In summary, the results of the measurements and the simulations demonstrate that orientation selection affects the DEER response. For az-1 it suggests that in solution more conformations are accessible than in the crystal, an aspect which is currently undergoing detailed analysis. With respect to the determination of distances in other systems, usually enough information about the relative g tensor orientation is known to allow the distances to be determined with sufficient accuracy. If the relative orientation is not known, the dipolar frequency n dip will differ by a factor of 2 between the two extreme angles q AB ¼ 08 and 908. The resulting difference in r AB of 25 % is therefore the maximum uncertainty expected from orientation effects. Finally, the study demonstrates that DEER can be applied to the determination of structures of compounds which contain paramagnetic transition-metal ions, and thus provides an additional method for structural investigations on biological systems. This is particularly important given the physiological significance of metal ions. Experimental SectionSpectra were measured on a Bruker Elexsys E680 spectrometer with modifications to the microwave bridge to introduce the second microwave frequency analogous to those described by Pannier et al. [3] using a HP synthesizer HP 83752B and a microwave amplifier (Microwave Amplifiers AL 16-9±10-15). The four-pulse DEER sequence [3] with pulse lengths of 16 ns for p/2, 32 ns for p pulses, and t ¼ 896 ns was employed. Pulse powers were adjusted to equal intensity at n obs and n pump . The maximum frequency separation of n obs and n pump was estimated by detuning the measurement frequency from the resonator frequency. At offsets of AE 37.5 mHz attenuation of the signal was observed, leading to the choice of n obs Àn pump of 75 MHz. At that separation, the echo is reduced by less than a factor of 2. The excitation bandwidth of the pulses is smaller than in the experiments described by Pannier et al., where a maximum of 25 MHz is given. [3] In the Fourier transformation (by using the Origin program (Microcal(TM) Northhampton, USA) of the modulation, a Gaussian background was subtracted from the experimental data, the resulting curve was filtered with a Hamming window, and filled with zeros to 1024 points to a total time of 3.2 ms.

Trialkylaluminum complexes, AlR 3 (R ) Me, Et), reacted with the sulfide-bridged bis-(phenol) tbmpH 2 (tbmpH 2 ) 2,2′-thiobis(6-tert-butyl-4-methylphenol)) in a 1:1 ratio to give monoalkyl compounds of the composition "[Al(tbmp)R]" with an unsymmetrical dinuclear structure with both six-and four-coordinate aluminum centers in the solid state. The reaction of 2 equiv of tbmpH 2 gave [Al(tbmp)(tbmpH)], which could be deprotonated with triethylamine to give [NEt 3 H][Al(tbmp) 2 ]. Upon reaction of [Al(tbmp)R] 2 with Lewis bases L (L ) Et 2 O, THF, propylene oxide), monomeric, five-coordinate adducts [Al(tbmp)R(L)] were formed, while 2-propanol gave the dinuclear complex [Al(tbmp)(µ-O i Pr)] 2 .

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