Characterization of conformers of non-ionized proline on the basis of topological and NBO analyses: Can nitrogen be a donor of hydrogen bond?

A detailed population analysis of 10 most stable conformers of neutral proline was undertaken by the natural bond orbitals (NBO) and the atoms in molecules (AIM) methods. The optimized geometries (at MP2/6-311++G(d,p) level) were employed to perform the NBO analysis and also to obtain the suitable wave function files for the AIM analysis. With the exception of OH⋯NH and CH⋯OH hydrogen bond critical points, corresponding to four conformers, the BCPs can be observed just for those which located at the interatomic paths that are defined by the covalent bonds. The charge transfer energy of n(N)→σO-H∗ interactions are 18.31 and 15.63 kcal/mol which are related to conformers that exhibit the OH⋯NH hydrogen bonds. Similar interactions higher than 0.5 kcal/mol threshold limit do not observe for other conformers. Thus, the NBO and AIM analyses do not confirm the presence of N-H⋯OC and N-H⋯O-H hydrogen bonds in the conformers of proline. On the other hand, improper hydrogen bonds (C-H⋯O-H) reveal in two conformers.