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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.

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slide1

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

slide2

Weak Interactions

  • We will discuss here about intermolecular weak interactions
  • The strong ‘bonds’ are: Covalent, Ionic and Metallic
slide3

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

slide4

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

slide5

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

slide6

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)
slide7

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

slide8

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

slide9

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