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.

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The possible uses of the ab initio spin-unrestricted Hartree–Fock (UHF) method for computing potential energy surfaces have been investigated, expanding the molecular orbitals from a basis set of Gaussian functions. In order to approximate spin-pure states, an exact spin projection operator, as well as an operator annihilating only the state of next higher multiplicity, has been used. Preliminary studies undertaken on the H2 and LiH molecules indicate that among the various methods employed, the unprojected UHF approach gives the most satisfactory shape for potential curves. The HNO molecule was then investigated, and five electronic states of the system were considered, namely A1,A″3,A″1 symmetry. The ground state is A′1. Its UHF energy was minimized by varying the three geometrical parameters independently. A value of 〈Ŝ2〉 of 0.32 was obtained at the experimental geometry. This system with an even number of electrons thus has the unexpected property of being unstable with respect to a splitting of the α and β orbitals. Application of the spin projection operator lowers the energy by 0.0265 a.u. The UHF wavefunctions give an excitation energy A″1←A′1 of 3860 cm−1. When pure spin states are produced, it becomes 11 660 cm−1 (experimental value 13 150 cm−1). The A″3 state practically coincides with the ground state in the UHF approximation, but after projection an excitation energy of 5850 cm−1 is obtained. A slight maximum is observed in the UHF potential energy curve of the A″ states as a function of the N–H distance, which we associate with the predissociation observed in the spectrum of the compound. Based upon projection at the experimental minimum for the ground state, and at infinite N–H separation, a set of adjusted curves has been generated for the system H + NO. The features of these curves are discussed in relation with the mechanism of the recombination reaction between hydrogen atoms and nitric oxide.