July 5, 2021
- Infrared spectroscopy is taught in Unit 3.4.11 for A2/A-level Chemistry
- It is an important technique used in analytical/instrumental chemistry
- It is suitable for both quantitative and qualitative analysis
- It helps identifying different types of covalent bonds and driving conclusions about the functional groups in a molecule
- the structure of organic compounds can be predicted by identifying peaks in the IR spectrum
How does IR Spectroscopy work?
- beam of infrared radiation passes through the sample of a chemical
- The radiation is absorbed by the covalent bonds in the molecules as their vibrational modes vary
- Bonds between various atoms have different bond strength, thus they absorb different frequencies of radiation
- Position of bonds in a molecule also absorb different frequencies of radiation
- the absorbance of frequencies will eventually form a spectrum with the aid of computer programs
IR Spectrum
Features
- X-axis: wavenumber expressed in cm-1
- Y-axis: percentage transmittance (Calculated from absorbance)
- The absorption signal are downward troughs
- Absorption peak are described as strong, medium, weak, broad, sharp...
- IR spectrum < 1500 cm-1 is called the fingerprint region
- fingerprint is not the most distinct part but can be used for further identification of compounds
Interpretation of IR Spectrum
- In the A2/A-level Chemistry scope, we tend to put more focus on more distinctive peaks in the spectrum
- region between 1500~4000 cm-1
- most prominent absorption signals can be matched with particular covalent bonds/functional groups
- functional group related signals are usually strong/ broad and sharp
- variation of peaks may occur as the bonding of atoms is different
- Most common signal: C-H peak - around 2800~3000 cm-1
- C=C peak - around 1600~1700 cm-1
- O-H peak: the wavenumber for alcohol is different from that in carboxylic acid
- we can check to see if there is any C=O peak in the spectra, if both O-H and C=O peaks are present, the possible functional group would be carboxylic acid (-COOH)
Fingerprint region
- refers to the region with wavenumber between 400~1500 cm-1
- fingerprint region as it consists of large number of peaks, making it unique for each compounds (just like our ID cards!!)
- scientists would use help with computer to search for the unknown compounds in the database
- if the fingerprint region matches up, then there is a high possibility of confirming the compound's identity
Image references:
- 1-hexanol infrared spectrum - https://www.masterorganicchemistry.com/2016/11/23/quick_analysis_of_ir_spectra/
- How to Find Functional Groups in the IR Spectrum - https://www.dummies.com/education/science/chemistry/how-to-find-functional-groups-in-the-ir-spectrum/
Drafted by Yoyo (Chemistry)