A2 AQA CHEM4 3.4.11

3.4.11 Structure Determination

Mass spectrometry

·         Analysis of gaseous mixtures of compounds and elements

·         Ionisation, acceleration, deflection, detection

·         Base peak is caused by the most abundant ion

·         Molecular ion peak is caused by the molecular ion of the compound (i.e. when one molecule of the compound loses an e- and becomes a cation)

o   If an element has isotopes, there is more than one molecular ion peak

o   Usually found furthest to the right

·         M_(g)-->M^+._(g) + e^-

·         This cation/free-radical can form a molecular ion and form fragments to produce other ions (which are detected) and free-radicals (which aren’t detected)

·         M^+. _(g)--> M⁺_(g) + e^-

·         M^+. _(g)-->X⁺_(g) + Y^._(g)

·         More stable cations give higher peaks- e.g.

o   acylium RCO⁺

o   carbocations RCH₂⁺ , R₂CH⁺ , R₃C⁺

Infrared spectroscopy

·         This gives us information on the bonds in molecules as different bonds absorb different frequencies of infrared radiation

·         Fingerprint region: pattern on absorption below 1500cm^-1 and it is unique to every compound. You can compare the fingerprint region of the unknown compound to a database and find one that matches exactly to find the compound identity

Nuclear magnetic resonance spectroscopy

·         Gives us information on the position of atoms in a molecule and it relies on the fact atomic nuclei spin

·         Atoms with a spin opposed to the magnetic field have high energy and those aligned have lower energy

·         Each atom absorbs a different frequency of EM radiationàresonance frequency

·         Chemical shift compares the resonance frequencies of ¹H and ¹³C to a reference…