Transcript chap13b - WordPress.com

 NUCLEAR-
Study of nuclear spins
 MAGNETIC-
Under the influence of applied
magnetic field
 RESONANCE-
Record the resulting resonance in
nuclear spin through the absorption
of RF
INTEGRATION
NMR Spectrum of Phenylacetone
O
CH2 C CH3
RECALL
from last
time
Each different type of proton comes at a different place .
You can tell how many different types of hydrogen
there are in the molecule.
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.
Integration = determination of the area
under a peak
The area under a peak is proportional
to the number of hydrogens that
generate the peak.
Benzyl Acetate
The integral line rises an amount proportional to the number of H in each peak
METHOD 1
integral line
integral
line
55 : 22 : 33
=
5:2:3
simplest ratio
of the heights
Benzyl Acetate (FT-NMR)
Actually :
5
58.117 / 11.3
= 5.14
2
21.215 / 11.3
= 1.90
3
33.929 / 11.3
= 3.00
O
CH2 O C CH3
METHOD 2
digital
integration
assume CH3
33.929 / 3 = 11.3
Integrals are
good to about
10% accuracy.
Modern instruments report the integral as a number.
DIAMAGNETIC ANISOTROPY
SHIELDING BY VALENCE ELECTRONS
Diamagnetic Anisotropy
The applied field
induces circulation
of the valence
electrons - this
generates a
magnetic field
that opposes the
applied field.
valence electrons
shield the nucleus
from the full effect
of the applied field
magnetic field
lines
Bo applied
B induced
(opposes Bo)
fields subtract at nucleus
Diamagnetic Anisotropy
The applied field
induces circulation
of the valence
electrons - this
generates a
magnetic field
that opposes the
applied field.
valence electrons
shield the nucleus
from the full effect
of the applied field
magnetic field
lines
Bo applied
B induced
(opposes Bo)
fields subtract at nucleus
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).
DOWNFIELD
Less shielded protons
appear here.
SPECTRUM
UPFIELD
Highly shielded
protons appear here.
It takes a higher field
to cause resonance.
CHEMICAL SHIFT
PEAKS ARE MEASURED RELATIVE TO TMS
Rather than measure the exact resonance position of a
peak, we measure how far downfield it is shifted from TMS.
reference compound
tetramethylsilane
“TMS”
CH3
CH3 Si CH3
CH3
Highly shielded
protons appear
way upfield.
TMS
shift in Hz
downfield
n
0
Chemists originally
thought no other
compound would
come at a higher
field than TMS.
REMEMBER FROM OUR EARLIER DISCUSSION
field
strength
frequency
hν =
γ B
o
2π
constants
ν = ( K) Bo
Stronger magnetic fields (Bo) cause
the instrument to operate at higher
frequencies (ν).
NMR Field
Strength
1.41 T
2.35 T
7.05 T
1H
Operating
Frequency
60 Mhz
100 MHz
300 MHz
HIGHER FREQUENCIES GIVE LARGER SHIFTS
The shift observed for a given proton
in Hz also depends on the frequency
of the instrument used.
Higher frequencies
= larger shifts in Hz.
TMS
shift in Hz
downfield
n
0
THE 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:
parts per
million
chemical
=
shift
δ
shift in Hz
=
spectrometer frequency in MHz
= ppm
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).
HERZ EQUIVALENCE OF 1 PPM
What does a ppm represent?
1H
Operating
Frequency
60 Mhz
100 MHz
300 MHz
7
6
1 part per million
of n MHz is n Hz
Hz Equivalent
of 1 ppm
n MHz (
60 Hz
100 Hz
300 Hz
5
4
3
2
1
1
= n Hz
)
6
10
0
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).
NMR Correlation Chart
-OH -NH
DOWNFIELD
DESHIELDED
UPFIELD
SHIELDED
CHCl3 , H
TMS
12
11
10
9
8
7
6
H
RCOOH
RCHO
C=C
5
4
CH2F
CH2Cl
CH2Br
CH2I
CH2O
CH2NO2
3
2
1
0
d (ppm)
CH2Ar
C-CH-C
CH2NR2
C
CH2S
C-CH2-C
C C-H
C=C-CH2 C-CH3
CH2-CO
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
R-CH2-R
R3CH
0.7 - 1.3
1.2 - 1.4
1.4 - 1.7
R-C=C-C-H
O
1.6 - 2.6
R-C-C-H
O
2.1 - 2.4
RO-C-C-H
O
2.1 - 2.5
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
R-C C-H
2.3 - 2.7
1.7 - 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
O
3.2 - 3.8
R-C-O-C-H
3.5 - 4.8
O2N-C-H
4.1 - 4.3
F-C-H
4.2 - 4.8
R-C=C-H
4.5 - 6.5
H
6.5 - 8.0
O
R-C-N-H
5.0 - 9.0
O
R-C-H
9.0 - 10.0
O
R-C-O-H
11.0 - 12.0
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
YOU DO NOT NEED TO MEMORIZE THE
PREVIOUS CHART
IT IS USUALLY SUFFICIENT TO KNOW WHAT TYPES
OF HYDROGENS COME IN SELECTED AREAS OF
THE NMR CHART
C-H where C is
CH on C
attached
to
an
aliphatic
acid
aldehyde benzene alkene
next to
C-H
COOH
CHO
CH
=C-H electronega- pi bonds
tive atom
X=C-C-H
X-C-H
12
10
9
7
6
4
3
2
0
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.
We will discuss these factors in the sections that
follow.
DESHIELDING BY
ELECTRONEGATIVE ELEMENTS
DESHIELDING BY AN ELECTRONEGATIVE ELEMENT
d-
Cl
d+
C
d-
electronegative
element
H
d+
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.
NMR CHART
“deshielded“
protons appear
at low field
highly shielded
protons appear
at high field
deshielding moves proton
resonance to lower field
Electronegativity Dependence
of Chemical Shift
Dependence of the Chemical Shift of CH3X on the Element X
Compound CH3X
Element X
Electronegativity of X
Chemical shift
d
most
deshielded
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
TMS
deshielding increases with the
electronegativity of atom X
Substitution Effects on
Chemical Shift
most
deshielded
most
deshielded
CHCl3 CH2Cl2 CH3Cl
7.27 5.30
3.05 ppm
-CH2-Br
3.30
-CH2-CH2Br
1.69
The effect
increases with
greater numbers
of electronegative
atoms.
-CH2-CH2CH2Br
1.25
ppm
The effect decreases
with incresing distance.
ANISOTROPIC FIELDS
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.
Ring Current in Benzene
Circulating  electrons
H
Bo
H
Deshielded
fields add together
Secondary magnetic field
generated by circulating 
electrons deshields aromatic
protons
ANISOTROPIC FIELD IN AN ALKENE
protons are
deshielded
Deshielded
fields add
H
shifted
downfield
C=C
H
Bo
H
H
secondary
magnetic
(anisotropic)
field lines
ANISOTROPIC FIELD FOR AN ALKYNE
H
C
C
H
Bo
Shielded
fields subtract
hydrogens
are shielded
secondary
magnetic
(anisotropic)
field
HYDROGEN BONDING
HYDROGEN BONDING DESHIELDS PROTONS
R
O
H
H
O
H
O 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).
R
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.
SOME MORE EXTREME EXAMPLES
O
H
O
C R
R C
O
H
O
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.
H3C O
O
H
O
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.