The Structure of N-Acetylprolinamide in the Solid State and in the Gasphase. A Remarkable Lattice Effect as the Result of a Competition Between Inter-and Intramolecular Hydrogen Bonds of Different Strengths

Abstract

Abstract In the solid state, the conformation of N-actetylprolinamide is stabilized by two intermolecular O···H bridges and one intramolecular N···H hydrogen bond. According to quantum chemical ab initio calculations with the 6-31+G* basis set at the one-determinant level, the intramolecular N ···H bond is not strong enough to maintain the solid-state molecular conformation the gas phase. The conformational changes predominantly consist in a rotation of the amide group about its C-C bond to the proline ring, resulting in a c/s-like conformation which is stabilized by a presumably stronger intramolecular O···H bond between one hydrogen atom of the NH2 group and the carbonyl oxygen of the acetyl subsituent bonded to the nitrogen atom of the five-membered ring. These confirmational changes cause a reduction of the molecular dipole moment by about 50%, indicating that the conformation in solution might be strongly solvent dependent. While both the MINDO/3 and the MNDO method in their standard parametrizations fail to reproduce the ab initio results, the lattice effect is reproduced at least qualitatively with the PM3 as well as with the AM1 method.