ABINITIO CALCULATION OF THE ELECTRON-DENSITY OF TETRAAZATETRAOXATRICYCLOTETRADECANE - AN EXPLANATION FOR THE DEFICIENCY OF CHARGE-DENSITY IN CERTAIN COVALENT BONDS
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The standard deformation density, molecular density minus spherical atom densities, may have features, such as density deficits or weak density accumulations in covalent bonds that seem inconsistent with conventional chemical theory. The interpretation of these unexpected features has been controversial. Using ab initio molecular orbital methods, we are able to reproduce all of the major features in the experimental contour maps of the standard deformation density for l,2,7,8-tetraaza-4,5,10,l l-tetraoxatricyclo[6.4.1.1]tetradecane by Dunitz and Seiler (J. Am. Chem. Soc. 1983, 105, 70577058). In particular, maps for the OO bond show an electron density deficit throughout the internuclear region, and density accumulations at each O in the ir regions perpendicular to the OO bond axis. Our analysis shows that these unexpected features arise because spherical atoms rather than valence-state atoms are substracted from the total density to form the deformation density. If one views bond formation in two steps, atom preparation and then bond formation, one easily sees the origin of the unexpected features in the standard deformation density. The density difference due to atom preparation (orientation of components of spherical O atom, and then promotion, polarization, and hybridization of all the atoms) is seen by subtraction of the electron densities of spherical atoms from that of optimally hybridized valence-state atoms. The density difference due to covalent bond formation (constructive interferences and charge transfer) is revealed by subtraction of the electron densities of optimally hybridized valence-state atoms from that of the molecule. The latter is dominated by a sum of two-electron bonding density differences, each of which can be isolated. for the 00 bond these reveal not only the strong accumulation of charge in the internuclear region but also the concomitant depletion of charge in the nonbonding regions which together are the signature of the covalent bond. The unexpected features arise from the O atoms preparation for bonding: orientation, promotion, and hybridization. Thus, we have used the concept of a valence-state atom to produce a useful partitioning scheme which reveals features related to chemical concepts that are not visible in the total density or in the standard deformation density and which has the potential to indicate the relative strength of various bonds. 1987, American Chemical Society. All rights reserved.
