Weak Interactions Weak Interactions Non-Covalent Interactions MATERIALS SCIENCE MATERIALS SCIENCE & ENGINEERING ENGINEERING Anandh Subramaniam & Kantesh Balani Materials Science and Engineering (MSE) Indian Institute of Technology, Kanpur- 208016 Email: [email protected], URL: home.iitk.ac.in/~anandh AN INTRODUCTORY E-BOOK AN INTRODUCTORY E-BOOK Part of http://home.iitk.ac.in/~anandh/E-book.htm A Learner’s Guide A Learner’s Guide
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We will discuss here about intermolecular weak interactions The strong ‘bonds’ are: Covalent, Ionic and Metallic
Weak 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)
How do we get a measure of non-covalent interactions?
NaCl → 1413 C
H2O → 100C
Ar → 186C
BrF → 20C
Boiling point
Dec
reas
ing
BP
EN = 4.0F 2.8Br = 1.2
Electronegativity difference
Polar covalentF
1.91
Br+
EN = 3.5O 2.1H = 1.4
Polar covalent
O
H+ H+
Schematics
Ar
Inter-molecular
Bonding
Intra-molecular
COVALENTHydrogen bond
Van der Waals
Etc.
, , …
Dipole-dipole
Dipole- Induced dipole
Instantaneous dipole-induced dipole London Dispersion
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
Noncovalent Interactions: A Challenge for Experiment and Theory, Klaus Müller-Dethlefs and Pavel Hobza, Chem. Rev. 2000, 100, 143−167
Further reading
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 H 2S), DNA, partly responsible for the secondary, tertiary, and quaternary structures of proteins and nucleic acids, Polymers
Hydrogen bond
O
H+ H+
O
H+ H+
Hydrogen bond
Schematics
Hydrogen bonded Ice crystal (hexagonal)
[0001]
Ice crystallizes in many polymorphic forms (12 crystal structures and 2 amorphous forms known)- we consider one form here
ValueLattice parameter(s) a = 4.52 Å c = 7.37 Å
Space Group p63/mmc
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)
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
FBr+ FBr+
Diplole-Dipole Interaction
Schematics
Van der Waals
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
Instantaneous dipole-induced dipole London Dispersion
Ar
Ar+
Schematics
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