17.2 Infrared spectroscopy
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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
What is the range for the absorbance peak in the IR spectrum of an aldehyde or ketone caused by the C=O bond
1630-1820cm^-1 Usually around 1700cm^-1
What is the range for the absorbance peak in the IR spectrum of a carboxylic acid caused by the C=O bond
1630-1820cm^-1 Usually around 1700cm^-1
What is the range for the broad peak in the IR spectrum of a carboxylic acid caused by the O-H group
2500-3330cm^-1
Why isn’t the C-O peak always reliable in the IR spectrum of a carboxylic acid
Because it is in the fingerprint region
How can pollutants be identified
By their IR spectral fingerprints Remote sensors analyse IR spectra of vehicle emissions to detect and measure carbon monoxide, carbon dioxide and hydrocarbons
How do IR-based breathalysers work
Pass a beam of IR radiation through captured breath in sample chamber and detect IR absorbance of compounds in breath Bonds present in ethanol are detected More IR absorbed, higher reading, more ethanol in breath
What does a typical sequence for identification include
Elemental analysis Mass spectrometry Infrared spectroscopy Once you have empirical formula and molecular mass, can determine molecular formula
What is elemental analysis
Use of percentage composition data to determine empirical formula
What is mass spectrometry
Use of the molecular ion peak from mass spectrum to determine molecular mass; use of fragment ions to identify sections of molecule