WHY THE ACCUMULATION OF ELECTRON-DENSITY APPEARS WEAK OR ABSENT IN CERTAIN COVALENT BONDS
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The standard deformation density, molecular density minus spherical atom densities, may appear small or even negative in certain covalent bonds. For example, in difluorine subtraction of the electron densities of spherically averaged F atoms, which include all components of the 2 P(2s 2 2p 5 ) ground state, results in the standard deformation density. Contour maps show an electron density deficit along the bond axis in both the internuclear bonding region and the lone-pair regions beyond the nuclei and accumulation in the regions perpendicular to that axis. This result resembles experimental maps of the deformation density of the covalent bond between electronegative atoms such as that between the O atoms of peroxides. Our analysis shows that one can interpret this type of deformation density as a combination of constructive interference (covalent bond formation) and atomic reorientation, polarization, promotion, and hybridization (atom preparation for bonding). Subtraction of the electron densities of one component of the 2 P ground state of the F atoms, oriented with singly occupied 2p 2 orbitals along the direction, results in maps with weak accumulation of charge in the bonding region between the nuclei, deep troughs of density deficit near the nuclei, no change along the regions, and accumulation of charge in the lone-pair regions beyond the nuclear centers. Subtraction of the electron densities of optimally hybridized valence-state F atoms matches the lone-pair densities of atoms and molecule, so that the total difference is dominated by a localized bonding orbital density difference. This map reveals not only the accumulation of charge in the internuclear region but also the concomitant depletion of charge in the nonbonding regions beyond the nuclear centers which together are the signature of the covalent bond. Thus, if one views bond formation in two steps, atom preparation then bond formation, one easily sees the origin of the loss of electron density in the bond region caused when two spherical F atoms form a bond. 1986, American Chemical Society. All rights reserved.