[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 and AP honors chemistry courses: Molecular spectroscopy analysis of cyclobutane [spectra page updated Mar 22nd 2026 *]

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Mass spectrometry - spectra index

Introductory note on the mass spectrum of cyclobutane

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

If M represents the cyclobutane 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 cyclobutane and only the formation of singly charged positive are considered for the mass spectrum of cyclobutane.

I've included a stick diagram and table of m/z ions for the mass spectrum of cyclobutane 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 cyclobutane.

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 cyclobutane, but the mass spectrometer software does!

Cyclobutane, C₄H₈ , , it is not planar, it is bent

Interpreting the fragmentation pattern of the mass spectrum of cyclobutane

[M]⁺ is the molecular ion peak (M) with an m/z of 56 corresponding to [C₄H₈]⁺, the original cyclobutane molecule minus an electron.

The small M+1 peak at m/z 57, corresponds to an ionised cyclobutane molecule with one ¹³C atom in it i.e. an ionised cyclobutane molecule of formula [¹³C¹²C₃H₈]⁺

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.

Cyclobutane has 4 carbon atoms, so on average, ~1 in 25 molecules will contain a ¹³C atom.

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

The base peak ion for the mass spectrum of cyclobutane is the m/z 28 ion [C₂H₄]⁺

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

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

Some of the possible positive ions, [molecular fragment]⁺, formed in the mass spectrometry of cyclobutane.

The parent molecular ion of cyclobutane is m/z 56: [C₄H₈]⁺

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

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: H = 1;  C = 12 (~1% 13)

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

Possible equations to explain the most abundant ion peaks of cyclobutane (tabulated above)

Formation of m/z 55 and other ions:

[C₄H₈]⁺  ===>  [C₄H₇]⁺  +  H

C-H bond scission of the parent molecular ion, hydrogen atom loss,

mass change 56 - 1 = 55.(M-1 ion peak)

The m/z 55 ion can then lose hydrogen atoms to give the m/z 54 to 50 ions.

The m/z 54 ion could be formed elimination of a hydrogen molecule from the parent molecular ion.

[C₄H₈]⁺  ===>  [C₄H₆]⁺  +  H₂

mass change 56 - 2 = 55.(M-2 ion peak)

Again, this fragment can also lose hydrogen atoms, retaining the C₄ atoms to give m/z ions 53 to 50 (see table).

Formation of m/z 41 ion:

[C₄H₈]⁺  ===>  [C₃H₅]⁺  +  CH₃

C-C bond scission of the parent molecular ion, proton rearrangement and a methyl group expelled.

mass change 56 - 15 = 41 (M-15 ion peak)

Although much less probable, note that the methyl group could be the fragment that is ionised giving the m/z 15 ion.

The m/z 41 ion can further lose hydrogen atoms to give m/z 40 to 37 ions (see table).

The m/z 42 ion could be formed in the same way but containing a ¹³C atom i.e. it has the structure  [¹³C¹²C₂H₅]⁺ rather the [C₃H₅]⁺ 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 are:

For m/z 42: [C₃H₆]⁺  = 42.0468  and  [¹³C¹²C₂H₅]⁺ =  42.0424, a difference of 0.0044 in relative ion mass.

Formation of m/z 25 to 29 ions:

[C₄H₈]⁺  ===>  [C₂H₄]⁺  +  C₂H₄

The elimination of ethene via a double C-C bond scission? of the parent molecular ion.

mass change 56 - 28 = 28 (M-28 ion peak)

The m/z 28 ion is the base peak ion, the most abundant and 'stable' ion fragment.

There will be several routes to the formation of the m/z 29 and 27 ions e.g.

[C₄H₈]⁺  ===>  [C₂H₅]⁺  +  C₂H₃

[C₄H₈]⁺  ===>  [C₂H₃]⁺  +  C₂H₅

The m/z 28/29 ions can lose hydrogen atoms to give the m/z ions 27 down to 25 (see diagram).

There are lots of possibilities!

The chemistry of ALKANES revision notes INDEX

The infrared spectrum of cyclobutane

The H-1 NMR spectrum of cyclobutane

The C-13 NMR spectrum of cyclobutane

Mass spectrometry index

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

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