Mass and IR spec Flashcards
What happens when infrared radiation is directed at
a compound?
Bonds within the molecule absorb
specific frequencies of radiation.
What happens when specific frequencies of radiation
are absorbed by bonds in a molecule?
The bonds vibrate. This causes the bond
to stretch or bend.
What can be identified using peaks on an infrared
spectrum? How?
The functional groups within a molecule
can be identified by comparing the peaks
to known values in a data book.
What is the fingerprint region on an infrared
spectrum?
The fingerprint region is the region typically
between 1500 cm-1 and 500 cm-1 that contains a
complicated series of absorptions. Every
compound has a unique fingerprint region.
Briefly describe what happens during mass
spectrometry
A vaporised sample of atoms of molecules is turned
into positive ions. The ions are accelerated,
deflected by a magnetic field and then detected. This
produces a graph with mass to charge ratio (m/z) on
the x axis and relative abundance on the y axis.
What is the molecular ion (M+) peak on a mass
spectrum?
The peak with the greatest mass to
charge ratio. The molecular mass of a
compound is equal to the m/z value of
this peak.
What is the M+1 peak on a mass spectrum? What is
it caused by?
A tiny peak 1 unit to the right of the molecular ion
peak. This is caused by the presence of the 13C
isotope (relative abundance 1.11%). 13C has one
more neutron than 12C meaning that the relative
formula mass is increased by 1.
How can the relative heights of the M+ and M+1
peaks be used to predict the number of carbon
atoms in a molecule?
n is the number of carbon atoms
When would an M+2 peak be seen on a mass
spectrum?
When a compound contains a chlorine or
bromine atom.
Describe how the presence of a chlorine atom in a
compound causes a M+2 peak on the mass spectrum
In these compounds, chlorine exists as two isotopes:
35Cl or 37Cl. Compounds containing the 37Cl isotope
will have a relative formula mass that is 2 units larger
than compounds containing the 35Cl isotope, causing
the M2+ peak.
What is the ratio of the peak heights of the M+ and
M+2 peaks on the mass spectrum of a compound
containing chlorine? Why?
3:1
Chlorine is 3 times more likely to be 35Cl
than 37Cl.
Describe how the presence of a bromine atom in a
compound causes an M+2 peak on the mass
spectrum
Bromine exists as two isotopes: 79Br or 81Br. Compounds
containing the 81Br isotope will have a relative formula
mass that is 2 units larger than compounds containing
the 79Br isotope, causing the M2+ peak.
Using the mass spectrum below, what is the relative
abundance of the isotope with a relative isotopic
mass of 80.1?
83%
How can a mass spectrum be used to deduce the
relative molecular mass?
The peak with the highest m/z value (the molecular
ion peak or M+ peak) is caused by the whole
molecule after one electron has been knocked off.
m/z value of M+ peak = molecular mass
How many molecular peaks will a compound
containing three chlorine atoms have?
Four molecular peaks. There are two isotopes of chlorine
so there a four different combinations:
35 + 35 + 35
35 + 35 + 37
35 + 37 + 37
37 + 37 + 37
Describe the formation of fragment ions during mass
spectrometry
Fragment ions form when an unstable
molecular ions break down into a positive
ion and an uncharged free radical.
M+ → X+ + Y•
What is a free radical?
A species with an unpaired electron.
Why are the free radicals formed during the
fragmentation of molecular ions not shown as peaks
on the mass spectrum?
These free radicals are uncharged. Only
charged particles are detected.
How can a fragment ion be identified from a mass
spectrum?
Compare the m/z value of the fragment
ion peak with the relative formula
masses of possible fragment ions.
