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Advanced Organic Chemistry: Mass spectrum of bromomethane

The mass spectrum of bromomethane (methyl bromide)

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses involving molecular spectroscopy analysing mass spectra of bromomethane

CH3Br mass spectrum of bromomethane fragmentation pattern of m/z m/e ions for analysis and identification of methyl bromide image diagram doc brown's advanced organic chemistry revision notes 

Bromomethane, CH3Br (methyl bromide)

Interpreting the fragmentation pattern of the mass spectrum of bromomethane

[M]+ is the molecular ion peak (M) with an m/z of 94 and 96 corresponding to [CH3Br]+, the original bromomethane molecule minus an electron.

There are two molecular ion peaks because bromine as two isotopes, 50.5% 79Br and 50.5% 81Br.

Their average relative mass is ~80, so the relative molecular mass for bromomethane is ~95.

However, this means any fragment carrying a bromine atom should show up as twin peaks, two mass units apart and approximately of equal height (intensities).

e.g. the twin peaks of the molecular ion correspond to m/z 94 [CH379Br]+ and m/z 96 [CH381Br]+.

The tiny M+1 peak at m/z 97, corresponds to an ionised bromomethane molecule with a 13C atom in it i.e. an ionised bromomethane molecule of formula [13CH381Br]+

There is also a nearly equal probability of m/z 95 ion [13CH379Br]+ being formed, but this m/z values applies to another fragment ion too (see table below), which is why the m/z 95 peak is higher than the m/z 97 peak.

Carbon-13 only accounts for ~1% of all carbon atoms (12C ~99%), but the more carbon atoms in the molecule, the greater the probability of observing this 13C M+1 peak.

Bromomethane has1 carbon atom, so on average, ~1 in 100 molecules will contain a 13C atom.

The most abundant ion of the molecule under mass spectrometry investigation (bromomethane) is usually given an arbitrary value of 100, called the base ion peak, and all other abundances ('intensities') are measured against it.

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of bromomethane.

Unless otherwise indicated, assume the carbon atoms in bromomethane are the 12C isotope.

m/z value of [fragment]+ 96 95 94 94 93 93 92
[molecular fragment]+ [CH381Br]+ [CH281Br]+ [CH379Br]+ [CH81Br]+ [C81Br]+ [CH279Br]+ [CH79Br]+
m/z value of [fragment]+ 91 82 81 80 79 15  
[molecular fragment]+ [C79Br]+ [H81Br]+ [81Br]+ [79BrH]+ [79Br]+ [CH3]+  

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of bromomethane

Atomic masses: C = 12;  H = 1; O = 16;  Cl = 35/37;  Br = 79/81; N = 14

Bond enthalpies kJ/mol:  C-H = 412; C-Br 276

Equations to explain the most abundant ion peaks of bromomethane

Formation of the base ion:

The m/z 94 ion [CH379Br]+ is the base peak ion, the most abundant and 'stable' ion fragment.

It also happens to be one of the parent molecular ion peaks formed in the initial ionisation, the other molecular ion being m/z ion 96 [CH381Br]+.

Formation of m/z 95 and 93 ions:

[CH3Br]+  ===>  [CH279Br]+  or  [CH281Br]  +  CH2

Proton loss from the parent molecular ion.

Further proton losses yield ions with m/z values of  94 to 91 - you will get peak overlap with different fragments with the same m/z value.

Formation of m/z 80 and 82 ions:

[CH3Br]+  ===>  [H79Br]+  or  [H81Br]  +  CH2

Elimination of hydrogen bromide from the parent molecular ion.

Formation of m/z 79 and 81 ions:

[CH3Br]+  ===>  [79Br]+  or  [81Br]  +  CH3

C-Br bond scission, where the bromine atom carries the positive charge.

Formation of m/z 15 ion:

[CH3Br]+  ===>  [CH3]+  +  Br

C-Br bond scission, where the methyl group carries the positive charge.


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