1. Lecture 2: Introductory lecture on gas phase
ion reactions using Electron Impact (EI) source

2. Interpretation of E.I. Mass Spectrometric Data
Fragmentation Region
Region A
Region B
Fragmentation
M+.

3. Interpretation of E.I. Mass Spectrometric Data
Ionization Process:
When an electron with a kinetic energy of 70 eV passes through the low pressure gas mixture in the ion source, it is very unlikely that it will actually collide with neutral molecules, since the valence electrons exert a repelling force, which can only be overcome when the kinetic energy of the passing electron is increased to ~ MeV.
The electron will therefore pass a neutral molecule within very close range.
A 70 eV electron travels at a speed of 4.2 x 108 cm/sec and it can be calculated that it will pass molecules with a diameter of a few Å in about sec.
During this interval, the strong electric field of the passing electron deforms the orbit of the valence electrons to such an extent that the neutral molecule may lose one electron and thus become ionized

4. Interpretation of E.I. Mass Spectrometric Data
An electron energy of 70 eV correspond to 6754 kJ /mole
(1eV = kJ/mole)

5. Interpretation of E.I. Mass Spectrometric Data
Excess of internal energy E = Eel + Evib + Erot
Excited ion can not colloid with the other ions due to high vacuum
Finally breakage of bonds

6. Interpretation of E.I. Mass Spectrometric Data
Fragmentation - General
The collision of a high energy electron with a molecule not only causes the loss of a valence electron, it imparts some of the kinetic energy of collision into the remaining ion
This energy typically resides in an increased vibrational energy state for the molecule – this energy may be lost by the molecule breaking into fragments
The time between ionization and detection in most mass spectrometer is 10-5 sec.
If a particular ionized molecule can “hold together” for greater than 10-5 sec. a M+ ion is observed

7. Interpretation of E.I. Mass Spectrometric Data
Fragmentation - General
Due to the low concentration of molecules in the ionization chamber, all fragmentation processes are unimolecular
Fragmentation of a molecule that is missing one electron in most cases results in a covalent bond breaking homolytically – one fragment is then missing a full pair of electrons and has a + charge and the other fragment is a neutral radical
Only the + charged ions will be observed; but the loss of a neutral fragment is inferred by the difference of the M+ and the m/e of the fragment
Fragmentation will follow the trends you have learned in organic chemistry – fragmentation processes that lead to the most stable cations and radicals will occur with higher relative abundances

8. Interpretation of E.I. Mass Spectrometric Data
Fragmentation Rule and Fragmentation Chemistry of Ions :
Common terminology used:
OE, EE
Radical induce dissociation, charge induce dissociation
Charge migration or charge retentions
Single bond cleavage, double bond cleavage, sigma bond cleavage

9. Interpretation of E.I. Mass Spectrometric Data
1-Preferable site for knock out electron:
-The most favored site in the molecule are assumed to arise by the loss of electron
of lowest ionization energy
-Order can be written as n >π >σ

10. Interpretation of E.I. Mass Spectrometric Data
Fragmentation :
Basic Mechanism of ion Fragmentation :
- Sigma bond dissociation
- Radical-site initiation (-cleavage)
- Charge site initiation (inductive cleavage, i)
-Rearrangements
-Decompositions

11. Interpretation of E.I. Mass Spectrometric Data
Basic Mechanism of ion Fragmentation :
- Sigma bond dissociation
Sigma bond cleavage occurs at a sigma bond that has given
up its electron during the ionization process
The more abundant ion will be the one better able
to stabilize the positive charge
Stevensons Rule: Among the two possibilities the tendency to
remain the un paired e should have higher IE, therefore the
fragment ion corresponding to the lower IE energy (more stable)

12. Interpretation of E.I. Mass Spectrometric Data
Basic Mechanism of ion Fragmentation :
- Radical-site initiation (-cleavage)
-This is accompanied by the homolytic cleavage of another bond to
that -atom, thus this is commonly called -cleavage reactions
-Radical initiation at the radical site arises from its strong
tendency for electro pairing. The odd electron is donated to
form a new bond to an adjacent atom.
-The radical site tendency to donate electrons:
N> S, O, π, R > Cl > Br > H

