Slides in this set
· In IR spectroscopy, substances are exposed to radiation in the frequency range
1014-1013 Hz ie. wavelengths 2.5-15 m.
· This makes vibrational energy changes in the molecule, which absorb infrared
radiation of specific frequencies.…read more
· Simple diatomic molecules such as HCl, HBr and HI can only vibrate by
stretching (atoms pull apart & then push together again).
· For these molecules, there is only one vibration IR absorption.
· Changing from their lowest vibrational energy state to the highest.
· Energy needed to excite a vibration depends on strength of the bond holding the
atoms together weaker bonds require less energy.
· So the frequencies of absorptions are different for each molecule.
· Bonds behave like springs of different strength holding the atoms.
Compound Bond enthalpy/kJ mol-1 IR absorption/cm-1
HCl +432 2886
HBr +366 2559
HI +298 2230…read more
More complex molecules have many vibrational modes. Eg: CO2
Symmetrical stretching Asymmetrical stretching
In-plane In-plane Out-of-plane Out-of-plane
scissoring rocking wagging twisting…read more
· IR radiation from a heated filament is split into two parallel beams, one of which
passes through the sample, the other through a reference chamber.
· This ensures that unwanted absorptions from water and CO2 in the air or from a
solvent are cancelled out.
· The beams are then directed by mirrors so that they follow parallel paths.
· Beams are analysed by passing them through NaCl prism (which is transparent
to IR radiation) or through a diffraction grating.
· Light of only one particular frequency will now be focused onto detector.
· Spectrum is produced by rotating the prism so that the detector scans the
frequencies and records their intensities.…read more