Spin density maps for the ferrimagnetic chain compound MnCu(pba)(H2O)(3)center dot 2H(2)O (pba equals 1,2-propylenebis(oxamato)): Polarized neutron diffraction and theoretical studies

Artikel i vetenskaplig tidskrift, 1997

This paper is devoted to the determination of the spin density in the ferrimagnetic ground state of the bimetallic chain compound MnCu(pba)(H2O)(3) . 2H(2)O, with pba = 1,3-propylenebis(oxamato). The crystal structure, previously determined at room temperature through X-ray diffraction, was redetermined at 10 K through unpolarized neutron diffraction (orthorhombic system, space group Pnma, a = 12.727(11) Angstrom, b = 21.352(19) Angstrom, c = 5.153(3) Angstrom, Z = 4). The experimental spin density has been deduced from polarized neutron diffraction data recorded at 10 K under 50 kOe. Positive spin densities were observed on the manganese side and negative spin densities on the copper side. The delocalization of the spin density from the metal centers toward the oxamato bridging ligand was found to be more pronounced on the copper side than on the manganese side, so that the nodal surface (of zero spin density) is closer to manganese than to copper. The experimental spin distribution has been compared to the theoretical distributions deduced from density functional theory (DFT) calculations, using both DGauss and DMol programs. The experimental results for the title chain compound have also been compared to the spin distribution for the binuclear compound [Mn(Me-6-[14]ane-N-4)Cu(oxpn)](CF3SO3)(2) with Me-6-[14]ane-N-4 = (+/-)-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, and oxpn = N,N'-bis(3-aminopropyl)oxamido, recently reported. The most striking difference between pair and chain compounds concerns the positive P+ and negative P- spin populations carried by the manganese and copper sides, respectively. For the pair compound P+ was found as 4.67(8) mu(B), and P- as -0.67(8) mu(B) while for the chain compound these values are 5.05(7) mu(B) and -1.05(10) mu(B), respectively. The spin distribution for the ferrimagnetic chain compound is very close to a Neel state (P+ = 5 mu(B) and P- = -1 mu(B)).