Weak InteractionsWeak Interactions

Non-Covalent Interactions

MATERIALS SCIENCEMATERIALS SCIENCE&&

ENGINEERING ENGINEERING

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

AN INTRODUCTORY E-BOOKAN INTRODUCTORY E-BOOK

Part of

http://home.iitk.ac.in/~anandh/E-book.htmhttp://home.iitk.ac.in/~anandh/E-book.htm

A Learner’s GuideA Learner’s GuideA Learner’s GuideA Learner’s Guide

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

Relative strengthsdispersion forces < dipole-dipole interactions < hydrogen bonds

Ion-dipole

Cation-Pi

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

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

water molecule.

Schematics