Some general rules for M+ and M+x peaks

H2 M+=2

CH4 M+ =16

NH3 M+=17

H2O M+ =18

H2S M+=34

SiH4 M+ =32

PH3 M+ =34

H2SO4 M+=98

Ph3PO M+=278

HCl M+=36

HBr M+=80

HI M+=128

BH3 M+=14

Nitrogen rule: if the M+ peak is an odd number, then there is an odd number ofnitrogen atoms in the molecule. A molecule with an even M+ peak either has nonitrogens or an even number of nitrogens.

N2 M+=28

Most atoms except N haveeither: a) odd valency and oddmass (H, B, P, halides) or (b)even valency and even mass(C, O, S, Si).

Finding M+

The M+ peak is usually the highest intensity peak in thecluster of peaks at highest m/z.

M+

M-1

Using isotopic ratios to determine formula

intensity of M/ M+1/M+2 peaks can indicate presence absence of certain atom types

Atoms can be categorized by their types of isotopes:

M+ atoms: only have one isotope that is important (H, F, P, I )M+1: significant amount of M+1 isotope (C, N, Si, S)M+2: significant M+2 isotope (O, Si, S, Cl, Br)

• M+ atoms do not make contributions to M+1 intensity• C, N only significantly contribute to M+2 peak if there are a lot of them• S and Si contribute significantly to both M+1 and M+2 peaks• O, Cl, Br contribute to only M+2 peak

M+2:M 0.2 - 3.3 suggests OM+2: M < 3.3 precludes S, Si, Cl, BrM+2: M < 4.4 precludes S, Cl, BrM+2: M < 32 precludes ClM+2: M < 97 precludes Br

Examples m/z RI

84.0 100.085.0 5.7

86.0 4.9

84/12 = 7

5.7/1.1 = 5 (max 6)

M+2 = 4.9 cannot be just from O (would need 24 O), nor Si+(6-8)O (6*16=96)

probably 1S (4.6) contributes 0.78 to M+1

C4H4S: predicted 84 (100), 85 (5.2), 86 (4.6)

Example 2m/z RI153.0 100.0154.0 8.2

(153-14)=139/12 = 11 (maximum #C based on weight) or

8.2/1.1 = 7.7 (maximum #C based on M+1) Guess: 7C + 1N, M+1= 7.7 + 0.38 = 8.0, mass 98

7C + 3N, M+1=8.8 (too much)6C + 5N, M+1= 8.3 (closest) mass 142

No M+2: no S, Si, Cl, Br. May be 1-4 oxygens (signal to noise)

C6N5H11 or C7H17NF2 or C7H7NO3 (M+1 = 8.1%)

Odd M+ peak suggests odd N (1,3, 5 etc.)

Reminder: structures from formulae• Degree of unsaturation, index of hydrogen deficiency

(2(#C) + 2 + (#N) - (#halides) - (#H))/2 = number of rings/π-bonds

e.g. pyridine: C5H5N5: (2(5) + 2 + 1 - 5)/2 = 4 cyclohexane: C6H12: (2(6) +2 -12)/2 = 1

C4H4S: (2(4) + 2 - 4)/2 = 3

C6N5H11 (2(6) + 2 + 5 - 11)/2 = 4 C7H17NF2 (2(7) + 2 + 1 - 2 -17)/2 = -1 (not possible!)C7H7NO3 (2(7) + 2 + 1 - 7)/2 = 5 NO2

O

NH2

NH2

NH2

H2N

H2N

S

Fragmentation

M+

M+ very low RI

Can we use fragmentation to uniquely identify molecules?

Mechanism of fragmentation

MMe-

2e-

radical

cation

M F1

F1 F2

F2+

+

+

Unimolecular fragmentation (pressure to low for bimolecular reactions)

loss of neutral evenelectron fragment,formation of new radicalcation

loss of neutral radicalfragment formation ofeven electron (EE) cation

How much fragmentation?

M+

Degree of fragmenation

• Fragmentation pattern depends on structure of M+•

which electron is removed?

• Stability of potential fragments and rate of theirformation (thermodynamics/kinetics)

Radicals: a brief overview

Resonance

Br

H

C

H H

ClCl

H

C

H H

Br

2 electron movement

Reaction mechanism (e.g. SN2)

“Fish-hook arrows”: 1 electron movement

Examples of Fish-hook use

Br Br Br Br

Homolytic bond cleavage: each atom takes 1 electron from bond

Br H H

HH Br

H

H

1 bond breaks, another forms

Initial radical formation

• The structure of the radical formed depends on the orbitals present andtheir relative energies.

• Electrons are preferentially removed from highest energy orbitals first

n > π > σC-C > σC-H

n

π

σC-C

σC-H

ener

gy

Electrons lost from lone pairs first

O O

then from π-bonds:

These M+ species are relatively stable since they σ-bonds remain intact.

Stable M+•, large M+ peak

Saturated compounds lose protons from σ-bonds

H C

H

C

H

H

H

H H C C

H

H

H

H

Hσ-bond loses electron:weakened skeleton

H C

H

H

H C

H

H

no charge (not detected by MS)

carbocation

m/z=30

m/z=15 (note: odd mass!)

More complex systems

C C

H

H

C C

H

H

C

H

H

H

H

C

H

C

HH

H

H

H

H

H

C C

H

H

C C

H

H

C

H

H

H

H

C

H

C

HH

H

H

H

H

H C C

H

H

C C

H

H

C

H

H

H

H

C

H

C

HH

H

H

H

H

H

C C

H

H

C C

H

H

C

H

H

H

H

C

H

C

HH

H

H

H

H

H

m/z=100

m/z=100 m/z=100

C C

H

H

C C

H

H

C

H

H

H

H

C

H

C

HH

H

H

H

H

HC C

H

H

C C

H

H

C

H

H

H

H

C

H

C

HH

H

H

H

H

H

C C

H

H

C C

H

H

C

H

H

H

H

C

H

C

HH

H

H

H

H

H

C C

H

H

C C

H

H

C

H

H

H

H

C

H

C

HH

H

H

H

H

H

m/z=85

C C

H

H

C C

H

H

C

H

H

H

H

C

H

C

HH

H

H

H

H

H

m/z=15

C C

H

H

C C

H

H

C

H

H

H

H

C

H

C

HH

H

H

H

H

H

C C

H

H

C C

H

H

C

H

H

H

H

C

H

C

HH

H

H

H

H

H

m/z=29

m/z=71

C C

H

H

C C

H

H

C

H

H

H

H

C

H

C

HH

H

H

H

H

H

C C

H

H

C C

H

H

C

H

H

H

H

C

H

C

HH

H

H

H

H

H

m/z=57

m/z=43

The actual spectrum (again)

C C

H

H

C

H

H

H

H

H

C C

H

H

H

H

C

H

C

H

H

H

H

C C

H

H

C

H

H

H

H

C

H

C

H

H

H

HC C

H

H

H

H

H

C

H

H

C C

H

H

C

H

H

H

H

C

H

C

H

H

H

HH C

H

H

Not all peaks are equal intensities: formed in unequal amounts.