Delicate Balance of Hydrogen
Bonding Forces in d‑Threoninol
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Abstract
The seven most stable conformers
of d-threoninol (2(<i>S</i>)-amino-1,3(<i>S</i>)-butanediol), a template
used for the synthesis of artificial nucleic acids, have been identified
and characterized from their pure rotational transitions in the gas
phase using chirped-pulse Fourier transform microwave spectroscopy. d-Threoninol is a close analogue of glycerol, differing by substitution
of an NH<sub>2</sub> group for OH on the C(β) carbon and by
the presence of a terminal CH<sub>3</sub> group that breaks the symmetry
of the carbon framework. Of the seven observed structures, two are
H-bonded cycles containing three H-bonds that differ in the direction
of the H-bonds in the cycle. The other five are H-bonded chains containing
OH···NH···OH H-bonds with different
directions along the carbon framework and different dihedral angles
along the chain. The two structural types (cycles and chains of H-bonds)
are in surprisingly close energetic proximity. Comparison of the rotational
constants with the calculated structures at the MP2/6-311++G(d,p)
level of theory reveals systematic changes in the H-bond distances
that reflect NH<sub>2</sub> as a better H-bond acceptor and poorer
donor, shrinking the H-bond distances by ∼0.2 Å in the
former case and lengthening them by a corresponding amount in the
latter. Thus revealed is the subtle effect of asymmetric substitution
on the energy landscape of a simple molecule, likely to be important
in living systems
