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Related Experiment Videos

The external-anomeric torsional effect.

Jenn-Huei Lii1, Kuo-Hsiang Chen, Glenn P Johnson

  • 1Center for Computational Chemistry, University of Georgia, Chemistry Annex, Athens, GA 30602-2526, USA.

Carbohydrate Research
|March 23, 2005
PubMed
Summary
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The external-anomeric torsional effect lowers methyl group rotation barriers in anomeric systems, particularly in carbohydrates. This conformational effect significantly impacts energy calculations in molecular mechanics for polysaccharides.

Area of Science:

  • Computational Chemistry
  • Organic Chemistry
  • Carbohydrate Chemistry

Background:

  • The rotational barrier of methyl groups in anomeric systems can be unexpectedly low.
  • The anomeric effect influences molecular conformation and properties.

Purpose of the Study:

  • To investigate and quantify the reduction in methyl group rotational barriers in anomeric systems.
  • To identify the underlying cause of this barrier lowering and its implications for carbohydrate chemistry.

Main Methods:

  • Density Functional Theory (DFT) calculations using the B3LYP/6-311++G(2d,2p) level of theory.
  • Conformational analysis of dimethoxymethane and methyl propyl ether.

Main Results:

  • Calculated rotational barrier for a methyl group in the trans-trans conformation of dimethoxymethane is 2.22 kcal/mol.

Related Experiment Videos

  • In a gauche conformation, the rotational barrier in dimethoxymethane is reduced to 1.52 kcal/mol.
  • This reduction, termed the external-anomeric torsional effect, is attributed to conformational changes in C-O bond order driven by the anomeric effect.

    • Conclusions:

    • The external-anomeric torsional effect significantly lowers rotational barriers in anomeric systems, by up to 2 kcal/mol.
    • This effect is prevalent in carbohydrates and must be considered for accurate molecular mechanics modeling of polysaccharide potential energy surfaces.