Intramolecular interactions in 2-aminoethanol and 3-aminopropanol.

Gas-phase vibrational spectra of 2-aminoethanol and 3-aminopropanol were recorded up to the third OH-stretching overtone using Fourier transform infrared spectroscopy, cavity ringdown spectroscopy, and intracavity laser photoacoustic spectroscopy. The experimental investigation was supplemented by local mode calculations, and the intramolecular interactions were investigated using atoms in molecules (AIM) and noncovalent interactions (NCI) theories. All calculations were performed at the CCSD(T)-F12a/VDZ-F12 level of theory. For both compounds the most abundant conformer has a structure that allows for hydrogen bond interaction from the OH group to the N atom of the amino group (OH-N). The spectra show signals from both hydrogen bonded and free OH stretches, implying the presence of several conformers. We observe hydrogen-bond-like interactions in both compounds. The red shift of the bonded OH-stretching frequency and intensity enhancement of the fundamental transition suggest that the hydrogen bond interaction is more pronounced in 3-aminopropanol. AIM analysis supports the presence of a hydrogen bond in 3-aminopropanol but not in 2-aminoethanol, whereas NCI analysis shows hydrogen bonding in both compounds with the stronger interaction found in 3-aminopropanol.