A QUANTITATIVE MOLECULAR-ORBITAL STUDY OF THE STRUCTURES AND VIBRATIONAL-SPECTRA OF THE HYDROGEN-BONDED COMPLEXES H2O.NH3, H2CO.NH3 AND (H2O)N, N = 2-4

Comparison of the calculated structures, interaction enthalpies, and vibrational spectra of water dimer, trimer, and tetramer, and the complexes formed between water/ammonia and NH3/H2CO at the OPLS/A molecular mechanics, semi-empirical AM1 and PM3 SCF-MO methods and ab initio theory at the SCF and MP2 levels shows the best agreement to occur between the correlated ab initio and the PM3 SCF-MO methods. The AM1 method generally predicts bifurcated hydrogen bonds, whilst the molecular mechanics force field does not reproduce the non-planar character of the water trimer and tetramer, nor the trend in the hydrogen-bond lengths as the size of the water cluster increases. Several specific errors in the PM3 method are identified, in particular concerning the interaction between N-H σ bonds and other lone pairs, which results in a calculated geometry and interaction energy for NH 3/H2CO which is significantly different from the ab initio values.