The proton position in amine-HX (X  Br, I) complexes

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Proton transfer potential energy surfaces are computed by ab initio methods for complexes of HBr and HI with NH 3 and mono, di, and trimethylamine. The fundamental natures of the SCF and MP n surfaces are distinct in that two separate minima are present in the SCF surfaces, corresponding to a neutral and ion pair, whereas only one minimum is encountered in most of the correlated surfaces. The single minimum in the MP2 surfaces of most of the complexes represents an ion pair. The first exception is BrH⋯NH 3 which exists as a neutral pair. The bridging proton in BrH⋯NH 2CH 3 is located approximately midway between the two molecules in a “shared” arrangement. The last exception is IH⋯NH 3, the surface of which may be best characterized as a long shallow valley for proton motion. The binding energies of these complexes rise as the amine becomes more basic due to progressive methylation.