Delicate Balance of Hydrogen Bonding Forces in d‑Threoninol

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 118; no. 35; pp. 7267 - 7273
• Main Authors: Vaquero-Vara, Vanesa, Zhang, Di, Dian, Brian C, Pratt, David W, Zwier, Timothy S
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 04.09.2014
• Subjects: Amino Alcohols - chemistry; Butylene Glycols - chemistry; Computer Simulation; Hydrogen Bonding; Microwaves; Models, Chemical; Molecular Structure; Rotation; Spectrum Analysis
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp410859n

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.