17.2 - Infrared Spectroscopy Flashcards
What happens when covalent bonds absorb IR radiation?
- Covalent bonds contain energy and naturally vibrate
- Absorption of IR makes them vibrate in 2 ways - bending or stretching.
What happens during bending or stretching of bonds?
Stretch - rhythmic movement that increases or decreases the distance between the 2 atomic centres
Bending - change in bond angle
What does the amount a bond bends or stretches depend on?
- Atom’s mass in bond - heavier=more vibration
- Bond strength - the stronger the bond the fast the vibration
How much radiation can a bond absorb?
Any bond can absorb as much radiation that has the same frequency as its natural frequency.
How do chemists measure the absorption of IR?
Frequency values are too large, so the reciprocal is used which is = to wave number
How are IR and atmospheric gases linked?
- Sun’s IR is absorbed by Earth
- Some IR is re-emitted in the form of longer wavelengths
- H2O, CH4, CO2 (greenhouse gases) absorb these wavelengths and re-emit this energy as radiation > cause for global warming as it heats up the Earth.
What is the purpose and process of Infrared Spectroscopy (for organic molecules)?
- Used to identify functional groups
1. Place sample in IR spectrometer
2. Fire IR beam at 200-4000cm^-1 at sample
3. Molecule absorbs some of IR and re-emits that beam - it is analysed to identify the absorbed frequencies
4. Graphs plotted of transmittance against wavenumber.
What is the fingerprint region?
<1500cm^-1 on an infrared spectrum is known as the fingerprint region
- It contains unique peaks that are used to identify a particular molecules by comparing to a booklet of published spectrums or by a computer.
What are the applications of IR radiation?
- Pollutants can be identified by checking published IR fingerprint regions
- IR-based breathalysers pass a beam of IR through the breath sample to detect IR-absorbance - characteristic ethanol bonds identified
