The IR spectra of nonionized glycine and its deuterated derivatives isolated in the low-temperature argon matrices have been measured, and for the first time the infrared spectral characteristics of the three most stable conformers have been determined and assigned. Correlated level ab initio and density functional theory (DFT) calculations of IR frequencies and intensities with extended basis sets were performed and their results were employed to separate the bands of the glycine conformers in the experimental spectra and to assist the assignment. The intramolecular interconversion, conformer III w conformer I, which is observed in the matrices at temperatures higher than 13 K, was found to cause a significant decrease of the band intensities of conformer III in the spectra. This phenomenon was used to distinguish the vibrational bands of this conformer from the bands of the other conformers. The reliability of the Møller-Plesset second-order perturbation theory (MP2) method and the DFT method with the three-parameter density functional, Becke3LYP, in the prediction of the IR spectra of the nonionized glycine conformers was examined. We found that the DFT/Becke3LYP method with aug-cc-pVDZ basis set yields vibrational frequencies of the glycine conformers very similar to the MP2 results. Both DFT and MP2 results are in the excellent agreement with the experimental data.

Two conformers of the nonionized α-alanine and its isotopomer N,N,O-d 3-alanine have been observed in low-temperature Ar matrixes. Their infrared spectra have been analyzed and assigned using DFT/B3LYP/aug-cc-pVDZ and MP2/aug-cc-pVDZ geometry and frequency theoretical calculations. Two different intramolecular H-bonds, bifurcated NH2···OC and N···H−O, were found in the observed α-alanine conformers, I and IIa. We found that the DFT/B3LYP/aug-cc-pVDZ method yields vibrational frequencies of the α-alanine conformers in excellent agreement with the experimental data.

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-d 2 ). 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‚‚‚HsO H bond (conformer IIa) and the second conformer with a NH‚‚‚OdC 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‚ ‚‚HsO 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 stronger then in other amino acids.

We present results of the first experimental observation of the nonionized natural amino acid valine. The study has employed the matrix-isolation IR spectroscopy and the density functional theory (DFT) and ab initio calculations. In the calculations geometries of nine low-energy valine conformers were optimized using the DFT method with the B3LYP parametrization and the 6-31++G** basis set. Additionally, the relative energies of the conformers were calculated at the MP2/6-31++G** level. The harmonic frequencies and IR intensities were calculated for all the minima found. These data were used to separate and assign the bands of the valine conformers in the experimental spectra. We found that two valine conformers are present in the Ar matrix: one with a bifurcated NH 2 ‚‚‚OdC H-bond (conformer Ia) and one with a N‚‚‚H-O H-bond (conformer IIa). A trace amount of a third valine conformer with NH 2 ‚‚‚O-C H-bond (conformer IIIb) was also detected. The conformational composition of the matrix-deposited nonionized valine was determined on the basis of observed and predicted IR intensities of the bands of different conformers. The composition is ∼94% of conformer Ia, ∼5% of conformer IIa, and less than 2% of conformer IIIb. The presence of three valine conformers in the Ar matrix results in broadening and/or in multiplex structures of some bands in the valine IR spectrum. Common features in the IR spectra of some nonionized natural aliphatic amino acids are discussed.

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