Molecular Repulsions, Attractions, and Hydrogen Bonding

Unlike charges attract, and like charges repel each other. The uneven distribution of electrons in a molecule can result in a pair of partial charges, represented by 6+ and 6- or by H— , a dipole moment arrow with the plus end indicated. Polar covalent bonds create 

Pi-complexes, also called donor-acceptor complexes, are often a weak association of an electron-rich molecule with an electron-poor species. The donor is commonly the electron cloud of a pi bond or aromatic ring the acceptor can be a metal ion, a halogen, or another organic compound. In the absence of solvent, as can occur in an enzyme cavity, the cation-pi interaction can be stronger than hydrogen bonding. The cation snuggles into the face of the aromatic pi cloud (see aromaticity. Sections 1.9.3 and 12.3). 

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