AM1 study of hydrogen bonded complexes of water

Abstract

The failure of AM1 to reproduce the alleged experimental geometry of the water dimer is reported in this paper. A second-order transition state, instead of a minimum, is found at the experimental geometry with a linear hydrogen bond. It collapses to a first-order transition state with a bifurcated hydrogen bond (similar to the ab initio one) from which the true AM1 minimum is obtainable. This structure has a stabilization energy of −5.47 kcal mol −1 and shows three hydrogen bond interactions. This failure of the method is attributed to an exceedingly weak bending potential of the hydrogen bond within AM1. The conclusions reached in the study of the dimers are further tested by the calculation of a water trimer and a water tetramer and the linear and bifurcated hydrogen bonded complexes between H 2O and NH 4 +. It is concluded that although it fails in the case of a dimer, AM1 may be useful in the treatment of larger clusters of hydrogen bonded molecules. All calculations are contrasted with the results of ab initio ones.