Determination of Unknowns Using Analytical Instrumentation

1
Determination of Unknowns Using Analytical
Instrumentation

• FT-IR
• GC/MS
• NMR

Colin Henck Spring 2006 Forensic Chemistry
2
No One Technique is Enough!

• FT-IR
• Offers data about the molecules functional groups
• Mass Spectrometry
• Offers mass data about the compound
• Often the molecular ion can be seen and also a
  fragmentation pattern that can offer insight into
  the structure of the molecule
• 1-D H1 NMR
• Offers structural information and geometric
  arrangement of atoms in the molecule

3
Identification of UnknownsCompound 1 of 2
4
FT-IR Spectrum
5
FT-IR SpectrumPeaks of Interest
3050
2990
2890
1750
6
Interpretation of IR Peaks
C-H (ar)
C-H
C-H
CO
7
Building a Molecule
Functional Groups
CHx
CO
This information is helpful, but not enough to
generate a structure. On too another technique.
8
Mass Spectrum
9
Interpreting Mass Spectrum
Questions Probable Molecular Weight?
Number of carbons in the Structure?
10
Interpreting Mass Spectrum
Probable Molecular Weight 136 amu
Number of carbons in the Structure 5/57 100
8.7 8.7/1.1 7.9 or 8 Carbons
57
5
11
Interpreting Mass Spectrum
Keeping in mind the functional groups, what
do these peaks correspond too?
Functional Groups
CHx
CO
28
16
15
12
Interpreting Mass Spectrum
Functional Groups
CHx
CO
Mass Losses 1 -H 28 -CO 15 -CH3 16 -O
28
16
15
13
Building a Molecule
All mass spec data is gained from cations.
Molecules will be shown as neutral for
clarity. What might account for mass 76?
14
Mass 76
or
or
What might account for mass 92?
15
Mass 92
Four hydrogen's not shown
What might account for mass 107?
16
Mass 107
or
What might account for mass 135?
17
Mass 135
or
What might account for mass 136?
18
Mass 136
or
The structures contain 8 carbon atoms!
19
Mass Breakdown Pattern 1
-H
-O
-CH3
-CO
135
107
92
76
136
Each contains four hydrogen atoms on the ring.
20
Mass Breakdown Pattern 2
-H
-O
-CH3
-CO
135
107
92
76
136
Each contains four hydrogen atoms on the ring.
21
Six Possible Structures
Of all compounds, this could be only one of six!
o-Methoxybenzaldehyde
m-Methoxybenzaldehyde
p-Methoxybenzaldehyde
o-Formic Acid Toluic Ester
m-Formic Acid Toluic Ester
p-Formic Acid Toluic Ester
22
Which is More Likely?
or
No Peak at 91!
Contains 4 Hydrogen's
23
H1 NMR Spectrum
Obtained from sigmaaldrich.com
or
CH3 Shift!
24
NMR Spectrum
CH3 Shift around 4 ppm
or
CH3 Shift around 2 ppm
25
Three Structures Left
o-Methoxybenzaldehyde
m-Methoxybenzaldehyde
p-Methoxybenzaldehyde
26
NMR Spectrum
Integration Shown in Red 4 Peaks 2 Doublets 2
Singlets
3
2
2
1
o-Methoxybenzaldehyde
m-Methoxybenzaldehyde
p-Methoxybenzaldehyde
27
Assigned NMR Spectra
a
c
b
d
a
b
b
c
c
d
p-Methoxybenzaldehyde
28
Identification of UnknownsCompound 2 of 2
29
FT-IR Spectrum
30
FT-IR SpectrumPeaks of Interest
31
Interpretation of IR Peaks
C-H(ar)
32
Building a Molecule
Functional Groups
O-H
CHx
CO
This information is helpful, but not enough to
generate a structure. On too another technique.
33
Mass Spectrum
34
Interpretation of Mass Spectrum
Questions Probable Molecular Weight?
Number of carbons in the Structure?
35
Interpretation of Mass Spectrum
Probable Molecular Weight 136 amu
Number of carbons in the Structure 6/78 100
7.7 7.7/1.1 7 or 7 Carbons
78
Same molecular weight as unknown 1! Different
Molecule!!
6
36
Interpretation of Mass Spectrum
Keeping in mind the functional groups, what
do these peaks correspond too?
Functional Groups
CHx
O-H
CO
28
17
15
37
Interpretation of Mass Spectrum
Functional Groups
CHx
O-H
CO
Mass Losses 17 -OH 28 -CO 15 -CH3
28
17
15
Mass 65 is common too the breakdown of toluene in
a mass spec.
38
Building a Molecule
Mass Losses (131 119) 17 -OH (119 91) 28
-CO (91- 76) 15 -CH3
Major Mass Peaks 131 119 91 76
Functional Groups
CHx
O-H
CO
All mass spec data is gained from cations.
Molecules will be shown as neutral for
clarity. What might account for mass 76?
39
Mass 76
Mass Losses (131 119) 17 -OH (119 91) 28
-CO (91- 76) 15 -CH3
Major Mass Peaks 131 119 91 76
Functional Groups
CHx
O-H
CO
or
or
What might account for mass 91?
40
Mass 91
Mass Losses (131 119) 17 -OH (119 91) 28
-CO (91- 76) 15 -CH3
Major Mass Peaks 131 119 91 76
Functional Groups
CHx
O-H
CO
What might account for mass 119?
41
Mass 119
Mass Losses (131 119) 17 -OH (119 91) 28
-CO (91- 76) 15 -CH3
Major Mass Peaks 131 119 91 76
Functional Groups
CHx
O-H
CO
What might account for mass 131?
42
Mass 131
Mass Losses (131 119) 17 -OH (119 91) 28
-CO (91- 76) 15 -CH3
Major Mass Peaks 131 119 91 76
Functional Groups
CHx
O-H
CO
Note The structure has 8 carbon atoms although
we calculated it should have 7!
43
Mass Breakdown Pattern
Mass Losses (131 119) 17 -OH (119 91) 28
-CO (91- 76) 15 -CH3
Major Mass Peaks 131 119 91 76
Functional Groups
CHx
O-H
CO
-CH3
-CO
-OH
119
91
76
136
Each contains four hydrogen atoms on the ring.
44
Three Possible Structures
Of all compounds, this could be only one of three!
o-Toluic Acid
m-Toluic Acid
p-Toluic Acid
45
NMR Spectrum
Integration Shown in Red 3 Peaks 2 Doublets 1
Singlet
3
2
2
o-Toluic Acid
m-Toluic Acid
p-Toluic Acid
Not consistent with any molecule?
46
NMR Spectrum
Integration Shown in Red 3 Peaks 2 Doublets 1
Singlet
a
b
c
a
b
b
c
c
p-Toluic Acid
Carboxylic acids can easily undergo deuterium
exchange!