CHARGE-TRANSFER COMPLEXES - NH3-O3, NH3-SO2, AND N(CH3)3-SO2
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The donor-acceptor complex between trimethylamine and sulfur dioxide is the strongest (in terms of its dissociation energy to N(CH3)3 plus SO2) such complex yet to be studied experimentally in the gas phase. Ab initio self-consistent-field theory has been applied to this and two related complexes, NH3O3 and NH3SO2. Minimum basis sets were used for all three complexes, while for NH3SO2 two considerably larger sets (double and double plus sulfur d functions) were employed. The equilibrium structure of these complexes was predicted by an investigation of many points on the respective potential energy surface. To a surprising degree, the O3 or SO2 molecule is found to lie in a plane nearly perpendicular to the amine C3c axis. Further, the central atom in O3 and SO2 is predicted to lie only slightly off the amine C3c axis. With these guidelines, the prediction of further structures of this type essentially reduces a one-dimensional search for the central atom-N distance. Using a minimum basis set, the three predicted binding energies are 2.24, 5.00, and 4.06 kcal/mol. Using the double basis set the NH3-SO2 dissociation energy is increased to 10.40 kcal/mol. When a set of sulfur d functions is added to the latter set, the binding energy becomes 9.30 kcal/mol in the absence of geometry reoptimization. 1976, American Chemical Society. All rights reserved.