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.

The qualifying features of a hydrogen bond can be contentious, particularly where the hydrogen bond is due to a constrained intramolecular interaction. Indeed there is disagreement within the literature whether it is even possible for an intramolecular hydrogen bond to form between functional groups on adjacent carbon atoms. This work considers the nature of the intramolecular interaction between the OH (donor) and NH2 (acceptor) groups of 2-aminoethanol, with varying substitution at the OH carbon. Gas-phase vibrational spectra of 1-amino-2-methyl-2-propanol (BMAE) and 1-amino-2,2-bis(trifluoromethyl)-2-ethanol (BFMAE) were recorded using Fourier transform infrared spectroscopy and compared to literature spectra of 2-aminoethanol (AE). Based on the experimental OH-stretching frequencies, the strength of the intramolecular hydrogen bond appears to increase from AE < BMAE ≪ BFMAE. Non-covalent interaction analysis shows evidence of an intramolecular hydrogen bond in all three molecules, with the order of the strength of interaction matching that of experiment. The experimental OH-stretching vibrational frequencies were found to correlate well with the calculated kinetic energy density, suggesting that this approach can be used to estimate the strength of an intramolecular hydrogen bond.

The intramolecular hydrogen bonding in methyl lactate was studied with Fourier transform infrared (FTIR) spectroscopy, intracavity laser photoacoustic spectroscopy, and cavity ring-down spectroscopy. Vapor phase spectra were recorded in the ΔvOH = 1-4 OH-stretching regions, and the observed OH-stretching transitions were compared with theoretical results. Transition frequencies and oscillator strengths were obtained using a one-dimensional anharmonic oscillator local mode model with potential energy and dipole moment surfaces calculated at the CCSD(T)-F12a/VDZ-F12 level. The three most abundant conformers of methyl lactate all appear to possess an intramolecular hydrogen bond, with the hydroxyl group forming a hydrogen bond with either the carbonyl or ester oxygen. The intramolecular hydrogen bonds were investigated theoretically by analyses based on electron density topology, natural bond orbital analysis, and visualization of the electrostatic potential energy.

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