Katherine Mills

Honors in Chemistry

Major: Chemistry

Supervisor: Ned Martin, Chemistry and Biochemistry

Computed NMR Shielding Values of Cyclic Hydrocarbons as a Measure of Aromaticity

Aromaticity is a well-known concept in chemistry, yet it is not clearly defined. It is considered to be multidimensional, embodying several characteristics, including structural, energetic, and magnetic characteristics. This research utilizes computed magnetic characteristics to explore the aromaticity and antiaromaticity of a series of fused aromatic/antiaromatic neutral and charged hydrocarbons. A diatomic hydrogen probe was placed in a grid at 1 Ã… increments at distances of 2.5, 3.0, and 4.0 Angstroms above the XY plane of each of these planar hydrocarbons in separate computations. NMR isotropic shielding values were calculated. The difference (Δε) between the shielding values calculated for the proximal hydrogen of the H2 probe molecule over the hydrocarbon being tested and the shielding value for the diatomic hydrogen by itself was used as a measure of aromaticity. These computed magnetic shielding increments were compared with other reported methods of measuring aromaticity. Plotting (Δε) against Cartesian coordinates of the probe allowed determination of the computed through-space shielding increment surfaces for these compounds. The characteristics of these shielding surfaces as well as the computed bond lengths and the computed charges on atoms are described and interpreted in terms of the aromatic or antiaromatic character of the molecules in the study.