Computed NMR Shielding Effects over Fused Aromatic / Antiaromatic Hydrocarbons

Through-space isotropic NMR shielding values of a perpendicular diatomic hydrogen probe moved in a 0.5 Å grid 2.5 Å above several polycyclic aromatic/antiaromatic ring and aromatic/aromatic hydrocarbons were computed with Gaussian 03 at the GIAO HF/6-31G(d,p) level. Combinations of benzene fused with cyclobutadiene, with the tropylium ion, and with the cyclopentadienyl anion were investigated. Subtraction of the isolated H2 isotropic value gave shielding increments (∆σ), which, when plotted against Cartesian coordinates of the probe over each hydrocarbon, gave representations of threedimensional isotropic shielding increment surfaces. The results are related to the degree of bond length alternation, the extent of π electron delocalization, and (for the ions) the NPA charge distribution. The shielding increment data are compared to NICS(1) values computed at the same level; both indicate the degree of aromaticity or antiaromaticity of the component rings.