S3.2.9 Infrared spectra (Higher Level Only) Notes

Infrared (IR) Spectra: Identifying Bonds in Molecules

How IR Spectroscopy Works.

What Determines Vibration Frequencies?

1. Two main factors influence how a bond vibrates:
   1. 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.
   2. Atomic Masses: Lighter atoms vibrate at higher frequencies compared to heavier ones.
2. 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).
3. 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:

1. Functional Group Region (4000–1500 cm⁻¹): This region contains peaks that correspond to specific functional groups, making it the most useful for identification.
2. 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:

Greenhouse Gases and IR Absorption

1. IR spectroscopy isn’t just for identifying molecules in the lab.
2. It also plays a critical role in understanding our atmosphere.
3. Greenhouse gases like carbon dioxide ($CO_2$), methane ($CH_4$), and water vapor ($H_2O$) absorb IR radiation, trapping heat and contributing to global warming.

Why Do Greenhouse Gases Absorb IR Radiation?

For a molecule to absorb IR radiation, its vibration must result in a change in dipole moment. Here’s how this works:

1. $CO_2$ absorbs IR radiation because its asymmetric stretching and bending vibrations change the dipole moment.
2. $H_2O$ is highly IR-active because of its polar nature and multiple vibration modes.
3. In contrast, diatomic molecules like $O_2$ and $N_2$ are symmetric and have no dipole moment, making them IR-inactive.

Using IR Data to Identify Compounds

When analyzing an IR spectrum, follow these steps:

1. Identify Key Peaks: Look for strong, characteristic peaks in the functional group region.
2. Match Peaks to Functional Groups: Use the IB data booklet to assign peaks to specific bonds.
3. Combine with Other Data: IR spectra provide functional group information but not the full molecular structure. Use techniques like mass spectrometry or NMR to complete the puzzle.

Limitations of IR Spectroscopy

While IR spectroscopy is a powerful tool, it has its limitations:

1. Cannot Determine Full Structure: IR spectra only reveal functional groups, not how atoms are connected.
2. Overlapping Peaks: Some functional groups, like $O–H$ and $N–H$, have similar absorption ranges.
3. Requires Complementary Techniques: For complete structural determination, IR spectroscopy must be combined with methods like mass spectrometry or NMR.