The role of weak hydrogen-bonds on the formation of short-lived molecular associations

Weak molecular homo- and hetero-associations among some ethene derivatives and several common solvents are studied using (1)H NMR spectroscopy. In connection with the aromatic-solvent induced shifts which is a special case of molecular association between aromatic solvents and polar molecules a model for association was suggested that involves the interaction of the positive end of the solute dipole with the aromatic pi-electrons. This model met with certain difficulties. An alternative model for configuration of associated molecules that explains the literature results and the results obtained in this paper is presented. The model is based on interaction of protons of the acceptor molecules with the lone-pair electrons or pi-electrons of the donor molecules. The present model removes the difficulties met with the former model and extends the concept of aromatic-solvent induced shifts to include non-polar molecules besides polar ones. In all cases, the interaction of protons of acceptor molecules with lone-pair electrons of donor molecules gives rise to a deshielding effect, while interaction with aromatic pi-electrons leads to shielding.