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Introductory note on the mass spectrum of 1,3,5-trimethylbenzene (mesitylene)

Students and teachers please note my explanation of the mass spectrum of 1,3,5-trimethylbenzene (mesitylene) is designed for advanced, but pre-university, chemistry courses.

If M represents the 1,3,5-trimethylbenzene (mesitylene) 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 1,3,5-trimethylbenzene (mesitylene) and only the formation of singly charged positive are considered for the mass spectrum of 1,3,5-trimethylbenzene (mesitylene).

I've included a stick diagram and table of m/z ions for the mass spectrum of 1,3,5-trimethylbenzene (mesitylene) 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 1,3,5-trimethylbenzene (mesitylene).

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 1,3,5-trimethylbenzene (mesitylene), but the mass spectrometer software does!

The molecular structure and naming of aromatic compounds

Interpreting the fragmentation pattern of the mass spectrum of 1,3,5-trimethylbenzene (mesitylene, C₆H₃(CH₃)₃)

[M]⁺ is the molecular ion peak (M) with an m/z of 120 corresponding to [C₉H₁₂]⁺, the original 1,3,5-trimethylbenzene (mesitylene) molecule minus an electron, [C₆H₃(CH₃)₃)]⁺

The small M+1 peak at m/z 121, corresponds to an ionised 1,3,5-trimethylbenzene (mesitylene) molecule with one ¹³C atom in it i.e. an ionised 1,3,5-trimethylbenzene (mesitylene) molecule of formula ¹³C¹²C₈H₁₂

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.

Mesitylene has 9 carbon atoms, so on average, ~1 in 11 molecules will contain a ¹³C atom.

The most abundant ion of the molecule under mass spectrometry investigation (1,3,5-trimethylbenzene) is usually given an arbitrary value of 100, called the base ion peak, and all other abundances ('intensities') are measured against it.

In this case the base ion peak for mesitylene is m/z 105 ion [C₈H₉]⁺

The molecular ion for mesitylene is the m/z ion 120  [C₉H₁₂]⁺   or  [C₆H₃(CH₃)₃)]⁺

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of 1,3,5-trimethylbenzene (mesitylene).

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 1,3,5-trimethylbenzene (mesitylene)

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.

Suggested equations to explain the most abundant ion peaks of 1,3,5-trimethylbenzene (mesitylene)

Atomic masses: C = 12 (~1 in 100 is 13);  H = 1

Bond enthalpies kJ/mol: C-C = 348;  C-H = 412;  aromatic = 518

Formation of m/z 119 ion

[C₆H₃(CH₃)₃)]⁺  ===>  [C₈H₉]⁺  +  CH₃

Loss of proton from the parent molecular ion.

mass change 120 - 1 = 119  (M-1 ion peak)

Formation of m/z 103 and 105 ions

[C₆H₃(CH₃)₃)]⁺  ===>  [C₈H₉]⁺  +  CH₃

The m/z 105 ion is the base peak ion, the most abundant and 'stable' ion fragment, formed by the loss of a methyl group from the parent molecular ion after C-C bond scission in the parent molecular ion.

mass change 120 - 15 = 105  (M-15 ion peak)

The m/z 106 ion could be [¹³C¹²C₇H₉]⁺ formed by the same process of methyl group loss from the parent molecular ion (instead of a [C₈H₁₀]⁺ ion).

The m/z 103 ion could be formed by hydrogen loss from the m/z 105 ion.

[C₈H₉]⁺  ===>  [C₈H₇]⁺  +  H₂

The m/z 104 ion could be [¹³C¹²C₇H₇]⁺ formed by the same process of methyl group loss from the parent molecular ion (instead of a [C₈H₈]⁺ ion).

Note that an accurate mass spectrometer can sort out (resolve) pairs of ions with the same integer m/z value because they can measure relative fragment ion masses to four decimal places,

e.g. using accurate relative isotopic masses:

¹H = 1.0078  ¹²C = 12.0000   ¹³C = 13.0034: you can then calculate (predict) that the accurate relative ion masses:

For m/z 104: [C₈H₈]⁺ = 104.0624  and  [¹³C¹²C₇H₇]⁺ = 104.0580, relative ion mass difference of 0.0044

For m/z 106: [C₈H₁₀]⁺ = 106.0780  * [¹³C¹²C₇H₉]⁺ =  106.0736, relative ion mass difference of 0.0044

The formation of the m/z 91 ion

[C₈H₉]⁺  ===>  [C₇H₇]⁺  +  CH₂

C-C bond fission and proton rearrangement from the m/z 105 ion.

mass change 105 - 14 = 91

The formation of the m/z 77 ion

[C₇H₇]⁺  ===>  [C₆H₅]⁺  +  CH₂

C-C bond fission and proton rearrangement from the m/z 91 ion.

Mass change 91 - 14 = 77

This is a phenyl cation and very characteristic in the mass spectrum of aromatic benzene compounds like 1,3-dimethylbenzene.

The formation of ions of m/z <77

The m/z 77 ion will fragment and other smaller ions will lose C/H atoms, most of which show up as relatively low intensity peaks.

The pattern often shows the equivalent of successive CH2 loss e.g. the m/z fragment ion sequence of 79 ==>  65  ==> 51  ==>  39

The infrared spectrum of 1,3,5-trimethylbenzene

The H-1 NMR spectrum of 1,3,5-trimethylbenzene

The C-13 NMR spectrum of 1,3,5-trimethylbenzene

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