GEOMETRIES OF O4+, O4-, AND N4+ BY AN APPROXIMATE SCF-MO THEORY WHICH CONSIDERS INTERMOLECULAR DIFFERENTIAL OVERLAP

We have found that the CNDO/2 (semiempirical SCF-MO) theory does not properly account for the long-range repulsive interactions in O 2-O2 (S=1/2) and O2-O2+ (S=1/2). This results in the O2-O2 interaction potential being zero at O-O separations of 2.3-3.5 Å and the O2-O 2+ bond energy being much too large. The CNDO/2 method has been modified by considering sp-type integrals to account for charge-quadrupole and charge-atomic polarization interactions. In addition, differential overlap is considered and multicenter integrals are approximated for intermolecular interactions. This is a semiempirical method in which the adjustable constants are fixed by the quadrupole moments of O2 and N2 and the O4+ bond energy. The interaction energies of O 4+, O4-, and N4 + are computed for structures of C2 symmetry as a function of θ and R. The equilibrium dissociation energies De (kcal) at Re (Å) and θ3 are as follows: O 4+, 11.66, 2.03, -67°; O4-, 8.49, 2.08, -71°; N4+, 34.7, 2.04, 0°. The computed O2-O2 interaction potential is in reasonable agreement with the experimental results for the repulsive part of the van der Waals potential.