Quantum-chemical studies of spin transition systems

Full text.The spin crossover phenomenon is an example of molecular bistability that can be used in creating molecular devices. Spin crossover complexes exhibit a transition from the singlet low-spin state (LS) to the quintet high-spin state (HS) with increasing temperature. In our presentation we will elucidate the case of a spin crossover complex with two conformation at the same temperature. Recently, synthesized new spin transition complex [FeII(DAPP)(abpt)](CIO4)2 with DAPP=[bis(3-aminopropyl)(2-pyridylmethyl)amine] and abpt=4-amino-3,5-bis(Pyridin-2-yl)-1,2,4-triazole persents some new features of the phenomenon. Two types of order-disorder transitions accompanying the thermal spin change are experimentally reached. The first in the perchlorate anions and the second in the DAPP ligand. FeII(DAPP)(abpt)]2+ gives the first example of the order-disorder in the ligand directly attached to the metal. We have performed ab-initio calculation with the Density Functional Theory methods on [FeII(DAPP)(abpt)]2+ cation. The calculation of the energy gap E(HS) - E(LS) for LS state structure and the two HS state structure was done. The impact of the disorder at high temperature on magnetic properties is discussed. The optimization of both HS state structures revealed that the disorder phenomena in this spin state is induced by the lattice effect. The calculation of the vibrational frequencies was performed. Our analysis reproduces the well known effect of decrease of vibrational frequencies in the HS state, which can be about 30%