C2H6O CH3OCH3 mass spectrum of methoxymethane fragmentation pattern of m/z m/e ions for analysis and identification of dimethyl ether image diagram doc brown's advanced organic chemistry revision notes

Advanced Organic Chemistry: Mass spectrum of methoxymethane CH₃OCH₃

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Interpreting the mass spectrum of methoxymethane (dimethyl ether)

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Introductory note on the mass spectrum of methoxymethane

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

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

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

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

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 the accurate ion masses, 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 methoxymethane, but the mass spectrometer software does!

C2H6O CH3OCH3 mass spectrum of methoxymethane fragmentation pattern of m/z m/e ions for analysis and identification of dimethyl ether image diagram doc brown's advanced organic chemistry revision notes

Methoxymethane (dimethyl ether)

Revision notes on the structure and naming (nomenclature) of aliphatic ALCOHOLS and ETHERS

Interpreting the fragmentation pattern of the mass spectrum of methoxymethane

[M]⁺ is the molecular ion peak (M) with an m/z of 46 corresponding to [C₂H₆O]⁺, the original methoxymethane molecule minus an electron, [CH₃OCH₃]⁺.

The small M+1 peak at m/z 47, corresponds to an ionised methoxymethane molecule with one ¹³C atom in it i.e. an ionised methoxymethane molecule of formula [¹³C¹²CH₆O]⁺

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.

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

In the mass spectrum of ethers, an M+1 ion can also be formed by a hydrogen atom radical combining with the molecular ion i.e. m/z 47 can originate from [M]⁺ + H• ===> [•MH]⁺

The most abundant ion of the molecule under mass spectrometry investigation (methoxymethane) 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 methoxymethane is the m/z 45 ion [CH₃OCH₂]⁺

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

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

Some of the possible positive ions, [molecular fragment]⁺, formed in the mass spectrometry of methoxymethane based on parent molecular ion, m/z 46 [CH₃OCH₃]⁺.

m/z value of [fragment]⁺	45 [C₂H₃O]⁺	31	29	15
[molecular fragment]⁺	[CH₃OCH₂]⁺	[CH₃O]⁺	[CHO]⁺	[CH₃]⁺

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

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); O = 16

Bond enthalpies = kJ/mol: C-H = 412; C-O = 360

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

Formation of m/z 45 ion:

[CH₃OCH₃]⁺ ===> [CH₃OCH₂]⁺ + H

Formed by hydrogen radical loss from the parent molecular ion of methoxymethane, involves scission of a C-H bond,

mass change 46 - 1 = 45 (M-1 ion peak)

The m/z 45 ion is the base peak ion, the most abundant and 'stable' ion fragment.

Formation of m/z 31 ion:

[CH₃OCH₃]⁺ ===> [CH₃O]⁺ + CH₃

Formed by scission of the C-O bond in the parent molecular ion,

mass change 46 - 15 = 31 (M-15 ion peak)

Formation of m/z 29 ion:

[CH₃O]⁺ ===> [CHO]⁺ + H₂

Elimination of hydrogen from the m/z 31 ion?

mass change 31 - 2 = 29

Formation of m/z 15 ion:

[CH₃OCH₃]⁺ ===> [CH₃]⁺ + CH₃O

Formed by scission of the C-O bond in the parent molecular ion (see note on m/z 31 ion).

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Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 2 isomers of C₂H₆O NOTE: The images are linked to their original detailed spectral analysis pages AND can be doubled in size with touch screens to increase the definition to the original ethanol (ethyl alcohol) and methoxymethane (dimethyl ether) image sizes.

INFRARED SPECTRA: Apart from the significant differences in the fingerprint region at wavenumbers 1500 to 400 cm^-1, the most striking difference is the broad O-H stretching band ~3400 cm^-1, found in the infrared spectrum of alcohols, but absent in the infrared spectrum of ethers.

MASS SPECTRA: Both ethanol and methoxymethane show some similarities in their mass spectra, but their base ion peaks are quite different - for ethanol it is m/z 31 and for methoxymethane it is m/z 45. Methoxymethane gives the m/z 29 [CHO]⁺ ion, which can be distinguished from the m/z 29 [C₂H₅]⁺ ion by high resolution spectroscopy. Methoxymethane has less abundant peaks for m/z ions 26, 27 and 43.

1H NMR SPECTRA: The 1H NMR spectra of ethanol and methoxymethane are quite significantly different. Ethanol gives 3 peaks in the proton ratio 3:2:1 (3 different chemical environments), whereas methoxymethane only gives one ¹H chemical shift peak (all 6 protons in the same chemical environment).

13C NMR SPECTRA: The 13C NMR spectra of ethanol and methoxymethane are different. Ethanol gives two ¹³C resonances, but methoxymethane only one (2 different ¹³C chemical environments and a ¹³C single chemical environment).

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What next? links associated with methoxymethane

The infrared spectrum of methoxymethane (dimethyl ether)

The H-1 NMR spectrum of methoxymethane (dimethyl ether)

The C-13 NMR spectrum of methoxymethane (dimethyl ether)

Mass spectrometry index

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