ORGANIC CHEMISTRY : Infra Red Spectroscopy

This is used commonly to identify functional groups and is based on the wavenumbers by which they absorb infrared radiation. For instance, the carbonyl groups show around 1800 to 1650 cm-1 depending on what is attached to it. The hydroxyl group is usually broad ranging at 3300 to 3000cm-1 . A hydrogen attached to an alkane carbon is usually at 3000cm-1 while that in alkene is 3200cm-1 and that for an alkyne is over 3300cm-1. This difference in wavenumbers is due to the stiffness or strength of the C-H bond.

Stronger and shorter bonds require more energy to vibrate and therefore have higher wavenumbers. This also explains slight differences in wavenumbers of O-H, N-H and C-H. The O-H is slightly shorter and stiffer than N-H, which is also stiffer and shorter than C-H due to atomic size and orbital overlap. The other factor involved is the reduced mass of the atoms forming the bond. If the bond is the same between two atoms, then the reduced mass takes effect. Atoms of lower mass or weight vibrate at higher wavenumbers than heavier atoms An example is stretching vibrations of O-H, N-H and C-H occur at higher frequencies than C-O, C-N and C-C.