Conformational equilibrium of aminoethanol in the gas phase and in solution driven by intra- and intermolecular interactions and solvent polarity

Abstract

The study on the conformational equilibrium of small flexible molecules under different environments is essential to understand the folding motives in complex molecules. Aminoethanol (NH2CH2CH2OH) is a typical flexible molecule with rich conformational isomerism, but its conformational diversity in the gas phase or solutions is not yet fully understood. Owing to well-separated peaks and the absence of intermolecular interactions, gas-phase Raman spectroscopy is a valuable tool for characterizing molecular conformations. Using temperature-dependent photoacoustic Raman spectroscopy (PARS), the gas-phase Raman spectra of aminoethanol were measured in the O–H and C–H stretching regions. The analysis of the observed gas-phase Raman spectra, together with theoretical calculations, allows us to identify one O–H⋯N intramolecular H-bonded conformer (g′Gg′) and four free-OH conformers (gGt and tGt, gGg and tGg). Furthermore, using the assigned gas-phase conformers, the conformational changes of aminoethanol were investigated in the nonpolar CCl4 and polar water solutions, respectively. It was shown that intramolecular H-bonds are unlikely to be a significant factor in the conformational equilibrium of liquid aminoethanol, whereas the gauche effect is clearly very important. In addition, the concentration-dependent Raman spectra in aqueous solution revealed that aminoethanol molecules prefer the tGt conformation upon dilution with water. This can be ascribed to the fact that polar water molecules push aminoethanol conformers toward a higher dipole moment rather than a lower one, which is in good agreement with recent molecular dynamic simulations. Our findings deepen the understanding of the effect of ruling conformational stability and shed light on molecular switches that are activated in different solvent environments.