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

Advanced Organic Chemistry: Mass spectrum of dimethylamine (CH₃)₂NH

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Interpreting and explaining the mass spectrum of dimethylamine (N-methylmethanamine)

[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 spectrometry - analysing the mass spectra of dimethylamine [spectra page updated April 3rd 2026 *]

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Introductory note on the mass spectrum of dimethylamine

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

If M represents the dimethylamine molecule, the initial ionisation to give the molecular ion is:

M_(g) + high KE e^- ==> [M•]⁺_(g) + 2e^- and 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 dimethylamine and only the formation of singly charged positive are considered for the mass spectrum of dimethylamine.

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

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 and compared the accurate ion masses if appropriate for dimethylamine. 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 dimethylamine, but the mass spectrometer software does!

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

Dimethylamine, N-methylmethanamine, C₂H₇N, (c) doc b , ,

The classification, structure and naming of organic nitrogen compounds

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.

The base ion peak for the mass spectrum of dimethylamine is m/z 44 ion [C₂H₆N]⁺

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.

The parent molecular ion of dimethylamine is the m/z 44 ion [C₂H₇N]⁺ or [CH₃NHCH₃]⁺

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

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); 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 (M-1 ion peak)

The m/z 44 ion can lose more hydrogen atoms to give m/z ions of 43 down from

[C₂H₅N]⁺ 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]⁺ or [CH₃NH]⁺ + CH₃

C-N bond fission of the parent molecular ion, loss of methyl group,

mass change 45 - 15 = 30 (M-1 ion peak)

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]⁺.

The m/z 28 ion is probably [CH₂N]⁺, but theoretically could be the [C₂H₄]⁺ ion, but not likely since no C-C bond in the original parent molecule molecular ion of dimethylamine.

Note that an accurate mass spectrometer can sort them out, it can measure relative fragment ion masses to four decimal places e.g. using very accurate relative isotopic masses,

¹H = 1.0078 ¹²C = 12.0000 ¹⁴N = 14.0031

For m/z 28: [C₂H₄]⁺ = 28.0312 and [CH₂N]⁺ = 28.0187, a relative mass difference of 0.0125.

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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