The region below 1500 cm⁻¹ is where the single bond vibrations occur. Here our hope that individual bonds may vibrate independently of the rest of the molecule is usually doomed to disappointment. The atoms C, N, and O all have about the same atomic weight and C–C, C–N, and C–O single bonds all have about the same strength.

In addition, C–C bonds are often joined to other C–C bonds with virtually identical strength and reduced mass, and they have essentially no dipole moments. The only one of these single bonds of any value is C–O, which is polar enough to show up as a strong absorption at about 1100 cm⁻¹. Some other single bonds, such as C–Cl (weak and with a large reduced mass, so appearing at low frequency), are quite useful at about 700 cm⁻¹. Otherwise, the single bond region is usually crowded with hundreds of absorptions from vibrations of all kinds used as a ‘fingerprint’ characteristic of the molecule but not really open to interpretation. Among those hundreds of peaks in the fingerprint region there are some of a quite different kind. Stretching is not the only bond movement that leads to IR absorption. Bending of bonds, particularly C–H and N–H bonds, also leads to quite strong peaks. These are called deformations. Bending a bond is easier than stretching it (which is easier, stretching or bending an iron bar?). Consequently, bending absorptions need less energy and come at lower frequencies than stretching absorptions for the same bonds. These bands may not often be useful in identifying molecules, but you will notice them as they are often strong (they are usually stronger than C=C stretches, for example) and may wonder what they are.

مواضيع ذات صلة

Forming bonds using 2s and 2p atomic orbitals: σ and π orbitals

Bond order

Why hydrogen is diatomic but helium is not

Breaking bonds

Molecular orbitals—diatomic molecules

Four short clarifications about orbitals before we go on

The phase of an orbital

s and p orbitals have different shapes

Electrons occupy atomic orbitals

Electrons have quantized energy levels

Atomic emission spectra

Introduction