INTEGRATION. NMR Spectrum of Phenylacetone. RECALL. Each different type of proton comes at a different place . You can tell how many different types of hydrogen there are in the molecule. from last time. INTEGRATION OF A PEAK. Not only does each different type of hydrogen give a
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Each different type of proton comes at a different place .
You can tell how many different types of hydrogen
there are in the molecule.
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.
Integration = determination of the area
under a peak
The area under a peak is proportional
to the number of hydrogens that
generate the peak.
The integral line rises an amount proportional to the number of H in each peak
of the heights
55 : 22 : 33 = 5 : 2 : 3
Actually : 5 2 3
21.215 / 11.3
33.929 / 11.3
58.117 / 11.3
33.929 / 3 = 11.3
good to about
Modern instruments report the integral as a number.
SHIELDING BY VALENCE ELECTRONS
The applied field
of the valence
electrons - this
that opposes the
shield the nucleus
from the full effect
of the applied field
fields subtract at nucleus
Less shielded protons
protons appear here.
It takes a higher field
to cause resonance.
PROTONS DIFFER IN THEIR SHIELDING
All different types of protons in a molecule
have a different amounts of shielding.
They all respond differently to the applied magnetic
field and appear at different places in the spectrum.
This is why an NMR spectrum contains useful information
(different types of protons appear in predictable places).
Rather than measure the exact resonance position of a
peak, we measure how far downfield it is shifted from TMS.
thought no other
come at a higher
field than TMS.
shift in Hz
REMEMBER FROM OUR EARLIER DISCUSSION
Stronger magnetic fields (Bo) cause
the instrument to operate at higher
hn = Bo
n = ( K) Bo
The shift observed for a given proton
in Hz also depends on the frequency
of the instrument used.
= larger shifts in Hz.
shift in Hz
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:
shift in Hz
= d =
spectrometer frequency in MHz
This division gives a number independent
of the instrument used.
A particular proton in a given molecule will always come
at the same chemical shift (constant value).
What does a ppm represent?
1 part per million
of n MHz is n Hz
of 1 ppm
n MHz = n Hz
60 Mhz 60 Hz
100 MHz 100 Hz
300 MHz 300 Hz
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).
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-N-C-H 2.2 - 2.9
R-CH2-R 1.2 - 1.4
4.5 - 6.5
R-S-C-H 2.0 - 3.0
R3CH 1.4 - 1.7
I-C-H 2.0 - 4.0
R-C=C-C-H 1.6 - 2.6
Br-C-H 2.7 - 4.1
6.5 - 8.0
Cl-C-H 3.1 - 4.1
R-C-C-H 2.1 - 2.4
RO-C-H 3.2 - 3.8
5.0 - 9.0
RO-C-C-H 2.1 - 2.5
HO-C-H 3.2 - 3.8
HO-C-C-H 2.1 - 2.5
9.0 - 10.0
R-C-O-C-H 3.5 - 4.8
N C-C-H 2.1 - 3.0
O2N-C-H 4.1 - 4.3
R-C C-C-H 2.1 - 3.0
11.0 - 12.0
F-C-H 4.2 - 4.8
C-H 2.3 - 2.7
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
1.0 - 4.0
R-C C-H 1.7 - 2.7
IT IS USUALLY SUFFICIENT TO KNOW WHAT TYPES
OF HYDROGENS COME IN SELECTED AREAS OF
THE NMR CHART
C-H where C is attached to an
CH on C
MOST SPECTRA CAN BE INTERPRETED WITH
A KNOWLEDGE OF WHAT IS SHOWN HERE
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.
We will discuss these factors in the sections that
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.
at low field
at high field
deshielding moves proton
resonance to lower field
Dependence of the Chemical Shift of CH3X on the Element X
CH3F CH3OH CH3Cl CH3Br CH3I CH4 (CH3)4Si
F O Cl Br I H Si
Electronegativity of X
4.0 3.5 3.1 2.8 2.5 2.1 1.8
Chemical shift d
4.26 3.40 3.05 2.68 2.16 0.23 0
deshielding increases with the
electronegativity of atom X
CHCl3 CH2Cl2 CH3Cl
7.27 5.30 3.05 ppm
-CH2-Br -CH2-CH2Br -CH2-CH2CH2Br
3.30 1.69 1.25 ppm
The effect decreases
with incresing distance.
DUE TO THE PRESENCE OF PI BONDS
The presence of a nearby pi bond or pi system
greatly affects the chemical shift.
Benzene rings have the greatest effect.
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.
Carboxylic acids have strong
hydrogen bonding - they
With carboxylic acids the O-H
absorptions are found between
10 and 12 ppm very far downfield.
In methyl salicylate, which has strong
internal hydrogen bonding, the NMR
absortion for O-H is at about 14 ppm,
way, way downfield.
Notice that a 6-membered ring is formed.