Infrared Spectroscopy
Follow instructions 4-9 on the faceplate of the IR machine for each run! The circular
sample plates are made from salt. Avoid contact with water!!!! Handle only by the edge
of the plate. DO NOT USE deuterated solvents for your thin film IR’s.
Preparation of Liquid samples: Put two to three drops of the liquid on one NaCl plate.
Put the other plate on top. Place both plates (“the sandwich”) in the metal sleeve and
screw the plastic piece onto the top. Place in sample holder.
Preparation of Solid Samples: There are three ways to do this. The first way is to make a
KBr pellet. The instructions for this are in the previous handout. This way is
problematic for several reasons and is not recommended as the best way to take IR
spectra.
The second method is to make a mull in Nujol (a type of mineral oil). With a
mull, C-H stretching and bending (1380, 1470, 2850-3000 cm-1)cannot be interpreted
because of the presence of C-H bonds in the mineral oil. To prepare, grind 10-20 mg of
solid in a mortar for 5-10 minutes. Add a drop of two of Nujol and grind further until
substance has consistency of petroleum jelly. Spread evenly on a single salt plate. Place
into the metal holder as before except this will be an open faced sandwich. If you have
too much sample (see below) some can be removed by smearing it onto a second plate.
Then run your original plate, now with less sample on it, as an open faced sandwich
(Lehman, p717).
The third method is thin film, which can be accomplished in one of two ways. If
your substance is readily soluble in a solvent, you can place a grain or two of it on a NaCl
plate, and add your solvent to the top of the plate. Wait for the solvent to evaporate. This
spreads out the sample evenly over the surface of the plate. If the sample on the plate is
grainy, it is not dissolving in your solvent before the solvent evaporates and you will need
to do your thin film the second way.
The second way is to take a few grains of your sample and dissolve it in some
solvent in a test tube. Take a pipet and place a few drops of the solution onto a single
NaCl plate and let the solvent evaporate. Repeat until you have a thin film of your
substance on the plate. Run as an open faced sandwich.
Potential problems with IR spectra
IR measures absorbance. If there is too much sample, then the peaks tend to max
out and flat line on the bottom of the piece of paper. If there is not enough sample, then
you tend to get no peaks or very small peaks. If it maxes out on the absorbance, remove
SOME, but not ALL of your sample and try again. If you don’t get much absorbance,
add more of your sample to the plate(s) WITHOUT removing the old sample.
NMR Spectroscopy
A solvent is always used with NMR. The previous instructions called for using
carbon tetrachloride (CCl4) as a universal solvent for organic compounds. Carbon tet is a
wonderful solvent for most organic compounds and has no protons itself, so it works just
swell as an NMR solvent. The problem is that carbon tetrachloride is a powerful ozone
depleting agent. You need to find another solvent that works. You will need to use a
deuterated solvent (one that has had its hydrogens removed and replaced with deuterium)
in the actual NMR run. However, deuterated solvents typically run around $10 a gram.
To test the solubility of your sample, use the NON-DUETERATED version of your
solvent. Try the following: acetone, methanol, water, DMSO, and chloroform in that
order. These solvents can typically be found in the flammable cabinet of the lab. Once
you have a solvent that works, you can use that one instead of carbon tet. If all of these
fail, only then can you use carbon tetrachloride. Use the amounts listed in the handout.
Place your sample directly into the NMR tube and then add the deuterated solvent to
avoid potential contamination. After using a deuterated solvent, IMMEDIATELY screw
the lid back on the stock bottle tightly. Air contains water. Water contains hydrogen.
Leaving these bottles open to the air contaminates these $70-$100+ bottles of solvent and
makes everyone else’s NMR look unprofessional at best and completely worthless at
worst.
Follow the instructions on the sheet for how to do NMR runs. Do not forget to
turn the NMR tube upside down a few times after adding TMS to mix the TMS into your
sample. Do not place tape on your NMR tube; write on the side with a sharpie pen.
When finished with your NMR, place in the waste jars in the hood. Wash with acetone
and place this wash acetone in the appropriate waste jar in the hood, as well. Dry out
your tube before your next use (use oven if necessary) as acetone does have hydrogens.