrillung zwischen den einzelnen Ringsystemen und damit auch ggf. der cis- oder trans-Orientierung der halftigen Spindichtezentren auf die Grosse der D-Parameter ermittelt. Schliesslich wird das Verhalten der vier hochsten besetzten, nahezu entarteten Molekulorbitale gegenuber strukturellen Veranderungen diskutiert.

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.

The light-induced radical cation of the primary electron donor, P+.960, in photosynthetic reaction centers (RCs) from Rhodopseudomonas viridis has been investigated by various magnetic resonance techniques (ESR, ENDOR, TRIPLE). Comparison of the measured hyperfine couplings with those of the cation radical of monomeric bacteriochlorophyll b (BChl b) and with RHF-INDO/SP molecular orbital calculations, performed on P+.960 using coordinates 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 derived spin density ratio is 2.8 in favor of the dimer half bound to the L protein subunit. The possible relevance of this result for the primary electron transfer step in the RC is briefly discussed.

A theory of stimulated nuclear polarization (SNP) is proposed for the first time. The calculations are performed by the method of summation of radical pair (RP) re-encounter contributions to the recombination of neutral radicals in high magnetic fields in the framework of a continuous diffusion model. Some rules for the sign of the SNP effect are formulated. Based on the calculations for model systems, we have studied the effect of different parameters of RPs on the SNP effects: the rate constants of RP recombination from singlet and triplet states, the rates of RP decay, e.g. due to the reaction of radicals with acceptors, and the effective in-cage RP lifetime. It is shown that in SNP the phenomenon of spin-looking manifests itself: when the amplitude of an applied microwave field exceeds the hf splitting in the ESR spectrum, the sign of polarization changes. The mutual effect of nuclei on the magnitude and sign of SNP is studied. For non-viscous liquids (η ≈ 1 cP), the analytical expressions for the SNP effects are obtained by perturbation theory. It is demonstrated that over a wide range of RP parameters, the results for SNP obtained on the basis of these analytical expressions are in good agreement with numerical results. The comparison of theoretical and experimental results exhibits their full conformity.

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.

Absolute g-tensor calculations for planar hydrocarbon and for non-planar phenyl substituted hydrocarbon radicals are reported. The relevant interactions determining g are discussed. Calculations are performed on the basis of a second-order perturbation expansion. The electronic wavefunctions are obtained from a simplified version of Hoffmann's extended Huckel model (SEH), where all valence electrons are taken into account explicitly. For planar systems the observed linear dependence of g on the energy of the half filled π orbital is well reproduced. A qualitative analysis of this dependence, making restrictive assumptions about the σ electrons, was given earlier by Stone. The calculations for non-planar model systems reproduce the g-factor anomalies which are observed for highly twisted phenyl substituted hydrocarbon radicals. The results show the necessity of direct π-σ mixing and are consistent with recent investigations of the proton hyperfine couplings in such systems.

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 possibility of applying ENDOR in studies of hyperfine interactions in stable biradicals in liquid solution has been explored. The ENDOR spectra of two phenoxy biradicals are reported. Attempts to record ENDOR of some nitroxide and verdazyl biradicals were unsuccessful. The general structure of biradicals spectra and the relaxation processes which may affect the ENDOR effect are briefly 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.

Die Synthese von Schlenk'schen und Tschitschibabin'schen Kohlenwasserstoffanaloga mit unterschiedlich verbru¨ckten Dibiphenylmethyl-Ha¨lften wird beschrieben. Mit Hilfe von EPR-Pulverspektren lassen sich bei den regellos orientierten Zweispinsystemen intramolekulare Elektronen-Spin-Spin-Kopplungen zu Triplettzusta¨nden erkennen. Aus der Gro¨sse der Nullfeldaufspaltung wird zuna¨chst anhand einfacher Moleku¨lmodellbetrachtungen auf die Moleku¨lkonformation geschlossen. Insbesondere werden jedoch mit Hilfe quantenmechanischer Modellrechnungen und durch Vergleich der Rechenergebnisse mit den experimentellen Daten detailliertere Strukturuntersuchungen durchgefu¨hrt. Dabei wird der Einfluss der Verdrillung zwischen den einzelnen Phenylringen und damit auch der cis- oder trans-Orientierung der ha¨lftigen Spindichtezentren auf die Gro¨sse der D-Parameter ermittelt. Eine Klassifizierung der einzelnen Zweispinsysteme nach ihren magnetischen Eigenschaften wird diskutiert.

