An observed exponential decay in the electron paramagnetic resonance linewidth of the nearly classical two-dimensional antiferromagnet as a function of temperature is shown to be the result of solitons interacting with magnons, and provides an experimental confirmation of these excitations. The temperature dependence of the linewidth is calculated using the dynamic spin correlation function derived from soliton-magnon scattering in the Born approximation. Data in the critical regime for (n-propylammonium) 2 tetrachloromanganese (II) are presented and compared to the theory.

The crystal structures of a series of trinuclear copper(II) halide salts have been determined. These all contain pseudoplanar, symmetrically bibridged Cu3Xg2" (la) or Cu3X7L" (lb) anions.

The syntheses, crystal structures, and powder magnetic studies of several new quasi-planar bibridged CunX2n+22õ ligomers (n = 3, 4, 6, and 7; X = Cl or Br) are reported, based on the 1 -methylpyridinium (CftHsN'U and 1,2-dimethylpyridinium (C7H|0N)+ cations. These include (C7HioN)2Cu3Br8, (C6H8N)2Cu4C1io, (C7H|0N)2Cu6-Clu, and (C7H|0N)2Cu7Bri6. Crystallographic data: (C7HioN)2Cu3Br8, triclinic, space group PI, a = 7.947(2) A,b = 8.799(2) A, c = 9.840(2) A, a = 86.95(2)°, = 76.23(2)°, y = 71.54(2)°, V = 633.6(3) A3, Z=2,dx = 2.78 g/cm3, and R = 0.0483; (CeHgN^CmClio, monoclinic, space group P2\!n, a = 11.759(2) A, b = 9.056(2)A, c = 12.048(3) k,J = 106.21(2)°, V = 1232.1(5) A3, Z = 2, dx = 2.15 g/cm3, and 7? = 0.0321; (C7HI0N)2-Cu6Cl|4, triclinic, PI, a = 8.997(3) A, b = 9.288(3) A, c = 11.540(4)A, a = 80.53(2)°, /? = 67.82(2)°, y = 60.22(2)°, V = 714.7(4) A3, Z = 1, dx = 2.34 g/cm3, and R = 0.0363; (C6H8N)2Cu7Bri6, triclinic, PI, a = 7.237(2) k,b= 10.880(2) A, c = 12.880(2) A, a = 89.47(2)°, /? = 75.08(2)°, y = 79.48(2)°, V = 962.7(3) A3, Z = 1, dx = 3.35 g/cm3, and R = 0.0520.A common feature of the structures is the existence of oligomers containing quasi-planar symmetric bibridged finite chains of edge-sharing CuXa (X = halide) monomeric units.The n = 3 oligomers aggregate into chains through the formation of asymmetric bibridged linkages between terminal copper ions on adjacent trimers. In the n = 4 salt, the oligomers aggregate into stacks in which pairs of the copper ions extend their coordination sphere by forming a long, semicoordinate bond to a halide ion from a neighboring oligomer. For both the n = 3 and n = 4 salts, the pyridinium cations lie parallel to and directly above and below the anionic oligomers, separating the chains. In the n = 6 and n = 7 salts, the stacks formed in this manner interdigitate, forming two-dimensional slabs. The slabs are separated by the organic cations. The magnetic properties of compounds are dominated by antiferromagnetic intraoligomer interactions. Thus, the n = 4 and n = 6 salts depopulate into singlet ground states at low temperature. In contrast, the n = 3 and n -7oligomers have S = '/2 ground states. Expressions for the magnetic susceptibility of the n = 6 and n = 1 oligomers were obtained by diagonalization of a nearest neighbor Heisenberg Hamiltonian. The data were fit to these expressions, with inclusion of a mean field correction for interoligomer exchange. The intra-oligomer exchange coupling constants are

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