Gina Winchester

Honors in Chemistry

Major: Chemistry Minor: Spanish

Supervisor: Ned Martin, Chemistry and Biochemistry

Computed NMR Shielding Increments Over Six-Membered Heterocyclic Compounds and Their Benzo-Analogs as a Measure of Aromaticity

Aromaticity can be described both quantitatively and qualitatively. Aromaticity is a multidimensional property that is not well defined. Aromatic molecules have greater stability, different reactivity, and special magnetic properties. Some of these can be measured quantitatively. Six-membered heterocycles with one, two, or three nitrogen atoms and their benzo- analogs were analyzed in attempts to better understand measures of predicting aromaticity. Utilizing ab intio molecular orbital theory, NMR shielding calculations were performed to determine the degree of through-space magnetic shielding exerted by these molecules. The GIAO-HF method within Gaussian 03 was used to calculate the isotropic shielding values of the proximal hydrogen of a diatomic hydrogen probe at various positions and at three different distances above the plane of each molecule. The calculated isotropic shielding value of the diatomic hydrogen by itself was subtracted from the calculated isotropic shielding value at each probe position to yield the magnetic shielding increment, delta sigma. This shielding increment, delta sigma, was plotted against the Cartesian coordinates of the probe relative to the center of the molecule to visualize the shielding surface above each molecule and to observe important trends. A significant amount of shielding was seen over the midpoint of each of the rings. The specific values that were measured at 2.5 Ã… above each ring midpoint (delta sigma 2.5) were compared to literature values of previously established methods of measuring aromaticity. The lack of strong correlation between delta sigma 2.5 and other established methods suggested that the isotropic shielding method is influenced by not just the degree of aromaticity of a molecule, but also polarity and stabilization effects.