Transition metal complexes, electron spin hyperfine coupling

For those familiar with NMR spectroscopy it may be helpful to realize that the ESR g-shift is comparable with the NMR chemical shift. Similarly, electron-nuclear hyperfine coupling can be compared with nuclear-nuclear spin-spin coupling in NMR. (In systems containing more than one unpaired electron per molecule, electron spin-electron spin coupling is, of course, important. For doublet-state radicals, this coupling does not arise it is of great importance in triplet state molecules and in many high-spin transition metal complexes.)... 

For polyatomic radicals in the gas phase the above Spin-Hamiltonian does not apply and four magnetic hyperfine coupling constants a, b, c, d are needed to describe the interaction between a nuelear and ftie elee-tron spin. These are defined and explained in the introduction to the tables on inorganic radicals. Polyradicals and certain radicals on transition metal complexes have unpaired electrons located on different molecular segments k. Their Spin-Hamiltonian is... 

At sufficiently low temperatures, usually below 10 20 K for transition metal complexes with 5 = and below 50 K for free radicals, transverse relaxation of electron spins is dominated by fluctuation of dipolar hyperfine fields from distant protons. These fluctuations are driven by coupling among the protons,... 

A large number of paramagnetic transition metal nitrosyl complexes have been studied using electron spin resonance (ESR) spectroscopy. Information on the electronic ground state can be derived from the g-value and the hyperfine coupling constants, and many [MLslNO)]" (see Table IV) and nitrosyl porphyrin complexes (99) have been studied in this way with a view to understanding their electronic structures. 

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