Calculation of theoretical NMR chemical shieldings in small molecules

• Main Author: England, James A.
• Published: University of Surrey 2002
• Subjects: 543; Physical chemistry
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.248028

In the following thesis the calculation of the NMR shieldings of the nuclei within a variety of molecules in differing situations is presented. A detailed introduction to modern quantum chemical methodology is initially laid down including descriptions of the Hartree-Fock method as well as electron correlation methods such as Configuration Interaction and Many Body Perturbation Theory. This is followed by a description of the common methods used in the calculation of NMR shielding values including Gauge Invariant Atomic Orbitals (GIAO), Individual Gauge for Local Orbitals (IGLO) and Localized Orbitals/Localized Origins (LORG), as well as the existing methods which account for electron correlation effects. The theoretical background is then completed with a discussion of the methods used in optimizing molecular geometries, both to minimum energy structures or first order saddle points (i.e. equilibrium and transition structures), as well as reaction path following and a brief description of frequency calculations. The remaining chapters are then devoted to the three studies of this thesis. The first study presented is that of the investigation of the effects of hydrogen bonding upon the 19F shielding in monofluorobenzene. It is found that the 19F shielding is sensitive to hydrogen bonding, which has implications for the use of fluoroaromatic compounds as probes into large biological molecules such as proteins. The second study is into the performance of a variety of basis sets with respect to the calculation of the 19F shielding in three fluorobenzenes, as well as an investigation into the convergence of the NMR shielding toward the Hartree-Fock limit. This study shows that the moderately sized Karlsruhe-TZF and 6-311G** basis sets offer a good compromise between size and accuracy, but the investigation into the shielding convergence is less conclusive. In the final study the effect of molecular inversion and Berry pseudorotation upon both the shielding and temperature dependent shielding of five small molecules is examined. It is found that in the case of four of the five molecules studied (NH3, NF3, PF3 and PF5) the contribution to the temperature dependent shielding is low, but in the case of one of the molecules (PH3) the contribution is large.