MOLECULAR-ORBITAL STUDIES OF MOLECULAR EXCIPLEXES .1. AM1 AND PM3 CALCULATIONS OF THE AMMONIA OXYGEN COMPLEX AND ITS SOLVATION BY WATER

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 (TN), whereas the polar excited state (TP) corresponding to NH3+·O 2-· 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 TP and ground-state (T0) triplet surfaces intersect at an N-O distance of 1.95 Å for n = 5, with TP 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+·nNH 3, and HO-·nH2O (n = 1-4) with known experimental values.