CAPE CHEMISTRY Module 2 Spectroscopy

Module 2: Spectroscopy Mass Spectroscopy

Process: 1) Vapourization by heated filament. 2) Atom/molecule bombarded by high energy electrons until one of its electrons is removed. Cat ion forms ([M+] peak ) ) which may fragment due to the addition of e nergy from the high-energy electrons. 3) Cations are accelerated through an electric field 4) Based on mass/charge ration fragments are deflected in variable magnetic field. 5) Fragmented cations are detected and are reflected on an ion detector screen where the current is proportional to the abundance of each ion fragment deflected on the m ass spectrum.

The abundance of each ion fragment is represented as a percentage of o f the most abundant(stable) fragment- the base peak.

N.B The mass spectra of compounds are as unique as fingerprints. They are used in oil refineries for comparison in the analysis of complex hydrocarbon mixtures.

Uses: 1) used in oil refineries for comparison in the analysis of complex hydrocarbon mixtures. 2) used in rockets to study the chemistry of outer space. 3) analysis of low concentration contaminants 4) drug testing of athlete’s

Applications: 1) Determining relative atomic mass 2) Determining relative molecular mass 3) Determining the number of carbon atoms in an organic molecule based on [m+1] peak 4) Identifying halogen compounds using [m+2] peak and [m+4] peak 5) To determine the position of reaction in a molecule by isotopic labeling.

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Control: a) Same Temperature b) Same ionizing voltage c) The same instrument

Figure 1 Base peak- largest peak

An [M+1] peak appears in the mass spectra of all organic compounds because some of the molecules contain the carbon-13 isotope.

Figure 2 [m+] (cation of organic compound) and [m+1] peak (due to carbon-13 isotope)



UV/Visible Spectroscopy 

A uv/vis spectrum measures the absorption of light in the ultraviolet and visible region of the spectrum in the form of a plot of intensity of transmitted light against wavelength.

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The uv/vis spectra arise from the transitions between e nergy levels which differ in their vibrational and rotational energies to higher electronic levels of slightly different energies when a molecule absorbs a photon of energy (quanta).

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UV/Vis spectroscopy is carried out in a solution and solvent that absorb at different wavelength as that of sample if possible.

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Some species will not absorb in the uv/vis re gion because the energy required for the electronic transition lies outside of this region. Eg in the Xray r egion instead.

The possible electron jumps that light might cause are:

In each possible case, an electron elect ron is given a quanta of energy promote it from a full bonding orbital into an empty anti-bonding orbital. Each jump takes energy from t he light, and a big jump obviously needs



Anti-bonding molecular orbitalorbital- It is a low low electron density density between between both nuclei, nuclei, which does little to to reduce the repulsion between the nuclei.

Electronic transitions possible in the uv/vis spectrum:  

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from pi bonding orbitals to pi anti-bonding orbitals; from non-bonding orbitals to pi anti-bonding orbitals; From non-bonding orbitals to sigma anti-bonding orbitals.

That means that in order to absorb light in the region from 200 - 800 nm (which is where the spectra are measured), the molecule must contain either pi bonds or atoms w ith nonbonding orbitals. Remember that a non-bonding orbital is a lone pair on, say, oxygen, nitrogen o r a halogen. Groups in a molecule which absorb light are known as chromophores.

N.B- the colour seen after absorption in the uv-vis region is the complementary colour of the one absorbed.

colour region wavelength (nm) violet

380 - 435

blue

435 - 500

cyan

500 - 520

green

520 - 565

yellow

565 - 590

orange

590 - 625

red

625 - 740

Uses of UV/Vis spectra: 1) For the identification of compounds 2) For finding the concentration of solutions eg iron in iron tablets by plotting plotting a calibration curve or using Beer-Lambert’s Law.

Beer-Lambert’s Law

Colours directly opposite each other on the colour wheel are said to be complementary c omplementary colours. Blue and yellow are complementary colours; red and cyan are complementary; and so are green and magenta. Mixing together two complementary colours of light will give you white light.



Infrared Spectroscopy 

All organic compounds absorb in the infrared region of the electromagnetic spectrum.

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Absorption in the infrared region arises from transitions from different vibrational modes within the same electronic level.

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Molecules vibrate as the bonds stretch and bend.

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The energy of vibrations are quantized.

A molecule must have a dipole moment to absorb in the IR region and the absorption of IR energy must change the dipole moment. The symmetrical stretch does not change the dipole mome nt.

Compounds may be examined as solids, liquids or gases:

SolidFinely ground 1mg of sample with a drop of hydrocarbon to form mull; mull pressed between NaCl discs and inserted into spectrometer.



SolutionCompound dissolved in tetrachloromethane or trichloromethane (neither dissolve strongly in IR re gion ). Solution placed in a cell made of NaCl and a similar cell, containing solvent only, is placed in the

reference beam.

GasesGas placed in 10cm long cell in the path of the IR beam. End walls of cell made of NaCl which is transparent to IR radiation.

USES OF IR SPEC: 1) Forensic science to identify the presence of certain substances 2) Monitoring air pollution 3) Following the course of a reaction eg reaction kinetics. .

CAPE CHEMISTRY Module 2 Spectroscopy

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