Infrared (IR) Spectra: Identifying Bonds in Molecules
How IR Spectroscopy Works.
Analogy- Consider the bonds between atoms in a molecule as tiny springs connecting weights.
- These "springs" can stretch, compress, and bend when they absorb energy.
- Infrared radiation provides just the right amount of energy to make these bonds vibrate at specific frequencies.
What Determines Vibration Frequencies?
- Two main factors influence how a bond vibrates:
- Bond Strength: Stronger bonds (like $C \equiv C$ triple bonds) vibrate at higher frequencies, while weaker bonds (like $C–C$ single bonds) vibrate at lower frequencies.
- Atomic Masses: Lighter atoms vibrate at higher frequencies compared to heavier ones.
- When a molecule absorbs infrared radiation, it creates an IR spectrum—a graph showing how much radiation is absorbed (intensity) at different wavenumbers (measured in cm⁻¹).
- The wavenumber is inversely related to the wavelength of the absorbed radiation.
Interpreting the Functional Group Region of an IR Spectrum
The IR spectrum can be divided into two regions:
- Functional Group Region (4000–1500 cm⁻¹): This region contains peaks that correspond to specific functional groups, making it the most useful for identification.
- Fingerprint Region (1500–400 cm⁻¹): This region is unique to each molecule and is often used for comparison rather than identifying functional groups.
Key Functional Group Absorptions
Using the IB data booklet, you can match absorption peaks in the functional group region to specific bonds. Here’s a handy reference:
- You’re analyzing an IR spectrum and notice a strong, sharp peak at 1720 cm⁻¹ but no broad peaks around 3200 cm⁻¹.
- This suggests the presence of a carbonyl group ($C=O$) and the absence of hydroxyl groups ($O–H$).
- The compound could be a ketone or an aldehyde.
Always have a look at the IB data booklet for precise wavenumber ranges when interpreting IR spectra.
Greenhouse Gases and IR Absorption
- IR spectroscopy isn’t just for identifying molecules in the lab.
- It also plays a critical role in understanding our atmosphere.
