Chemical bonding and aromaticity in furan, pyrrole, and thiophene: a magnetic shielding study.

Aromaticity and bonding in furan, pyrrole, and thiophene are investigated through the behavior of the isotropic shielding σiso(r) within the regions of space surrounding these molecules. HF-GIAO/6-311++G(d,p) and MP2-GIAO/6-311++G(d,p) (Hartree-Fock and second-order Møller-Plesset perturbation theory utilizing gauge-including atomic orbitals) σiso(r) contour plots are constructed using regular two-dimensional 0.05 Å grids in the molecular plane, in horizontal planes 0.5 and 1 Å above it, and in a vertical plane through the heteroatom. The nucleus-independent chemical shifts (NICS) calculated at the ring centers and at 0.5 Å and 1 Å above these centers, NICS(0), NICS(0.5), and NICS(1), respectively, support the widely accepted order of aromaticities thiophene > pyrrole > furan. The results suggest that accurate NICS calculations benefit more from the use of an extended basis set than from the inclusion of dynamical electron correlation effects. The different extents of σiso(r) delocalization observed in the horizontal contour plots and other features of σiso(r) are also consistent with an aromaticity reduction of the order thiophene > pyrrole > furan. It is suggested that the extent of σiso(r) delocalization in σiso(r) contour plots in planes 1 Å above the molecular plane could be used for comparing the relative aromaticities of a wide range of aromatic systems.