A Density-Functional Study of Heterometallic Cr-Based Molecular Rings

Abstract

We present a density-functional theoretical investigation of the electronic and magnetic properties of octametallic Cr-based molecular antiferromagnetic rings. The presence of the divalent magnetic ion M unbalances the charge and the spin of the parent Cr(8) ring, leading to a finite total spin in the molecules. Results are presented for Cr(8), i.e., [Cr(8)F(8)(O(2)CH)(16)] (1), and for Cr(7)M rings belonging to two different derivatives, i.e., [Me(2)NH(2)][Cr(7)MF(8)(O(2)CH)(16)], with M = Ni, Mn, Fe, and Cu, and Me=CH(3) (2, "green" derivative), and [Cr(7) NiF(3)(C(6)H(10)NO(5))(O(2)CH)(15) (H(2)O)] (3, "purple" derivative). Exchange interaction parameters have been extracted from broken-symmetry calculations and compared with the available experiments; in agreement with them, we find that exchange parameters are rather similar in the two derivatives, although somewhat larger in the "purple" derivative. The analysis of the electronic properties shows some differences depending on M, in particular in the size of the highest occupied molecular orbital to lowest unoccupied molecular orbital (HOMO-LUMO) gaps. For all the "green" rings we observe that the HOMOs are localized on the divalent ion site, while the HOMOs for the "purple" Cr(7)Ni have a more delocalized nature; LUMOs, instead, are, except for "green" Cr(7)Cu, localized on the Cr atoms opposite to the M site. We discuss how these findings may show up in terms of an asymmetric I-V curve in molecular junctions working in the sequential tunneling regime, or help in discerning the orientation of the molecules with respect to a surface, in scanning tunneling experiments.

DFT Estimation of Exchange Coupling Constant of Cr8Molecular Ring using the Hybrid Functional B3LYP

“…It is important that our calculations, performed with dierent basis set with respect to that in Ref. [9] conrmed the earlier predictions found for the B3LYP functional so that it can be trusted more. Moreover, the B3LYP functional not only improves the coupling J systematically with respect to the PBE counterpart, but also enhances the HOMO-LUMO gaps, which suggests a relation between these two parameters.…”

“…and majority (maj.) channels are presented in Table. The previous B3LYP H-L majority gap [9] calculated for the AFM conguration amounts to 4.0 eV. 2.36 eV 2.15 eV with that found in literature (J = 3.2 meV), using the Gaussian basis functions and the same B3LYP functional implemented in the NWChem package [9].…”

“…We will particularly address the persisting problem of coupling constant signicantly overestimated in previous calculations for Cr 8 ring [3,4,914] as well as for other chromium based molecules [5,9,15,16].…”

Fundamental Gaps in Cr_{8}, Cr_{7}Ni and Cr_{7}Cd Molecules

“…For heteronuclear rings the plots for N particle systems shows that LUMO states are mainly delocalized over a large portion of the ring as shown in Ref. [13] for Cr 7 Ni. Furthermore, the picture for N + 1 system leads to the important conclusion that the LUMO orbital of the Cr ion located oppositely to the doping ion will be occupied rst.…”

“…In Ref. [13] the HOMO-LUMO gaps, calculated by the B3LYB functional for Cr 8 and Cr 7 Ni, are presented with the following values, 4.0 eV and 4.2 eV, respectively. The last inconsistency should be veried by other theoretical studies and may be related to the estimates of magnetic couplings.…”