WM - Analytical techniques Flashcards
What does infrared spectroscopy help you identify?
Organic molecules
What happens in infrared spectroscopy?
A beam of IR radiation is passed through a sample of chemical.
It is absorbed by the covalent bonds in the molecules, increasing their vibrational energy (i.e. they vibrate more).
Bonds between different atoms absorb different frequencies of IR radiation. Bonds in different places in a molecule absorb different frequencies too.
What (in IR spec) absorbs different wavelengths of IR radiation?
Bonds between different atoms absorb different frequencies of IR radiation. Bonds in different places in a molecule absorb different frequencies too.
What absorbs the IR radiation in IR spec?
It is absorbed by the covalent bonds in the molecules, increasing their vibrational energy (i.e. they vibrate more).
What happens when the covalent bonds in the molecules absorb the IR radiation in IR spec?
Their vibrational energy is increased (i.e. they vibrate more).
What does an infrared spectrometer produce?
A spectrum that shows you what frequencies of radiation the molecules are absorbing.
What can you use the spectrum produced by an infrared spectrometer for?
Identifying the functional groups in a molecule.
What can you tell if you can use an IR spectrum to identify the functional groups in a molecule?
You can tell if a functional group has changed during a reaction.
What can mass spectrometry help to identify?
Compounds.
What can you get a mass spectra for?
Molecular samples.
What is a mass spectrum produced by?
A mass spectrometer.
What does a mass spectrometer produce?
A mass spectrum.
What happens in mass spec?
The molecules in the sample are bombarded with electrons, which remove an electron from the molecule to form a molecular ion, M+ (g).
What do you look at to find the relative molecular mass of a compound on a mass spectrum?
Molecular ion peak (the M peak).
What can you find by looking at the molecular ion peak (the M peak)?
The relative molecular mass of a compound.
How can you find out the relative molecular mass of a compound by looking at the molecular peak (the M peak)?
The mass/charge value of the molecular ion peak is the molecular mass.
What is assumed when we say that the mass/charge value of the molecular ion peak is the molecular mass of the compound?
Assuming the ion has a 1+ charge, which it normally will have.
What is the y-axis on an IR spectrum?
Transmittance %.
What is the x-axis on an IR spectrum?
Wavenumber (cm^-1)
What do the peaks on an IR spectrum show?
Where radiation is being absorbed.
Which way are the peaks on IR spectra?
Upside-down (they point downwards).
What is wavenumber?
The measure used for the frequency (it’s just 1/wavelength in cm.
What is the y-axis on a mass spectrum?
The relative abundance of ions (often as a percentage).
What is the x-axis on a mass spectrum?
Mass/charge (m/z)
For most organic compounds, which peak is the M peak?
The one with the second highest mass/charge ratio.
What is the smaller peak to the right of the M peak called?
The M+1 peak.
What is the M+1 peak caused by?
The presence of the carbon isotope 13C.
What can the molecular ion be broken down into?
Smaller fragments.
What makes the molecular ions break up into fragments?
The bombarding electrons.
What is shown on the mass spectrum?
The fragments that are ions - making a fragmentation pattern.
What can you use fragmentation patterns to do?
Identify molecules and even their structure.
Why aren’t free radicals shown on the mass spectrum?
They are ‘lost’.
How do you work out the structural formula from the mass spectrum?
You have to work out what ion could have made each peak from its m/z value.
What do you assume when working out the structural formula from a mass spectrum?
You assume that the m/z value of a peak matches the mass of the ion that made it.
What is the molecular mass of the common fragment CH3+?
15
What is the molecular mass of the common fragment C2H5+?
29
What is the molecular mass of the common fragment CH3CH2CH2+?
43
What is the molecular mass of the common fragment OH+?
17
What common fragment makes up the molecular mass 15?
CH3+
What common fragment makes up the molecular mass 29?
C2H5+
What common fragment makes up the molecular mass 43?
CH3CH2CH2+
What common fragment makes up the molecular mass 17?
OH+
What can you combine IR spectroscopy and mass spectrometry to do?
Identify a compound.
How do you combine IR spectroscopy and mass spectrometry to identify a compound?
1) Work out what functional groups are in a compound from its infrared spectrum.
2) Use the mass spectrum to work out the structure of the molecule.
What is meant by molecular ion?
When an electron have been removed from the original molecule.
Why do most organic compounds have a peak at about 3000 cm^-1 on IR spectra?
This is caused by the C-H groups.
