Module 4 - [ch17] (spectroscopy)
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what is the molecular ion
when the organic compound is placed into the mass spectrometer it loses and electron to form a positive ion
where do you find M+ peak on a mass spectrum
at highest m/z value however, often before the very highest due to the M+1 peak
what are the axis on a mass spectrum graph
relative intensity against m/z
why does the M+1 peak exist
1.1% of carbon is present as the C13 isotope.
what is fragmentation
some molecular ions break down into smaller pieces known as fragments fragment ions cause the other peaks in a mass spectrum
What is the simplest way a molecular ion can fragment
fragment cation + radical
4 common fragment ions and their corresponding m/z values
CH3+ = 15 C2H5+ = 29 C3H7+ = 43 C4H9+ = 57
what happens when a molecule absorbs infrared radiation
covalent bonds absorb energy and vibrate about a central point more in a stretching and bending motion
what are the types of vibration
stretch increases and decreases the distance between the atomic centres bend results in a change of bond angle
Which factors affect the amount of vibration of a bond
mass of atoms in the bond ( heavier atoms vibrate slower than lighter ones ) strength of bond ( stronger bonds vibrate faster than weaker ones )
what is the condition for the radiation absorbed by a particular bond
has to have the same frequency as the natural frequency of the bond
why is wavenumber used?
frequency values are very large wavenumber is proportional to frequency vibrations of most bonds are observed between wavenumber 200cm^-1 to 4000cm^-1
greenhouse effect
IR radiation emitted by sun absorbed by earth’s surface, some re-emitted in the form of longer-wavelength IR radiation greenhouse gases absorb this radiation vibrating bonds in these molecules re-emit this energy as radiation which increases the temp of the earth’s atmosphere near the surface - global warming
how does IR spectroscopy work
• the sample is placed inside an IR spectrometer • A beam of IR radiation is passed through the sample • the molecule absorbs some of the IR frequencies, the emerging beam is analysed to identify the frequencies that are missing (that have been absorbed) • spectrometer connected to a computer which plots a graph of transmittance against wavenumber
where is the fingerprint region
below 1500cm^-1
what does the alcohol (O-H) peak look like
smooth peak (looks like a positive quadratic) 3200 - 3600cm^-1
what to look for when searching for an aldehyde or ketone
C = O bond peak close to 1700cmm^-1
what to look for when searching for a carboxylic acid
broad peak at O-H and C=O peak
applications of infrared spectroscopy
pollutants can be identified by the IR spectral fingerprints IR based breathalysers, pass a beam of IR radiation through breath
what happens in a mass spectrometer
sample vaporised and ionised to form molecular ions excess energy from ionisation causes bonds to vibrate more vibration causes bonds to weaken molecular ion splits by fragmentation
how do you determine the Mr of a compound using a mass spectrum
molecular masses of the molecular ions is equal to the Mr of the compound
what are the advantages of mass spectrometry (2)
cheap small quantities of sample needed
what is the main advantage of using mass spectrometry
the sample is destroyed