Conformers of Nonionized Proline. Matrix-Isolation Infrared and Post-Hartree−Fock ab Initio Study

Abstract

Matrix-isolation IR spectroscopy and ab initio calculations performed at the DFT, MP2, MP4, and CCSD(T) levels of theory were employed to investigate the conformational topology of the nonionized amino acid proline and its deuterated derivative, N,O-dideuteroproline (proline-d2). In the calculations, equilibrium structures of 15 low-energy proline conformers were obtained using the DFT/B3LYP/aug-cc-pVDZ and MP2/aug-cc-pVDZ methods. The harmonic frequencies and IR intensities of the conformers were calculated for the DFT geometries, and these data was used to account for the zero-point vibration energy correction and to assist the analysis of the experimental matrix-isolation IR spectra. Two proline conformers were found to be present in the Ar matrix. They are the lowest energy conformer with a N···HO H bond (conformer IIa) and the second conformer with a NH···OC H bond (conformer Ia). We found that the DFT/B3LYP and MP2 methods are not capable of predicting the relative energies of the proline conformers with a quantitative accuracy. Both methods provide the energy difference between the IIa and Ia conformers of 7−8 kJ mol-1, thus suggesting that only conformer IIa should be present in the matrix. However, strong bands due to the two conformers are observed in the experimental spectra and their intensities indicate approximately equal presence of the two systems in the matrix. To explain the descrepancy between the MP2 and DFT results and the experiment, calculations were performed at the CCSD(T) level of theory. The relative energy difference obtained at this level of 3.9 kJ mol-1 better agrees with the experiment because it is less than kT at the matrix preparation temperature. The observed low-frequency shift of the OH stretching vibration due to the intramolecular N···HO H bond in the proline conformer IIa of 534 cm-1is much larger than the ones found for other amino acids (340−360 cm-1). It demonstrates that the intramolecular H bonding in proline is much stronger then in other amino acids.