Infrared spectroscopy
reveals what
functional groups are on an
organic molecule by shining
infrared light through a
sample at various
frequencies.
There are two major types of
instrument: the more traditional kind, which compares the
beam that went through a sample with a
reference beam that is bent around it with
mirrors (and typically takes a while to make a
spectrum), and the newer, more
sophisticated FTIR (
Fourier Transform Infrared Spectrometer).
The FTIR works by first taking a reference shot just through
air and
storing it on the
computer. Then the sample is placed in front of the beam, and light of many frequencies is shined through the sample all at once, and the computer uses the
Fourier Transform to sort it all out, separating the different frequencies into separate peaks. This means the spectrum takes only a few seconds to
generate. Then you
mark the
peaks and
print out the spectrum.
The peaks signify different functional groups. For example, if you want to tell if your sample is an
alcohol, you can look at around 3400
wavenumbers. If there's a huge, fat, strong peak (heh, that's kind of amusing), then there's an alcohol. If you see a peak thats of more medium
intensity at closer to 3200, perhaps multiple peaks, you've probably got an amine. You can learn a lot about what it is. For example, one time, I learned that I had taken an IR of
ethanol instead of the unknown. On the plus side, I can now even more readily determine whether the punch is spiked (this in practice is a bad idea, as we shall see below, because of the water found in punches of high water-alcohol content).
When taking a spectrum of a
liquid, typically the sample is placed between two solid
NaCl plates (care must be taken to avoid getting
water on these, as we all know what happens to
table salt in water) in very small
quantity (less than half a drop is a good amount). For
solids, one must mix a tiny bit of sample with KBr (
potassium bromide) and crush it in a crushing device to form a clear
pellet (this is very
difficult and takes much
practice).
In addition to the requirement that a
dipole exist for a
vibration to show up most of the time (exceptions being double bonded
carbons, for example, or even a
benzene ring), there are other items of interest. For example, the frequency at which the sample absorbs light is related to the difference in
mass between the
atoms which are
bonded together. The frequency is roughly described by the folowing
equation (units of inverse
centimeters, as is usual in IR
spectroscopy):
vbar = 4.12 * (K / u) ^ (1/2)
where K is about 5 * 10^5
dynes per centimeter per
bond (10 * 10^5 for a double bond, for example)
And u = M1 * M2 / (M1 + M2), M1 and M2 being the
atomic
weights of the elements involved. In addition to this sort of bond, there are characteristic out of
plane bendings and all sorts of things that can tell you more about what kind of
molecule you're looking at. A great place to look for more detailed information as well as some examples is
Introduction to Spectroscopy by Pavia et al.