Selectivity descriptors for the Michael addition reaction as obtained from density functional based approaches.

Density functional (DF) based numerical approaches for computing orbital and atomic reactivity indices were employed in the study of selectivity descriptors for the 1,4 Michael addition reaction. To this aim, atomic and orbital Fukui indices and atomic softnesses for 2-arylmethylene-1,4-butanolides and N,N-disubstituted phenylacetamides were computed. Further on, these local selectivity descriptors have been rationalized in terms of the Pearson's hard-soft-acid-base principle to explain the observed regioselectivity. It is shown that the methods employed for local (atomic and orbital) reactivity index computations are useful and reliable for prediction of the regioselectivity upon conjugate addition of ambident nucleophiles to 2,3-unsaturated carboxylic esters. All the results reveal similar degree of localization/hardness of the 1,4-butanolides C4 and active alpha-carbon belonging to the N,N-dimethyl-phenylacetamide, while the soft alpha-carbon in LiCH2CN reacts with the soft C2 1,4-butanolide center.

Hydrophilic interactions between organic and water molecules as models for monolayers at the gas/water interface.

Model clusters of surfactant prototypes with small number of water molecules are calculated at different levels of theory. All approaches used yield correct trends in the variation of the dipole moment upon tail elongation or polar headgroup variation. Models including one, two, or more water molecules are optimized. The most stable structures are those with maximum number of atoms involved in hydrogen bonding. The normal components of the dipole moment prove to be less sensitive to the nature (aliphatic or aromatic) of the hydrophobic tail, in accord with findings from the phenomenological models. Values of the dipole moment approaching the experimental estimates required inclusion of sufficient aqueous environment (>20 water molecules per hydrophilic head) and of lateral intersurfactant interactions into the model.

A theoretical study of model lipid monolayers.

Studies of pure phospholipid monolayers or various well defined lipid mixtures have greatly contributed to the current knowledge of the relationship between monolayer composition and its properties and to understand how their physicochemical properties, e.g. refraction, polarization, are controlled by structural variations at the molecular level. Therefore, an attempt was made to investigate model lipid molecules adsorbed on the air/water interface. Semi-empirical (AM1) quantum chemical calculations were performed for several clusters and were compared with experimental data. The optimized acidic molecules show a marked tendency to group forming domains. An increase in the number of surfactant molecules in the cluster leads to a decrease in the effective headgroup area. On the other hand, the area grows with the increase of the degree of ionization. Both relations are in accordance with the experimental trends and the electrostatic theory. The employed theoretical approach proves to be applicable to the study of monolayers, thus providing a reliable description at molecular level of the traditionally phenomenological investigations in the field.