Quantum chemical study of three polymorphs of the mononuclear spin-transition complex [Fe(DPPA)(NCS)2].

The calculations of the high spin (HS) and low spin (LS) states of the [Fe(II)(DPPA)(NCS)(2)] complex have been performed at three experimentally observed geometries corresponding to three synthesized polymorphs with different spin-transition behavior. The structure optimization leads to a single molecular structure, suggesting that the existence of three geometries is not an intrinsic phenomenon but is induced by the crystal lattice. The structural difference between three forms can be reproduced by introducing the Madelung field of the crystal lattice. However, the calculations show that the differences in magnetic behavior of the three polymorphs cannot be attributed only to variations of the energy gap between two spin states.