NMR 3 Interpretation

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NMR Interpretation 
1. Number of signals 
2. Integration 
3. Chemical Shift 
4. coupling 
 
 
INTEGRATION 
CH2 C
O
CH3
Each different type of proton comes at a different place . 
Can tell how many different types of hydrogen 
 there are in the molecule. 
NMR Spectrum of Phenylacetone 
The area under a peak is proportional 
to the number of hydrogens that 
generate the peak. 
Integration = determination of the area 
 under a peak 
INTEGRATION OF A PEAK 
Not only does each different type of hydrogen give a 
distinct peak in the NMR spectrum, but we can also tell 
the relative numbers of each type of hydrogen by a 
process called integration. 
55 : 22 : 33 = 5 : 2 : 3 simplest ratio of the heights 
The integral line rises an amount proportional to the number 
 of H in each peak 
Benzyl Acetate 
Method 1 
Benzyl Acetate (FT-NMR) 
assume CH3 
33.929 / 3 = 11.3 
33.929 / 11.3 
= 3.00 
21.215 / 11.3 
= 1.90 
58.117 / 11.3 
= 5.14 
Actually : 5 2 3 
METHOD 2 
digital 
integration 
Modern instruments report the integral as a number. 
CH
2
O C
O
CH
3
Integrals are 
good to about 
10% accuracy. 
 
