some general rules for m+ and m+x peaksvancew/chem380/lecture5.pdf · some general rules for m+ and...
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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
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
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.
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