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Advanced Organic Chemistry: Mass spectrum of dimethylamine

Interpreting the mass spectrum of dimethylamine (N-methylmethanamine)

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 dimethylamine

mass spectrum of dimethylamine C2H7N CH3NHCH3 fragmentation pattern of m/z m/e ions for analysis and identification of N-methylmethanamine image diagram doc brown's advanced organic chemistry revision notes 

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Interpreting the fragmentation pattern of the mass spectrum of dimethylamine

[M]+ is the molecular ion peak with an m/z of 45 corresponding to [C2H7N]+, the original dimethylamine molecule minus an electron, [CH3NHCH3]+.

The small M+1 peak at m/z 46, corresponds to an ionised dimethylamine molecule with one 13C atom in it i.e. an ionised dimethylamine molecule of formula [13C12CH7N]+

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.

Dimethylamine has 2 carbon atoms, so on average, ~1 in 50 molecules will contain a 13C atom.

The most abundant ion of the molecule under mass spectrometry investigation (dimethylamine) 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 dimethylamine.

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

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

m/z value of [fragment]+ 44 43 42 41 40 30 28 27 15
[molecular fragment]+ [C2H6N]+ [C2H5N]+ [C2H4N]+ [C2H3N]+ [C2H2N]+ [CH4N]+ [CH2N]+ [CHN]+ [CH3]+

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

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 dimethylamine (tabulated above)

Formation of m/z 44 ion:

[CH3NHCH3]+  ===>  [C2H6N]+  +  H

N-H or C-H bond scission to lose a hydrogen atom, mass change 45 - 1 = 44.

The m/z 44 ion can lose more hydrogen atoms to give m/z ions of 43 down to 38, [C2N]+.

The m/z 44 ion is the base peak ion, the most abundant and 'stable' ion fragment.

Formation of m/z 30 ion:

[CH3NHCH3]+  ===>  [CH4N]+  +  CH3

C-N bond fission of the parent molecular ion, loss of methyl group, mass change 45 - 15 = 30.

The formation of this m/z 30 ion is very characteristic of aliphatic amines.

Successive hydrogen atom loss gives m/z ions of 29, 28, 27 and 26 [CN]+.

Formation of m/z 15 ion:

[CH3NHCH3]+  ===>  [CH3]+  +  CH4N

C-N bond fission, loss of methyl group, mass change 45 - 30 = 15.

The less likely, but alternative ionisation resulting from the C-N bond scission of the parent molecular ion of dimethylamine.

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