A. A. K. Klaassen

14N-coordination to VO2+ in reduced vanadium bromoperoxidase, an electron spin echo study

Vanadium bromoperoxidase from the brown seaweed Ascophyllum nodosum was studied with electron spin echo envelope modulation (ESEEM) spectroscopy. After comparing the Fourier transformed (FT) ESEEM spectra with those of a number of vanadyl model compounds, it could be concluded that nitrogen is present in the equatorial plane of the vanadyl cation of reduced bromoperoxidase (14N frequencies occurred at 3.1, 4.2, 5.3 and 8.1 MHz). Furthermore, the FT‐ESEEM spectra of reduced bromoperoxidase exhibited an intense 1H modulation (13.8 MHz), which was completely replaced by a deuterium modulation at ∼2 MHz when bromoperoxidase was dissolved in D2O, instead of H2O. These latter data confirm earlier EPR experiments on reduced bromoperoxidase [(1988) Biochemistry 27, 1629–1635], showing that the oxo‐vanadium (IV) ion is coupled to exchangeable protons.

Na43+ clusters in sodalite

Abstract Anion-sodalites with the general formula Na 8 [(AlO 2 ) 6 (SiO 2 ) 6 ]A 2 , where A = Cl, Br or I, were reacted with the vapour of alkali metals. In all cases the ionic cluster Na 4 3+ was formed, together with a metallic cluster. The ionic cluster was investigated by electron spin resonance, electron spin echo and pulse-electron nuclear double resonance techniques. A modulation was observed in the electron spin echo envelope due to interaction with distant Na + ions. These Na + ions have hyperfine splitting constants between 0.24 and 1 MHz and are situated ∼ 5 A from the centre of the sodium cluster.

Structure of alkali biphenyl ion pairs in solution and in the solid state

NMR experiments on sodium biphenyl (NaBp), potassium biphenyl (KBp) and rubidium biphenyl (RbBp), dissolved in various solvents are summarized with emphasis on the structural information which can be obtained from such experiments. The crystal structures of NaBp.2Tg (Tg = triglyme), KBp.2Ttg and RbBp.2Ttg (Ttg = tetraglyme) are discussed, in relation to the NMR results obtained for these systems in solution. In the solid state these crystals may be considered to be built up of solvent separated ion pairs. Magnetic experiments are reported for single crystals of the three systems. Susceptibility measurements revealed a ferromagnetic coupling in NaBp.2Tg and KBp.2Ttg and an antiferromagnetic coupling in RbBp.2Ttg. All crystals exhibit an exchange narrowed ESR line, with an orientation dependent linewidth. From the resonance positions the molecular g tensor of the biphenyl anion could be derived. The orientation dependent linewidth is due to dipolar interactions between the spins. A quantitative explanation of it could not be given at present.

Proton ENDOR study of copper(II) bis(dithiocarbamate)

The proton hyperfine coupling tensors of the methylene protons in methyl-deuterated copper(II) bis(N,N-diethyldithiocarbamate) in a diamagnetic host crystal of the corresponding nickel complex have been measured by ENDOR spectroscopy. Two intermolecular and all four intramolecular proton coupling tensors could be determined. With the aid of spin densities, obtained from extended Huckel molecular orbital calculations, the anisotropic part of the tensors can be reproduced quantitatively, taking into account all two- and three-centre contributions. Comparison of the transition frequencies which are computed from the theoretical tensors with the experimental transitions enables the tracing of another five tensors which cannot be completely determined experimentally.

The detection of radicals in strained, high-modulus polyethylene fibres

Abstract It is shown that loading ultra-high molecular weight polyethylene (UHMW-PE) fibres up to about 50% of their theoretical strength causes bond scission to occur as corroborated from the number of radicals monitored. In addition, a creep test on UHMW-PE fibres also showed an ongoing production of radicals in the polymer. Although precise quantification of the radical concentration is difficult, a first estimate shows that there is a significant number (of the order of 4 × 10 13 spins) compared with the total number of polymer chains (10 14 ) that run through the cross-section of the PE rope employed in the experiments.

