Infrared Spectroscopy IR Dr. 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) intensity as a function of frequency Most commercial instruments separate and measure IR radiation using dispersive spectrometers 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 .