Forslund, Eva Linnea; (2003) Computational studies of the electronic structure of transition metal and p-block compounds. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

A series of calculations, using time-dependent density functional theory as implemented in the Amsterdam Density Functional (ADF) program, have been carried out on 2,3-dialkynyl-1,4-diazabuta-1,3-diene palladium molecules and their complexes in order to determine their electronic excitation energies for comparison with experimental UV/Vis absorption spectra. A molecular orbital explanation is presented for the bathochromic shift which occurs when hydrogen is substituted for a dimethyl amino-group in the para position of the aryl rings of the free ligands. The near infrared (NIR) absorption in the free diazabutadiene is found to be a HOMO→LUMO transition, and the bathochromic shift was found to be due to a destabilising an-tibondirig interaction between the NNMe₂ Pπ and the aryl ring in the HOMO. It was found that palladium stabilises the LUMO and hence coniplexation reduces the HOMO-LUMO gap, causing a further bathochromic shift of the NIR absorption. The bond energies of the diatomic halogens (F₂→I₂) have been studied, using the ADF program, to gain an understanding of why F2 has an unusually low bond energy. The low F-F bond energy was found to be the result of a lower than expected electrostatic energy at the equihbrium bond length. This in turn is due to large electron-electron repulsion of F charge clouds. The gain in the electrostatic energy that occurs when the bond length is decreased from equilibrium is, however, outweighed by the increase in Pauli repulsion energy which is greater in F₂ than in the heavier halogens due to the more rapidly varying orbital overlap. The potential energy surface of the ClO+HO₂ reaction has been studied using the ADF program and the results compared with published data obtained using various ab initio and hybrid-DFT methods. The reaction was found to take place either on a singlet surface to form HCl and O₃ via a transition state, or on the triplet surface to form HOCl and O₂(³Σ) without any activation barrier being present. No other transition state besides the one mentioned above could be found due to a variety of computational problems. Similar problems occurred when using the Gaussian 98 program, suggesting that DFT calculations on these type of radical reactions should be treated with caution.

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