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Weak Interactions Non-Covalent Interactions Part of MATERIALS SCIENCE & A Learner’s Guide ENGINEERING AN INTRODUCTORY E-BOOK Anandh Subramaniam & Kantesh Balani Materials Science and Engineering (MSE) Indian Institute of Technology, Kanpur- 208016 Email: [email protected], URL: home.iitk.ac.in/~anandh http://home.iitk.ac.in/~anandh/E-book.htm Weak Interactions We will discuss here about intermolecular weak interactions The strong ‘bonds’ are: Covalent, Ionic and Metallic How do we get a measure of non-covalent interactions? Boiling point (being the temperature at which vapor pressure of substance equals the ambient pressure) is a better measure of non-covalent forces as compared to Melting point (which not only influenced by Attractive forces but also the crystal packing) Boiling point Electronegativity difference Decreasing BP NaCl → 1413 C H2O → 100C O DEN = 3.5O 2.1H = 1.4 Polar covalent BrF → 20C DEN = 4.0F 2.8Br = 1.2 Polar covalent Ar → 186C H+ Br+ H+ F Ar 1.91 Schematics Bonding Intra-molecular Inter-molecular COVALENT Hydrogen bond , , … Van der Waals Dipole-dipole Dipole- Induced dipole Instantaneous dipole-induced dipole London Dispersion Ion-dipole Etc. Cation-Pi Pi-Pi Relative strengths dispersion forces < dipole-dipole interactions < hydrogen bonds The term ‘Van der Waals forces’ is sometimes used for a specific type (London Dispersion) rather than the class We will describe briefly a few of these (only) here Further reading Noncovalent Interactions: A Challenge for Experiment and Theory, Klaus Müller-Dethlefs and Pavel Hobza, Chem. Rev. 2000, 100, 143−167 Hydrogen bond The covalent boding between a hydrogen atom and a strongly electronegative atom becomes ‘polar’covalent The ‘charged’ hydrogen ‘ion’ can be attracted to a electronegative atom, such as nitrogen, oxygen or fluorine hydrogen bond should not be confused with a covalent bond to hydrogen. Types of hydrogen bonds: Intermolecular (between molecular) Intramolecular (within a molecule) E.g. of hydrogen bonding: water (responsible for the high boiling point of water compared to say H2S), DNA, partly responsible for the secondary, tertiary, and quaternary structures of proteins and nucleic acids, Polymers O Hydrogen bond H+ O H+ Schematics H+ H+ Ice crystallizes in many polymorphic forms (12 crystal structures and 2 amorphous forms known)- we consider one form here Hydrogen bonded Ice crystal (hexagonal) [0001] Packing fraction → ~0.34 Note the low packing fraction in spite of having the same space group as HCP crystals c/a ratio → 1.628 (very near ideal ratio of 1.633) Value Lattice parameter(s) a = 4.52 Å c = 7.37 Å Space Group p63/mmc Van der Waals Dipole- Dipole interactions In the covalent bonding between two atoms of very different electronegativity the bond becomes highly polar (introducing partial charges on the species) This dipole can interact with other permanent dipoles This interaction is stronger than dispersion forces Diplole-Dipole Interaction Br+ Schematics F Br+ F Instantaneous dipole-induced dipole London Dispersion Instantaneously generated dipole (due to asymmetry in electron charge distribution around the nucleus) on one atom leads to slight polarization of the atom (→ quantum induced instantaneous polarization) This induces a dipole on the neighbouring atom (temporarily) The force between these two dipoles is called the London dispersion forces The force is very weak and is temporally varying Can operate between non-polar molecules (H2, Cl2, CO2 etc.) The strength of the dispersion forces will increase with number of electrons in the molecule Ar Schematics + Ar Ion-Dipole Permanent dipole interacts with an ion. This explains for example the solubility of NaCl in water. The figure below shows the interaction of Na+ and Cl ions interacting with the permanent dipoles in a water molecule. Schematics