The external-anomeric torsional effect.
The rotational barrier for a methyl group at the end of an anomeric system is sometimes lower than we might have anticipated. Thus, in the trans-trans conformation of dimethoxymethane, the barrier to methyl rotation is calculated (B3LYP/6-311++G(2d,2p)) to be 2.22 kcal/mol, just slightly smaller than the corresponding barrier to rotation of the methyl group in methyl propyl ether of 2.32 kcal/mol. However, if the methyl being rotated in dimethoxymethane is placed into a gauche conformation, that rotational barrier is reduced to 1.52 kcal/mol. This substantial (0.80 kcal/mol relative to methyl propyl ether) reduction in barrier height in the latter case is attributed mainly to the change in the bond order of the C-O bond to which the methyl is attached, as a function of conformation, which in turn is a result of the anomeric effect. We have called this barrier lowering the external-anomeric torsional effect. This effect is apparently widespread in carbohydrates, and it results in the changing of conformational energies by up to about 2 kcal/mol. If polysaccharide potential surfaces are to be accurately mapped by molecular mechanics, this effect clearly needs to be accounted for.
Lii
Jenn-Huei
JH
Center for Computational Chemistry, University of Georgia, Chemistry Annex, Athens, GA 30602-2526, USA.
Chen
Kuo-Hsiang
KH
Johnson
Glenn P
GP
French
Alfred D
AD
Allinger
Norman L
NL
D002236:Carbohydrate Conformation; D002475:Cellobiose / Q000031:analogs & derivatives / Q000737:chemistry; D005019:Ethyl Ethers / Q000737:chemistry; D008738:Methyl Ethers / Q000737:chemistry*; D008956:Models, Chemical; D008968:Molecular Conformation
D005019:Ethyl Ethers; D008738:Methyl Ethers; C505394:ethoxymethoxymethane; D002475:Cellobiose; C042581:dimethoxymethane