DIAMAGNETIC ANISOTROPY 
SHIELDING BY VALENCE ELECTRONS 
valence electrons 
shield the nucleus 
from the full effect 
of the applied field 
B induced 
 (opposes Bo) 
Bo applied 
magnetic field 
lines 
The applied field 
induces circulation 
of the valence 
electrons - this 
generates a 
magnetic field 
that opposes the 
applied field. 
Diamagnetic Anisotropy 
fields subtract at nucleus 
All different types of protons in a molecule 
have a different amounts of shielding. 
This is why an NMR spectrum contains useful information 
(different types of protons appear in predictable places). 
They all respond differently to the applied magnetic 
field and appear at different places in the spectrum. 
PROTONS DIFFER IN THEIR SHIELDING 
UPFIELD DOWNFIELD 
Highly shielded 
protons appear here. 
Less shielded protons 
appear here. 
SPECTRUM 
It takes a higher field 
to cause resonance. 
CHEMICAL SHIFT 
PEAKS ARE MEASURED RELATIVE TO TMS 
TMS 
shift in Hz 
0 
Si CH3CH3
CH3
CH3
tetramethylsilane 
 “TMS” 
reference compound 
n 
Rather than measure the exact resonance position of a 
peak, we measure how far downfield it is shifted from TMS. 
Highly shielded 
protons appear 
way upfield. 
Chemists originally 
thought no other 
compound would 
come at a higher 
field than TMS. 
downfield 
hn = Bo 
g 
2p 
constants 
frequency 
field 
strength 
Stronger magnetic fields (Bo) cause 
the instrument to operate at higher 
frequencies (n). 
FROM EARLIER DISCUSSION LARMOUR FREQUENCY 
NMR Field 
Strength 
1H Operating 
 Frequency 
60 Mhz 
100 MHz 
300 MHz 7.05 T 
2.35 T 
1.41 T 
n = ( K) Bo 
TMS 
shift in Hz 
0 n 
downfield 
The shift observed for a given proton 
in Hz also depends on the frequency 
of the instrument used. 
Higher frequencies 
= larger shifts in Hz. 
HIGHER FREQUENCIES GIVE LARGER SHIFTS 
chemical 
 shift 
= d = 
shift in Hz 
spectrometer frequency in MHz 
= ppm 
This division gives a number independent 
of the instrument used. 
parts per 
million 
CHEMICAL SHIFT 
The shifts from TMS in Hz are bigger in higher field 
instruments (300 MHz, 500 MHz) than they are in the 
lower field instruments (100 MHz, 60 MHz). 
We can adjust the shift to a field-independent value, 
the “chemical shift” in the following way: 
A particular proton in a given molecule will always come 
at the same chemical shift (constant value). 
0 1 2 3 4 5 6 7 ppm 
Hz Equivalent 
 of 1 ppm 
1H Operating 
 Frequency 
60 Mhz 60 Hz 
100 MHz 100 Hz 
300 MHz 300 Hz 
HERZ EQUIVALENCE OF 1 PPM 
Each ppm unit represents either a 1 ppm change in 
Bo (magnetic field strength, Tesla) or a 1 ppm change 
in the precessional frequency (MHz). 
1 part per million 
of n MHz is n Hz 
n MHz = n Hz 1 
106 
( ) 
What does a ppm represent? 
NMR Correlation Chart 
12 11 10 9 8 7 6 5 4 3 2 1 0 
-OH -NH 
CH2F 
CH2Cl 
CH2Br 
CH2I 
CH2O 
CH2NO2 
CH2Ar 
CH2NR2 
CH2S 
C C-H 
C=C-CH2 
CH2-C- 
O 
C-CH-C 
C 
C-CH2-C 
C-CH3 
RCOOH RCHO C=C 
H 
TMS 
HCHCl3 , 
d (ppm) 
DOWNFIELD UPFIELD 
DESHIELDED SHIELDED 
Ranges can be defined for different general types of protons. 
This chart is general, the next slide is more definite. 
APPROXIMATE CHEMICAL SHIFT RANGES (ppm) FOR SELECTED TYPES OF PROTONS 
R-CH3 0.7 - 1.3 
R-C=C-C-H 1.6 - 2.6 
R-C-C-H 2.1 - 2.4 
O 
O 
RO-C-C-H 2.1 - 2.5 
O 
HO-C-C-H 2.1 - 2.5 
N C-C-H 2.1 - 3.0 
R-C C-C-H 2.1 - 3.0 
C-H 2.3 - 2.7 
R-N-C-H 2.2 - 2.9 
R-S-C-H 2.0 - 3.0 
 I-C-H 2.0 - 4.0 
 Br-C-H 2.7 - 4.1 
 Cl-C-H 3.1 - 4.1 
 RO-C-H 3.2 - 3.8 
 HO-C-H 3.2 - 3.8 
R-C-O-C-H 3.5 - 4.8 
O 
R-C=C-H 
H 
6.5 - 8.0 
R-C-H 
O 
9.0 - 10.0 
R-C-O-H 
O 
11.0 - 12.0 
O2N-C-H 4.1 - 4.3 
 F-C-H 4.2 - 4.8 
R3CH 1.4 - 1.7 
R-CH2-R 1.2 - 1.4 4.5 - 6.5 
R-N-H 0.5 - 4.0 Ar-N-H 3.0 - 5.0 R-S-H 
R-O-H 0.5 - 5.0 Ar-O-H 4.0 - 7.0 
R-C-N-H 
O 
5.0 - 9.0 
1.0 - 4.0 R-C C-H 1.7 - 2.7 
IT IS NECESSARY TO KNOW WHAT TYPES 
OF HYDROGENS COME IN SELECTED AREAS OF 
THE NMR SPECTRUM BETWEEN 0 & 12 PPM 
aliphatic 
 C-H 
CH on C 
next to 
pi bonds 
C-H where C is 
attached to an 
electronega-
tive atom 
alkene 
=C-H 
benzene 
 CH 
aldehyde 
 CHO 
acid 
COOH 
2 3 4 6 7 9 10 12 0 
X-C-H 
X=C-C-H 
MOST SPECTRA CAN BE INTERPRETED WITH 
A KNOWLEDGE OF WHAT IS SHOWN HERE 
DESHIELDING AND ANISOTROPY 
Three major factors account for the resonance 
positions (on the ppm scale) of most protons. 
1. Deshielding by electronegative elements. 
2. Anisotropic fields usually due to pi-bonded 
 electrons in the molecule. 
3. Deshielding due to hydrogen bonding. 
 DESHIELDING BY 
ELECTRONEGATIVE ELEMENTS 
highly shielded 
protons appear 
at high field 
“deshielded“ 
protons appear 
at low field 
deshielding moves proton 
resonance to lower field 
C H Cl 
Chlorine “deshields” the proton, 
that is, it takes valence electron 
density away from carbon, which 
in turn takes more density from 
hydrogen deshielding the proton. electronegative 
element 
DESHIELDING BY AN ELECTRONEGATIVE ELEMENT 
NMR SPECTRUM 
d- d+ 
d- d+ 
Electronegativity Dependence 
of Chemical Shift 
Compound CH3X 
Element X 
Electronegativity of X 
Chemical shift d 
CH3F CH3OH CH3Cl CH3Br CH3I CH4 (CH3)4Si 
F O Cl Br I H Si 
4.0 3.5 3.1 2.8 2.5 2.1 1.8 
4.26 3.40 3.05 2.68 2.16 0.23 0 
Dependence of the Chemical Shift of CH3X on the element X 
deshielding increases with the 
electronegativity of atom X 
TMS most 
deshielded 
Substitution Effects on Chemical Shift 
CHCl3 CH2Cl2 CH3Cl 
7.27 5.30 3.05 ppm 
 -CH2-Br -CH2-CH2Br -CH2-CH2CH2Br 
 3.30 1.69 1.25 ppm 
most 
deshielded 
most 
deshielded 
The effect decreaseswith increasing distance. 
The effect 
increases with 
greater numbers 
of electronegative 
atoms. 
ANISOTROPIC FIELDS 
DUE TO THE PRESENCE OF π BONDS 
 The presence of a nearby π bond or π system 
 greatly affects the chemical shift. 
•Benzene rings have the greatest effect. 
 
