C-H Stretch region

As mentioned, the more s character a bond has, the stronger the bond and the higher its stretching frequency.

The natural frequency of a vibration of a bond is given by:

In this case, an increase in s character increases K, the force constant.

Except for an aldehyde hydrogen, absorptions of less than 3000 cm^-1 usually implies a saturated compound. Absorption between 3000 and 3150 implies aromatic or vinyl hydrogen. Cyclopropane rings also absorb here, since they have extra s character, because more P character goes into the C-C bonds to reduce angle distortion. They can be distinguished by cross-referencing the C=C and C-H out of plane regions.

The aldehyde C-H stretch is even lower than saturated ones, normally consists of two peaks at 2850 (often a shoulder on the other C-H stretches) and 2750 (often too weak and missed). It is also possible to distinguish between methine, methylene and methyl groups, but this is rarely worth the effort: