  The quantum entanglement measure is determined, for the first time, for
antiferromagnetic trimer spin-1/2 Heisenberg chains. The physical quantity
proposed to measure the entanglement is the distance between states by adopting
the Hilbert-Schmidt norm. The method is applied to the new magnetic Cu(II)
trimer system, 2b.3CuCl_2.2H_2O, and to the trinuclear Cu(II) halide salt,
(3MAP)_2Cu_2Cl_8. The decoherence temperature, above which the entanglement is
suppressed, is determined for the both systems. A correlation among their
decoherence temperatures and their respective exchange coupling constants is
established.