Secondary magnetic field
generated by circulating p
electrons deshields aromatic
protons
Circulating p electrons
Ring Current in BenzeneRing Current in Benzene
Bo
Deshielded
H H
fields add together 
C=C 
H H 
H H 
Bo 
ANISOTROPIC FIELD IN AN ALKENE 
protons are 
deshielded 
shifted 
downfield 
secondary 
magnetic 
(anisotropic) 
field lines 
Deshielded 
fields add 
Bo 
secondary 
magnetic 
(anisotropic) 
field 
H 
H 
C 
C 
ANISOTROPIC FIELD FOR AN ALKYNE 
hydrogens 
are shielded 
Shielded 
fields subtract 
Magnetic Anisotropy 
 
• extreme case of this is 
provided by 18-C ring 
system annulene. 
• The 12 outside protons are 
deshielded and appear at 
8.9 ppm. The 6 inside 
protons are shielded and 
appear at negative ppm, -
1.8 ppm. 
 
HYDROGEN BONDING 
HYDROGEN BONDING DESHIELDS PROTONS 
O H
R
O R
HHO
R The chemical shift depends 
on how much hydrogen bonding 
is taking place. 
Alcohols vary in chemical shift 
from 0.5 ppm (free OH) to about 
5.0 ppm (lots of H bonding). 
Hydrogen bonding lengthens the 
O-H bond and reduces the valence 
electron density around the proton 
- it is deshielded and shifted 
 downfield in the NMR spectrum. 
O
C
O
R
H
H
C
O
O
R
Carboxylic acids have strong 
hydrogen bonding - they 
form dimers. 
With carboxylic acids the O-H 
absorptions are found between 
10 and 12 ppm very far downfield. 
O
O
O
H
CH
3
In methyl salicylate, which has strong 
internal hydrogen bonding, the NMR 
absorption for O-H is at about 14 ppm, 
a 6-membered ring is formed. 
SOME MORE EXAMPLES