17.2 Infrared spectroscopy Flashcards
What happens when bonds absorb infrared radiation
Makes them bend or stretch more
What is a stretch
A rhythmic movement along the line between the atoms so that the distance between the two atomic centres increases and decreases
What is a bend
Results in change in bond angle
What does the amount that a bond stretches or bends depend on
The mass of the atoms in the bond - heavier atoms vibrate more slowly than lighter atoms
The strength of the bond - stronger bonds vibrate faster than weaker bonds.
What is the scale called wavenumber
Proportional to frequency
Frequency values of a bond
The vibrations of most bonds are observed in the IR wavenumber range of…
…200cm^-1 to 4000cm^-1
What is some Infrared radiation re-emitted from the Earth as
Longer-wavelength Infrared radiation
The greenhouse effect and longer wavelength IR radiation
Greenhouse gases absorb this as it has the same frequency as the natural frequency of their bonds
Vibrating bonds in these molecules re-emit this energy as radiation which increases the temperature of the atmosphere close to the Earth’s surface
What are the three most abundant greenhouse hases
Water vapour
Carbon dioxide
Methane
Infrares spectroscopy to identify functional groups in organic molecules
- Sample under investigation placed inside IR spectrometer
- Beam of IR in range 200-4000cm^-1 passed through sample
- Molecule absorbs some IR frequencies and emerging beam of radiation analysed to identify frequencies that have been absorbed by sample
- IR spectrometer usually connected to computer that plots graph of transmittance against wavenumber
What is the fingerprint region of a spectrum
Contains unique peaks which can be used to identify particular molecule under investigation
Using computer software or comparing to booklets of published spectra
WHAT DO YOU NEED TO KNOW HOW TO IDENTIFY (FUNCTIONAL GROUPS) FROM AN IR SPECTRUM IN THE EXAM
O-H groups in alcohols
C=O group in aldehydes, ketones and carboxylic acids
COOH group in carboxylic acids
What region do all organic compounds produce a characteristic peak at from the presence of C-H bonds
2850 to 3100cm^-1
OFTEN CONFUSED WITH O-H peak in alcohols
What is the range for the absorbance peak in the IR spectrum of an alcohol caused by the O-H bond
3200-3600cm^-1
What is the range for the absorbance peak in the IR spectrum of an alcohol caused by the C-O bond
1000-1300cm^-1