infrared spectroscopy

Cards (16)

  • What happens when a molecule absorbs infrared radiation?
    it makes the covalent bond vibrate more in a stretching or bending motion
  • What factors affect the amount of vibration of a bond?
    bond strength
    bond length
    mass of each atom in the bond
  • How does infrared spectroscopy work?
    every bond has a unique vibration frequency in the infrared region of the electromagnetic spectrum
    bonds absorb radiation that has the same frequency as their frequency of vibration
    infrared radiation emerged from a sample is missing the frequencies that have been absorbed -> this information can be used to identify the compound's functional group
  • What do the peaks on an infrared spectrum represent?
    absorbance of energy from the infrared radiation
  • What are the 2 peaks that must be present to identify a substance as a carboxylic acid?
    very broad peak at 2500-3300 cm-1 = O-H group
    sharp peak at 1680-1750 cm-1 = C=O group
  • What is resonance?

    If an organic molecule is irradiated with infra-red energy that matches the natural vibration frequency of its bonds, it absorbs some of that energy and the amplitude of vibration increases.
  • All covalent bonds act like springs, like springs the bonds can vibrate in a number of different ways
    the frequency of vibration occurs in the infra-red region of the electromagnetic spectrum
  • What is infrared spectroscopy?
    a technique used to identify compounds based on changes in vibrations of atoms when they absorb infra-red of certain frequencies
  • How does infrared spectroscopy work?
    a spectrophotometer irradiates the sample with infra-red radiation and then detects the intensity of IR radiation absorbed by the molecule
    infra-red energy is absorbed only if a molecule has a permanent dipole that changes as it vibrates, so symmetrical molecules like O2 are IR inactive
  • What is resonance frequency?
    the specific frequency at which the bonds will vibrate
  • What units do an IR spectrum show?
    wavenumber, this is the reciprocal of the wavelength and has units of cm-1
  • How can infrared spectroscopy be used to identify pollutants in vehicle emissions in the air?

    • Sensors detect and measure the amount of pollutants such as carbon monoxide, carbon dioxide and unburnt hydrocarbons
    • This commonly occurs on motorways and in busy town centres to monitor localised pollution
  • How can infrared spectroscopy be used to measure alcohol levels using roadside breathalysers?

    • A ray of infrared radiation is passed through the breath that is exhaled into the breathalyser chamber
    • The characteristic bonds of ethanol are detected and measured - the higher the absorbance of infrared radiation, the more ethanol in the person's breath
  • What is the fingerprint region?
    • The region below about 1500 cm-1 is called the fingerprint region and is unique to every molecule
    • It has many peaks that can be difficult to assign
    • These peaks represent the complex vibrational interactions that occur between different bonds within a molecule
  • What is valuable about the fingerprint region?
    • being able to compare the IR spectrum to a known compound from a database and coming up with an exact match
    • This is particularly useful, for example, in identifying a specific member of a homologous series
    • All members of the series will show the same type of bonds present, but no two molecules will have the same fingerprint region
  • What other analytical techniques are infrared spectroscopy most commonly used in combination with?
    • Elemental analysis - to determine the empirical formula
    • Mass spectrometry - to determine the molecular mass and fragment ions from the whole molecule
    • (NMR spectroscopy is also included in the techniques but this is not covered as part of the AS course)