Infrared (IR) Spectroscopy
Basic Principle
• IR radiation causes bonds to vibrate (stretching/bending).
• Each type of bond absorbs specific wavenumbers (cm⁻¹), depending on:
o Bond strength
o Masses of atoms
o Type of vibration (stretch/bend)
IR Spectrum Overview
• X-axis: Wavenumber (cm⁻¹)
• Y-axis: Transmittance (%)
• Peaks point downwards – a dip in transmittance means IR absorption.
• The region between 4000–500 cm⁻¹ is most important.
o Above 1500 cm⁻¹: Functional group region (used for identification).
o Below 1500 cm⁻¹: Fingerprint region (unique to each compound;
compare with databases).
How to Identify Peaks (Key Bonds)
Wavenumber
Bond Type Peak Shape & Notes
(cm⁻¹)
O–H (alcohol) 3200–3600 Broad, smooth peak
O–H (carboxylic
2500–3300 Very broad, overlaps with C=O
acid)
N–H (amine) 3300–3500 Sharp, less broad than O–H
C–H (alkane) 2850–2960 Multiple narrow peaks
C=O (carbonyl) 1680–1750 Strong, sharp peak (very diagnostic)
Medium/narrow peak, not always
C=C (alkene) 1620–1680
intense
C–O (ester,
1000–1300 Strong but in the fingerprint region
alcohol)
Tip: Always look for the C=O and O–H peaks first—they’re strong and distinctive!
Basic Principle
• IR radiation causes bonds to vibrate (stretching/bending).
• Each type of bond absorbs specific wavenumbers (cm⁻¹), depending on:
o Bond strength
o Masses of atoms
o Type of vibration (stretch/bend)
IR Spectrum Overview
• X-axis: Wavenumber (cm⁻¹)
• Y-axis: Transmittance (%)
• Peaks point downwards – a dip in transmittance means IR absorption.
• The region between 4000–500 cm⁻¹ is most important.
o Above 1500 cm⁻¹: Functional group region (used for identification).
o Below 1500 cm⁻¹: Fingerprint region (unique to each compound;
compare with databases).
How to Identify Peaks (Key Bonds)
Wavenumber
Bond Type Peak Shape & Notes
(cm⁻¹)
O–H (alcohol) 3200–3600 Broad, smooth peak
O–H (carboxylic
2500–3300 Very broad, overlaps with C=O
acid)
N–H (amine) 3300–3500 Sharp, less broad than O–H
C–H (alkane) 2850–2960 Multiple narrow peaks
C=O (carbonyl) 1680–1750 Strong, sharp peak (very diagnostic)
Medium/narrow peak, not always
C=C (alkene) 1620–1680
intense
C–O (ester,
1000–1300 Strong but in the fingerprint region
alcohol)
Tip: Always look for the C=O and O–H peaks first—they’re strong and distinctive!
