Organic Analysis: Infrared Spectroscopy Flashcards
What is infrared spectroscopy?
- Analytical technique that identifies the functional groups present in organic compounds
How does infrared spectroscopy work?
- It measures wavenumbers (frequencies) of IR radiation beams that pass through a chemical sample
- (Covalent) bonds in a molecule absorb infrared radiation at characteristic wavenumbers (frequencies)
What factors affect the frequency absorbed by a certain bond?
- The bond enthalpy (strength of bond)
- The masses of atoms
How does bond enthalpy affect the frequency absorbed by a certain bond?
- Stronger bonds vibrate faster so absorb at higher frequencies
How does the masses of atoms affect the frequency absorbed by a certain bond?
- Heavier atoms vibrate more slowly so absorb at lower frequencies
- E.g heavier guitar string = lower frequency
Name different types of bond vibrations
- Bending
- Stretching
- Symmetric stretching
- Asymmetric stretch
Explain how radiation affects bond vibrations
- IR radiation is absorbed by covalent bonds increasing their vibrational energy
- Different covalent bonds have different strengths due to masses of different atoms at either end of bond
- So, bonds vibrate at different frequencies
- There are different types of bond vibrations
What does an infrared spectrometer look like?
- Wavenumber on x-axis; transmittance on y-axis
- Shows what frequencies of radiation the bond in molecules are absorbing
- The peaks show you where radiation is being absorbed
- The peaks are upside down
How do you interpret an infrared spectrometer?
- Look at wavenumber of a particular peak
- Look at data sheet and identify group that includes the peak’s wavenumber
Why are some peaks broader than others?
- Some organic compounds form hydrogen bonds between molecules which makes peak broader
What is the fingerprint region?
- Region between 1500-400cm^-1 is known as ‘fingerprint’ region
- Each compound has a unique fingerprint region
- Compound can be identified by comparing spectrum with sample spectra stored in a database
- Spectrum - the entire range of wavelengths of electromagnetic radiation
How do greenhouse gases contribute to global warming?
- The Sun emits UV radiation which is absorbed by the Earth’s surface and re-emitted as IR radiation
- Molecules of greenhouse gases have bonds that are good at absorbing infrared energy
- If amount of greenhouse gases in atmosphere increase, more IR radiation is absorbed leading to global warming
Name examples of greenhouse gases
- Carbon dioxide
- Methane
- Water vapour