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

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LN–2Dipole-Induced Dipole Interaction:δ+ δ- δ+ δ-Figure 11Induced dipole interaction is important in aqueous solutions and very effectiveduring adsorption of inert molecules on active solid substrates.3. London Dispersion Forces: It is a well-known fact that all substances, includingrare gases and hydrogen, assume liquid state at finite temperature, an indicationof the existence of attractive interatomic and intermolecular forces, even in theabsence of permanent dipole systems. The origin of this force has been proposedby F. London in 1930. Accordingly, orbiting electrons will at any instance generatea “temporary dipole”, the configuration of which changes as the electrons move.Since all atoms of a given system similarly experience temporary instantaneousdipole moments, their effect is expected to be cancelled because of thestatistically random orientation of dipoles. It is evident that, should the dipoles besynchronized in a given assembly of atoms, then a net attractive force wouldresult (fig. 12). But since such an attractive force constitutes a lowering of theenergy of a given system, synchronization can and will take place because allsystems will attempt to assume minimum energy configuration. The Londondispersion force can be formulated as:+*K a2E Londonr 6The attractive London forces are small, as manifested by the very low boilingpoints of the smaller rare gases, of hydrogen and nitrogen.4. Hydrogen Bonding: The short and long range dipole interactions calculated frommolecular dipole moments are inadequate in explaining a multitude of phenomenain organic as well as some inorganic systems. L. Pauling studied such28
LN–2London Dispersion (Fluctuating Dipole Interaction)Electron motion in atoms generates fluctuating dipoles. Since the polarization is randomattractions are cancelled by repulsions.No net attractionNet attractionLondon:Upon synchronization of electron motioninter atomic attraction will be establishedin an assembly of atoms.Since this attraction constitutes a loweringof the energy of the system - attraction bysynchronization will take place - as allsystems attempt to assume lowest energyconfigurationFigure 12phenomena and concluded the existence of highly specific attractive interactionbetween hydrogen that is acidic (carries a fractional positive charge) and theelements O, F, N and, to a lesser extent, S in both organic and inorganicmolecules. This interaction, which can as yet not be formulated, is referred to ashydrogen bonding; its magnitude, ranging up to 40 kJ/mole, is significantly largerthan that of any other secondary bonding type. Hydrogen bonding is consideredinstrumental in controlling most properties of water, is a key element in thestructure of nucleic acid and thought to be an essential component in memoryfunctions of the human brain.29
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3.091 â Introduction to Solid State Chemistry Lecture Notes No. 2 ...

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