Angular selection method

The angular selection method established by Rist and Hyde [47] for the analysis of ligand ENDOR of metal complexes in powders has been further developed and applied for biological systems. Measurements at X- and Q-band are often adequate due to an appreciable -anisotropy. We refer to recent reviews for further account of this application [48]. 

Powder ENDOR Hyperfine couplings obtained by the angular selection method with the field set at anisotropic gx, gy and gz features can give single-crystal-like ENDOR spectra from randomly oriented samples. The enhanced resolution of g-anisotropy at high magnetic field increases orientation selectivity of ENDOR spectra in amorphous systems. 

Several factors unique for ENDOR affect the intensities, i.e. magnetic relaxation, hyperfine enhancement, and angular selection. The two first effects also affect spectra of liquid and crystalline samples, while the third is typical for powder spectra of species with anisotropic g-values. Methods that take the two latter effects into account have been developed and are usually incorporated in software developed for the simulation of ENDOR spectra in the solid state. Simulations that take magnetic relaxation effects into account have been employed only to analyse ENDOR spectra in the liquid state [2]. It is possible that the commonly observed poor agreement between experimental and simulated intensities in the solid state is at least in part due to relaxation effects that are not taken into account in any software we are aware of. 

Fig. 3.30 Simulated powder ENDOR spectrum (in absorption) of NO-ligated ferrocytochrome c heme a3, at the field setting (g = 2.079) marked in the X-band (v = 9.32 GHz) ESR spectrum. The parameters g = (2.082, 1.979, 1.979) A( N-His) = (16.5, 16.1, 19.3) MHz, Q(> N-His) = (+0.67, -1.12, + 0.45 ) MHz, A( N-NO) = (30.56, 30.56 59.90) MHz, Q( N-NO) = (+1.03, -0.51, -0.52) MHz were employed for the simulation, using a method teiking angular selection into account. For experimenUil spectra see [R. LoBrutto et aL, J. Biol. Chem. 258 (1983) 7437], for simulation with an exact method see [49]. The spectrum is adapted from [R. Erickson, Chem. Phys. 202, 263 (1996)] with permission from Elsevier...

While the dial-indicator and optical methods differ in the equipment and/or equipment setup used to align machine components, the theory on which they are based is essentially identical. Each method measures the offset and angularity of the shafts of movable components in reference to a pre-selected stationary component. Each assumes that the stationary unit is properly installed and that good mounting, shimming, and bolting techniques are used on all machine components. 

This method is equally applicable to atoms 26) and to molecules 22). In molecules the Zeeman splitting depends on the quantum number / of the total angular momentum and therefore the fluorescence from a single rotational level (v, f) need be observed. Because of this necessarily selective excitation, these molecular level-crossing experiments can be performed much more easily with lasers than with conventional light sources and have been sucessfully performed with Naj 2 > and NaK 29). 

A number of techniques have been used previously for the study of state-selected ion-molecule reactions. In particular, the use of resonance-enhanced multiphoton ionization (REMPI) [21] and threshold photoelectron photoion coincidence (TPEPICO) [22] has allowed the detailed study of effects of vibrational state selection of ions on reaction cross sections. Neither of these methods, however, are intrinsically capable of complete selection of the rotational states of the molecular ions. The TPEPICO technique or related methods do not have sufficient electron energy resolution to achieve this, while REMPI methods are dependent on the selection rules for angular momentum transfer when a well-selected intermediate rotational state is ionized in the most favorable cases only a partial selection of a few ionic rotational states is achieved [23], There can also be problems in REMPI state-selective experiments with vibrational contamination, because the vibrational selectivity is dependent on a combination of energetic restrictions and Franck-Condon factors. 

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