Complexes of ammonia with propane and cyclopropane: electrostatic guidelines for ab initio treatment

A model based on the molecular electrostatic potential (MESP) is employed f or the investigation of structures and energies of complexes of ammonia wit h propane and cyclopropane. The electrostatic model geometries are employed as starting points for an ab initio investigation at the self-consistent h eld and second-order Moller-Plesset (MP2) levels. The most stable structure s of C3H6..NH3 and C3H8..NH3 complexes have the interaction energies of 10. 07 kJ/mol and 8.15 kJ/mol, respectively, at the MP2/6-31G(d,p) level. The e nergy rank order of the structures is not altered with the use of the 6-31+G(d,p) basis set, and the basis set superposition error has little effect. The interaction energy decomposition analysis shows that the electrostatic component is dominant over the other ones. MESP topography thus seems to o ffer valuable hints for predicting the structures of weakly bonded complexe s.