| Summary: | 碩士 === 國立臺灣大學 === 化學研究所 === 106 === Nature abounds with helical conformations in all scales ranging from nanoscale DNA structures to macroscopic tendrils in plants. The related hemihelical conformation that contains left-hand and right-hand helical segments simultaneously, i.e. the reversal of chirality – sometimes also referred to as a helical perversion, in the same chain, also occurs frequently and has been studied particularly in plant tendrils since the 19th century. The molecular analog of helical perversion has only been found recently by Peng and his coworkers in a special type of extended metal atom chains (EMACs), a family of multinuclear coordination complexes consisting of a string of metal atoms connected directly by metal-metal bonding and four surrounding organic ligands in left- or right-handed helical formations. The particular EMACs that show the possibility of helical perversions in its ligands have the general structural formula Mo2MMo2(tpda)4X2, where M=Ni, Co, … and X is the axial ligand. We hope to address the issue on the origin and conditions of their formations in this work. By using the DFT study, we demonstrate that the formation of helical ligands can be attributed to the unbalance of the bond lengths of central metal-metal bonds and peripheral carbon-nitrogen bonds. Starting from trinuclear EMACs, especially the unique nonhelical npo ligand EMACs, we illustrate the relationship between metal-metal bond length and the N-M-M-N dihedral angle, and the effect of ligand helicity on electronic structures. Furthermore, we analyze the Mo2MMo2(tpda)4(NCS)2 system directly to clarify the origin of its helical perversion on the surrounding ligands. Particularly, as the Mo-Ni bond length at the center is increased, the hemihelical conformation in the surrounding ligands is stabilized gradually. Furthermore, we built up a generalized Su-Schreiffer-Heeger model with linked springs representing the surrounding ligands in EMACs so demonstrate the coupling between the central chain longitudinal conformation and the ligand rotational conformation.
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