Download Weak Interactions

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 → 100C
O
DEN = 3.5O  2.1H = 1.4
Polar covalent
BrF → 20C
DEN = 4.0F  2.8Br = 1.2
Polar covalent
Ar → 186C
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