Conformers of nonionized proline. Matrix-isolation infrared and post-Hartree-Fock ab initio study

Matrix-isolation IR spectroscopy and ab initio calculations performed at th e DFT, MP2, MP4, and CCSD(T) levels of theory were employed to investigate the confomational topology of the nonionized amino acid proline and its deu terated derivative, N,O-dideuteroproline (proline-d(2)). In the calculation s, 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 DF T geometries, and these data was used to account for the zero-point vibrati on energy correction and to assist the analysis of the experimental matrix- isolation IR spectra. Two proline conformers were found to be present in th e Ar matrix. They are the lowest energy conformer with a (NH)-H-...-O H bon d (conformer IIa) and the second conformer with a (NHO)-O-...=C H bond (con former Ia). We found that the DFF/B3LYP and MP2 methods are not capable of predicting the relative energies of the proline conformers with a quantitat ive accuracy. Both methods provide the energy difference between the IIa an d la conformers of 7-8 kJ mol(-1), thus suggesting that only conformer Ha s hould be present in the matrix. However, strong bands due to the two confor mers are observed in the experimental spectra and their intensities indicat e approximately equal presence of the two systems in the matrix. To explain the descrepancy between the MP2 and DFT results and the experiment, calcul ations were performed at the CCSD(T) level of theory. The relative energy d ifference obtained at this level of 3.9 kJ mol(-1) better agrees with the e xperiment 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-. H-..-O H bond in the proline conformer IIa of 534 cm(-1 ) is 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 stro nger then in other amino acids.