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Title: Computational studies of paddlewheel complexes in isolation and incorporated into metal organic frameworks
Author: Alzahrani, Khalid
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2017
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The aim of the work presented in this thesis is to develop computational approaches to the modelling of zinc and copper paddlewheel complexes both in isolation and when incorporated into metal organic frameworks (MOFs). We considered both ‘ab initio’ and empirical force field methods based mainly on DFT and ligand field molecular mechanics (LFMM) respectively. A new all-atom first-principles force field (FF) is constructed for the bimetallic, four-bladed zinc paddlewheel (ZPW) motif. Zinc-ligand interactions are described via Morse functions and the angular geometry at the metal centres is modelled with a pure ligand-ligand repulsion term. The ZPW-FF is principally based on 21 DFT-optimized model systems of general formula ZnPR.nL, where ZnP is the base Zn2(O2CR)4 unit, R=H, CH3 or CF3, L=NH3, pyridine, or water and n = 0, 1 or 2. It correctly generates the distorted tetrahedral coordination of the uncapped [Zn2(O2CR)4] species in their ground states as well as giving reasonable structures and energies for the higher symmetry D4h transition state conformations. The zinc-ligand Morse function reference distance, r0, is further refined against some experimental complexes located in the Cambridge Structural Database and this FF is applied to pore models of the flexible MOF [Zn(bdc)2(dabco)]n and also used to assess the system under water vapour. A single pore model reproduces the unit cell of the evacuated MOF system while a 3×3 grid model is necessary to provide good agreement with the observed pronounced structural changes upon adsorption of either dimethylformamide or benzene. The ZPW-FF is also applied to 2D and 3D crystal systems of MOF-2 which comprises Zn(bdc)(H2O).(dmf) building units and provides good results. In the second part of this thesis, our density functional theory calculations on four-bladed copper paddlewheel (CPW) systems [Cu2(O2CR)4L2] reveal a change in ground state with increasing Cu–L bond strength. For L = N-heterocyclic carbene (NHC), the Jahn–Teller axis switches from parallel to orthogonal to the Cu–Cu vector and the copper coordination geometry becomes highly flexible. While the calculated dimer/monomer equilibrium for isolated complexes slightly favours monomers, the preformed paddlewheel units embedded in many metal organic frameworks are potential targets for developing novel materials. Therefore, the preliminary LFMM parameters for constructing a generic CPW-FF are reported. However, a definitive version of the CPW-FF remains a task for the future.
Supervisor: Not available Sponsor: Jāmiʻat al-Malik ʻAbd al-ʻAzīz ; Wizārat al-Maʻārif ; Saudi Arabia
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QD Chemistry