Electron nuclear double resonance
In the standard continuous wave (cwENDOR) experiment, a sample is placed in a magnetic field and irradiated sequentially with a microwave followed by radio frequency.
The changes are then detected by monitoring variations in the polarization of the saturated electron paramagnetic resonance (EPR) transition.
[4] ENDOR is illustrated by a two spin system involving one electron (S=1/2) and one proton (I=1/2) interacting with an applied magnetic field.
The right figure illustrates the energy diagram of the simplest spin system where a is the isotropic hyperfine coupling constant in hertz (Hz).
In a steady state ENDOR experiment, an EPR transition (A, D), called the observer, is partly saturated by microwave radiation of amplitude
This technique was established by Hyde[7] and is especially useful for separating overlapping EPR signals which result from different radicals, molecular conformations or magnetic sites.
EI-EPR spectra monitor changes in the amplitude of an ENDOR line of the paramagnetic sample, displayed as a function of the magnetic field.
This technique was first introduced by Cook and Whiffen[10] and was designed so that the relative signs of hf coupling constants in crystals as well as separating overlapping signals could be determined.
This technique was developed by Schweiger and Gunthard so that the density of ENDOR lines in a paramagnetic spectrum could be simplified.
[11] Polarization modulated ENDOR (PM-ENDOR) uses two perpendicular rf fields with similar phase control units to CP-ENDOR.
[5] In polycrystalline media or frozen solution, ENDOR can provide spatial relationships between the coupled nuclei and electron spins.
Such spatial arrangements require that the ENDOR spectra are recorded at different magnetic field settings within the EPR powder pattern.
[13] The origin, for purposes of this explanation, can be thought of as the position of a molecule's localized unpaired electron.
The electron nuclear distance (R), in meters, along the direction of the interaction is determined by point-dipole approximation.
Isolation of R gives the distance from the origin (localized unpaired electron) to the spin active nucleus.
Point-dipole approximations are calculated using the following equation on the right: ENDOR technique has been used to characterize of spatial and electronic structure of metal-containing sites.
paramagnetic metal ions/complexes introduced for catalysis; metal clusters producing magnetic materials; trapped radicals introduced as probes for disclosing the surface acid/base properties; color centers and defects as in ultramarine blue and other gems; and catalytically formed trapped reaction intermediates that detail the mechanism.
Such advantages are the generation of distortion-less line shapes, manipulation of spins through a variety of pulse sequences, and the lack of dependence on a sensitive balance between electron and nuclear spin relaxation rates and applied power (given long enough relaxation rates).
[12] HF pulsed ENDOR is generally applied to biological and related model systems.
Applications have been primarily to biology with a heavy focus on photosynthesis related radicals or paramagnetic metal ions centers in matalloenzymes or metalloproteins.
Incorporation of transition metal complexes into the framework of molecular sieves is of consequence as it could lead to the development of new materials with catalytic properties.
ENDOR as applied to trapped radicals has been used to study NO with metal ions in coordination chemistry, catalysis and biochemistry.
