1,2,3-Dithiazolyl and 1,2,35-Dithiadiazolyl Radicals as Spin-Bearing ...

Chapter 1 – General Introduction
1.1 Theoretical Consideration for Organic Systems
1.1.1 Molecular Magnetism
When two spin centers, such as organic radicals, are brought within close proximity
to one another, there will be some degree of communication between them. This occurs
through a process referred to as indirect dipole-dipole coupling which arises from the
influence of electrons on the magnetic field of the nuclei they are associated with. The
magnetic coupling term J provides a clear picture of the magnetic interactions between
any two interacting magnetic components in a system. Ferromagnetic coupling occurs
when the spins of the unpaired electrons are aligned in a parallel fashion in an applied
magnetic field
and this is represented by a positive J term whereas antiferromagnetic
coupling defines spins which are aligned anti-parallel and is denoted by a negative J term
in some definitions. If J = 0 then there is no energy difference between the two possible
alignments of the spin vectors and so neither of these coupling patterns are preferred
which results in a random distribution of spin orientations.
These definitions hold true for compounds which are paramagnetic, in that they
possess unpaired electrons and are, by their nature attracted by an externally applied
magnetic field. Most organic systems however are diamagnetic, such that all of their
electrons are spin paired as core electrons, lone pairs or by their incorporation in
chemical bonds. A diamagnetic compound by contrast, is repelled by an externally
applied magnetic field.
Other than diamagnetism and paramagnetism, all forms of magnetism are reliant on a
critical temperature, above which magnetic moments are independent of one another.
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