Weak exchange in the Heisenberg linear chain: Structure and EPR of [N( n ‐Bu) 4 ] 2 [Cu(mnt) 2 ]
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We report magnetic and x-ray diffraction studies on a single crystal of [N(n-Bu)4]2[Cu(mnt)2], where mnt = maleonitriledithiolato (C4N2S2)-2. The planar paramagnetic (S = 1/2) [Cu(mnt)2]-2 anions crystallize in stacks with crystallographically equivalent sites, and the stacks are isolated by the cations. The compound crystallizes in space group C i1-P1 of the triclinic system, with unit cell dimensions a = 11.149(5), b = 13.201(8), c = 9.403(6) Å, α = 105.44(5)°, β = 114.92(5)°, and γ = 72.09(9)°, with Ζ = 1. Static susceptibility measurements show exchange to be weak. Single crystal EPR studies yield a rich variety of multilined spectra, in spite of the magnetically concentrated nature of [N(n-Bu)4]2[Cu(mnt)2]. Analysis of the EPR spectrum in terms of a one-dimensional exchange, J 0 = 0.0107 cm-1, at 4.2°K between successive [Cu(mnt)2]-2 ions along the c axis yields a surprisingly detailed picture of one-dimensional exchange in a hitherto unobserved, weak coupling regime. The temperature dependence of J0 and the angular dependence of the Cu hypertine splitting K(θ) permit spectra with both J0 < K(θ) and J0 > K(θ). In the former case, the magnetic inequivalence of [Cu(mnt)2]-2 sites with different nuclear spin projection leads to a complicated EPR spectrum that can be quantitatively simulated to yield the value of J0. In the latter case, a typical single, exchange-narrowed EPR signal is observed. A single theoretical description based on one-dimensional exchange describes all the angular and temperature dependences of the [N(n-Bu)4] 2[Cu(mnt)2] EPR spectra. Copyright © 1975 American Institute of Physics.
author list (cited authors)
Plumlee, K. W., Hoffman, B. M., Ibers, J. A., & Soos, Z. G.