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In mass spectrometry it is possible to determine the stability of an ion in the gas phase using e.g. collision induced dissociation (CID). In CID, we fragment an ion by colliding it with a neutral buffer gas. By gradually increasing the collision energy, it is thereby possible to get a relative measurement of the stability of the ion in the gas phase (so called survival yield analysis).

Now, I have the following question:

The picture below illustrates an effect that is commonly observed in CID namely that bigger ions tend to be more stable as they have more rotational degrees of freedom available to handle the collision energy transferred using CID. This can lead to counterintuitive effects, as for example in this case, one would expect acetic acid to be more acidic (and thus a more stable anion), than propanoic and decanoic acid, but it is the least stable.

enter image description here

In my work, I wanted to show that increasing chain lengths of the ligand increase the stability of a complex by weak interactions but I don't know how to factor in the effect that the increasing chain length has on the stabilization by the increased rotational freedom. Obviously I want to rule out that the increase in stability is due to the longer chains absorbing the collision energy better. How large could that effect be? Is there any systematic approach to take this effect into account? Is there any literature on the subject?

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    $\begingroup$ I have a number of concerns with your question, and maybe some suggestions. (a) In the definition of the stability of your molecule, you tend to focus on the acidity whereas in a survivor ion analysis, stability is defined relative to the fragmentation pathway (difference between starting product and fragments or starting product and transition state). Thus, unless the fragments or TS are very close in stability, you cannot focus your analysis based purely on the acidity carboxylic acids as the basis of your analysis. $\endgroup$ – PLD Jan 3 '17 at 15:13
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    $\begingroup$ (b) How do you consider the energy deposition in your molecule? Energy transfer in CID is dependent on the mass of the ion if you do not consider energies in the center of mass reference frame. In the laboratory frame, you will observe a decrease in energy deposition as ion mass increases. This would apply for a single collision conditions. If your conditions are multi-collisional conditions, things get even more complicated. $\endgroup$ – PLD Jan 3 '17 at 15:20
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    $\begingroup$ (c) The displacement in fragmentation energy as size increases is generally known as "kinetic shift", as it involves a reduction in the fragmentation kinetics when the degrees of freedom of a system increase. I guess that this is what you refer to as the number of rotational degrees of freedom. RRKM theory could probably be used to model this effect, but calculation times would probably scale up rapidly with the size of the systems under study. $\endgroup$ – PLD Jan 3 '17 at 15:23
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    $\begingroup$ I see some thermodynamic rules of thumb that could fit, but in this case it is a kinetic effect, so it would also depend on the exact experimental setup (time frame of the fragmentation event). And yes, other activation methods (IRMPD, UVPD, ...) will suffer from similar experimental artefacts. One option that would work would be to allow long fragmentation times such as in an FT-ICR cell, or in the same vein, to use BIRD experiments. $\endgroup$ – PLD Jan 3 '17 at 16:02
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    $\begingroup$ By the way, for the acidity of carboxylic acids in the gas phase, you should consider having a look at this J. Phys. Chem A article by McMahon et al. pubs.acs.org/doi/abs/10.1021/jp9908202 which shows that gas phase acidities of carboxylic acids increase with chain lengths due to intramolecular solvation of the charge. $\endgroup$ – PLD Jan 3 '17 at 16:15

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