Hello. I have met a significant number of publications on the study of the laws of mass fragmentation using DFT. Researchers are building huge schemes where they show in which direction the decay is taking place, for example, the one that I attach to the message.
Based on the calculations, it is concluded that fragmentation proceeds in one way or another. The problem is that according to my data, the energy difference at each stage is 70-750 kJ / mol, and the ionization potential is 70 eV, which is much higher than the received energies. Even if we assume that the molecule cannot receive more energy than the energy difference between the molecule and the molecular ion, it still turns out to be about 890 kJ / mol. What is more than the difference between [M]+* and H+ + [M]* (752 kJ / mol).
Question: How correct is it to assume that fragmentation does not occur? There is no signal [M-H] on the real mass spectrum.
First of all, the 70 eV is not the ionization potential, it is the incident energy. Out of these, only ca. 6 eV end up in the molecule (see e.g. the works of Julia Laskin in the field).
The relevant point for the fragmentation probability is not the reaction energy, it is the activation energy for the reaction. In principle you would even have to go further and use RRKM.
Maybe I'll write more later.
Thank you for your reply. I'm not very good in English, of course I had in mind incident energy. In any case, 6 eV is much more than 700 kJ / mol. Part of the energy goes with the electron, but it seems to me that it is still enough for deprotonation.
I would very much like to be wrong, because then I can find a very interesting object for research.
OK, now with a bit more time. I have done some experiments regarding the fragmentation of aromatic systems myself (the paper on it has actually been submitted last week).
There are some works on H vs. H2 loss, see e.g.
https://www.aanda.org/articles/aa/abs/2018/08/aa33220-18/aa33220-18.html
H2 loss is always favored over H loss since there are two things going for it:
1) It forms a stable product (H2)
2) the main fragment has an aryne-like stabilization if there is a concerted reaction step
Regarding the energies: 1 eV is 96.5 kJ/mol, so 6eV is not more than 700 eV but actually less. By the way, I now had time to look up the Laskin article I was talking about:
Article Ion–surface collisions in mass spectrometry: Where analytica...
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