Burkhard Kirste

Methods for automated analysis and simulation of electron paramagnetic resonance spectra

Abstract Three computer programs for the simulation and iterative least-squares fitting of high-resolution electron paramagnetic resonance (EPR) spectra are described. The program eprft assumes constant linewidths, the programs hffit and hffits allow for linewidth variations. The program eprft offers three minimization techniques: evolutionary Monte Carlo, simplex and Marquardt. Procedures for an automated analysis of EPR spectra are tested, using the combination of a search technique and iterative least-squares fitting.

Least-squares fitting of EPR spectra by Monte Carlo methods

Abstract An iterative least-squares fitting procedure for the simulation of EPR spectra is described. It is based on a systematic application of Monte Carlo methods; i.e., the parameters are varied by small but random amounts, and the new set is retained if a better fit is obtained. The method is applied to different kinds of problems, including isotropic spectra, alternating linewidth effects, and powder spectra. A description of the computer programs is given. In most cases the Fourier transform method is employed for the convolution of the lineshape function.

1H and 13C endor on phenoxyl biradicals randomly oriented in frozen solution

Abstract ENDOR measurements have been performed on 13 C labelled biradicals, namely Yangs 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 Yangs biradical must be negative. This may be due to a spin-orbit coupling contribution to the zero field splitting parameter.

Electron paramagnetic resonance and ENDOR study of semiquinones in reversed micelles

A series of substituted p-benzosemiquinone radical anions have been investigated in reversed micelles by EPR and ENDOR spectroscopies. Compared with EPR studies in homogeneous aqueous phases or in alcoholic solutions, small but significant changes in g values and hyperfine splittings were observed. Particularly interesting is the observation of asymmetric line-broadening effects in reversed-micellar solutions. Computer simulations of the EPR spectra demonstrate that different molecular positions make distinctly different contributions to this effect, and the ENDOR spectra exhibit a selective broadening of some of the signal pairs. It is concluded that the semiquinone anion radicals are immobilized at the surfactant–water interface. Models for the preferred location and anisotropic motion are discussed. The effect of bulky alkyl groups such as t-butyl on the spin density distribution in semiquinones in polar protic solvents is rationalized in terms of a hydrophobic effect.

E.S.R. investigation of helix inversion in induced cholesteric phases

Abstract The phenomenon of a reversible helix inversion in induced cholesteric mesophases caused by variation of the temperature has been investigated by E.S.R. spectroscopy using the nitroxide radical tempone as spin probe. Chiral esters of optically active (R)-(+)-2,2′-dihydroxy-1,1′-binaphthyl were added as dopants to the nematic mixture RO-TN 404. The inversion temperatures and the handedness of the helices were determined. In addition to the distortion parameter which is a measure of the helix pitch, order parameters and rotational correlation times of the spin probe were obtained from line-shape analyses.

Endor on photoexcited triplets randomly oriented in solid solution

Abstract It is shown that hyperfine interactions in photoexcited triplets randomly oriented in solid solution can be measured with the aid of ENDOR. Results of measurements on naphtalene and zinc tetraphenylporphyrin triplets in polymethyl-methacrylate are presented. Preliminary results obtained with Zn(CN)4TPP show that ENDOR spectra of triplets with short lifetimes (less than 5 ms) can also be obtained due to the signal enhancement resulting from electron spin alignment.

Continuous-wave electron spin resonance studies of porphyrin and porphyrin-quinone triplet states

CW ESR studies of the photoexcited triplet states of a series of porphyrins and covalently linked porphyrinquinones have been performed in isotropic and anisotropic (liquid-crystalline) frozen solutions. In frozen nematic solutions, the appearance of the spectra is strongly dependent on the orientation of the sample relative to the external magnetic field. This fact allows a differentiation between the z and x, y axes and hence a more accurate determination of the zero-field splitting parameters D and E. Spectrum simulation and fitting aided in the interpretation of the experimental results and the extraction of the relevant parameters. In the case of some porphyrin-quinones, steady-state triplet ESR signals cannot be observed, which is ascribed to singlet-electron transfer enhanced by folding of the quinone acceptor over the porphyrin donor. The doublet signal of porphyrin or chlorin cation radicals, which is also observed, appears in emission under certain conditions, indicative of the radical triplet pair mechanism.

13 ENDOR studies of organic radicals in natural isotopic abundance

Abstract 13 C ENDOR studies of phenoxyls, galvinoxyls, triphenylmethyl radicals, nitroxides, and cyclosilane and semiquinone radical anions with natural isotopic distribution are reported. The method is described, and it is shown that 13 C coupling constants can be measured precisely; in favorable cases even the determination of signs is possible by general TRIPLE resonance. Studies of the relaxation behavior of 13 C ENDOR signals or measurements of hyperfine shifts in liquid-crystalline solutions yield information about dipolar hyperfine interactions and hence π spin populations which is of aid in assignments to molecular positions. Complete sets of 13 C coupling constants have been determined for 2,4,6-tri- tert -butylphenoxyl and Coppingers radical. For the central carbon atoms of tert -butyl groups, a Q parameter of Q τ -Bu C = −34 MHz is proposed, and for a 29 Si atom in trimethylsilyl groups, Q TMS Si = +49 MHz. Favorable conditions for natural-abundance 13 C ENDOR experiments, e.g., small hyperfine anisotropies and use of deuterated compounds, and limitations of the method are discussed.

Application of Fourier transformation techniques for sensitivity and resolution enhancement of continuous wave EPR spectra

Abstract It is shown that time-domain data manipulation methods routinely used in NMR spectroscopy can also be applied in cw EPR and ENDOR spectroscopy. Convolution difference and Lorentzian-Gaussian transformation of the EPR pseudo FID obtained after a forward Fourier transformation of the experimental cw data lead to a significant improvement of the spectral quality with respect to resolution and/or signal-to-noise ratio. Some examples demonstrate the usefulness of these methods.

ENDOR studies of alkyl substituted p-Benzosemiquinones in reversed micelles and in 2-propanol

Various substituted p-benzosemiquinone radical anions, inter alia ubisemiquinone and derivatives, have been investigated in 2-propanol and in reversed micelles by EPR and ENDOR spectroscopy. Unsymmetrical semiquinones, with respect to the oxygen atoms, experience remarkable hyperfine shifts depending on the medium. This effect even allows differentiation between stereoisomers. Immobilization of the semiquinone molecules at the water-surfactant interface in reversed micelles gives rise to pronounced asymmetric linewidth effects. In the case of 2-cyclohexyl-3-methyl-1,4-benzosemiquinones, mixtures of two species (conformers) have been observed.