16.3 Infrared spectroscopy Flashcards
What is infrared spectroscopy used for
It is used by organic chemists to help identify compounds
How does the vibrations of atoms affect infrared spectroscopy
Do stronger or weaker bonds have a higher frequency
How does the mass of the atom affect the frequency
. A pair of atoms joined by a chemical bond is always vibrating
. System behaves as two balls (atoms) joined by a spring (the bond)
. Stronger bonds vibrate faster (at a higher frequency)
. Heavier atoms make the bonds vibrate more slowly (at a lower frequency)
Every bond has its own unique natural frequency that is in the infrared region of electromagnetic spectrum
How can you get a reading from the vibrating atoms
. Shine a beam of infrared radiation through the sample
. So the bonds absorb energy from the IR and vibrate more
. However, any particular bond can only absorb radiation that has the same frequency as the natural frequency of the bond
. Therefore the radiation that emerges from the sample will be missing the frequencies that correspond to the bonds in the sample
What occurs in the infrared spectrometer
. Beam of infrared radiation containing a spread of frequencies is passed through the sample
. The radiation that emerges is missing the frequencies that correspond to the types of bonds found in the sample
. The instrument plots a graph of the intensity of the radiation emerging from the sample, called the transmittance, against the frequency of radiation
The frequency is expressed as a wavenumber, measured in cm^-1
How can we use the infrared spectrum to help us identify functional groups in the compound
- The graph has peaks (which are actually dips in the graph) which represent particular bonds
- The OH bond produces a broad peak between 3230 and 3500cm-1
This is found in alcohols ROH - The OH bond produces a very broad peak between 2500 and 3000cm^-1 in carboxylic acids RCOOH
whilst the C=O bond produces a peak between 1680 and 1750cm^-1
This is found in aldehydes, ketones and carboxylic acids
So a table will tell you the frequencies, that will correspond to different bonds
What is the fingerprint region of infrared spectrum
The area of infrared spectrum below 1500 has many peaks caused by complex vibrations of the whole molecule
This shape is unique for any particular substance, it can be used to identify the chemical
Eg how people can be identified by their fingerprints
Chemists use a computer to match the fingerprint region of a sample with those on a database of compounds
An exact match confirms identification of the sample
How does the fingerprint region affect propan-1-ol and propan-2-ol
The molecules are very similar down until under 1500, and then they have different frequencies
