Introductory note on the mass spectrum of cyclohexene

cyclohexene , ,  , 

Interpreting the fragmentation pattern of the mass spectrum of cyclohexene

[M]⁺ is the parent molecular ion peak (M) with an m/z of 82 corresponding to cyclo [C₆H₁₀]⁺, the original cyclohexene molecule minus an electron.

Unless otherwise stated, C means a ¹²C atom, if not, the isotopic carbon atom ¹³C will be indicated.

The small M+1 peak at m/z 83, corresponds to an ionised cyclohexene molecule with one ¹³C atom in it i.e. an ionised cyclohexene molecule of formula [¹³C¹²C₅H₁₀]⁺

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 ion of m/z 83.

Cyclohexene has 6 carbon atoms, so on average, ~1 in 17 molecules will contain a ¹³C atom.

This sort of argument also applies to fragment ions from the parent molecular ion of cyclohexene - though the ratio will be greater e.g. m/z 68 could be [C₅H₈]⁺  or  [¹³C¹²C₄H₇]⁺.

The most abundant ion of the molecule under mass spectrometry investigation (cyclohexene) 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 ion peak for the mass spectrum of cyclohexene is the m/z 67 ion [C₅H₇]⁺

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

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

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

The mass spectrum parent molecular ion of cyclohexene is m/z 82: [C₆H₁₀]⁺

Data table of some of the ions formed in the fragmentation pattern of the mass spectrum of cyclohexene

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

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;  C=C = 612

Possible equations to explain some of the most abundant ion peaks in the mass spectrum of cyclohexene (tabulated above)

Formation of m/z 81 ion:

[C₆H₁₀]⁺  ===>  [C₆H₉]⁺  +  H

Loss of a hydrogen atom from the parent molecular ion,

mass change 82 - 1 = 81 (M-1 ion peak)

The m/z 81 or 82 ions can lose hydrogen atoms/molecule to give the m/z ions of 81 down to 77 (see data table and mass spectrum diagram).

Formation of m/z 67 ion:

[C₆H₁₀]⁺  ===>  [C₅H₇]⁺  +  CH₃

Formed by the loss of CH₃ (CH₂ + adjacent H) by ring scission of the parent molecular ion,

mass change 82 - 15 = 67 (M-15 ion peak)

The m/z 67 ion is the base peak ion, the most abundant and 'stable' ion fragment.

The m/z 68 peak could have the same structure as the m/z ion 67, formed by the same process, but be  [¹³C¹²C₄H₇] with a carbon-13 atom in it.

The m/z 67 ion can lose hydrogen atoms to give the m/z 65 ion [C₅H₅]⁺

Formation of m/z 54 ion

[C₆H₁₀]⁺  ===>  [C₄H₆]⁺  +  C₂H₄

Perhaps loss of an unionised ethene molecule from the parent molecular ion?,

mass change 82 - 28 = 54 (M-28 ion)

This is the 2nd most abundant fragment in the spectrum.

The m/z 55 peak could have the same structure as the m/z ion 54, formed by the same process, but be  [¹³C¹²C₃H₅] with a carbon-13 atom in it.

The m/z 55 ion can lose hydrogen atoms to give the m/z ions from 53, 52, 51 and 50.

Formation of m/z 38 to 42 ions

[?]⁺  ===>  [C₃H_x]⁺  +  ?  (where x = 2 to 6)

Remainder of some C₄ to C₆ fragment ion?

There are several possibilities for some of these [C₃H_x]⁺ ions e.g. involving some including a ¹³C isotope.

An accurate mass spectrometer sorts this out, measuring relative fragment ion masses to four decimal places e.g. using very 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 40: [¹³C¹²C₂H₃]⁺ =  40.0268, [C₃H₄]⁺ = 40.0312,  a difference of 0.0044 in relative ion mass.

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

Formation of m/z 27 ion

[?]⁺  ===>  [C₂H₃]⁺  +  ?

Remainder of some C₃ to C₆ fragment ion?

My spectra data source https://sdbs.db.aist.go.jp/Disclaimer.aspx for my m/z ion data

The infrared spectrum of cyclohexene

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