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Delicate balance of hydrogen bonding forces in D-threoninol

Vaquero-Vara, V.; Zhang, D.; Dian, B.C.; Pratt, D.W.; Zwier, T.S.

Journal of Physical Chemistry. a 118(35): 7267-7273

2014


ISSN/ISBN: 1520-5215
PMID: 24392821
DOI: 10.1021/jp410859n
Accession: 052478271

The seven most stable conformers of D-threoninol (2(S)-amino-1,3(S)-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 NH2 group for OH on the C(β) carbon and by the presence of a terminal CH3 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 NH2 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.

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