Noncovalent coulombic attractions

Ions bound by very strong Coulomb attraction (after partial loss of molecules from the ion solvating shell) or by some other noncovalent specific, short-range attraction build up the so-called Stem layer. 

Interactions become shorter-ranged and weaker as higher multipole moments become involved. When a monopole interacts with a monopole. Coulomb s law says u r) oc r But when a monopole interacts with a distant dipole, coulombic interactions lead to u r) oc r (see Equation (21.26)). Continuing up the multipole series, two permanent dipoles that are far apart interact as u(r) oc r Such interactions can be either attractive or repulsive, depending on the orientations of the dipoles. Table 24.2 gives typical energies of some covalent bonds, and Table 24.3 compares covalent to noncovalent bond strengths. 

Noncovalent interactions occurring between molecules that are charged, have a dipole moment, or are polarizable come in many different flavors. If both molecules carry an opposite charge, then the electrostatic interaction between them will be attractive and follow Coulomb s law. The Coulomb energy Ec depends on the charges (Qi and Q2), the distance r between them, and the dielectric constant of the medium. 

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