3.091 – Introduction to Solid State Chemistry Lecture Notes No. 2 ...

LN–2:δ+ δ-LSchematic presentation of a dipole, exhibiting a Dipole-moment (µ)which is given by:µ= L.∂qwhere L is the distance of separation of the fractional electronic charges∂q+ and ∂q- (in abbreviated form given as δ+ and δ−). In molecules thedipole moment is given by the vector sum of the polar bonds. Thus, becauseof molecular geometry (symmetry), molecules such as CCl4 exhibit nodipole-moment.Dipole-Dipole Interaction: δ+ δ-δ+ δ-rδ+ δ-δ+ δ-Figure 102 m4 E +* x 1Dipole*Dipole 3 Dr 6 kTwhere µ is the dipole moment, r is the distance of approach of the oppositelycharged molecular portions, k is the Boltzmann constant (see P/T) and T is theabsolute temperature in K.The molar energies of attraction associated with dipole-dipole interaction rangefrom 0 to about 10 kJ/mole. These forces are primarily responsible for the liquidstate (at room temperature) of most polar organic molecules; they are acontributing factor for H 2 O to be liquid at room temperature, and are responsiblefor alcohol being a liquid.2. Dipole-Induced Dipole Interaction: A dipole in one molecule can interact with andpolarize the electrons of a neighboring non-polar molecule, thus generating aninduced dipole which will experience an attractive force with the polarizing, polardipole (fig. 11). P. Debye showed that in a molecule with a “polarizability” of (α) theattractive potential arising from dipole-induced dipole interaction is given as:2am 2E Dipole*Induceddipole+*Dr 6 27