[pavlosmelas]

Hello,

As an assignment I need to explain all fragments with high intensity, the molecule is 2,8-decadiyne (see structure made with chemdraw).
 


The MS spectrum:


The molecular ion is 134 m/z, the fragments with 133, 119, 67, 53 and 39 m/z I figured out already but I just can not figure out the fragments at 106, 105, 91, 79, 77, 41 and 27 m/z. Especially the base peak of 91 m/z does not make sense for me, I am new at analytical chemistry and my teacher told me that if you see 91 m/z you should think first about tropylium cation but I think this is not possible since this is no aromatic molecule but a diyne.

I hope someone can help, thank you very much.

[wildfyr]

What if your teacher phrased it this way: Can you think of a way to make a trophylium cation from this system? If I squint I think I can, though I'm no MS fragmentation expert.

[Babcock_Hall]

My advice is even more general, which is not to be too surprised by one's mass spectral results.  It is based on my limited knowledge of MS of carotenoids.  There is one rearrangement that splices out the center of the carotenoid and joins the two ends.  I certainly would not have predicted it.

[pavlosmelas]

Thank you both for the reply.
Babcock_Hall, I followed your advise and I got the following fragment for the base peak, it got a molar mass of 91 so I think I got it! If it is right then I can easily also do the other fragments, it makes a lot of sense actually.

[wildfyr]

I think you can actually make the trophyllium ion by creating a ring from this di-yne. I dont see a good way to make the molecule you have there.

Babcock's reply is a quite unusual rearrangement. One you wouldn't be expected to arrive at in undergrad level work.

[Babcock_Hall]

Yes, my example was not meant to be interpreted mechanistically.  It was more like, "Hey, weird things happen in mass spectrometry."