[Author ©  Dr Phil Brown PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses & US K12 grade 11, grade 12 & AP honors chemistry courses: Molecular spectroscopy of bromomethane [updated Mar 20th 2026 *]

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Introductory note on the mass spectrum of bromomethane

Students and teachers please note my explanation of the mass spectrum of bromomethane is designed for advanced, but pre-university, chemistry courses.

If M represents the bromomethane molecule, the initial ionisation to give the molecular ion is:

M_(g) + high KE e^-  ==> [M•]⁺_(g) + 2e^- and for fragmentation equations assume [M]⁺ is the start of the processes and all species are in a gaseous state.

I've not usually shown an unpaired electron on e.g. an ion or a non-ionised alkyl radical R e.g.

[M•]⁺ ==> [X]⁺  +  R•, but you should be aware this is a more accurate depiction of some processes.

I've used simplified equations to show how some of the ions that might be formed in the fragmentation pattern for the mass spectrum of bromomethane and only the formation of singly charged positive are considered for the mass spectrum of bromomethane.

I've included a stick diagram and table of m/z ions for the mass spectrum of bromomethane and doing the mass spectrum analysis under standard conditions, databases can be compiled based on complex fingerprint patterns, often involving the relative intensities of many fragment ions, and used to identify compounds including bromomethane.

In selected cases, where two different fragment ions have the same integer m/z value, I've pointed out that modern mass spectrometers can measure relative ion mass to four decimal places. So, using accurate isotopic masses, I've calculated the accurate ion masses, BUT strictly speaking, 0.0005 should be deducted for singly charged ions to account for the loss of the electron in their formation. I have NOT done this for bromomethane, but the mass spectrometer software does!

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. One peak is slightly shorter for the molecular ion with the less abundant heavier bromine isotope.

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. twin peaks of the molecular ion correspond to m/z 94 [CH₃⁷⁹Br]⁺ (the M ion peak) and m/z 96 [CH₃⁸¹Br]⁺ (the M+2 ion peak), because the latter has the slightly less abundance of the heavier bromine isotope.

It also means any fragment carrying a bromine atom should show up as twin peaks, two mass units apart and approximately of equal height (intensities), though intensity of ions with the heavier bromine isotope are slightly smaller..

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.

The base ion peak is one of the molecular ion peaks for bromomethane, the m/z ion 94 [CH₃⁷⁹Br]⁺

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.

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

PLEASE NOTE I have found it difficult to find 'authentic' equations to explain mass spectra fragmentation patterns and it is complex chemistry! I've identified the formulae of the ionised fragments on the mass spectrum diagram, but the equations are from the internet or my conjecture as to how the ions might be formed - please take care in using the information, especially for assignments at university or pre-university level.

Atomic masses: C = 12 (~1% 13);  H = 1; Br = 79 and 81

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

Possible equations to explain the most abundant ion peaks from the fragmentation of ionised bromomethane molecules.

Formation of the base ion

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

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]⁺ (M+2 ion peak)

Where R is alkyl, the double RBr peaks of roughly 1 : 1 abundance ratio are characteristic of organo-bromine compounds (one m/z ion peak is slightly shorter than the other, technically 50.7 : 49.3).

Formation of m/z 95 and 93 ions

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

Proton loss from the parent molecular ion (M-1 ion peak).

Further proton losses yields 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.

Low probability, very low intensity peaks.

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.

Low probability, very low intensity peaks.

Formation of m/z 15 ion

[CH₃Br]⁺  ===>  [CH₃]⁺  +  Br

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

This ion is much more abundant the m/z 79 and 81 ions because the electronegative bromine atom tends to be less easily ionised.

The infrared spectrum of bromomethane

The H-1 NMR spectrum of bromomethane

The C-13 NMR spectrum of bromomethane

The chemistry of HALOGENOALKANES (haloalkanes) revision notes INDEX

 Mass spectrometry - introduction and mass spectra index

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

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