The one topic from Ch. 13 of Smith that I have been confused about, is how to read peaks and intensities on the IR spectrum. At first, they all just looked like a lot of spikes and dips on a graph. One of the most helpful tools I found to determine what molecule a peak may represent, is the four regions of the IR spectrum. In the first region, the wavenumbers range between 4000 and 2500. Bonds in this region typically involve lighter atoms and hydrogen bonding. An molecule peaking in this region is probably a hydrocarbon. Molecules containing triple bonds are usually found in the 2500-2000 range. These include C-C and C-N triple bonds. The third region contains molecules with double bonds. The fourth region is also referred to as the fingerprint region and includes frequencies from 1500-400. In other words, the molecules found in the fingerprint region have relatively long wavelengths and are lower in energy.
The intensity of a peak is, in part, determined by that molecule's hybridization. Molecules having a hybridization of sp3 has a much greater intensity than that of a molecule who's hybridization is sp. This means that molecules with lower sp hybridizations contain stronger bonds, which, in turn, lower the intensity of the peak.
Applying this information to the IR spectrum, can help one to determine the types of atoms in a molecule and how they are bonded together. This, in turn, will help to determine the over-all shape of the molecule so it may be identified.
Once you learn to read the spectrum, the deciphering of molecules is not only much easier, but kind of neat!
