Elucidating the intermolecular hydrogen bonding interaction of proline with amides—quantum chemical calculations

The hydrogen-bonded complexes formed between proline and amides have been investigated completely by the use of computational methods such as Atoms In Molecules (AIM), Natural Bond Orbitals (NBO), and Molecular Electrostatic Surface Potential (MESP). All computations are based on structural models previously generated at wB97XD/aug-cc-pVDZ level. The amide molecule interacts with proline, in a pair of amide C=O···H-O Pro and amide N-H···O=C Pro hydrogen bonds (HBs), forming eight-membered ring structure in the most stable complexes. The substitution of N-methyl in formamide strengthens the amide C=O···H-X (X = O, N of proline) HBs, whereas weakens the amide N-H···O/N Pro and amide C-H···O/N Pro HB interactions. The tendency of N-H of amide to act as HB donor is found to be far better than the N-H of amino group of proline. On the basis of vibrational frequency calculations, the red and blue shifting of proton donor fragment X-H (X = O, N, C) has also been analyzed.