A spectroscopically effective molecular mechanics model for the intermolecular interactions of the hydrogen-bonded N-methylacetamide dimer

An MP2/6-31 + G * calculation of the N-methylacetamide dimer shows that it has two minimum energy structures, both hydrogen bonded with peptide planes roughly perpendicular to each other. A complete molecular mechanics optimi zation of the dimer has been done, using a model for the intermolecular int eractions consisting of charges, atomic dipoles, and van der Waals interact ions and the methodology of our spectroscopically determined force field fo r the intramolecular interactions. The two structures are satisfactorily re produced, as are their interaction energies, their dipole moments, and, fro m the point of view of our goal of a spectroscopically accurate force field , their six intermolecular normal mode frequencies. (C) 1999 Elsevier Scien ce B.V. All rights reserved.