IR Spectroscopy

Bond Stretches and Bends

Stretches:

1. Symmetric Stretch

2. Asymmetric Stretch

Bends:

1. Symmetric In-Plane Bend (Scissor)

2. Asymmetric In-Plane Bend (Rock)

3. Symmetric Out-of-Plane Bend (Twist)

4. Asymmetric Out-of-Plane Bend (Wag)

Hooke’s Law

μ = corrected mass (m1*m2)/m1+m2

so

And divide by speed of light in cm to get wavenumber (3.00 × 10¹⁰)

Effect of Mass on Wavenumber

higher mass = smaller wavenumber

Lighter atoms absorb at higher wavenumbers

Effect of Bond Order on Wavenumber

Higher bond number = stronger spring = larger k

larger k = higher wavenumber

Higher bond numbers absorb at higher wavenumbers

Effect of Resonane on Wavenumber

depends

If resonance removes double bond and gives higher single-bond character then the resonance

• weakens spring

• lowers k

• reduces wavenumber

Effect of Hydrogen Bonding on Wavenumber

pulls on OH bonds (or NH/etc), making the bonds weaker causing minimal differences

Hydrogen bonding “removes” electronegativity, leading to lower wavenumber (slightly)

• different if dilute or concentrated hydrogens

Also explains some broad peaks; H-bonding changes bond length, and so variable stretch

Effect of Hybridization on Wavenumber

More s-character means shorter bonds meaning stronger springs = higher wavenumber

Wavenumber: sp > sp2 > sp3

Important Note about Amine and Amide Stretches

Primary amines/amides have a double peak at their NH stretch (because they have 2 NHs)

Secondary amines/amides only have a single peak

Tertiary amines/amides have no NH stretch, and must be found in the fingerprint region