# Fragmentation of phenyl radical cations in mass spectrometry

What are the main fragmentation pathways and products of a phenyl radical cation in mass spectrometry ? I notice sometimes that there's a m/z = 51 peak after the m/z = 77 for phenyl radical cation; I assume due to loss of acetylene.

• Can you give more details? What type of ionization? What is the source of the phenyl radicals? What types of collision cells etc. are used in the mass spectrometry you're thinking of? – Curt F. Apr 9 '15 at 14:55
• Phenyl radical source: benzylic acid; I'm not familiar with the experimental method. – J. LS Apr 9 '15 at 14:58

Without more experimental details, it isn't 100% clear that the small peak is exclusively derived from the phenyl radical cation. It also isn't clear that observed the "phenyl" radical cation is actually cyclic! It could be another isomer of $\ce{C6H5^{+.}}$. With benzilic acid as the ion source, it is possible that the small peak contains an oxygen atom, although searching for possible formulas with appropriate "exact" masses makes me think this isn't true (see below)

The fragmentation spectrum of benzoic acid shows similar peaks. The "exact" mass of the observed ion 51.0237 Daltons, within a milliDalton of the mass of $\ce{C4H3+}$. The negative mode fragmentation spectrum of benzoic acid shows a corresponding ion at 49.0084, corresponding to $\ce{C4H1-}$ within a mDa.

I think the existence of something like $\ce{C4H1-}$ in negative mode supports Klaus's answer of cyclic structure because it's hard to see how else such an "unsaturated" cluster of carbon atoms can form. Perhaps a tetrahedrane-type structure is even possible?

The easiest way to get more information on this ion's structure would be to fragment a variety of partially deuterated benzene rings.

Update: I found a paper talking about fragmentation of fluorobenzene, and they argue the "phenyl" cation fragment derived from that molecule may not be cyclic after all:

In combining these approaches, several mechanistic variants can be excluded, while a new, hitherto not considered pathway involving the formation of acyclic $\ce{C6H5+}$ is proposed.

I don't know if the same thing happens during the fragmentation of benzilic acid, but if so, then I wonder if the observed peaks could be protonated butadiyne (positive mode) or deprotonated butadiyne (negative mode).

What about $\ce{phenyl^{*+} -> Dewar\ benzene^{*+} -> cyclobutadiene^{+*} + C2H2}$?

• That's definitely a plausible pathway, thanks ! – J. LS Apr 9 '15 at 18:00