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.

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Changes induced by stepwise substitution of hydrogen for fluorine in neutral and protonated dimethyl ethers are analyzed in the light of the Atoms in Molecules (AIM) theory. AIM atomic and bond properties were computed by using B3LYP/6-31++G(d,p)//B3LYP/6-31G(d,p) wave functions. The effects brought about by fluorine substitution on the atomic and bond properties of the C-O-C chain were analyzed. Fluorine substitution was found to strengthen the C-H bonds. The computed proton affinities were related to the charge and energy of the proton. Reorientation of the CF n H 3-n groups upon protonation was explained in terms of the balance between anomeric and steric interactions. This interpretation and lacking of F-H bond paths allow the rejection of the previously proposed hydrogen bonding O-H‚‚‚F linkages in these systems. The fluorine substitution destabilizes C atoms in neutral and protonated forms, whereas the O is stabilized in the neutral molecule and destabilized in the cation.

The integrated values of the electron population, electron energy, nucleus–electron potential energy interaction, dipole moment and volume of the oxygen atoms, and the main properties of the O—C bond critical points, were determined by employing the theory of atoms in molecules and 6-31++G**//6-31G* wave functions for a series of 25 unbranched alkyl monoethers. These results were used to assess the degree of approximate transferability of the oxygen atom along this series in terms of the particular alkyl radicals bonded to it. It has been found that a set of six different oxygen atoms is necessary to classify all the computed values. It can be established that the oxygen atoms bonded to propyl and larger radicals can be treated, in practice, as a transferable fragment, while those bonded to at least one smaller radical are specific. Though the total HF energy and the available experimental heats of formation are well fitted by a traditional additivity scheme that distinguishes only among O, CH2, and CH3 units, it has been found that the energy properties are influenced by the size of the molecule.Key words: transferability, AIM theory, ethers.

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