Conformers and Properties of Proline

A total of 15 conformers of Proline were found and their geometrical structures, relative energies, harmonic frequencies, dipole moments, rotational constants, polarizabilities, and the difference in energies between HOMO and LUMO were calculated at the X3LYP/6-311++G(d, p) and the PBE1PBE/6-311++G(d, p) levels. Accurate relative energies were obtained at the X3LYP/6-311++G(3df, 3pd)//X3LYP/6-311++G(d, p) level of theory, which were in good agreement with high-level ab initio methods using large basis sets. The results of X3LYP are superior to those of PBE1PBE method. The characteristic H-bonding types for all the conformers were classified. The four most stable conformers had intramolecular H-bondings of N…H-O and N-H…O=C, and the dipole moments of conformers 1 and 2 were the largest and the polarizabilities were the smallest. An additional H-bonding C-H…O=C was found. In combination with the principles of statistical mechanics, conformational distributions at room temperature were computed.