Breakdown of the\nTotal Dipole Moments of Diatomic\nMolecules into Individual Orbital Contributions

Quantum chemical results at the CCSD(T)/def2-QZVPP and\nBP86/def2-QZVPP\nlevels are reported for the neutral and charged diatomic molecules\nEF, EO<sup>–</sup> (E = B–Tl), EF<sup>+</sup>, EO, EN<sup>–</sup> (E = C–Pb), EO<sup>+</sup>, and EN (E = N–Bi).\nThe theoretically predicted bond lengths and dipole moments are in\ngood agreement with each other and with the available experimental\nvalues. It is shown that the total dipole moment of the molecules\ncan be nicely separated into the contributions of the individual occupied\nmolecular orbitals. The σ lone-pair orbital has a dominating\ninfluence on the total dipole moment in the lighter EX systems, where\nE is an atom of the first or second octal row of the periodic table,\nbut it becomes less influential for the heavier species. The HOMO\nof the heavy cations PbF<sup>+</sup>, SbO<sup>+</sup>, and BiO<sup>+</sup> is the degenerate π-bonding orbital, and the σ\nlone-pair orbital is the HOMO–2. The orbital energies of the\n(<i>n</i>–1)d AOs of the heavier atoms are in the\nsame range as those of the lowest lying genuine valence orbitals,\nso the division into nuclear and valence orbitals is not so clear.