A quantitative test of ENDOR theory for free radicals in solution is performed on the 14N nucleus of the nitrobenzene radical anion in dimethoxyethane. Dominant relaxation contributions are caused by modulation of the electron nuclear dipolar interaction. They can be calculated from the known G and 14N hyperfine tensors of the nitrobenzene radical anion. The effects of spin-rotation interaction and Heisenberg spin exchange at different concentrations are investigated experimentally and theoretically. The concentration and temperature dependences of the 14N ENDOR amplitudes are found to agree well with theoretical predictions.

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.

ESR powder spectra of several differently bridged bis-galvinoxyl biradicals have been recorded. The spectra can be interpreted in terms of a weak electron-electron dipolar coupling between the unpaired spins of randomly oriented triplet molecules. Using MO-theory and a point charge model for atomic orbitals a detailed structural analysis is performed on the basis of quantum mechanical calculations. Taking into account the twist angles between the various aryl rings the experimental data can be explained satisfactorily.

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.

The nuclear-nuclear coherence effect in electron-nuclear double resonance (ENDOR) experiments is studied in the case of the perinaphthenyl radical in solution. It is shown that in certain cases this coherence effect can be used to determine the number of protons contributing to an ENDOR line. In the case of ion-radicals it turns out that NMR fields as high as 35 gauss amplitude (rotating frame) are necessary to produce significant effects. By this method it was possible to determine all the 12 proton hyperfine splitting constants for the ion-radicals 1-phenylnaphthalene and 2-phenylnaphthalene. They are interpreted in terms of π-electron theories of Huckel, McLachlan, and Amos-Snyder. The large discrepancies between experimental and calculated splitting constants indicate that π-σ separability is no longer permissible for these nonplanar molecules.

The E.S.R. spectrum of the o-dimesitoylbenzene anion-alkali cation radical shows unusually large isotropic alkali hyperfine splitting constants. We report a solution ENDOR study of this radical in which both alkali (85,87Rb) and proton ENDOR spectra were recorded. Both the alkali and proton intensities showed a strong dependence on the metal ion nuclear spin quantum number of the E.S.R. line being saturated. This dependence is attributed to strong flip-flop cross-relaxation induced by modulation of the isotropic alkali hyperfine splitting. The powder E.S.R. spectrum of the complex reveals a small anisotropy of the Rb hyperfine splitting tensor. This indicates a small metal non-s-contribution to the half-filled molecular orbital, which is consistent with the observed relaxation behaviour and the small g shift. The intensity variations in the alkali and proton ENDOR spectra were used to determine the relative signs of all hyperfine splitting constants, and the absolute signs of the hyperfine splitting const...

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.

Zusammenfassung Von den negativen und positiven Radikalionen des Rubrens (Tetraphenyl tetracen) wurden ENDOR-Spektren in Losung aufgenommen. Die eindeutige Zuordnung der Protonen -hfs-Kopplungskonstanten wurde durch Vergleich mit den ENDOR-Spektren verschiedener teildeuterierter Rubrene ermoglicht. Das positive Vorzeichen einer Phenylkopplungskonstante wurde mit NMR nachgewiesen. Die gemessene Reihenfolge der Phenylkopplungskonstanten |aHmeta|⪢ |aHpara| = |aHortho| widerspricht den Voraussagen von reinen π-MO-Theorien. Die Ergebnisse lassen sich mit einer direkten Delokalisation des ungepaarten π-Elektrons in das σ-System der Phenyl- substituenten deuten. Die Synthese der teildeuterierten Verbindungen wird beschrieben.

The role of tryptophan (W) in the quinone reduction in the reaction centers of R. viridis and Rb. sphaeroides is studied by a quantum chemical calculation of the electronic coupling matrix elements within the bacteriopheophytin-W-quinone subsystem, which is based on the X-ray structural data. The results favor the superexchange mechanism via W over the direct coupling. Uncertainties in the available X-ray coordinates and structural relaxation accompanying the formation of the P+H− state result in large changes in the relative contributions of the electronic couplings due to electron and hole superexchange. Aromatic amino acid residues may serve as essential functional components in electron transfer.

We present a critical scrutiny of the models for the primary charge separation process in bacterial photosynthesis in the light of recent experimental data and theoretical analysis. The sequential mechanisms are fraught with difficulties due to their incompatibility with magnetic data for the first observable radical pair in conjunction with the activationless nature of the electron transfer process. The implications of the unistep, superexchange mediated nonadiabatic electron transfer mechanism are examined.