The extended Huckel method has been applied to nitrogen dioxide for a series of bond angles extending from 90 to 180" and for three values (1.50, 1.75, and 2.00) of the parameter K appearing in the Wolfsberg-Helmholtz formula. For K = 2.00, very good agreement with experiment is obtained for the bond angle of NO2-, NO2, and NO2+, while the experimental bending force constants are reproduced in a satisfactory fashion. Comparison is made between Walsh's diagram for AB2 molecules and the curves obtained for the orbital energies. The adiabatic ionization potential of NO2 is found equal to 10.98 ev, which coincides with a recent experimental determination by photoelectron spectroscopy.2 The first excited state of NO2+ is predicted to be bent. It is found that upon dimerization all the occupied b2 orbitals of NO2 give rise to some destabilization; this is suggested to be the reason for the low dissociation energy of the dimer. The loss of color associated with the dimerization appears to be linked with the stabilization incurred by the 4al orbital of NO2.

The excitation energies, potential-energy surfaces, and oscillator strengths computed in the previous paper are discussed in light of experimentally reported spectroscopic data, namely absorption, fluorescence, chemiluminescence, predissociation, photolysis, and the absence of phosphorescence. It is found that the main components of the visible spectrum are due to the transitions B21 ← 2A1, and B22 ← 2A1, with the B22 state being responsible for the observed fluorescence emission. The anomalous fluorescence lifetime is also discussed.

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Rotationally inelastic and hyperfine resolved cross sections for OH-H2 collisions. Calculations using a new ab initio potential surfaceThe spin-unrestricted Hartree-Fock (UHF) method has been applied to the H2NO radical, using as basis functions Gaussians centered on the various atoms. The system is found to be nonplanar, with an out-of-plane angle of 26° and an HNH angle of 116°. Bond lengths of 1.34 and 0.99 A are predicted for the NO bond and the NH bonds, respectively. The nonplanarity of the system is found to be due to the steep slope of the highest occupied orbital (of b1 symmetry and NO antibonding). This last point is in agreement with the prediction of Walsh. As for the other orbitals, little similarity is observed with the Walsh orbital energy diagram. Hyperfine coupling constants have been computed for both Nand H atoms, using the UHF wavefunction as well as carrying out a single annihilation and a complete projection. The UHF method provides the most satisfactory results, namely, 11.2 G for Nand -8.8 G for H. These are to be compared with the experimental results (absolute values) of 11.9 G for both atoms. The importance of averaging over vibrational states is indicated.

The accurate molecular wave functions for H20 and NH3 recently calculated by Ellison and Shull, and by Higuchi are transformed to equivalent orbitals and then used to calculate lone-pair moments and bond moments. These latter have both longitudinal and transverse components, of which the latter are much the more important in determining 3p/3a, where cc is the valence angle. The significance of this in discussions of bond properties is stressed.