Atoms in molecules interpretation of the anomeric effect in the OCO unit

Abstract

The conformational preferences of two model compounds for the O--CH2--O anomeric unit: methanediol and dimethoxymethane analyzed within the framework of the QTAIM theory provide a new interpretation of the anomeric effect. The characteristic stabilization of the gauche conformers of these compounds is accompanied by a progressive reduction of the electron population of the hydrogens of the central methylene as the number of their gauche interactions to lone pairs rises. The electron population removed from these atoms during the conformational change is gained in the gauche conformers by atoms of larger atomic number, which results in a more negative molecular energy. Also, the variations displayed by atomic populations and the QTAIM delocalization indexes are not keeping in line with the hyperconjugative model of the anomeric effect.

Theoretical study of morphine and heroin: Conformational study in gas phase and aqueous solution and electron distribution analysis

“…when a proton or methyl cation are formally added to a ternary nitrogen, as previously found by us for other systems [39]. Also, it has previously found variations of QTAIM charges obtained at different levels are very similar [39][40][41][42].…”

Topological characterisation of intermolecular lithium bonding

“…1, upon LB formation, the electron population flows from the base to the Li-F monomer, the hydrogens of the base providing this charge and that gained by the oxygen or sulphur atom. This active role of hydrogens as reservoirs of charge is found in other processes, such as protonation [37] or anomeric effect [38]. The increase of N(Li) after LB formation is accompanied by an energetic stabilisation.…”

Ligand Close Packing, Molecular Compactness, the Methyl Tilt, Molecular Conformations, and a New Model for the Anomeric Effect

“…This means that we can predict which conformer of any of the molecules we have studied has the lowest energy simply by looking at the structures of the conformers to see which of the conformers has the H or C atom of the 1,4 O···H or O···C contact pointing toward the lone-pair side of the oxygen atom and is therefore shorter than the 1,4-distance between the 1,4-H atom and the bonding side of the oxygen atom. For example, we can predict by simply looking at the structures of vinyl alcohol and vinyl methyl ether (Figure 6), as determined by Eskandar and Mosquera [17] that for both the alcohol and the ether, the syn conformers have a lower energy than the anti conformers. Our calculations show that the syn conformers are 5.06 and 7.68 kJ mol À1 , respectively, lower in energy than the anti conformers.…”

“…Criticism and doubts about the negative hyperconjugation explanation contimue to be made, such as those expressed in two recent papers. [17] The authors of these papers used QTAIM [18] to study differences in the electron density between the conformations of anomeric molecules of the type R-O-C-O-R' and R-N-C-N-R'. They claim that the electron density differences and the corresponding changes in the atomic charges between the different conformations are not compatible with the negative hyperconjugation model but are compatible with an explanation based on the differences in the interatomic attractions and repulsions between the atoms as a consequence of the differences in the atomic charges in different conformations.…”