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Advanced Organic Chemistry: Mass spectrum of methylamine

Interpreting the mass spectrum of methylamine

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 methylamine

mass spectrum of methylamine CH5N CH3NH2 fragmentation pattern of m/z m/e ions for analysis and identification of methylamine image diagram doc brown's advanced organic chemistry revision notes 

Methylamine  CH5N   (c) doc b   (c) doc b   (c) doc b   (c) doc b

Interpreting the fragmentation pattern of the mass spectrum of methylamine

[M]+ is the molecular ion peak with an m/z of 31 corresponding to [CH5N]+, the original methylamine molecule minus an electron, [CH3NH2]+.

The tiny (M+1) peak at /z 32 may be due to the 1% of methylamine molecules with a 13C atom instead of a 12C atom.

The most abundant ion of the molecule under mass spectrometry investigation (methylamine) is usually given an arbitrary abundance 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 methylamine.

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

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

m/z value of [fragment]+ 30 29 28 27 16 15
[molecular fragment]+ [CH4N]+ [CH3N]+ [CH2N]+ [CHN]+ [NH2]+ [?]+

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

Atomic masses: H = 1;  C = 12; N = 14

Bond enthalpies kJ/mol: C-C = 348;  C-H = 412;  C-N = 305;  N-H = 391

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

Formation of m/z 30 ion:

[CH3NH2]+  ===>  [CH4N]+  +  H

Hydrogen atom loss, due to C-H or C-N bond scission, mass change 31 - 1 = 30.

The m/z 30 ion is the base peak ion, the most abundant and 'stable' ion fragment.

This ion can lose hydrogen atoms to give light ions of m/z 29, 28 and 27, in fact there is a tiny peak at m/z 26 corresponding to CN.

Formation of m/z 16 ion:

[CH3NH2]+  ===>  [NH2]+  +  CH3

C-N bond scission, mass change 31 - 15 = 16.

Formation of m/z 15 ion:

[CH3NH2]+  ===>  [CH3]+  +  NH2

C-N bond scission, mass change 31 - 16 = 15.

Note the ionisation of both fragments from the C-N bond scission of methylamine, either can ionise, but only one at a time.


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