Conformational Isomerizations by Rotation around C–C or C–N Bonds: A Comparative Study on Matrix-Isolated Glycolamide and N-Hydroxyurea Excited with Near-IR Laser Light

Abstract

Conformers and near-IR-induced conformational transformations were studied for monomers of glycolamide isolated in low-temperature matrixes. Two conformational isomers of the compound, Tt and Cc, were trapped from the gas phase into solid Ar matrixes. Selective near-IR excitation of glycolamide molecules adopting the Tt form led to the Tt → Cc conformational conversion. Analogously, selective near-IR excitation of Cc conformers resulted in the Cc → Tt transformation. Monochromatic near-IR light, generated by frequency-tunable laser sources, was used for irradiation of matrix-isolated monomers. Near-IR-induced Tt → Cc and Cc → Tt conformational transformations occurred upon excitation of 2νOH, 2ν a NH 2 , and 2ν s NH 2 overtones, as well as upon excitation of ν a NH 2 + ν s NH 2 combination modes. In spite of the structural similarity of glycolamide and N-hydroxyurea, no conformational conversions were observed for monomers of the latter compound isolated in Ar matrixes and excited with near-IR light. The comparison of the effects of near-IR excitations of glycolamide and N-hydroxyurea demonstrates that transformations involving rotation of molecular fragments around a single C−C bond occur much easier than transformations involving rotation of the fragments around a C−N bond. The efficiency of the latter conversions is extremely low.

Conformational isomerizations triggered by vibrational excitation of second stretching overtones

“…S5, ESI †). The identification of Tt and Cc conformers of GAm is unequivocal, not only due to the excellent agreement between the experimental and computed IR spectra but also based on the comparison with previous data for these two conformers isolated in argon matrices 31 (Tables S6 and S7, ESI †). Interestingly, we found that the Tt -Cc conformational isomerization induced upon excitation at the second OH stretching overtone is site-selective.…”

“…They are simple molecules where vibrational excitations to the first OH stretching overtone produce stable high-energy conformers under cryogenic conditions. [29][30][31] High-energy conformers of simple carboxylic acids such as formic and acetic acids are known to decay fast (few minutes or less) by tunneling. 1,7 Moreover, HA, GAc and GAm have strong infrared absorptions in the fundamental OH stretching and welldefined first OH stretching overtone absorptions, measured under matrix-isolation conditions, which suggests the possibility to detect experimental bands due to their second OH stretching overtones.…”

“…GAm is known to exist in two different conformations (Cc and Tt; Scheme 2) when isolated in cryogenic matrices, with both forms contributing to the mid-IR spectrum. 31 The most stable Cc conformation is observed with slightly larger population in the matrix and is characterized by a relatively broad n(OH) band at 3481 cm À1 , whose low frequency results from an intramolecular interaction with the carbonyl group. This feature makes it very difficult to identify the corresponding band due to the second OH stretching overtone [3n(OH)].…”

“…The abbreviations in bold correspond to the identification of the respective conformers according to the convention adopted in previous studies. [29][30][31] Numbers in parentheses show the relative energies (kJ mol À1 , zero-point vibrational energy [ZPVE] included) calculated at the B3LYP/SNSD level of theory. of Tt is due to the existence of two different matrix environments, arguably resulting from two different argon cavities or sites surrounding the molecules.…”