integration. each different type of proton comes at a different place. you can tell how many...
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INTEGRATIONINTEGRATION
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CH2 C
O
CH3
Each different type of proton comes at a different place .You can tell how many different types of hydrogen there are in the molecule.
NMR Spectrum of PhenylacetoneNMR Spectrum of Phenylacetone
RECALLfrom lasttime
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The area under a peak is proportionalto the number of hydrogens thatgenerate the peak.
Integration = determination of the area under a peak
INTEGRATION OF A PEAKINTEGRATION OF A PEAKNot 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 aprocess called integration.
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55 : 22 : 33 = 5 : 2 : 3
The integral line rises an amount proportional to the number of H in each peak
METHOD 1integral line
integralline
simplest ratioof the heights
Benzyl AcetateBenzyl Acetate
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Benzyl Acetate (FT-NMR)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.
CH2 O C
O
CH3
Integrals aregood to about10% accuracy.
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DIAMAGNETIC ANISOTROPYDIAMAGNETIC ANISOTROPY
SHIELDING BY VALENCE ELECTRONS
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valence electronsshield the nucleusfrom the full effectof the applied field
B induced (opposes Bo)
Bo applied
magnetic fieldlines
The applied fieldinduces circulationof the valenceelectrons - thisgenerates amagnetic fieldthat opposes theapplied field.
Diamagnetic AnisotropyDiamagnetic Anisotropy
fields subtract at nucleus
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All different types of protons in a moleculehave 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 SHIELDINGPROTONS DIFFER IN THEIR SHIELDING
UPFIELDDOWNFIELD
Highly shielded protons appear here.
Less shielded protonsappear here.
SPECTRUM
It takes a higher fieldto cause resonance.
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CHEMICAL SHIFTCHEMICAL SHIFT
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PEAKS ARE MEASURED RELATIVE TO TMSPEAKS 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 shieldedprotons appearway upfield.
Chemists originallythought no other compound wouldcome at a higherfield than TMS.
downfield
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h = Bo
2
constants
frequency
fieldstrength
Stronger magnetic fields (Bo) causethe instrument to operate at higherfrequencies ().
REMEMBER FROM OUR EARLIER DISCUSSION
NMR Field Strength
1H Operating Frequency
60 Mhz
100 MHz
300 MHz7.05 T
2.35 T1.41 T
= ( K) Bo
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TMS
shift in Hz
0n
downfield
The shift observed for a given protonin Hz also depends on the frequencyof the instrument used.
Higher frequencies= larger shifts in Hz.
HIGHER FREQUENCIES GIVE LARGER SHIFTSHIGHER FREQUENCIES GIVE LARGER SHIFTS
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chemical shift
= = shift in Hz
spectrometer frequency in MHz= ppm
This division gives a number independent of the instrument used.
parts permillion
THE CHEMICAL SHIFTTHE CHEMICAL SHIFTThe 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).
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01234567 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 PPMHERZ 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 millionof n MHz is n Hz
n MHz = n Hz 1
106( )
What does a ppm represent?
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NMR Correlation ChartNMR Correlation Chart
12 11 10 9 8 7 6 5 4 3 2 1 0
-OH -NH
CH2FCH2ClCH2BrCH2ICH2OCH2NO2
CH2ArCH2NR2
CH2SC C-HC=C-CH2
CH2-C-O
C-CH-C
C
C-CH2-CC-CH3
RCOOH RCHO C=C
H
TMS
HCHCl3 ,
(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.
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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 - 2.5O
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 - 1.7R-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 - 4.0R-C C-H 1.7 - 2.7
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YOU DO NOT NEED TO MEMORIZE THE PREVIOUS CHART
IT IS USUALLY SUFFICIENT TO KNOW WHAT TYPESOF HYDROGENS COME IN SELECTED AREAS OFTHE NMR CHART
aliphatic C-H
CH on Cnext to pi bonds
C-H where C is attached to anelectronega-tive atom
alkene=C-H
benzene CH
aldehyde CHO
acidCOOH
2346791012 0
X-C-HX=C-C-H
MOST SPECTRA CAN BE INTERPRETED WITH A KNOWLEDGE OF WHAT IS SHOWN HERE
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DESHIELDING AND ANISOTROPYDESHIELDING 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.
We will discuss these factors in the sections thatfollow.
3. Deshielding due to hydrogen bonding.
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DESHIELDING BY DESHIELDING BY ELECTRONEGATIVE ELEMENTSELECTRONEGATIVE ELEMENTS
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highly shieldedprotons appearat high field
“deshielded“protons appear at low field
deshielding moves protonresonance to lower field
C HClChlorine “deshields” the proton,that is, it takes valence electron density away from carbon, whichin turn takes more density fromhydrogen deshielding the proton. electronegative
element
DESHIELDING BY AN ELECTRONEGATIVE ELEMENTDESHIELDING BY AN ELECTRONEGATIVE ELEMENT
NMR CHART
-
-
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Electronegativity Dependence Electronegativity Dependence
of Chemical Shiftof Chemical Shift
Compound CH3X
Element X
Electronegativity of X
Chemical shift
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 theelectronegativity of atom X
TMSmostdeshielded
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Substitution Effects on Substitution Effects on Chemical ShiftChemical Shift
CHCl3 CH2Cl2 CH3Cl
7.27 5.30 3.05 ppm
-CH2-Br -CH2-CH2Br -CH2-CH2CH2Br
3.30 1.69 1.25 ppm
mostdeshielded
mostdeshielded
The effect decreaseswith incresing distance.
The effect increases withgreater numbersof electronegativeatoms.
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ANISOTROPIC FIELDSANISOTROPIC FIELDSDUE 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.
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Secondary magnetic field
generated by circulating electrons deshields aromaticprotons
Circulating electrons
Ring Current in BenzeneRing Current in Benzene
Bo
Deshielded
H H fields add together
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C=CHH
H H
Bo
ANISOTROPIC FIELD IN AN ALKENEANISOTROPIC FIELD IN AN ALKENE
protons aredeshielded
shifteddownfield
secondarymagnetic(anisotropic)field lines
Deshielded
fields add
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Bo
secondarymagnetic(anisotropic)field
H
HC
C
ANISOTROPIC FIELD FOR AN ALKYNEANISOTROPIC FIELD FOR AN ALKYNE
hydrogensare shielded
Shielded
fields subtract
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HYDROGEN BONDINGHYDROGEN BONDING
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HYDROGEN BONDING DESHIELDS PROTONSHYDROGEN BONDING DESHIELDS PROTONS
O H
R
O R
HHO
RThe chemical shift dependson how much hydrogen bondingis taking place.
Alcohols vary in chemical shiftfrom 0.5 ppm (free OH) to about5.0 ppm (lots of H bonding).
Hydrogen bonding lengthens theO-H bond and reduces the valence electron density around the proton- it is deshielded and shifted downfield in the NMR spectrum.
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O
CO
RH
HC
O
OR
Carboxylic acids have stronghydrogen bonding - theyform dimers.
With carboxylic acids the O-Habsorptions are found between10 and 12 ppm very far downfield.
O
OO
H
CH3
In methyl salicylate, which has stronginternal hydrogen bonding, the NMRabsortion for O-H is at about 14 ppm,way, way downfield.
Notice that a 6-membered ring is formed.
SOME MORE EXTREME EXAMPLESSOME MORE EXTREME EXAMPLES