Unit 9.2 Analytical Chemistry

3.2.8 3.2.9 3.2.10 3.2.11 3.2.12

Mass spectrometer

Can be used to find the relative molecular mass of a compound.

Fragmentation patterns

The ionization process in the mass spectrometer involves an electron from an electron gun hitting the incident species and removing an electron. The collisions can be so energetic that it causes the molecule to break up into different fragments. The peak with the largest mass/charge value in the mass spectrum corresponds to the molecular ion which has passed through the instrument without fragmenting.

Infrared spectroscopy

Used to identify the type of bond present in a molecule.

Radio waves

Can be absorbed by certain nuclei, causing them to reverse their nuclear spin. They are used in nuclear magnetic resonance (NMR) and can give information about the environment of certain atoms.

Microwaves

Cause molecules to increase their rotational energy. This can give information about bond lengths.

Infrared radiation

Absorbed by certain bonds causing them to stretch or bend. This gives information about the bonds in a molecule.

Visible light and ultraviolet light

Can produce electronic transitions and give information about the elctronic energy levels within the atom or molecule.

X-rays

Are produced when electrons make transitions between inner energy levels. They have wavelengths of the same order of magnitude as the inter atomic distances in crystals and produce diffraction patterns which provide direct evidence of molecular and crystal structure.

Absorption of infrared radiation effect on bonds

The absorption of infrared radiation excites the bonds in a molecule, to make them vibrate with a greater amplitude. A bond in a diatomic molecule will only interact with the IR radiation if it is polar.

Natural frequency of a chemical bond

Each bond vibrates and bends at a natural frequency which depends on the bond strength and the masses of the atoms.

Stretching and bending in a polyatomic molecule

In polyatomic molecules such as water, it is more correct to consider the molecule stretching and bending as a whole, rather than considering the individual bonds.

Relationship between wavenumbers and bonds

The absorptions of particular wavenumbers of IR radiation can help identify the bonds in a molecule. The precise position of the absorption depends on the environment of the bond, so a range of wavenumbers is used to identify different bonds.

H NMR spectoscopy

A technique for finding the structure and shape of molecules