Determination of conformational composition of proline in the gas phase

Extensive exploration of the conformational landscapes of neutral and terminally blocked prolines in the gas phase: A density functional theory study

“…Similarly, ν exp ≈ 1110 cm −1 cannot be properly assigned to δ(OH) in the study by Stepanian et al 34 and in that by Ten and Shcherbakova. 37 It is of practical interest to emphasize that conformations with similar H-bonds also produce similar IR spectra, as is well-documented in the literatures, 52 and is also seen in Figure 3 for conformers 5 and 7. That is, as far as the IR spectrum is concerned, the ensemble average may be approximated by a mix of a few conformers with a proper coverage of characteristic H-bonds.…”

“…Similarly, ν exp ≈ 1110 cm −1 cannot be properly assigned to δ(OH) in the study by Stepanian et al 34 and in that by Ten and Shcherbakova. 37…”

“…The vibrational frequencies of the proline conformations are calculated at the B97D/6-311++G** level. Based on the scaling factors suggested for B3LYP/aug-cc-pvdz 37 and through benchmarking tests, the B97D/6-311++G** frequencies are scaled with a scaling factor of 0.97 for the OH, NH, and CH stretching modes, and with a scaling factor of 1.01 for all other vibrations. The infrared spectrum for the conformational ensemble is calculated from the equilibrium compositions and smoothed with a 5-point Savitzky-Golay smoothing function.…”

“…However, the three conformers do not include the global minimum of the conformation ensemble of Czinki and Császár. 36 Not surprisingly, the comparison between the computational result of Ten and Shcherbakova 37 and the experimental spectrum is of low quality.…”

“…However, their search did not consider the variations of the dihedral angles (DAs) for the main chain C α –C and C–OH bonds, and some energy minima may be easily missed in their search. Recently, Ten and Shcherbakova 37 performed B3LYP/6-311++G (d, p) calculations of the IR spectra of three proline conformers and attempted to attribute the experimental spectrum to two of the three proline conformers. However, the three conformers do not include the global minimum of the conformation ensemble of Czinki and Császár.…”