Ab initio computations of zero-field splitting parameters and effective exchange integrals for single-molecule magnets (Mn12- and Mn11Cr-acetate clusters)

Abstract

In this study, the zero-field splitting parameters (D, E) and effective exchange integrals (J) for a Mn12/Mn11Cr mixed crystal were evaluated using ab initio molecular orbital (MO) computations based on the hybrid density functional theory (DFT) methods for Mn11Cr∗-acetate cluster (Mn12 and Mn11Cr mixing). In this study, Mn12-acetate, Mn11Cr-acetate, and Mn12−nCrn-acetate clusters were focused on. The zero-field splitting (ZFS) parameters are obtained by UB3LYP methods, i.e. −0.339cm−1 (Mn12-acetate) and −0.351cm−1 (Mn11Cr-acetate). The energy barriers were estimated to be 53.7K and 50.4K, which showed good agreement with previously reported experimental values of 68.8K and 56.8K, respectively. The origin of such anisotropy is assumed to be the eight MnIII ions with d4-electrons in their outer ring subunits, where elongated conformation along the axial direction becomes very important and leads to negative D values. The spin networks in these clusters were discussed by using the effective exchange integral sets. The Js value, as estimated by the UB3LYP method revealed that the J2, J3, and J3′ interactions played important roles in describing the parallel spin ordering in cubane and ring subunits as well as the antiferromagnetic coupling between two subunits. The other interactions, i.e. J1, J1′, J4, and J4′ somewhat disturbed the spin ordering in the Mn12-acetate cluster. The substitution of MnIV by CrIII did not change the spin network. However, the total S value changed after the substitution.