Infrared Spectroscopy

IRDr. Wael Abu-Dayyih

Introduction

Mass spectroscopy === Molecular formula

Infrared spectroscopy === Functional groups

UV spectroscopy === Extent of conjugation

NMR spectroscopy === Map of C-H framework

Introduction

All molecules have a certain amount of energy

(quantized )and are in constant motion

stretching Scissoring Twisting

Introduction

Introduction

Theory

exploits the fact that molecules absorb specific frequencies that are characteristic of their structure

Theory

The result of this energy absorption is an increased amplitude for the vibration

Molecular Motion

Translational movement

Rotational movement

Vibrational movement

Theory

In vibrational motion :

Each atom has three degrees of freedom

For molecules with N atoms in them, linear molecules have 3N – 5 degrees of vibrational modes, whereas nonlinear molecules have 3N – 6 degrees of vibrational modes

Theory

Carbon dioxide has 3 x 3 - 5 = 4 vibrations

Interpreting the spectra

IR spectra are obtained by detecting changes in transmittance )or absorption( intensityas a function of frequency

Most commercial instruments separate and measure IR radiation using dispersivespectrometers or Fourier transform spectrometers

Example

Toluene

Example

Sample preparation

It is possible to obtain an IR spectrum from samples in many different forms, such as

liquid, solid, and gas.