Self-assembled Transition Metal Coordination Frameworks of ...

_g 7 Introduction
is associated with a spin angular momentum m, = +‘/2, leading to a doubly degenerate
spin state in the absence of a magnetic field. When a sufficient magnetic field is
applied, this degeneracy is removed, and the energy difference between two resultant
states is given [91] by,
AE = hv = g/3H
where h is Planck’s constant, v is the frequency, g is the Lande splitting
factor, ,6 is the electron Bohr magneton and H is the magnetic field.
The static spin Hamiltonian used to describe the energies of states of a
paramagnetic species in the ground state with an effective electron spin S and m nuclei
with spins I is given by [92]
F1 -1? +151 ..+H +1-Al. +1?
0 _ 1-tz zrs HF .-\-z A-"Q
Where, UH - electron Zeeman interaction, H ZFS - zero-field splitting, H HF ­
hyperfine interactions between the electron spins and the m nuclear spins, H NZ ­
nuclear Zeeman interactions and H NQ - nuclear quadrupole interactions for I > ‘/2.
EPR is an important spectroscopic tool in experimental studies of systems
containing unpaired electrons. The traditional application areas for EPR include
studies of transition metal complexes, stable organic radicals, transient reaction
intermediates, as well as solid state and surface defects. EPR spectroscopy measures
differences between magnetic energy levels and this is the principal difference from
the magnetic susceptibility measurements, which measures the Boltzmann occupation
of all energy levels [93]. In many cases, the extreme sensitivity of EPR allows
experimental access to electronic structure and molecular environment parameters,
which would be impossible to measure otherwise. The structure of the EPR spectrum
depends upon (a) the g-tensor anisotropy, (b) the presence of the hyperfine interaction
of the central atom nuclear spin with the electron spin (the AM-i€11SOI‘), (c) the presence
of the superhyperfine interaction of the ligand donor atom nuclear spins with the
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