Structural Characterization of Isolated Siloxaalkane Molecular Gyroscopes via DFTB-based Quantum Mechanical Model

Abstract

Despite many more improvements in the mathematical formalisms of the DFT model, it is still unable to compute ground state electronic structure of the crystalline macrocyclic molecular compounds (CMMCs) with closed topology at low computational cost. In the recent years, a less computationally complex and relatively fast plus efficient self-consistent-charge density functional based tight binding (SCC-DFTB) scheme has been heavily extended to CMMCs under both periodic and isolated molecular conditions mainly for computing as reliable molecular structures as standard time-dependent DFT. In this study, the SCC-DFTB method is employed to characterize the molecular structures of three different molecular gyroscopes (MGs) encapsulating dihalogen substituted (e.g. ROT-2F and ROT-2Cl) and non-substituted (e.g. ROT-2H) phenylene segments under isolated condition. We realized the SCC-DFTB method a cheap yet decent quantum mechanical model in reproducing one or more X-ray observed equilibrium structures of all these three MGs as well as in deriving as consistently good structural information as DFT method. While elucidating experimentally observed structural deformation mainly in the ROT-2F and ROT-2Cl, the SCC-DFTB computed free-volume units present around their central phenylene segments are also found to be in quantitatively good agreement with those derived by the X-ray and DFT. The results presented here enlighten us about the SCC-DFTB method for adopting it directly in crystalline condition to predict crystal structures and rotational dynamics of these MGs. This theoretical insight will promote SCC-DFTB studies on gyroscopic nanostructures and giant molecular assemblies that attract increasing attention for molecular machinery world.