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First-principles studies of transition metal doped systems and hyperfine coupling constants of muoniated butyl radicals Chen, Yakun

Abstract

The first part of the thesis examines, using density functional theory (DFT) calculations, the effects of introducing transition metals (TMs) into different systems, including small Au clusters; carbon nanotubes (CNTs); pristine and defected boron nitride nanotubes (BNNTs). The results show that the frontier molecular orbitals of the TM modified systems are usually localized around the doping site and the reactivities of these systems are often improved. In the case of small TM clusters, both PtAum and Aun tend to be planar in their ground state. N₂ and O₂ adsorption onto these clusters results in different adsorption configurations due to different orbital interactions. With regard to the TM modified CNTs, the endo-TM-doped CNTs are less stable than the corresponding exo-doped isomers due to the large geometric strain caused by deformation. The exo-doped SWCNTs are better electron donors than their endo-doped counterparts. As for the Pt modified BNNTs, binding energy analysis revealed that a Pt atom can move freely on a pristine BNNT. But the Pt atom is trapped between the B B bond at the defect site if a Stone-Wales defect exists. In both cases, the hosting BNNTs are wide-gap semiconductors with slightly improved reactivities. In comparison, BNNTs doped with Pt atoms are narrow-gap semiconductors with greatly enhanced reactivities. Both MP2 EPR-III and B3LYP EPR-III calculations were used to optimize butyl isomers and calculate hyperfine coupling constants (HFCCs) to explain experimental data. The C-Mu distance was elongated to 1.076 times the corresponding equilibrium C-H bond length to mimic vibrationally average β-muoniated radical geometries so as to calculate the muon HFCCs. Some muon HFCCs and most proton HFCCs were satisfactorily reproduced by the B3LYP calculations due to error cancellations, whereas other cases were better predicted by the MP2 calculations. The torsional potential energy surface (PES) of the sec-butyl radical was also studied. The cis conformation, which was observed in experiments, was unobtainable using some common DFT functionals, but can be identified by calculations using wavefunction theory or some modified hybrid functionals. Changes in basis set only modify the shape of the PES slightly.

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