Weak Interactions

1. MATERIALS SCIENCE & ENGINEERING Part of A Learner’s Guide AN INTRODUCTORY E-BOOK Anandh Subramaniam & Kantesh Balani Materials Science and Engineering (MSE) Indian Institute of Technology, Kanpur- 208016 Email:anandh@iitk.ac.in, URL:home.iitk.ac.in/~anandh http://home.iitk.ac.in/~anandh/E-book.htm Weak Interactions Non-Covalent Interactions

2. Weak Interactions • We will discuss here about intermolecular weak interactions • The strong ‘bonds’ are: Covalent, Ionic and Metallic

3. 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 O NaCl → 1413 C DEN = 3.5O 2.1H = 1.4 H2O → 100C H+ H+ Polar covalent Decreasing BP DEN = 4.0F 2.8Br = 1.2 F BrF → 20C Br+ Polar covalent Ar → 186C Ar 1.91 Schematics

4. 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 Relative strengths dispersion forces < dipole-dipole interactions < hydrogen bonds Pi-Pi 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

5. 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 O H+ H+ H+ H+ Schematics

6. 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)

7. 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 F F Br+ Br+ Schematics

8. 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 Ar +  Schematics

9. 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