A study of the magnetic, molecular and electronic structure of some alkali biphenyl ion pairs in the solid state

A magnetic resonance and susceptibility study of the molecular and magnetic structure of some alkali biphenyl (Bp) polyglyme single crystals and powders has been performed. For RbBp · 2Ttg (Ttg=tetraglyme), space group C2/c, and NaBp · 2Tg (Tg=triglyme), space group P21, the average g tensors have been measured. From the average g tensor and the known crystal structure of RbBp · 2Ttg the molecular g tensor of the Bp anion could be derived. The molecular g tensor was calculated with Stones theory. Excellent agreement was found with the experimental values. With the aid of the molecular g tensor of the Bp anion and the average g tensor of NaBp · 2Tg, for which the crystal structure is unknown, the orientation of the symmetry axes of Bp with respect to the crystal axes could be calculated. Susceptibility measurements reveal ferromagnetic coupling in both KBp · 2Ttg and NaBp · 2Tg, whereas antiferromagnetic coupling exists in RbBp · 2Ttg. The results of alkali N.M.R. experiments are related to the molecular ...

Application of time-variable feedback to the input amplifier of pulse magnetic resonance spectrometers: Experimental studies

Experimental research on the improvement of the sensitivity and time resolution of pulsed magnetic resonance spectrometers is discussed. It is shown that application of a time-variable feedback of a signal to the input of the receiver amplifier can decouple, the “fixed” relationship between the quality factorQ and the ringdown time of the resonance system. Experiments were performed with low-frequency, radio-frequency and microwave pulse-type magnetic resonance receivers. Modifications of an S/C-band electron spin echo modulation spectrometer carried out to check the “time-variable feedback” performance are described. It is demonstrated that the application of a time-variable feedback can significantly reduce the ringdown time and improve the recovery properties of the magnetic resonance receiver system. It is also demonstrated that the time-variable feedback can improve the overall receiver sensitivity due to the fact that the working bandwidth of the resonance system can be optimized separately for the transmitting and the receiving mode. Signal values could be increased at least three times and the signal-to-noise ratio about 1.5–2 times. The largest improvement is achieved with the initially overcoupled resonator. Experimental spectra of test samples for different settings of the time-variable feedback are shown.

A cw electron nuclear double resonance resonator for single crystal studies

The design of an electron nuclear double resonance (ENDOR) resonator suitable for single crystal studies is reported. An outline is given of the mechanical and electronic construction of the resonator. This paper will focus on the construction suitable for studies at 4.2 K, but the design can be easily adapted to a flow cryostat for variable temperature measurements. The rf‐coil efficiency has been measured and is high and fairly constant over the frequency range from 0.3 to 20 MHz. Measurements on a test compound, show that the baseline stability is good over this frequency range. Even for low abundant nuclei as 13C, the signal to noise ratio is very good.

Pulsed EPR Study of the Trapping Process of Radicals in Polyethylene

It has been suggested that irradiation of polyethylene (PE), with γ or X-rays, creates radicals in spurs, i.e. regions with high local concentrations. Both, Nunorae c.s.1 and Shimada c.s.2 applied microwave power satuaration experiments in order to verify this model. This technique results in the determination of a (not absolute) value for the product of the electron spin-lattice (T1) and the electron spin-spin (T2) relaxation times. In order to draw conclusions from changes in the magnitude of this product (by changing the irradiation dose, the irradiation temperature or by heat treatment after the irradiation) both groups of authors assumed that T2 is a measure of the local radical concentration and that T1 is independent of the history before the time of measurement. In order to test these assumptions, it was decided to measure the effect of heat treatments on T1 and T2 by using Electron Spin Echo spectroscopy.

REVERSAL OF SINGLET AND TRIPLET STATES IN AROMATIC DIANIONS WITH TRIGONAL SYMMETRY

Dianions of molecules with a three- or sixfold symmetry axis are usually in the triplet groundstate; examples of this are the dianions of triphenylene (Tp), coronene, 1,3,5-triphenylbenzene (Tpb), 2,4,6-triphenyl-sym-triazine and decacyclene. However, when these anions are associated with cations the singlet state may become the groundstate. NMR, ESR and susceptibility experiments have proven that at room temperature and in apolar solvents the dianions of Tp and Tpb occur in the singlet groundstate, whereas at liquid nitrogen temperature the triplet state is the lowest. For this reversal of lowest states a satisfactory explanation can be given for Tpb 2− , but not for Tp 2− . The theoretical calculations always predict a triplet groundstate for the latter dianion. The observed zero-field splitting parameters of the unperturbed triplet dianions and their associated species could be satisfactorily interpreted.