An ab initio study of the potential energy surface of methylamine dimer

The potential energy surface of methylamine dimer was studied on the basis of ab initio calculations performed by using the HF, DFT/B3LYP and MP2 methods in conjunction with the aug-cc-pvdz basis set. An overall six stationary points were located two of which were found to be minima and the other four transition structures between them. The most stable structure exhibits an N-(HN)-N-... hydrogen bond with an interaction energy of -15.1 kJ/mol as calculated with the MP2 method. Both minima possess a dipole moment of ca. 1.9 D (MP2), which is 28% greater than the combined molecular dipoles. Two of the transition structures identified correspond to the conversion between equivalent minima, where the molecules simply exchange their roles of acceptor and donor in the hydrogen bond. The conversion takes place by rotation about the C-N bonds, with an energy barrier of 1-2 kJ/mol. The other transition structures connect two different minima and the conversion takes place by alteration of the (CNNC)-N-... torsion, which entails overcoming a barrier of 0.9 kJ/mol. The potential surface is very flat near the minima and is subject to the effects of the basis set superposition error (BSSE). Using the counterpoise method to correct for BSSE results in an even flatter surface and in the disappearance of the less stable minimum, so the corrected surface for methylamine dimer exhibits a single minimum. (C) 2002 Elsevier Science B.V. All rights reserved.