13. Interpretation of E.I. Mass Spectrometric Data
Basic Mechanism of ion Fragmentation :
- Charge site initiation (inductive cleavage, i)
-Initiation of a cleavage reaction by the positive charge
involves attraction of an electron pair.
-The tendency for the formation of R+ from RY is:
halogens > O, S >> N, C; for elements of the same row of the
periodic table this tendency parallels the inductive effect (i) of Y .
- Heteroylytic cleavages require migration of the charge,
they are generally less favored than radical-site reactions

14. Interpretation of E.I. Mass Spectrometric Data
Basic Mechanism of ion Fragmentation :
- Charge-site initiation (inductive cleavage, i)

15. Interpretation of E.I. Mass Spectrometric Data
Basic Mechanism of ion Fragmentation :
Rearrangements
That produce ions whose atoms have not retained the connectivity relationship of the original molecule
Example 1:
-γ-H rearrangement to an unsaturated group
with -cleavage (Mc-lafferty rearrangement)

16. Interpretation of E.I. Mass Spectrometric Data
Basic Mechanism of ion Fragmentation :
Rearrangements
Example 2:
Nonhyderogen rearrangements
Example 3:
Charge base rearrangements

17. Interpretation of E.I. Mass Spectrometric Data
Basic Mechanism of ion Fragmentation :
Rearrangements
Example 4:
Rearrangements of two hydrogen atoms

18. Interpretation of E.I. Mass Spectrometric Data
Basic Mechanism of ion Fragmentation :
Decompositions
Particularly observed in cyclic structure and involve the cleavage
of two bonds in a ring, i.e necessary to produced a fragment ion
Example:
RDA

19. Interpretation of E.I. Mass Spectrometric Data
Basic Mechanism of ion Fragmentation: 2nd dimension to look
(a)-One bond s-cleavages:
cleavage of C-C
cleavage of C-heteroatom

20. Interpretation of E.I. Mass Spectrometric Data
Basic Mechanism of ion Fragmentation: 2nd dimension to look
(a)-One bond s-cleavages:
-a-cleavage of C-heteroatom

21. Interpretation of E.I. Mass Spectrometric Data
Basic Mechanism of ion Fragmentation: 2nd dimension to look
(b)-Two bond s-cleavages/rearrangements:
-Elimination of a vicinal H and heteroatom:
-Retro-Diels-Alder (see from the back slide)
(c)- Two bond sigma-cleavage:
-McLafferty Rearrangement (see from the back slide)

22. Interpretation of E.I. Mass Spectrometric Data
Basic Mechanism of ion Fragmentation: 3rd dimension to look
Cleavage by Odd Electron Ions OE+.
Odd electron ions ( or a fragment ) can decompose by:
(i) loss of a radical to give a cation (even electron fragment):
(ii) by loss of a neutral molecule (even electron) to give an odd electron fragment
(i)
(ii)

23. Interpretation of E.I. Mass Spectrometric Data
Basic Mechanism of ion Fragmentation: 3rd dimension to look
Cleavage by Even Electron Ions EE+
Even-electron ions always lose even-electron molecules
generating another even-electron ion
Fields rule: In EE+ ion decomposition forming the same EE+ product,
the tendency of a neutral to leave without the charge is greater
for molecules of lower proton affinity (PA)

24. Interpretation of E.I. Mass Spectrometric Data
Basic Mechanism of ion Fragmentation: 3rd dimension to look (summary)
b = Not favored

25. Interpretation of E.I. Mass Spectrometric Data
Basic Mechanism of ion Fragmentation (Summary)
When deducing any fragmentation scheme:
The even-odd electron rule applies: “thermodynamics dictates that even electron ions cannot cleave to a pair of odd electron fragments”
The order of carbocation/radical stability is
benzyl/3° > allyl/2° > 1° > methyl > H
Fragment ion stability is more important than fragment radical stability
Fragmentation mechanisms should be in accord with the even-odd electron rule

26. Interpretation of E.I. Mass Spectrometric Data
References:
INTRODUCTION TO MASS SPECTROMETRY, Third edition , J. Throck Watson
A BEGINNER'S GUIDE TO MASS SPECTRAL INTERPRETATION, Terrence A. Lee
MASS SPECTROMETRY PRINCIPLES AND APPLICATIONS, Edmond de Hoffmann
UNDERSTANDINGMASS SPECTRA, Second Edition, R. Martin Smith