What are the most common factors besides clustering and spin-spin interactions of paramagnetic atoms that lead to poor spectra without expected hyperfine?
besides spin-spin interaction (intermolecular: T2 and intramolecular: T1) the intrisic properties of the elecronic systems (a) and the nuclear spin of atoms (b) might be a problem:
a) spin systems with S = 1/2 should be generally detectable, higher spin systems such as S = 1 can be critical. E.g. in my opinion there is no "real" EPR spectrum of a Ni(II) ion (S =1) in the literature.
b) large quadrupolar moments of atoms can broaden lines tremendously. These quadrupolar moments occur frequently for atoms with larger nuclear spins such as Cl and Br. You can look up the quadropolar (and higher moments) usally in tables (e.g. from the Bruker company)
Both phenomena might be overcome by varying the temperature (usally better at low T) or the field (high field EPR often helps) - but there is no guarantee as the Ni(II) example shows (see also attached literature).
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