Breakdown of the Total Dipole Moments of Diatomic Molecules into Individual Orbital Contributions

Quantum chemical results at the CCSD(T)/def2-QZVPP and BP86/def2-QZVPP levels are reported for the neutral and charged diatomic molecules EF, EO^- (E = B-Tl), EF⁺, EO, EN^- (E = C-Pb), EO⁺, and EN (E = N-Bi). The theoretically predicted bond lengths and dipole moments are in good agreement with each other and with the available experimental values. It is shown that the total dipole moment of the molecules can be nicely separated into the contributions of the individual occupied molecular orbitals. The σ lone-pair orbital has a dominating influence on the total dipole moment in the lighter EX systems, where E is an atom of the first or second octal row of the periodic table, but it becomes less influential for the heavier species. The HOMO of the heavy cations PbF⁺, SbO⁺, and BiO⁺ is the degenerate π-bonding orbital, and the σ lone-pair orbital is the HOMO-2. The orbital energies of the (<i>n</i>-1)d AOs of the heavier atoms are in the same range as those of the lowest lying genuine valence orbitals, so the division into nuclear and valence orbitals is not so clear.