Spectroscopy

Spectroscopy – Getting Ready What happens when an electron

absorbs energy? What kind of energy can cause this

to happen? Why do different elements

emit/absorb different wavelengths? Which electron transitions produce

red light?

Key Terms Electromagnetic spectrum Wavelength Standard Calibration curve Ionisation Vibrational energy level Magnetic resonance

The Electromagnetic Spectrum

Using Light A number of instruments have

been developed that work on the ability of substances to absorb or emit certain wavelengths of light:

Colourimetry UV-Visible Light Spectroscopy Atomic Absorption Spectroscopy Atomic Emission Spectroscopy

Colourimetry Shine light of a certain colour through a

coloured solution (colour must be complimentary to the colour of the solution).

The greater the absorption, the greater the concentration.

A set of standards must be used.

UV-Visible Spectroscopy A lot like colourimetry except:

Specific wavelengths are selected, not just colours.

Can detect absorption of UV light.

Extra step in the process: A scan of the pure substance is taken to

see which wavelengths are absorbed the most.

Atomic Absorption Spectroscopy Developed by the CSIRO. Used for analysing metals.

Atomic Absorption Spectroscopy

N2O (laughing gas) can also be used as an oxidant in place of air (O2).

AAS is selective. You don’t need to separate the components to analyse them.

It’s also very sensitive, measuring concentrations in ppm and even ppb.

Atomic Emission Spectroscopy Note – In AAS the flame does not

heat the solution enough to make the metals EMIT light. The flame only serves to make the solution a vapour.

In AES the flame DOES heat the solution to a point where the atoms begin emitting their own light.

Mass Spectroscopy Measures the mass of atoms and

molecules.

Mass Spectroscopy1. Sample is injected as a gas into the

ionisation chamber.

2. Sample then bombarded with high energy electrons, knocking electrons out of the atoms.

3. The positive ions formed are accelerated to high speeds by an electric field.

Mass Spectroscopy3. Ions then passes through a magnetic

field, deflecting the ions according to their mass to charge ratio (m/e).

4. Number of ions detected.

5. The data generated can be presented graphically as a mass spectrum.

Mass Spectroscopy

Infra-Red Spectroscopy Identifies the functional groups attached to

molecules.

Can identify: Methyl groups Hydroxy groups Carboxylic acids Double and triple bonds

Can you remember what the above groups look like?

Infra-Red Spectroscopy In a similar way to electrons

absorbing particular amounts of energy that cause them to jump shells, covalent bonds have certain vibrational energy levels.

That means that certain types of covalent bonds absorb certain wavelengths of infra-red light.

Infra-Red Spectroscopy

Symmetricalstretching

Antisymmetricalstretching

Scissoring

Rocking Wagging Twisting

Infra-Red Spectroscopy Measures transmittance, not

absorbance, so graphs look upside down.

Units are wave number per cm, not wavelength.

Readings below 1000cm-1 are ‘whole molecule’.

Almost all organic molecules will have a peak at 2950cm-1 as it is a C-H bond.

Infra-Red SpectroscopyBond Wave

NumberC-Cl 700-800

C-C 750-1100

C-O 1000-1300

C=C 1610-1680

C=O 1670-1750

O-H (as part of an acid)

2500-3300

C-H 2850-3300

O-H (as part of an alkanol)

3200-3550

N-H 3350-3500

Nuclear Magnetic Resonance Spectroscopy (NMR) Measures the ‘spin’ of H or C nuclei. Spin is the direction of the magnetic

field in the nucleus. Two types of spin – up and down. Magnetic energy is absorbed by the

nucleus which changes the direction of its spin

The energy is then released and detected.

NMR Spectra A hydrogen attached to a carbon atom

will have a different magnetic resonance to a hydrogen attached to a nitrogen atom.

The elements that surround an atom are referred to as its ‘environment’.

NMR Spectra

CNMR Spectra # of peaks = # of environments Chemical shift = type of environment

(lower electron density, further downfield)

Peak area = number of similar atoms

CNMR Spectra Refer to the table of C NMR data to interpret the

following spectrum. Empirical formula is C4H10O and it is an alkanol.

CNMR Spectra Molecular formula of C3H6O2. Does not

react with NaOH.

HNMR Spectra In high resolution proton (H) NMR, the

number of split peaks gives the number of hydrogens on a neighbouring carbon.

HNMR Spectra Molecular formula of C2H4Br2.

Image Credits EM Spectrum properties edit, By Inductiveload, NASA (http://

upload.wikimedia.org/wikipedia/commons/c/cf/EM_Spectrum_Properties_edit.svg), via Wikimedia Commons

AASBlock, By K05en01 (Own work) [Public domain] (http://upload.wikimedia.org/wikipedia/commons/0/08/AASBLOCK.JPG), via Wikimedia Commons

‘FlammenAAS’, By Talos at de.wikipedia (http://upload.wikimedia.org/wikipedia/commons/5/51/FlammenAAS.jpg), from Wikimedia Commons

Image Credits Acetone MS, By NIST (

http://webbook.nist.gov/cgi/cbook.cgi?Name=acetone) [Public domain], via Wikimedia Commons

Ethanol infrared spectrum, By Mfomich (Own work) (http://upload.wikimedia.org/wikipedia/commons/b/bb/Ethanol_IR_Spectrum.png), via Wikimedia Commons