Dissociation of Even-Electron Ions

Transcription

1 Dissociation of Even-Electron Ions Andrea Raffaelli CNR Istituto di Fisiologia Clinica Via Moruzzi, 1, Pisa. Web: A Simple? ESI Spectrum 370 [M+NH 4 ] + Glucosidic Compound MW 352 [M+Na] [M+FA+K] [M+H] [M+FA+Na] + [M+K] [M+TFA+K] [M+TFA+Na]

2 EI Odd-Electron Ions Fragmentation vs. Cold Even-Electron Ions Fragmentation EI MS deals with oddelectron, high internal energy ions. Fragmentation patterns have been completely described and rationalized. High internal energy induces extended fragmentation. Soft ionization MS (ESI, APCI, MALDI ) deals with even-electron, low internal energy ions. Fragmentation patterns are still waiting a complete rationalization. Fragmentation can be completely missing so that we need MS-MS techniques. Comparison between the EI-MS and the ESI-MS-MS Spectra of Cocaine EI 70 ev ESI-MS-MS QqQ, CE 20 ev

3 Moreover Independently of the instrument type, EI spectra are quite similar. The EI source is always the same (at least, very similar). Spectra are acquired under the same experimental conditions (70 ev). MS-MS instruments can be quite different (quadrupoles, traps, TOFs, ). Collision cells can operate at different levels (high energy vs. low energy) Collision energy can be, however, adjusted by the operator. Effect of the Collision Energy CE 10 ev CE 20 ev 124 Intensity, cps O N O N H N N 124 CE 30 ev CE 50 ev theophyllin CE, Volts

4 Also: Positive vs. Negative Ion Mode 107 Positive Ion MS-MS m/z, Da 143 Negative Ion MS-MS m/z, Da And How About Fragmentation Pattern? In contrast with EI fragmentations, even-electron fragmentations have not been completely rationalized yet. Owing to the low internal energy, these ions tend to behave in a more chemical fashion. Even if the factors affecting the fragmentation processes are roughly the same, probably the bond energy here gets more important. Considering that the behavior is more like normal organic chemistry, the interpretation of the spectra can be obtained in such a way.

5 Let s Look to an Example: Theophyllin 124 -C=O -CH 3 -N=C=O -CH N Fragmentation of Even-Electron Ions + Even-Electron positive ions (cations, EE + ) - Even-Electron negative ions (anions, EE - )

6 1. Even-Electron Cations Let s Try to Put Some Order As opposed to radical cations, which are met with almost exclusively in mass spectrometry, the even electron cations are common in classical chemistry. Their reactions, hence, seem much more familiar to the chemist. These molecular species are generally more stable than the radical cations produced by electron ionization. The spectra are, usually, much simpler, but the rearrangements are more frequent and more varied. As a consequence, these spectra yield less information and are more difficult to interpret than EI spectra. Actors in the Fragmentation Plot EE + OE + M R r Even-Electron Cation Odd-Electron Cation Neutral Molecule Radical Rearrangement

7 Fragmentation Reactions EI: Odd-Electron Ions Soft: Even-Electron Ions * # *: This process is usually highly endotermic and thus improbable. A statistical estimate based on many spectra indicate that EE ions yield EE fragments in about 95% of cases. # : These processes, more common, obey the Parity Rule McLafferty Classification of EE Ions Reactions Obeying the Parity Rule 1. Cleavage of a bond with charge migration 2. Cleavage of a bond with cyclization and charge migration 3. Cleavage of two bonds in a cyclic ion with charge retention 4. Cleavage of two bonds with rearrangement and charge retention McLafferty, F.W. Organic Mass Spectrometry 1980, 15, 114.

8 Cleavage of a Bond with Charge Migration The reaction occurs more easily when the protonated site is less basic. For instance, for Bu-XR molecules, we can observe: -XR NH 2 SH OH I Br Cl [(M+H)-HXR] + /(M+H) >250 >250 PA (kcal/mol) Cleavage of a Bond with Cyclization and Charge Migration It is the same reaction as above, but it is assisted by the presence of a heteroatom in a convenient position. This also provides anchimeric assistance to the expulsion of a neutral fragment. The basicity of the protonation site is important also in this case.

