Determination of the molecular structure of gaseous proline by electron diffraction, supported by microwave and quantum chemical data

The molecular structures of the four most stable conformers of neutral proline (pyrrolidine-2-carbonic acid) were investigated by means of gas-phase electron diffraction. The results were used in conjunction with the experimental rotational constants in combined structural analysis. Small differences between similar structural parameters were constrained on the basis of theoretical calculations at the M062X/aug-cc-pVTZ level of theory. Because intramolecular hydrogen bonds play an important role in the molecular structure and conformational stability of amino acids, the hydrogen bond descriptors calculated by natural bond orbital, atoms in molecules, interacting quantum atoms and noncovalent interactions methods were included.