Browsing by Author "Alkan, Fahri"
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Item Calculation of chemical-shift tensors of heavy nuclei: a DFT/ZORA investigation of 199Hg chemical-shift tensors in solids, and the effects of cluster size and electronic-state approximations(Royal Society of Chemistry, 2014-06-02) Alkan, Fahri; Dybowski, Cecil; Fahri Alkan and C. Dybowski; Alkan, Fahri; Dybowski, C.Calculations of the nuclear magnetic resonance chemical-shielding tensors of a suite of mercury-containing materials using various cluster models for the structures provide a stringent test of the procedures for forming models and for calculation with various methods. The inclusion of higher co-ordination shells in the molecular clusters permits quantum chemical calculations of 199Hg chemical-shielding tensor elements within 3% of the experimental values. We show that it is possible to reduce the size of computationally expensive molecular-cluster calculations with limited effect on calculated NMR parameters by carefully introducing the frozen core approximation. The importance of the relativistic Hamiltonian for accurate predictions of chemical-shielding values is demonstrated within the molecular cluster approach. The results demonstrate that careful design of a cluster to represent the solid-state structure, inclusion of relativistic components in the Hamiltonian at least at the spin–orbit level, and judicious use of approximations are essential to obtain good agreement with experimental results.Item Chemical-shift tensors of heavy nuclei in network solids: a DFT/ZORA investigation of 207Pb chemical-shift tensors using the bond-valence method(Royal Society of Chemistry, 2015-09-02) Alkan, Fahri; Dybowski, Cecil; Fahri Alkan and C. Dybowski; Alkan, Fahri; Dybowski, CecilCluster models are used in calculation of 207Pb NMR magnetic-shielding parameters of α-PbO, β-PbO, Pb3O4, Pb2SnO4, PbF2, PbCl2, PbBr2, PbClOH, PbBrOH, PbIOH, PbSiO3, and Pb3(PO4)2. We examine the effects of cluster size, method of termination of the cluster, charge on the cluster, introduction of exact exchange, and relativistic effects on calculation of magnetic-shielding tensors with density functional theory. Proper termination of the cluster for a network solid, including approximations such as compensation of charge by the bond-valence (BV) method, is essential to provide results that agree with experiment. The inclusion of relativistic effects at the spin–orbit level for such heavy nuclei is an essential factor in achieving agreement with experiment.Item Relativistic DFT calculations of magnetic shielding tensors for spin ½ heavy nuclei(University of Delaware, 2016) Alkan, FahriA computational study of magnetic-shielding tensors of spin-½ heavy nuclei in solids has been carried out by employing relativistic DFT and cluster models. The performance of various theoretical treatments and cluster models has been investigated by comparing the agreement between theory and experiment as a metric of the goodness of the calculation. ☐ A significant amount of effort in this study has been dedicated to the development of cluster models for accurate calculation of magnetic-shielding tensors in the solid state. The performance of cluster models of various sizes, symmetries, as well as clusters with different net charges and with preparation by different truncation methods have been studied. The convergence of calculated principal components with cluster size is monitored in benchmark calculations. The results suggest that inclusion of higher coordination shells in the molecular cluster is generally necessary for quantitative predictions of magnetic-shielding tensors. However, it has been found possible to reduce the size of these computationally expensive molecular-cluster calculations with limited effect on the calculated NMR parameters by carefully introducing the frozen core approximation and locally dense basis sets. ☐ For network solids, a new formalism, which employs pseudo-atoms with altered nuclear charges and parameters obtained from bond-valence theory, is proposed for the truncation of clusters. This model has been applied to a large selection of systems with success. The performance of the cluster models in network solids is also compared to models that account for the full translational symmetry of the extended system (periodic boundary conditions with GIPAW) for lighter nuclei such as 29Si and 31P. ☐ The importance of treating a system with the relativistic Hamiltonian for accurate prediction of principal components of the magnetic-shielding tensor of heavy nuclei (207Pb, 199Hg, 125Te and 119Sn) is demonstrated within the cluster approach. The results demonstrate that inclusion of the spin-orbit component in the ZORA Hamiltonian is essential to obtain good agreement with experimental results. It is shown that spin-orbit effects on the principal components are strongly dependent on the oxidation state and coordination geometry about the NMR nuclei. ☐ Finally, the performance of hybrid functionals (B3LYP and PBE0) is examined for the prediction of magnetic-shielding tensors of 207Pb, 125Te and 119Sn. The results show that employing hybrid functionals improves agreement between theory and experiment, compared to GGA functionals. This improvement is more noticeable in the case of 207Pb than it is for 125Te and 119Sn.