1 research outputs found
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 NH<sub>2</sub> groups, hydrogen-bonded cycles (3 H-bonds)
and chains (2 H-bonds) remain close in energy, no matter what the
OH/NH<sub>2</sub> 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 NH<sub>2</sub>), 2-amino-1,3-propanediol (2 OH, 1 NH<sub>2</sub>), and 1,3-diamino-2-propanol (1 OH, 2 NH<sub>2</sub>), H-bonded
cycles are most highly populated, followed by curved chains (3 OH
or 2 OH/1 NH<sub>2</sub>) or extended chains (1 OH/2 NH<sub>2</sub>). In propane-1,2,3-triamine (3 NH<sub>2</sub>), H-bonded cycles
are pushed higher in energy than both curved and extended chains,
which carry all the observed population. The NH<sub>2</sub> 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)