Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

Spectroscopy and Analytical Techniques
The three main analytical techniques you study at A-Level are infrared spectroscopy, NMR
spectroscopy, and mass spectrometry
...
At A2 you will learn about two types of NMR
spectroscopy: one using the isotope carbon-13, and the other using hydrogen
...
Questions on analysis tend to be long answer questions, worth at least 6 marks, therefore
it is very important that you note down ALL evidence, even if you can work out the structure without
it
...
A mass
spectrometer measures the ratio of mass-to-charge of ions, and uses this to measure the mass of a
sample
...
The m/z value is the mass to charge ratio, where m = mass and z =
the charge on the ion
...

Firstly, a sample is entered into the machine at the inlet
...
These
ions then enter a mass analyser after being separated by their respective m/z values
...
e
...
The ions are recorded by the detector, and this is output to a mass
spectrum displayed on a computer screen by specialist software
...
In the case of elements,
mass spectrometry is used to measure the m/z values of isotopes of the element present in the
sample
...
The
number of peaks on the spectrum corresponds to the number of isotopes present, and the heights
of the individual peaks correspond to the relative abundance of each isotope
...

Questions on the mass spectrometry of compounds are generally more common
...
In order to understand the mass spectrometry of compounds, you must first
understand what ions will be formed and why
...

Every compound analysed will produce a molecular ion, and, since it has the same mass as the
original sample molecule, it will result in a peak at the m/z value equal to the mass of the original
sample molecule
...
The peak produced by the molecular ion is always the peak
with the highest m/z value on the spectrum (rather obviously, as it is not possible to obtain a
measurement with a mass higher than that of the full molecule)
...


Less relevant to A-Level, but still important, are fragment ions
...
Fragment ions can be any part of the
sample molecule, if we go back to ethanol, some possible fragment ions that can be formed are
CH3+, OH+, and H+
...

These will produce peaks at lower m/z values on the spectrum, and are less useful at A-Level for
questions involving the analysis of an unknown compound, however, it is still important that you are
aware of them
...
All chemical molecules absorb infrared radiation, which will in turn cause their
bonds to vibrate either by stretching or by bending
...
In an infrared spectrometer, a beam of infrared energy containing
the full range of infrared frequencies is passed through a sample
...
Along the x-axis is the frequency, measured in
wavenumbers with the unit cm-1, and along the y-axis is the transmittance, measured in %
...

For exam purposes, infrared spectroscopy is used to determine the functional groups of a sample
molecule
...
g
...
Some of these
absorptions overlap, so it is important that you take into account every possibility when interpreting
an infrared spectrum
...
In a nucleus, protons and neutrons have a “spin”
...
If a nucleus has an
odd number of particles in the nucleus, the odd one out will not be able to pair up with a particle
with an opposite spin, which generates a magnetic field
...
The amount of energy the particle absorbs depends on nuclear shielding, which involves
the electrons and nearby atoms surrounding the nucleus
...

An NMR spectrum is displayed as a series of peaks
...
A measurement along the x-axis
will be written using the units δ/ppm, e
...
40 δ/ppm
...

Each TMS molecule is unreactive and has 12 equivalent protons, which gives rise to a distinct, easily
identifiable signal on an NMR spectrum, and so this is defined as the reference point δ = 0 ppm,
which all other measurements are taken relative to
...


Samples are carried out in solution and so a solvent is required, however, since we are measuring
using hydrogen and carbon isotopes, this solvent cannot contain carbon or hydrogen or else it would
produce a signal of its own
...
g
...
This does not produce a signal as there are an even
number of particles in the nucleus, so all particles with opposite spins are paired up
...

In carbon-13 NMR spectroscopy, the carbon isotope is used to identify the different environments
that carbons in a sample are in
...

Environments are only the same when everything around them is exactly identical, not just the
atoms bonded to the carbon: this will only happen in molecules which are symmetrical
...
A chain of five carbons with an OH group at one end will produce 5 different peaks, as
each carbon is a different distance away from the OH group, and therefore each carbon will have a
unique environment
...
A molecule with 3 different environments can only correspond with a spectrum with 3
different peaks
...

NMR spectroscopy using protons (1H nuclei) investigates hydrogen environments, as opposed to
carbon environments
...
The heights of the peak are proportional to the
number of protons in the environment, so the peak heights can be used to determine the ratio of
hydrogens in each environment
...
As you know, protons can spin either up or down, and normally particles with
opposite spins would pair up
...
g
...
These different magnetic fields generated will cause what would otherwise
be a single peak to be split into several, depending on the number possible combinations
...
In other words, if there are n protons on a carbon adjacent to a particular
environment, there will be n+1 peaks generated by that environment
...

The last key feature of proton NMR spectroscopy is the spectra of samples containing OH and NH
groups
...
Peaks generated by the protons in these groups would make it very difficult to read a
spectrum without mistakes, so these peaks are filtered out using D2O, deuterium oxide
...
As you already know, deuterium
will not produce a peak, as it has an even number of particles in the nucleus
...
First a proton
NMR spectrum is run as normal, and then a small amount of D2O is added
...




---