IR spectroscopy allows you to identify what functional groups are present in a compound. The IR spectrum is created by recording the frequencies at which a polar bond's vibration frequency is equal to the infrared light's frequency.

The fingerprint region is separate from the function group region, and generally corresponds to carbon-carbon or carbon-hydrogen interactions. While the spectrum can show what groups are present in a compound, it cannot be used to find the position of these groups or provide a carbon skeleton.

IR spectroscopy is most commonly used to determine the functional groups found in the molecule being observed. This is done by observing the vibration frequencies between atoms in the molecule. It does not easily reveal the size or shape of the molecule's carbon skeleton. Although the fingerprint region is unique for every molecule, it is very difficult to read when attempting to determine the molecule's functional groups. Most functional group peaks are observed in the functional group region adjacent to the fingerprint region.

Infrared (IR) spectroscopy takes advantage of the electrical difference between atoms in a polar bond. These dipole moments, when exposed to infrared radiation, stretch and contract in what appears to be a vibrating motion between the atoms. The different vibrational frequencies in the molecule allow for the compound to be "read" using IR spectroscopy.

An alcohol (-ROH) exhibits a strong, broad absorbance peak at about 3500cm^-1. A nitrile's (-RCN) characteristic absorbance peak is at about 2200cm^-1. Carbonyl groups have strong, sharp peaks from 1700cm^-1 to 1750cm^-1, depending on the type of carbonyl group. For instance, an ester (-RCO₂R'-) has an absorbance at about 1750cm^-1, while a ketone (-ROR'-) has an absorbance at around 1710cm^-1.

There are a couple of key functional group spectra that you must memorize. A carbonyl group will cause a sharp dip at about 1700cm^-1, and an alcohol group will cause a broad dip around 3400cm^-1.

An ester has a characteristic IR absorption at about 1750cm^-1. A saturated ketone has an absorption at about 1710cm^-1, while an unsaturated ketone has an absorption between 1650cm^-1 and 1700cm^-1. A nitrile has an IR frequency of about 2200cm^-1, while an alcohol has a strong, broad peak at about 3400cm^-1.

Carbonyl compounds all have peaks between roughly 1650cm^-1 and 1750cm^-1. Ketone peaks are generally observed at the lower end of this range, while aldehydes and esters are toward the higher end of the range.

Treating acetone, a secondary carbonyl, with a reducing agent, such as sodium borohydride (NaBH₄), will yield a secondary alcohol as the product.

When using IR spectroscopy, carbonyl (C=O) groups display characteristic peaks at approximately 1700cm^-1, while alcohol groups (O-H) display characteristic peaks around 3300cm^-1. The acetone would, therefore, initially have a characteristic peak at roughly 1700cm^-1. After the reduction reaction is complete, the resulting 2-propanol would display a characteristic peak roughly at 3300cm^-1.

It is important to memorize a couple key functional groups, and where they are located on an IR spectrum. If you see a sharp peak near 1700cm^-1, you can assume it is made by a carbonyl group. 

Similarly, a wide peak around 3000cm^-1 will be made by a hydroxyl group.

Transmittance () is the fraction of incident light transmitted through an analyte. Absorbance () is the amount incident light that is absorbed by the analyte. The equation that governs this relationship is:

Where  is the power of the incident radiation and  is the decreased power of the incident radiation due to the interactions between the absorbing analyte particles and the power of the incident radiation. So, as the percent transmittance increases the absorbance decreases. In this case, peak  has the lowest transmittance, therefore it has the highest absorbance.

There are two equations we can use to solve this question:

And

Let's show that each give us the same correct answer: