Question

What is the effect of water if present on the IR spectrum of a compound? what is the reason if a IR peak appears flat atthe bottom?

Answer 1

Water and carbon dioxide are two components in the ambience or as an impurity in the compound, that are active to IR spectroscopy.

Water produces intense vibrations corresponding to O-H bond deformations around 3600, 3700 and 1500cm^-1. IR spectroscopy is used to determine the functional groups present in an organic molecule or a coordination complex and produces peaks around the same areas of that of water corresponding to C-H and O-H vibrations. So, the presence of water in a compound, if unwanted will effectively mask the vibrations in the compound that indicate the presence of aliphatic C-H, O-H and carbonyl C=O functionalities, rendering the identification of the compound's actual functionalities impossible.

Unlike other spectra, IR spectrum arrives as the percentage of that wavelength of light transmitted to the detector while other techniques render the spectra as the measure of percentage absorbance. Since transmittance and absorbance are reciprocals of each other, an absorbance spectrum arrives as peaks similar to stalagmites while IR spectra arrive as their reciprocals, with peaks "hanging from the top" similar to stalactites.

So, if an IR spectrum arrives flat at the bottom and is a transmittance spectrum, it means that the sample is a blank or that there is no IR active sample at all in the holder. If however the spectrum arrives flat at the bottom and is an absorbance spectrum, it shows that all of the light arriving from the source has been absorbed, implying that the sample is totally dark to IR radiation or that the path of the light source to the detector is completely blocked.