This thesis aims to provide theoretical information by using ab initio calculations as a\ud bridge between theory and experiment. This thesis presents calculations of potential\ud energy curves for the diatomic molecular ions NF+, PCl+, AsCl+ and BBr+ and\ud preliminary potential energy surfaces for HC + NO.\ud For the diatomic molecular ions, this work presents the low-lying electronic states of\ud NF+, PCl+, AsCl+ and BBr+ correlating with the lowest dissociation asymptotes, namely\ud N+(3p) + F(2p), P+(3P) + Cl(2p),\ud As+(3p) + Cl(2p),\ud B+(1S) + Br(2P) and B(2p) + Br+(3p).\ud In this work CASSCF and CI calculations have been performed with the correlation-consistent\ud valence quadruple-zeta (VQZ) basis sets for NF+ and PCl+, averaged atomic\ud natural orbital (ANO) basis sets for AsCl+ and BBr+, and the correlation-consistent\ud valence double-zeta (VDZ) basis sets for HC + NO. From the calculations of\ud potential energy curves four bound states, namely X2Π, A2Π, 14Σ and 14Π, are found\ud for NF+, PCl+ and AsCl+, and there are many more bound states, namely 12Σ+, 12Π, 22Π, 22Σ+, 12Δ, 12Σ-, 4Σ+, 4Δ, and 14Σ-, for BBr+. For all the bound states\ud spectroscopic constants have been calculated. For NF+, PCl+ and AsCl+ our results\ud are in good agreement with the available spectroscopic data. However, for BBr+ the\ud theoretical values are in disagreement with experimental data. It is suggested that a\ud reinvestigation of the experimental spectrum of this species would be worthwhile.\ud Calculations of transition moments for bound-bound transitions have been carried out.\ud For HC + NO the linear structures for 1Σ+, 1Σ-, 3Σ+ and 3Σ- states and the bent structure\ud for 1A' state have been calculated. These calculations were intended to obtain\ud potential energy surfaces for the reaction of HC with NO. However, due to the lack\ud of time left, only preliminary calculations are completed. Reaction without an\ud activation barrier may occur for a non-collinear approach of HC to NO
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