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
