If you look back at the X–H regions (3000–4000 cm⁻¹) of the four spectra on pp. 66–67. you’ll notice something that at fi rst sight seems odd. The N–H and O–H absorptions are stronger than the C–H absorptions at 3000 cm⁻¹, despite there being more C–H bonds in these molecules than O–H or N–H bonds. The reason for this is that the strength of an IR absorption varies with the change of dipole moment (see the box below for a defi nition) when the bond is stretched. If the bond is perfectly symmetrical, there is no change in dipole moment and there is no IR absorption. Obviously, the C=C bond is less polar than either C=O or N=O and its absorption is less intense in the IR. Indeed, it may be absent altogether in a symmetrical alkene. By contrast the carbonyl group is very polarized, with oxygen attracting the electrons away from carbon, and stretching it causes a large change in dipole moment. C=O stretches are usually the strongest peaks in the IR spectrum. O–H and N–H stretches are stronger than C–H stretches because C–H bonds are only weakly polarized.

Dipole moments

Dipole moment depends on the variation in distribution of electrons along the bond and also its length, which is why stretching a bond can change its dipole moment. For bonds between unlike atoms, the larger the difference in electro negativity, the greater the dipole moment and the more it changes when stretched. For identical atoms (C=C, for example) the dipole moment, and its capacity to change with stretching, is much smaller. Stretching frequencies for symmetrical molecules can be measured using an alternative method known as Raman spectroscopy. This is an IR-based technique using scattered light that relies on the polarizability of bonds. Raman spectra are outside the scope of this book.

This is a good point to remind you of the various deductions we have made so far about IR spectra.

● Absorptions in IR spectra