17 - Spectroscopy Flashcards
Functions of mass spec
• Can be used to identify the molecular mass of an organic compound and to gain further information about its structure.
How does the molecular ion peak work
organic compound will lose an electron and form a positive ion = molecular ion (M+).
How does mass spec work - overall
detects the mass to charge ratio (m/z) which gives the molecular mass of the compound.
Mass spec always…
PRODUCES POSITIVE CHARGE
How to identify molecular ion peak
• Clear peak at the highest m/z value (on the right-hand side) of the mass spec
What is the m+1 peak
very small peak one unit after the M+ peak
Why does the m+1 peak exist
as 1.1% of carbon is present as the carbon-13 isotope
How does the number of carbon atoms effect m+1 peak
• The larger the number of carbon atoms present, the larger the M+1 peak.
Formular to find number of carbon atoms
How does mass spec work - full
- Atom is ionized to form positive ion
- Ions accelerated to same kinetic energy
- Ions deflected by a magnetic field - The amount of deflection depends on their mass
- Ions are detected.
What would make the ion deflect more
More mass and more positive charge = more deflection
Uses of mass spec
• Analysing molecules in space
• Detecting banned substances such as steroids in athletes
• Detecting traces of toxic chemicals in contaminated marine life
• Determine the abundance of each isotope of an element
• Identifying unknown compounds
• Gain further info about structure and chemical properties of molecules
What is fragmentation
• Molecular ion breaks up into smaller pieces
What are other smaller peaks caused by in mass spec
fragment ions
What does fragmentation break molecular ion into
positively charged fragment ion and a radical
Are the positive ions picked up by mass spec
Yes
Are radicals picked up by mass spec
No
Common Fragment ions - 15
Common Fragment ions - 17
Common Fragment ions - 29
Common Fragment ions - 43
Common Fragment ions - 57
Characteristics of covalent bonds
possess energy and vibrate (bend or stretch) around a central point.
Atoms in molecules are…
In constant motion
When does the vibration increase in covalent bonds
When temp increase
What also makes bonds vibrate more
absorb IR radiation
What is resonance
• The frequency of vibration occurs in the infra-red region of the electromagnetic spectrum
• If an organic molecule is irradiated with infra-red energy that matches the natural vibration frequency of its bonds, it absorbs some of that energy and the amplitude of vibration increases
Define stretch
distance between two atomic centres increase + decrease
Define bend
results in a change in bond angle
The amount a bond vibrates depends on what
o The mass of the atoms – heavier atoms vibrate more slowly
o The strength of the bond – stronger bonds vibrate faster
Any particular bond can only…
absorb radiation that has the same frequency as the natural frequency of the bond. (Specific functional groups absorb at specific frequencies)
Why do we use wave number instead of frequency
• Frequency scale is very large
How does the greenhouse effect work
• Most of the suns visible and UV radiation passes through the atmosphere to the Earth’s surface, where most of it is absorbed. Some is re-emitted as IR radiation.
• Greenhouse gases absorb this longer wavelength IR radiation as it has the same frequency as the natural frequency of the bonds.
• Eventually the vibrating bonds re-emit this energy as radiation, which increases the temp of the atmosphere (global warming)
Purpose of IR spec
identify the functional groups present in organic molecules
IR process
Sample placed into IR spectrometer
- Beam of IR radiation in the range of 200-4000cm-1 fired through sample
- Some IR radiation is absorbed by the sample, the emerging beam is analysed to work out the frequencies that have been absorbed
- IR spec. connected to a computer that plots a graph of transmittance against wavenumber.
The dips on an IR spec are
Peaks
Analysis of IR spec
• Each peak is observed at a wavenumber that corresponds to a particular bond in the molecule.
What is the fingerprint Region
• Contains unique peaks which can be used to identify a particular molecule. Compare to computer database/ published spectrum booklets.
Where is the fingerprint region
Below 1500
Uses of IR spec
• Breathalysers – detects the characteristic bonds present in ethanol. The more IR radiation absorbed, the more ethanol in the breath.
• Remote sensors that analyse vehicle emissions – detects pollutants (CO/CO₂/hydrocarbons).
