Infrared spectroscopy

Question 1

What determines the frequency that bonds vibrate?

Answer 1

a pair of atoms joined by a chemical bond is always vibrating

stronger bonds vibrate faster (high freq.)

heavier atoms make the bond vibrate more slowly (lower freq.)

every bond has its own unique frequency

Question 2

How is IR radiation absorbed?

Answer 2

when IR radiation causes the bonds to absorb energy and vibratmore

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

Question 3

What happens in the infrared spectrometer?

Answer 3

1. a beam of infrared radiation containing a spread of frequencies is passed through a sample
2. the radiation that ermerges is missing the frequencies that correspond to the types of bonds found in the sample
3. the instrument plots a graph of the intensity of the radiation emerging from the sampled, called the transmittance, against the frequences of radiation
4. the frequency is expressed as a wavenumber, measured in cm^-1

Question 4

What are the dips in the graph?

Answer 4

called the peaks

represent particular bonds

Question 5

What is the fingerprint region?cuse this in questions!!!!

Answer 5

the area of an infrared spectrum below 1500cm^-1 usually has many peaks caused by complex vibration of the whole molecule

this shape is unique for any particular substance

it can be used to identify the chemical, using a computer to match the fingerprint region of a sample with those on a database of compounds

Question 6

How can impurities be identified?

Answer 6

peaks that should not be there in the pure compound