Chagas Edson F, Rapp Raul E, Rodrigues Daniel E, Casado Nieves M C, Calvo Rafael
Instituto de Física, Universidade Federal do Rio de Janeiro, CP 68528, Rio de Janeiro 21941-972, RJ, Brazil.
J Phys Chem B. 2006 Apr 20;110(15):8052-63. doi: 10.1021/jp056838w.
The magnetic properties of the Cu(II)-peptide compounds (L-tyrosyl-L-leucinato)Cu(II) and (L-tryptophyl-glycinato)Cu(II), to be identified as Cu(II)Tyr-Leu and Cu(II)Trp-Gly, respectively, have been investigated by specific heat (0.08 < T < 28 K), dc magnetization (2 < T < 80 K, with B(0) = mu(o)H < or = 9 T), and ac magnetic susceptibility (with B(0) = 0 for 0.03 < T < 3 K and B(0) up to 9 T for 2 < T < 80 K) measurements. Above approximately 1 K, the specific heat and magnetization of both compounds display a ferromagnetic (FM) spin chain behavior sustained by syn-anti carboxylate bridges connecting equatorially Cu(II) ions at about 5 A. To model this behavior, we calculated the eigenvalues of Heisenberg chains with up to 20 spins 1/2 and used the method of Bonner and Fisher. A global fit of the model to the specific heat and magnetization data gives 2J(0)/k(B) = 3.60(5) K and 2.59(5) K for the intrachain exchange interactions in Cu(II)Tyr-Leu and Cu(II)Trp-Gly, respectively (H(ex)(i,j) = -2J(0) S(i).S(j)). These values of 2J(0) are discussed in terms of structural properties of the carboxylate bridges in the two compounds. Using the parameters obtained from the global fit, we calculated isothermal susceptibilities in agreement with the ac susceptibilities measured with small applied dc magnetic fields. However, the ac susceptibility measured with applied dc fields larger than 1 T lie between the values calculated for the isothermal and adiabatic susceptibilities. At 0.16 K for Cu(II)Tyr-Leu and 0.53 K for Cu(II)Trp-Gly, the observed specific heat and magnetic susceptibility display peaks associated to three-dimensional magnetic phase transitions. The interchain exchange couplings 2J(1) producing the 3D magnetic order are ferromagnetic and have magnitudes 2J(1)/k(B) approximately 0.015 and 0.073 K for Cu(II)Tyr-Leu and Cu(II)Trp-Gly, respectively.
已通过比热（0.08 < T < 28 K）、直流磁化强度（2 < T < 80 K，B(0) = μ₀H ≤ 9 T）和交流磁化率（0.03 < T < 3 K时B(0) = 0，2 < T < 80 K时B(0)高达9 T）测量，对分别被鉴定为Cu(II)Tyr - Leu和Cu(II)Trp - Gly的Cu(II) - 肽化合物（L - 酪氨酰 - L - 亮氨酸铜(II)和L - 色氨酰 - 甘氨酸铜(II)）的磁性进行了研究。在大约1 K以上，两种化合物的比热和磁化强度表现出铁磁（FM）自旋链行为，这种行为由连接赤道面上约5 Å处的Cu(II)离子的顺 - 反羧酸盐桥维持。为模拟这种行为，我们计算了多达20个自旋1/2的海森堡链的本征值，并使用了邦纳和费舍尔方法。将该模型与比热和磁化强度数据进行全局拟合，得到Cu(II)Tyr - Leu和Cu(II)Trp - Gly中链内交换相互作用的2J(0)/k(B)分别为3.60(5) K和2.59(5) K（H(ex)(i,j) = -2J(0) S(i)·S(j)）。根据这两种化合物中羧酸盐桥的结构性质对这些2J(0)值进行了讨论。使用从全局拟合获得的参数，我们计算了等温磁化率，其与在小直流磁场下测量的交流磁化率一致。然而，在大于1 T的直流磁场下测量的交流磁化率介于等温磁化率和绝热磁化率计算值之间。对于Cu(II)Tyr - Leu在0.16 K以及对于Cu(II)Trp - Gly在0.53 K时，观察到的比热和磁化率显示出与三维磁相变相关的峰值。产生三维磁有序的链间交换耦合2J(1)是铁磁性的，对于Cu(II)Tyr - Leu和Cu(II)Trp - Gly，其大小分别为2J(1)/k(B)约为0.015 K和0.073 K。
