Chapter 4 Novel Theoretical Prediction Method of the Haldane Gap among the Azido-Bridged Compounds by DV-Xα Molecular Orbital Calculation

The electronic structures of the Haldane gap compounds that include Ni^II ions having s=1 spin are reported. The azido-bridged Haldane gap compounds, such as NINAZ ([Ni(NH₂(CH₂)₃NH₂)₂N₃]ClO₄), NDMAZ ([Ni(NH₂CH₂C(CH₃)₂CH₂NH₂)₂N₃]ClO₄), and NDMAP ([Ni(NH₂CH₂C(CH₃)₂ CH₂NH₂)₂N₃]PF₆) have been synthesized by substituting different in-plane ligands and counteranions. The most powerful advantage of the metal complex based Haldane gap compounds is to control the antiferromagnetic interaction by changing the combination of the counteranions and the in-plane and the bridging ligands. After carrying out a high-precision X-ray structural analysis of NINAZ, the differences of the molecular structures and the electronic structures among these three azido-bridged Haldane gap compounds can be compared in order to clarify the mechanism of the energy difference between the singlet ground state and the triplet excited state. From the result of the energy difference between the singlet and the triplet states using the DV-Xα calculation method, we have observed a trend that the compound having a large Haldane gap energy, such a NINAZ, has a large energy difference between two spin states, implying the larger energy difference should be the harder spin inversion from anti-parallel singlet to the parallel triplet state. Our previous manifestation of the way to predict the value of the Haldane gap by using the analysis of the bond overlap population by means of a DV-Xα molecular orbital calculation was not so suitable for general Haldane gap compounds including the NO₂-bridged one-dimensional (1D) chain structure compounds. Therefore in this study the novel way using the analysis of the energy level structures should be reported.