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π‐Bonding contribution to restricted internal rotations in saccharides
Author(s) -
Llano Jorge,
Montero Luis A.
Publication year - 1996
Publication title -
journal of computational chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.907
H-Index - 188
eISSN - 1096-987X
pISSN - 0192-8651
DOI - 10.1002/(sici)1096-987x(199608)17:11<1371::aid-jcc10>3.0.co;2-q
Subject(s) - chemistry , bond order , single bond , sextuple bond , anomer , bond length , anomeric effect , crystallography , molecular geometry , triple bond , bent bond , computational chemistry , stereochemistry , molecule , double bond , group (periodic table) , organic chemistry , crystal structure
Extensive semiempirical SCF‐MO calculations confirm that the exo‐anomeric effect in methyl O‐, N‐ and S‐glycosides deals with an interaction of π‐character along the C 1 (SINGLE BOND)Y 1 bond in a X 5 (SINGLE BOND)C 1 (SINGLE BOND)Y 1 (SINGLE BOND)Me moiety (where X = O, S; Y = O, NH, S). The bond‐order between orbitals of pπ symmetry on C 1 and Y 1 serves as a measure of all significant molecular orbital interactions responsible for the exo‐anomeric stabilization. The set of simpler compounds X(SINGLE BOND)CH 2 (SINGLE BOND)Y (X = OH, SH, SeH, TeH; Y = OH, SH, SeH, TeH, NH 2 ) on which the anomeric effect has been well studied was also calculated and it is noticeable that the π‐bond‐orders accord with the results of other analyses of the ab initio wave function accounting for the anomeric effect. Although the AM1 and the PM3 parameterizations of MNDO do not accurately reproduce the anomeric effect energetic, they do reproduce accordingly the expected variations in the molecular conformations of complex carbohydrates, and thus it follows that there are maximal π‐bond‐orders for the synclinal arrangement around the C 1 (SINGLE BOND)Y 1 bond. In addition, the π‐bond‐orders show the same behavior for conformational preferences around the C 1 (SINGLE BOND)C′ 1 and the C 5 (SINGLE BOND)C 6 bonds in methyl C‐glycosides and in the hydroxymethyl group of α‐ D ‐glucose, respectively. © 1996 by John Wiley & Sons, Inc.