Interpretation
of Electron Delocalization in Benzene,
Cyclobutadiene, and Borazine Based on Visualization of Individual
Molecular Orbital Contributions to the Induced Magnetic Field
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Abstract
The magnetic response of the valence
molecular orbitals (MOs) of
benzene, cyclobutadiene, and borazine to an external magnetic field
has been visualized by calculating the chemical shielding in two-dimensional
grids of points on the molecular plane and on a plane perpendicular
to it, using gauge-including atomic orbitals (GIAOs). The visualizations
of canonical MO contributions to the induced magnetic field (CMO-IMF)
provide a clear view of the spatial extension, the shape, and the
magnitude of shielding and deshielding areas within the vicinity of
the molecule, originating from the induced currents of each valence
orbital. The results are used to investigate the delocalization of
each valence MO and to evaluate its contribution to the aromatic character
of systems under study. The differentiation of the total magnetic
response among the three molecules originates exclusively from π-HOMO
orbitals because the magnetic response of the subsets of the remaining
MOs is found to be almost identical. Borazine is classified as nonaromatic
as the four electrons that occupy the π-HOMO are found to be
strongly localized on nitrogen centers. CMO-IMF can clarify the interpretation
of various NICS indexes and can be applied for the investigation of
various types of electron delocalization
