Multi-frequency single-crystal and powder electron paramagnetic resonance spectroscopy of [Cu-2(chp)(4)] (chp equals 6-chloro-2-pyridonate)

Powder and single-crystal X- and Q-band EPR spectra show evidence for both intra- and inter-dimer exchange in [Cu-2(chp)(4)] (chp = 6-chloro-2-pyridonate). The antiferromagnetic intradimer exchange gives rise to a singlet ground state and a thermally accessible S = 1 excited state which is described by the spin-Hamiltonian parameters (from single-crystal measurements at 120 K) g(zz) = 2.30, g(xx) = 2.07, g(yy) = 2.05, \D\ = 0.275 cm(-1) and \lambda\ (= \E/D\) = 0.01. D-zz and g(zz) are colinear with the Cu-Cu vector. Interdimer exchange between neighbouring triplet state molecules is apparent by both a highly temperature dependent linewidth and an 'extra' feature at g(eff) approximate to 2.1, the intensity of which rapidly falls with decrease in temperature. Analysis of the linewidth variation with temperature from single-crystal measurements at X-band leads to an interdimer exchange parameter of J' approximate to 0.15 cm(-1). The extra feature arises from averaging of the S = 1 signals of individual dimers under the action of interdimer exchange and occurs over only a narrow range of orientations in the single crystal.