Broadband Microwave Spectroscopy of Prototypical Amino Alcohols and Polyamines: Competition between H‑Bonded Cycles and Chains

The rotational spectra of the amino alcohols d-allo-threoninol, 2-amino-1,3-propanediol, and 1,3-diamino-2-propanol and the triamine analog, propane-1,2,3-triamine, have been investigated under jet-cooled conditions over the 7.5–18.5 GHz frequency range using chirped-pulsed Fourier transform microwave spectroscopy. Microwave transitions due to three conformers of d-allothreoninol, four conformers of 2-amino-1,3-propanediol, four conformers of 1,3-diamino-2-propanol, and four conformers of propane-1,2,3-triamine have been identified and assigned, aided by comparison of the fitted experimental rotational constants with the predictions for candidate structures based on an exhaustive conformational search using force field, <i>ab initio</i> and DFT methods. Distinctions between conformers with similar rotational constants were made on the basis of the observed nuclear quadrupole splittings and relative line strengths, which reflect the direction of the permanent dipole moment of the conformers. With three adjacent H-bonding substituents along the alkyl chain involving a combination of OH and NH₂ groups, hydrogen-bonded cycles (3 H-bonds) and chains (2 H-bonds) remain close in energy, no matter what the OH/NH₂ composition. Two families of H-bonded chains are possible, with H-bonding substituents forming curved chain or extended chain structures. Percent populations of the observed conformers were extracted from the relative intensities of their microwave spectra, which compare favorably with relative energies calculated at the B2PLYP-D3BJ/aug-cc-pVTZ level of theory. In glycerol (3 OH), d-allothreoninol (2 OH, 1 NH₂), 2-amino-1,3-propanediol (2 OH, 1 NH₂), and 1,3-diamino-2-propanol (1 OH, 2 NH₂), H-bonded cycles are most highly populated, followed by curved chains (3 OH or 2 OH/1 NH₂) or extended chains (1 OH/2 NH₂). In propane-1,2,3-triamine (3 NH₂), H-bonded cycles are pushed higher in energy than both curved and extended chains, which carry all the observed population. The NH₂ group serves as a better H-bond acceptor than donor, as is evidenced by optimized structures in which H-bond lengths fall into the following order: <i>r</i>(OH···N) ≈ <i>r</i>(OH···O) < <i>r</i>(NH···N) ≈ <i>r</i>(NH···O)