2016-03-31 What is the possible origin of the 195Pt NMR shielding in the solid state? X-ray diffraction, solid-state NMR studies of deceptively simple C2[PtX6] salts (C = various cations, X = Cl- and Br-)

by James Arthur Odendal

Institution: Stellenbosch University
Department: Chemistry and Polymer Science
Degree: PhD
Year: 2015
Keywords: function of temperature, salt, electronic structures of salt
Record ID: 1446653
Full text PDF: http://hdl.handle.net/10019.1/96596


ENGLISH ABSTRACT: The primary objective of the work presented in this thesis is to investigate the sensitivity and nature of the 195Pt chemical shift in the solid-state, as a function of temperature. We have carried out a series of experimental single crystal X-ray diffraction (SCXRD), solid state nuclear magnetic resonance (SSNMR) and density functional theory (DFT) experiments and calculations on various dication hexachloridoplatinate(IV) salts to paint a picture of the nature of the 195Pt shielding in the solid state. The results obtained from SCXRD analyses performed as a function of temperature on Rb2PtCl6, (NH4)2PtCl6 and [(N(CH3)4]2PtCl6 salts give a detailed account on the changing inter- and intramolecular interactions in dication hexachloridoplatinate(IV) salts upon lattice expansion, as a function of increasing temperature. The 195Pt SSNMR measurements of these salts have a peculiar 195Pt chemical shift sensitivity to changes in temperature. The 195Pt chemical shift trends performed as a function of temperature of the Rb2PtCl6 salt resulted in a linear-like trend with the slope of δ195Pt/T = 0.362 (0.008) ppm/K. The (NH4)2PtCl6 salt trend showed some non-linear characteristics at lower temperatures and tended towards linearity at a higher temperature with the slope δ195Pt/T = 0.228 (0.0126) ppm/K. The 195Pt chemical shift of the [(N(CH3)4]2PtCl6 had quite a different trend due to its well-known structural phase transitions as a function of temperature. The expected sensitivity range of the 195Pt chemical shift, predicted from experimental temperature dependent (100 K and 400 K) SCXRD measurements of Rb2PtCl6 and (NH4)2PtCl6 salts coincide well with that of experimental 195Pt chemical shift sensitivity. The atomic coordinates obtained from SCXRD analyses performed as a function of temperature were used to conduct DFT calculations to extract SSNMR parameters together with electrostatic potentials in order to study the local molecular and electronic structures of Rb2PtCl6 and (NH4)2PtCl6 salts. The computed 195Pt SSNMR parameters gave an account of chemical shift parameters and quadrupolar coupling parameters, which show excellent agreement with experimental 195Pt chemical shift SSNMR measurements. These calculations shed some light on the 35Cl SSNMR parameters, which to date are still unobtainable by means of conventional experimental SSNMR measurements. The computed 35Cl SSNMR shielding parameters showed that the electronic environments of the Cl- ion in dication hexachloridoplatinate(IV) salts are central to the understanding and interpretion of experimental 195Pt chemical shift sensitivity. These computational analyses gave the means to propose a theory whereby the : “polarization of electronic charge density from the PtIV ion towards the Cl- ion as the interaction between the anion and cation are being weakened due to lattice expansion as a function of increasing temperature”. This thesis shows that the combination of these three usually independently used analytical …