Dissociation of Even-Electron Ions€¦ · • The fragmentation patterns are not been completely...

Dissociation ofEven-Electron Ions

Andrea RaffaelliCNR – Istituto di Fisiologia Clinica

Via Moruzzi, 1, 56124 Pisa.E-Mail: [email protected]: http://raffaelli.ifc.cnr.it

A …Simple? ESI Spectrum

300 350 400 450 500 550

353

370

375

391 421

437

489

505

Glucosidic CompoundMW 352

[M+H]+

[M+NH4]+

[M+Na]+

[M+K]+

[M+FA+K]+

[M+TFA+K]+

[M+TFA+Na]+

[M+FA+Na]+

19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015

EI Odd-Electron Ions Fragmentation vs. “Cold” Even-Electron Ions Fragmentation

• EI MS deals with odd-electron, 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


10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 3100

50

100

15 32

42

5159 68

77

82

94105

122140 152 166

182

198 272

303

EI70 eV

40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 3100

50

100

44

82

91105 119 150

182

272

304

ESI-MS-MSQqQ, CE 20 eV

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

19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 201525 50 75 100 125

CE, Volts

Intensity, cps N

N

O

O N

HN

theophyllin

181

CE 10 eV

181

124CE 20 eV

124

96

69 18142

CE 30 eV6942

94109

CE 50 eV

… Also:Positive vs. Negative Ion Mode


50 100 150 200 250m/z, Da

107

135

11991 165 229183

211

50 100 150 200 250m/z, Da

143

185

22715941

11769 93

Positive IonMS-MS

Negative IonMS-MS

… 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.


- C=O

… Let’s Look to an Example: Theophyllin


124

96

69 181

42

- CH3-N=C=O

-CHN

Fragmentation of Even-Electron Ions


+ Even-Electron positive ions (cations, EE+)

- Even-Electron negative ions (anions, EE-)

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+ Even-Electron Cation

OE+ Odd-Electron Cation

M Neutral Molecule

R Radical

r Rearrangement

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 migration2. Cleavage of a bond with cyclization and charge

migration3. Cleavage of two bonds in a cyclic ion with charge

retention4. Cleavage of two bonds with rearrangement and

charge retention


McLafferty, F.W. Organic Mass Spectrometry 1980, 15, 114.

Cleavage of a Bond withCharge Migration


The reaction occurs more easily when the protonated site is less basic. For instance, for Bu-XR molecules, we can observe:

-XR NH2 SH OH I Br Cl[(M+H)-HXR]+/(M+H)+ 0.04 0.15 19.5 240 >250 >250PA (kcal/mol) 207 175 164 145 141 140

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.

The Case of -Aminoalcohols


n 2 3 4 5 6[(M+H)-H2O]+/(M+H)+ 0.42 0.21 0.20 0.15 0.07

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.

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

Fragmentation Reactions not Obeying the Parity Rule

• The production of odd-electron ions from even-electron 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%.


44 7286

113130

148306

321

363

381

399510

528

50 100 150 200 250 300 350 400 450 500m/z, amu

10

20

30

40

50

60

70

80

90

% In

tens

ity

ESI-MS-MS Spectrum of Daunorubicine

O

OH

H3N

OH

-15


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 nucleophilicsubstitution/displacement and skeletal rearrangement reactions.

19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015Bowie, J.H. Mass Spectrom. Rev. 1990, 9, 349.

… Some Examples


H Loss

R Loss

Formation of anion-neutral complex

… More …


Rearrangement involving cyclyzation:

… And so on

… 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 IonsPositive Adducts

• NH4: 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-, CH3COO- 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 counter-indication to the use of TFA in the mobile phase.


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…19° Corso di Spettrometria di Massa – Pontignano, 16-20 Marzo 2015

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.


Molte grazie per la cortese attenzione e…

Arrivederci a…

Tutti!


Dissociation of Even-Electron Ions€¦ · • The fragmentation patterns are not been completely...

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