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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.

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.

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.

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.

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.

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.

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 .