IR Spectrum

Percentage transmittance (%T) is the ratio of radiant power passing through a sample to the power from the radiation source, multiplied by 100. Values range from 100% (no absorption) to 0% (complete absorption). Lower transmittance values indicate the molecule has absorbed infrared energy, allowing us to obtain the IR spectrum.

The diagnostic region comprises the left two-thirds of the spectrum where absorption bands from most functional groups appear. The fingerprint region on the right is unique to each compound, showing complex vibrations like bond rocking, bending, and twisting. Although two compounds may share the same functional group, they display distinct patterns in the fingerprint region due to differences in their chemical environment.

Downward-pointing peaks, called absorption bands, indicate that the molecule has absorbed infrared radiation at specific wavenumbers. Each absorption band corresponds to energy absorption at a particular frequency. For example, decane shows absorption bands around 3000, 1465, and 1375 cm-1, representing distinct molecular vibrations at those frequencies.

In an IR spectrum, wavenumber values range from 400 to 4000 cm-1. Signals on the left correspond to higher-energy, higher-frequency radiations, while signals on the right denote lower-energy, lower-frequency radiations. This arrangement allows chemists to quickly identify which molecular vibrations absorb high or low energy based on peak position.

Although 1-butanol and 2-butanol contain similar functional groups, they display different absorption band patterns in the fingerprint region below 1500 cm-1. This unique pattern arises from differences in their chemical environment and molecular structure. The fingerprint region acts like a molecular signature, making it useful for distinguishing between structural isomers.

When a molecule exhibits 100% transmittance, it has not absorbed any infrared energy, so no absorption spectrum is produced. Only molecules with lower transmittance values absorb energy and generate observable IR spectra. The degree of transmittance directly reflects the extent of energy absorption by the molecule.

An IR spectrum is plotted as a graph with percentage transmittance on the ordinate (y-axis) and wavenumber on the abscissa (x-axis). Absorption bands appear as downward signals at specific wavenumber values. This format allows researchers to visualize which frequencies the molecule absorbs and the intensity of absorption at each frequency.