9 The Case of -Aminoalcohols n [(M+H)-H 2 O] + /(M+H) It must be noted that the other possible fragmentation, loss of ammonia, uccurs only when n = 4 or 5. Cleavage of Two Bonds in a Cyclic Ion with Charge Retention This rearrangement reaction, in contrast with most of the odd-electron ones, is observed especially when it involves a four-centre transition state.

10 Cleavage of Two Bonds with Rearrangement and Charge Retention Also this reaction is observed especially when it involves a four-centre transition state. Cleavage of Two Bonds with Rearrangement and Charge Retention: Examples * *: Note that in CI a ketone can lose water, which never occurs under EI conditions

11 Fragmentation Reactions not Obeying the Parity Rule The production of odd-electron ions from evenelectron ions is, as stated before, much rarer and more difficult to predict. They are observed sometimes in ions with extended systems. The driving force is commonly the formation of very stable species, where the unpaired electron can be extensively delocalized. The occurrence of these reactions is, however, lower than 5%. ESI-MS-MS Spectrum of Daunorubicine % Intensity O OH H 3 N OH m/z, amu

12 2. Even-Electron Anions Even Harder to Put Some Order Even-Electron anions are normally more stable than the cations. Fragmentation reactions, hence, require usually higher collision energy values with respect to positive ion mode. The parity rule does not apply: both homolytic and heterolytic breakages can occur: Even-Electron anions can yield, hence, both Even- Electron and Odd-Electron fragments. Even-Electron Anions Fragmentations Simple homolytic cleavage reactions where loss of a radical forms a stable radical anion Reactions that occur by initial formation of an anion complex which may then undergo a variety of reactions involving the bound anion, including direct displacement of the anion, and reactions involving the bound anion, including deprotonation, elimination processes, etc. Reactions that are not specifically directed by the first-formed deprotonated species, but where proton transfer to that species forms a new anion that may then fragment as above. Rearrangement reactions, including internal nucleophilic substitution/displacement and skeletal rearrangement reactions. Bowie, J.H. Mass Spectrom. Rev. 1990, 9, 349.

13 Some Examples H Loss R Loss Formation of anion-neutral complex More Rearrangement involving cyclyzation: And so on

14 A Few Words about Adducts Soft ionization techniques provide often adduct ions, i.e. ions produced by ion-molecule reactions. Sometimes these adduct are predominant of even exclusive. Adduct ions are even-electron ions as well. Their use for getting structural information by MS- MS fragmentations need to face some peculiar behavior. In particular, their composition includes a strong and stable pre-formed ion, not related to the structure of the analyte. MS-MS of Adduct Ions Positive Adducts NH 4 : OK. They fragment with loss of ammonia affording the protonated ion. Na, K: Almost useless: they fragment badly, in an unpredictable mode and very often the only fragment is the metal ion. Other adducts and supramolecular assemblies: structurally dependent. Negative Adducts In general they can be somehow useful. Their fragmentation tends to provide the original ion (Cl -, HCOO -, CH 3 COO - and so on), but also some useful fragments. The only exception is the trifluoroacetate anion: TFA adducts provide TFA anion as the only product ion. This as another counterindication to the use of TFA in the mobile phase.

15 Conclusions Fragmentation of even-electron ions can be different with respect to odd-electron ions. Α-cleavage, very important for odd-electron ions is completely missing for even-electron ones. Positive and negative ions behave differently. The fragmentation patterns are not been completely rationalized yet. They occur, however, in a very similar way with respect to traditional solution organic reactions. A good knowledge of organic chemistry and a good practice with calculators allow in general a good spectral interpretation in the case of known analytes. For unknown analytes the situation is different Some Help from Libraries and Fragmentation Pattern Interpretation Software? Recent releases of the most common mass spectral libraries begin to include MS-MS even-electron fragmentation spectra. For instance, NIST 14 MS library package contains 234,284 Spectra (121,586 in the NIST 11) of 45,298 ions (15,180 previously). Most MS instrumentation vendors can provide specific library and search algorithms for their data systems. Several MS fragmentation software packages are available on the market (Mass Frontier), and they can take care also of even-electron ions reactions.

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