13.6 : IR Absorption Frequency: Hybridization

The IR stretching frequency of a C–H bond depends on the hybridization of the carbon atom.

An sp³ hybridized carbon displays the lowest C–H stretching frequency. The frequency increases for the C–H with an sp² carbon, followed by an sp carbon.

This is because an sp-hybrid orbital has maximum s character, with its electron density very close to the nucleus.

This makes the sp C–H bond shorter and stronger than the sp² and sp³ C–H bonds, which is reflected in the observed stretching frequency trend.

For instance, the IR spectra of alkanes, alkenes, and alkynes, in the 2600 to 3400 cm⁻¹range_,  show the alkane C–H stretch below 3000 cm⁻¹. The C–H stretches for alkenes and alkynes appear around 3100 cm⁻¹ and 3300 cm⁻¹, respectively.

Tetra substituted alkenes, however, do not have a band at 3100 cm⁻¹due to a lack of C–H bonds.

Similarly, internal alkynes do not exhibit a band at 3300  cm⁻¹.