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Advanced Organic Chemistry: Mass spectrum of trimethylamine

Interpreting the mass spectrum of N,N-dimethylmethanamine (N,N-dimethylmethylamine, trimethylamine)

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mass spectrum of N,N-dimethylmethanamine (trimethylamine) C3H9N (CH3)3N fragmentation pattern of m/z m/e ions for analysis and identification of N,N-dimethylmethylamine image diagram doc brown's advanced organic chemistry revision notes 

N,N-dimethylmethanamine, N,N-dimethylmethylamine, trimethylamine, (c) doc b ,(c) doc b

Interpreting the fragmentation pattern of the mass spectrum of N,N-dimethylmethanamine (trimethylamine)

[M]+ is the molecular ion peak with an m/z of 59 corresponding to [C3H9N]+, the original N,N-dimethylmethanamine (trimethylamine) molecule minus an electron, [(CH3)3N]+.

The small M+1 peak at m/z 60, corresponds to an ionised N,N-dimethylmethanamine (trimethylamine) molecule with one 13C atom in it i.e. an ionised N,N-dimethylmethanamine (trimethylamine) molecule of formula [13C12C2H9N]+

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.

N,N-dimethylmethanamine (trimethylamine) has 3 carbon atoms, so on average, ~1 in 33 molecules will contain a 13C atom.

The most abundant ion of the molecule under mass spectrometry investigation (N,N-dimethylmethanamine (trimethylamine)) 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 N,N-dimethylmethanamine (trimethylamine).

Unless otherwise indicated, assume the carbon atoms in N,N-dimethylmethanamine (trimethylamine) are the 12C isotope.

Some of the possible positive ions, [molecular fragment]+, formed in the mass spectrometry of N,N-dimethylmethanamine (trimethylamine).

m/z value of [fragment]+ 58 57 56 45 44 43
[molecular fragment]+ [C3H8N]+ [C3H7N]+ [C3H6N]+ [C2H7N]+ [C2H6N]+ [C2H5N]+
m/z value of [fragment]+ 42 41 40 30 29 28 15
[molecular fragment]+ [C2H4N]+ [C2H3N]+ [C2H2N]+ [CH4N]+ [CH3N]+ [CH2N]+ [CH3]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of N,N-dimethylmethanamine (trimethylamine)

Atomic masses: H = 1;  C = 12;  N = 14

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

Possible equations to explain the most abundant ion peaks of N,N-dimethylmethanamine (trimethylamine) (tabulated above)

Formation of m/z 58 ion:

[(CH3)3N]+  ===>  [C3H8N]+  +  H

C-H bond scission, loss of hydrogen atom, mass change 59 - 1 = 58

The m/z 58 ion is the base peak ion, the most abundant and 'stable' ion fragment.

The m/z 57 and 56 ions will be formed by further hydrogen atom loss.

Formation of m/z 44 ion:

[(CH3)3N]+  ===>  [C2H6N]+  +  CH3

C-N bond scission, loss of methyl group from the parent molecular ion, mass change 59 - 15 = 44.

The m/z 43, 42 and 41 ions will be formed by further hydrogen atom loss.

Formation of m/z 30 ion :

[C2H7N]+  ===>  [CH4N]+  +  CH3

C-N bond scission, loss of methyl group from the m/z 45 ion ?

The m/z 29 and 28 ions will be formed by further hydrogen atom loss.

This is a characteristic ion formed in the mass spectrum of aliphatic amines.

Formation of m/z 15 ion:

[(CH3)3N]+  ===>  [CH3]+  +  C2H6N

C-N bond scission, loss of methyl group from the parent molecular ion, but it carries the positive charge, mass change 59 - 44 = 15.

The positive methyl ion could be formed from other fragments too.


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