Ab Initio CI Determination of the Exchange Coupling Constant of Doubly-Bridged Nickel(II) Dimers

Angeli

Taratiel

et al. 2009

117

179

In magnetic coordination compounds and solids the magnetic orbitals are essentially located on metallic centers but present some delocalization tails on adjacent ligands. Mean field variational calculations optimize this mixing and validate a single band modelization of the intersite magnetic exchange. In this approach, due to the Brillouin's theorem, the ligand to metal charge transfer ͑LMCT͒ excitations play a minor role. On the other hand the extensive configuration interaction calculations show that the determinants obtained by a single excitation on the top of the LMCT configurations bring an important antiferromagnetic contribution to the magnetic coupling. Perturbative and truncated variational calculations show that contrary to the interpretation given in a previous article ͓C. J. Calzado et al., J. Chem. Phys. 116, 2728 ͑2002͔͒ the contribution of these determinants to the magnetic coupling constant is not a second-order one. An analytic development enables one to establish that they contribute at higher order as a correlation induced increase in the LMCT components of the wave function, i.e., of the mixing between the ligand and the magnetic orbitals. This larger delocalization of the magnetic orbitals results in an increase in both the ferroand antiferromagnetic contributions to the coupling constant.

Section: ͑8͒mentioning

confidence: 90%

Section: Introductionmentioning

confidence: 99%

Analysis of the magnetic coupling in binuclear systems. III. The role of the ligand to metal charge transfer excitations revisited

Angeli

Taratiel

et al. 2009

117

179

“…A significant reduction in the CI space results while the amplitude of the magnetic coupling constant is almost not affected. 25,[34][35][36][37] In order to elucidate the mechanisms governing the exchange coupling in ͑1͒, two complementary tools have been used. First, LOs were built from the triplet CASSCF MOs following the procedure reported in literature.…”

Section: Computational Detailsmentioning

confidence: 99%

Microscopic origins of the ferromagnetic exchange coupling in oxoverdazyl-based Cu(II) complex

Rota

Train

et al. 2010

The exchange channels governing the experimentally reported coupling constant ͑J expt =6 cm −1 ͒ value in the verdazyl-ligand based Cu͑II͒ complex ͓Cu͑hfac͒ 2 ͑imvdz͔͒ are inspected using wave function-based difference dedicated configuration interaction calculations. The interaction between the two spin 1/2 holders is summed up in a unique coupling constant J. Nevertheless, by gradually increasing the level of calculation, different mechanisms of interaction are turned on step by step. In the present system, the calculated exchange interaction then appears alternatively ferromagnetic/ antiferromagnetic/ferromagnetic. Our analysis demonstrates the tremendously importance of some specific exchange mechanisms. It is actually shown that both parts of the imvdz ligand simultaneously influence the ferromagnetic behavior which ultimately reaches J calc = 6.3 cm −1 , in very good agreement with the experimental value. In accordance with the alternation of J, it is shown that the nature of the magnetic behavior results from competing channels. First, an antiferromagnetic contribution can be essentially attributed to single excitations involving the network localized on the verdazyl part. In contrast, the ligand-to-metal charge transfer ͑LMCT͒ involving the imidazole moiety affords a ferromagnetic contribution. The distinct nature / of the mechanisms is responsible for the net ferromagnetic behavior. The intuitively innocent part of the verdazyl-based ligands is deeply reconsidered and opens new routes into the rational design of magnetic objects.

“…The second order expansion gives rather satisfactory results provided that the variational complete active space is large enough and properly chosen. 18 -21 In the recent past, an alternative configuration interaction ͑CI͒ technique has been designed for a direct evaluation of vertical energy differences, the difference dedicated configuration interaction ͑DDCI͒, 22 which has been successfully employed in a wide series of studies, concerning molecular [23][24][25][26] as well as solid state magnetic materials. [27][28][29][30] An error smaller than 10 cm Ϫ1 is typical for a value of Jϳ100 cm…”

mentioning

confidence: 99%

Analysis of the magnetic coupling in binuclear complexes. I. Physics of the coupling

Cabrero

Malrieu

et al. 2002

288

233

Accurate estimates of the magnetic coupling in binuclear complexes can be obtained from ab initio configuration interaction ͑CI͒ calculations using the difference dedicated CI technique. The present paper shows that the same technique also provides a way to analyze the various physical contributions to the coupling and performs numerical analysis of their respective roles on four binuclear complexes of Cu (d 9 ) ions. The bare valence-only description ͑including direct and kinetic exchange͒ does not result in meaningful values. The spin-polarization phenomenon cannot be neglected, its sign and amplitude depend on the system. The two leading dynamical correlation effects have an antiferromagnetic character. The first one goes through the dynamical polarization of the environment in the ionic valence bond forms ͑i.e., the M ϩ¯MϪ structures͒. The second one is due to the double excitations involving simultaneously single excitations between the bridging ligand and the magnetic orbitals and single excitations of the environment. This dispersive effect results in an increase of the effective hopping integral between the magnetic orbitals. Moreover, it is demonstrated to be responsible for the previously observed larger metal-ligand delocalization occurring in natural orbitals with respect to the Hartree-Fock ones.