7.2 - Infrared Spectroscopy

• how does an IR spectometer work?

  a beam of IR radiation is passed through a sample which is absorbed by the covalent bonds, increasing vibrational energy causing bond stretching or bending
• bond stretching

  bond length increases or decreases
• bond bending

  bond angle increases or decreases
• what frequency of IR absorbed by a covalent bond depends on

  atoms on either side of the bond

  the position of the bond in a molecule
• features of a single beam spectometer

  radiation source

  sample

  detector

  computer

  printer
• what molecules interact with IR

  molecules that have their bonds stretched asymmetrically so change polarity

  their absorptions are detected so an absorption spectrum can be produced
• absorption spectrum

  a plot showing how strongly a chemical absorbs radiation over a range of frequencies
• transmittance

  y-axis on an absorption spectrum

  percentage radiation that passes through the sample
• wavenumbers

  x-axis on an absorption spectrum

  400-1500 cm^-1

  number of waves in 1cm
• how hydrogen bonds affect peaks in absorption spectra

  in alcohols, O-H peaks are more broad

  even more broad in carboxylic acids because the O-H absorption overlaps with the C-H absorption
• why are wavenumbers given as ranges?

  the strength of a particular bond varies in different molecules due to atoms or groups around it so will absorb slightly different frequencies despite bonds being the same
• fingerprint region

  complex region of the absorption spectrum below 1500cm^-1 due to lots of single bonds stretching and bending vibrations unique to each molecule

  so used to identify unknown compounds
• how is a fingerprint region used to identify unkown compounds

  comparing pattern in fingerprint region to that of recorded IR spectra in database

  an exact match will identify the compound
• way to interpret IR spectra

  to predict absorptions in the spectrum of an organic compound

  deduce functional groups in a compound from a list of wavenumbers

  deduce the structure of a compound from wavenumbers and molecular formula