Abstract

CUEVAS, Gabriel; TENORIO, Julieta  and  CORTES, Fernando. Evaluación computacional del efecto anomérico absoluto. Rev. Soc. Quím. Méx [online]. 2000, vol.44, n.1, pp.42-48. ISSN 0583-7693.

The Absolute Anomeric Effect (AAE, Eq. 3), is proposed as a computational alternative in the evaluation of the excess in the axial preference shown by electronegative substituents located at the α position to the anular heteroatom of an heterocyclic compound (anomeric position), in both the presence and the absence of electron ic delocalization, retaining the same molecular geometry. The aim of the determination of the anomeric effect is computationally accessi ble by applying the natural bond orbital analysis (NBO), in order to compare the energy of the hypothetical molecules lacking electronic delocalization (Lewis molecules, in which the electrons are strictly located in bonds and lone pairs) with the fully delocalized molecules retaining the same geometry, and to evaluate the anomeric effect in terms of equation 3. The role of the Lewis molecules is the same as when cyclohexane is used experimentally, with the advantage that they are really stereoelectronically inert. By applying this methology to cyclic molecules at B3LYP/6-31G(d,p) and HF/6-31G(d,p)// B3LYP/6-31G(d,p) levels of theory, we found out that the anomeric effect shown by Cl in 1,3-dioxane; F, Cl, SMe, PH₃ and CO₂Me groups in 1,3-dithiane is of stereoelectronic nature, while the preference of F, OMe and NH₂ in 1,3-dioxane and P(O)Me₂ group in 1,3-dithiane is not; besides, this methology shows that anomeric effects with no stereoelectronic origin can modify the molecular geometry, in agreement with the geometric pattern required by double bond-non bond model, as has been recently proposed by Perrin.

Keywords : Anomeric effect; absolute anomeric effect; conformat ional analysis; computational chemistry; natural bond orbitals; population analysis.