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Aspirin has the following structure: enter image description here

What would the fragment structure be for m/z = 92?

Even though $\ce{C6H4O}$ is a molecular structure with mass 92, I don't think this will be formed since two bonds needs to be broken. Also the 92 peak is a major peak and the probability of two bonds breaking to lead to major peak is unlikely. The mass spectrum is as follows enter image description here

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    $\begingroup$ C6H4O has appropriate mass not C4 BTW there's no problem at all with losing two groups, there are further losses too, and peak 43 is even higher. $\endgroup$ – Mithoron Jun 2 '18 at 17:11
  • $\begingroup$ @Mithoron I have edited the question to change the C4H4O to $\ce{C6H4O}$ $\endgroup$ – Raoul Kessels Jun 4 '18 at 21:18
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This spectrum is recorded using electron impact. That means that the molecule is positively charged by losing one electron. Since that is a very unstable structure, the molecule starts fragmenting losing neutral fragments. More processes occur, but these are the ones that we see in the spectrum. The peaks are the positively charged fragments.

A fragment with m/z of 92, 93 or 94 is typical for phenolic compounds. The aromatic $\ce{C-O}$ bond is very strong and therefore first all the rest is lost. In this case the fragmentations are:

  1. Loss of the acetyl group: $180-42=138$
  2. Loss of water from the acid group: $138-18=120$
  3. Loss of $\ce{CO}$ from the acid group: $120-28=92$
  4. Loss of $\ce{CO}$ from the phenolic group: $92-28=64$

These are all typical fragmentations. The height of the peaks indicate the relative stability of the positively charged fragments.

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  • $\begingroup$ Just a comment on the first paragraph: it is generally not true to state that "Since that is a very unstable structure". In general, radical cations are minimums on the potential energy surface and some energy must be brought in to allow fragmentation. The point is that electron ionization deposits an extremely variable amount of energy in the system. The sequential fragmentation in the case of aspirin is in itself a nice illustration of this spread in internal energy: some ions are formed with enough energy to break 4 bonds, whereas others remain unfragmented as the molecular ion m/z 180. $\endgroup$ – PLD Jun 9 '18 at 15:48

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