303 02 Molec. Assoc blurb05

303 02 Molec. Assoc blurb05 - Chemistry 303 2005 HANDOUT 2...

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Chemistry 303 2005 HANDOUT 2 Nonbonding Interactions Between Molecules--Molecular Association If no molecular association, permanent gas even at 0 o K. Every molecule has molecular association. Even He atoms are liquid at absolute zero. How does structure correlate with attractions between molecules? Non-bonded interactions can be repulsive electron--electron repulsion Steric Interactions : Repulsion when the structure forces atoms to approach within their van der Waals radius (size of electron cloud around atom) Non-bonded interactions can be attractive: larger molecules have higher bp molecules with larger atoms have higher bp more polar molecules have higher bp molecules with more hydrogen bonding have higher bp for solvents and solutes, "Like dissolves like" [alike in molecular association features] enzymes can hold specific substrates in position for favorable reactions biological receptors can associate with organic molecules and trigger chemical message polymers can be soft, hard, brittle, low melting, ceramic, etc due to variable molecular association biological membranes are formed by self-association of molecules to form structure without forming new covalent bonds. Four forces (in order of increasing strength) 1. van der Waals (London dispersion) forces 2. dipole-dipole interactions 3. hydrogen bonding 4. ionic attraction [ (+) and (-) ions] Common measure of molecular attraction: melting point- -breaking down interaction between molecules in crystal lattice (complicated by issues of symmetry and packing forces) boiling point- -breaking down all interaction between the molecules to vaporize (simple and direct measure of intermolecular association in a liquid) solubility --competition between association between molecules of solute with themselves and with the solvent molecules. Van der Waals Forces : Temporary polarization leads to momentary attraction (difficult to visualize). Two electron clouds approach, and, from a distance one electron cloud induces a displacement of the other. Electrostatic attraction for an instant before the electron cloud relaxes again. This is sometimes referred to as an induced temporary dipole. Any single interaction is very weak, but all atoms can participate and the effects add up. The magnitude decreases with the distance between the molecules to the sixth power--it is very sensitive to the distance. [Simple coulombic attraction scales as the second power of the distance between the charges.] The picture in Figure 1 is highly oversimplified, but may help visualize the effect. More -CH
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This note was uploaded on 04/20/2008 for the course CHM 303 taught by Professor Martinf.semmelhack,henryl.gingrich during the Fall '07 term at Princeton.

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303 02 Molec. Assoc blurb05 - Chemistry 303 2005 HANDOUT 2...

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