Molecular orbital studies of molecular exciplexes. Part 1: AM1 and PM3 calculations of the ammonia–oxygen complex and its solvation by water

Abstract

Ground- and excited-state calculations of the triplet O₂·NH₃·nH₂O complex as a model for solution photo-oxygenation of amines by triplet oxygen are reported at the AM1 and PM3 SCF–MO levels. For n < 3, the first excited AM1 triplet state corresponds to local excitation on the ammonia (T_N), whereas the polar excited state (T_P) corresponding to NH₃⁺·O₂^–· becomes the lowest excited triplet state when n≥ 3. In contrast, the lowest excited PM3 triplet state is ionic even for n= 0, and the T_P and ground-state (T₀) triplet surfaces intersect at an N–O distance of 1.95 Å for n= 5, with T_P becoming a local minimum on ground triplet state surface at shorter N–O distances. The reliability of these models are discussed in terms of known errors and properties of the AM1 and PM3 methods, and by a comparison of the solvation enthalpies and structures of systems such as NH₄⁺·nH₂O, NH₄⁺·nNH₃, and HO^–·nH₂O (n= 1–4) with known experimental values.