Ab initio double-ζ (D95) valence bond calculations for the ground states of NO2, O3, and ClO2

The results of some double-ζ D95 valence-bond (VB) calculations are reported for the ground states of nitrogen dioxide, NO2 (17 valence electrons), ozone, O3 (18 valence electrons), and chlorine dioxide, ClO2 (19 valence electrons). The Mulliken, Löwdin and Hiberty structural weights are reported for nine (NO2), six (O3), and three (ClO2) Lewis structures that differ in the locations of the π electrons. The most important Lewis structures for NO2 are the uncharged spin-paired diradical structure VII and the two equivalent structures that carry no formal charges (II and V). For O3 and ClO2, the primary Lewis structures are, respectively, the uncharged singlet diradical structure III, and I with the odd electron located in the chlorine 3pπ atomic orbital. For ClO2, the results of some STO-6G calculations, with 16 canonical Lewis type structures included, give a much smaller value for the chlorine odd-electron charge than does the D95 vb2000 calculation with a Hartree–Fock core. However, the structural weights obtained from the STO-6G calculations do reflect the expectation that small atomic formal charge separations, together with some Cl–O covalent σ-bonding, are associated with large structural weights. Graphical abstract The results of some double-ζ D95 valence-bond (VB) calculations are reported for the ground states of nitrogen dioxide, NO2 (17 valence electrons), ozone, O3 (18 valence electrons), and chlorine dioxide, ClO2 (19 valence electrons).