Lecture Five - Mass spectrometry, IR spectroscopy and NRM spectroscopy Flashcards
What is Mass Spectromotry?
Used to determine the molecular weight of a compound.
The output form the detector is called a mass spectrum, plot of signal intensity agains mass:charge (m/z) ratio.
You can have high and low resolution mass spec. High resolution MS is important for distinguishing between compound compositions of similar masses.
How can you find the molecular formula using m/z and percentage abundances of the atoms in the molecule? Elemental analysis or microanalysis.
Divide all percentages by the atoms atomic number.
Divide by the smallest number.
This finds the empirical formula, if the sum of all atomic weights equal the molecular weight, it is also the molecular formula. If not, the values may have to be doubled/ tripled etc. to find the molecular formula.
What is the index of hydrogen deficiency?
Also known as double bond equivilance.
Information about the presence of rings or unsaturation can be obtained from the molecular formula.
Index of hydrogen deficiency = (H reference - H molecule) / 2.
What is IR spectroscopy?
The absorbance of infrared radiation provides information regarding functional groups in a molecule.
For a bond to be IR active it must have a dipole moment.
Records the transmittence of a functional group = the opposite of absorbance.
What causes the transmittance readings in IR spectroscopy?
The simplest vibrational motions in molecules giving rise to the absorption of intrared radiation are stretching and bending molecules.
What are the transmittance regions for common functional groups in IR spectroscopy?
Note that the exact frequencies depend on the strength of the bond.
What is NMR Spectroscopy?
Nuclear Magnetic Resonance spectroscopy is a powerful technique that gives us three types of information.
- Fucntional groups.
- Number of different groups.
- Conectivity of different groups.
Allows you to find information about the different hydrogen and cabron environments in a molecule.
NMR can only ‘read’ atoms with nuclei which are uneven.
How is NMR measured (spin)?
A spinning charge creates an associated magnetic field.
If a nucleus of 1H is placed in a strong ecternal magnetic field, its magnetic moment will line up with the field.
The moment can be parallel or antiparallel with the field.
Being parallel with the field is more stable.
When in NMR, the spin is flipped, making more nuclei anti-parallel.
Absorption of the radio-frequency radiation of the appropriate energy flips the nuclear spin.
When the spin is flipped back to its original state, energy is released and this energy is measured and this is what makes up the NMR spectrum.
Each nucleus that generates a unique magnetic field so that we can differentiate between the different hydrogen and carbon envirnments within a molecule.
How are NMR readings standardised?
Chemists have adopted a quantity called Chemical Shift (
