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.

The molecular structure of pyruvic acid was investigated by matrix isolation FTIR spectroscopy, density functional theory (DFT), and ab initio calculations performed at the RHF, MP2, MP4(SDQ), and CCSD(T) levels of theory with the aug-cc-pVDZ basis set. In these calculations, the geometries of the three lowest energy conformers of pyruvic acid were fully optimized at the DFT/B3LYP/aug-cc-pVDZ and MP2/aug-ccpVDZ levels. Additionally, the relative energies of the conformers were calculated at the MP4, CCSD, and CCSD(T) levels. Harmonic frequencies and IR intensities were then calculated for these three conformers and were used to account for the zero point vibrational energy corrections and to assist the assignment of the observed bands to the different forms. We found that two conformers are present in the Ar matrix, and both forms exhibit a planar framework with the carbonyl bonds in a trans arrangement but differ in the orientation of the hydroxyl hydrogen. By varying the temperature of the pyruvic acid vapor prior to matrix deposition we were able to separate the bands due to the two conformers and measure their enthalpy difference. The spectral signature of the second pyruvic acid conformer has been identified for the first time. Experimental enthalpy difference between the two most stable conformers was found to be 8.7((15%) kJ mol -1 , which is in good agreement with the theoretical result.

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.

A comparative conformational study of two related systems, methyl cyanoacetate (MCA) and cyanoacetic acid (CAA), is presented. Ab initio calculations predicted that both systems have two nearly isoenergetic conformers separated by similar low energy barriers (about 3 kJ mol À1 ). In xenon matrixes deposited at temperatures above 40 K for MCA and above 20 K for CAA only one conformer was observed for each of the two systems. However, below those temperatures two MCA and two CAA conformers were trapped into the matrixes. Conformational cooling was found responsible for this behavior. Factors contributing to this effect are discussed.

The conformational behavior and infrared spectrum of L-phenylalanine were studied by matrix-isolation infrared spectroscopy and DFT [B3LYP/6-311++G(d,p)] calculations. The fourteen most stable structures were predicted to differ in energy by less than 10 kJ mol -1 , eight of them with abundances higher than 5% at the temperature of evaporation of the compound (423 K). Experimental results suggest that six conformers contribute to the spectrum of the isolated compound, whereas two conformers (IIb 3 and IIIb 3 ) relax in matrix to a more stable form (IIb 2 ) due to low energy barriers for conformational isomerization (conformational cooling). The two lowest-energy conformers (Ib 1 , Ia) differ only in the arrangement of the amino acid group relative to the phenyl ring; they exhibit a relatively strong stabilizing intramolecular hydrogen bond of the O-H‚‚‚N type and the carboxylic group in the trans configuration (OdC-O-H dihedral angle ca. 180°). Type II conformers have a weaker H-bond of the N-H‚‚‚OdC type, but they bear the more favorable cis arrangement of the carboxylic group. Being considerably more flexible, type II conformers are stabilized by entropy and the relative abundances of two conformers of this type (IIb 2 and IIc 1 ) are shown to significantly increase with temperature due to entropic stabilization. At 423 K, these conformers are found to be the first and third most abundant species present in the conformational equilibrium, with relative populations of ca. 15% each, whereas their populations could be expected to be only ca. 5% if entropy effects were not taken into consideration. Indeed, phenylalanine can be considered a notable example of a molecule where entropy plays an essential role in determining the relative abundance of the possible low-energy conformational states and then, the thermodynamics of the compound, even at moderate temperatures. Upon UV irradiation (λ > 235 nm) of the matrix-isolated compound, unimolecular photodecomposition of phenylalanine is observed with production of CO 2 and phenethylamine.

Structural transformations were induced in conformers of glycolic acid by selective excitation with monochromatic tunable near-infrared laser light. For the compound isolated in Ar matrixes, near-IR excitation led to generation of two higher-energy conformers (GAC; AAT) differing from the most stable SSC form by 180° rotation around the C-C bond. A detailed investigation of this transformation revealed that one conformer (GAC) is produced directly from the near-IR-excited most stable conformer. The other higher-energy conformer (AAT) was effectively generated only upon excitation of the primary photoproduct (GAC) with another near-IR photon. Once these higher-energy conformers of glycolic acid were generated in an Ar matrix, they could be subsequently transformed into one another upon selective near-IR excitations. Interestingly, no repopulation of the initial most stable SSC conformer occurred upon near-IR excitation of the higher-energy forms of the compound isolated in solid Ar. A dramatically different picture of near-IR-induced conformational transformations was observed for glycolic acid isolated in N2 matrixes. In this case, upon near-IR excitation, the most stable SSC form converted solely into a new conformer (SST), where the acid OH group is rotated by 180°. This conformational transformation was found to be photoreversible. Moreover, SST conformer, photoproduced in the N2 matrix, spontaneously converted to the most stable SSC form of glycolic acid, when the matrix was kept at cryogenic temperature and in the dark.