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Published on: July 6, 2012
The Effect of Hyperconjugation and Hydrogen Bonding on the Conformers of Methylated Monosaccharides
Juliette C Whiteside1, Aran Insausti1,2,3, Jiarui Ma1
1Department of Chemistry, University of Alberta, 11227 Saskatchewan Dr NW, T6G 2G2, Edmonton, AB, Canada.
Abstract:
The conformational landscapes of four 1-O-methylated monosaccharides-methyl α-glucose, methyl β-glucose, methyl α-galactose, and methyl β-galactose-were characterized using jet-cooled broadband rotational spectroscopy, supported by density functional theory calculations. A newly designed, simple pulsed nozzle assembly was used to introduce the sugar samples into a jet expansion without thermal degradation, eliminating the need for a complex and expensive laser ablation system. Ten conformers were experimentally identified by assigning their rotational spectra, and the intricate methyl internal rotation splittings were analysed. Notably, methylation alters the directionality of intramolecular hydrogen bonding of α-galactose highlighting its impact on structural preference. Natural bond orbital, intrinsic bond strength, and non-covalent interaction analyses were conducted to explore the interplay between hydrogen bonding and hyperconjugation. A set of σ to σ* neutral hyperconjugative interactions was found to override a strong hydrogen bond, driving a preference for the gauche conformers.
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