Ability of the PM3 quantum‐mechanical method to model intermolecular hydrogen bonding between neutral molecules

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Abstract
The PM3 semiempirical quantum‐mechanical method was found to systematically describe intermolecular hydrogen bonding in small polar molecules. PM3 shows charge transfer from the donor to acceptor molecules on the order of 0.02–0.06 units of charge when strong hydrogen bonds are formed. The PM3 method is predictive; calculated hydrogen bond energies with an absolute magnitude greater than 2 kcal mol−1 suggest that the global minimum is a hydrogen bonded complex; absolute energies less than 2 kcal mol−1 imply that other van der Waals complexes are more stable. The geometries of the PM3 hydrogen bonded complexes agree with high‐resolution spectroscopic observations, gas electron diffraction data, and high‐level ab initio calculations. The main limitations in the PM3 method are the underestimation of hydrogen bond lengths by 0.1–0.2 Å for some systems and the underestimation of reliable experimental hydrogen bond energies by approximately 1–2 kcal mol−1. The PM3 method predicts that ammonia is a good hydrogen bond acceptor and a poor hydrogen donor when interacting with neutral molecules. Electronegativity differences between F, N, and O predict that donor strength follows the order F > O > N and acceptor strength follows the order N > O > F. In the calculations presented in this article, the PM3 method mirrors these electronegativity differences, predicting the F‐H‐‐‐N bond to be the strongest and the N‐H‐‐‐F bond the weakest. It appears that the PM3 Hamiltonian is able to model hydrogen bonding because of the reduction of two‐center repulsive forces brought about by the parameterization of the Gaussian core–core interactions. The ability of the PM3 method to model intermolecular hydrogen bonding means reasonably accurate quantum‐mechanical calculations can be applied to small biologic systems. © 1993 John Wiley & Sons, Inc.