Browsing by Author "Dybowski, Cecil"
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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 Coordination geometry of lead carboxylates – spectroscopic and crystallographic evidence(Royal Society of Chemistry, 2014-12-14) Catalano, Jaclyn; Murphy, Anna; Yao, Yao; Yap, Glenn P. A.; Zumbulyadis, Nicholas; Centeno, Silvia A.; Dybowski, Cecil; Jaclyn Catalano, Anna Murphy, Yao Yao, Glenn P. A. Yap, Nicholas Zumbulyadis, Silvia A. Centeno and Cecil Dybowski; Catalano, Jaclyn; Murphy, Anna; Yao, Yao; Yap, Glenn P. A.; Dybowski, CecilDespite their versatility, only a few single-crystal X-ray structures of lead carboxylates exist, due to difficulties with solubility. In particular, the structures of long-chain metal carboxylates have not been reported. The lone electron pair in Pb(II) can be stereochemically active or inactive, leading to two types of coordination geometries commonly referred to as hemidirected and holodirected structures, respectively. We report 13C and 207Pb solid-state NMR and infrared spectra for a series of lead carboxylates, ranging from lead hexanoate (C6) to lead hexadecanoate (C18). The lead carboxylates based on consistent NMR parameters can be divided in two groups, shorter-chain (C6, C7, and C8) and longer-chain (C9, C10, C11, C12, C14, C16, and C18) carboxylates. This dichotomy suggests two modes of packing in these solids, one for the short-chain lead carboxylates and one for long-chain lead carboxylates. The consistency of the 13C and 207Pb NMR parameters, as well as the IR data, in each group suggests that each motif represents a structure characteristic of each subgroup. We also report the single-crystal X-ray diffraction structure of lead nonanoate (C9), the first single-crystal structure to have been reported for the longer-chain subgroup. Taken together the evidence suggests that the coordination geometry of C6–C8 lead carboxylates is hemidirected, and that of C9–C14, C16 and C18 lead carboxylates is holodirected.