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TYPES OF INFORMATION FROM NMR SPECTRUM
1. Each different type of hydrogen gives a peak or group of peaks (multiplet).
3. The integral gives the relative numbers of each type of hydrogen.
2. The chemical shift (δ, in ppm) gives a clue as to the type of hydrogen generating the peak (alkane, alkene, benzene, aldehyde, etc.)
4. Spin-spin splitting gives the number of hydrogens on adjacent carbons.5. The coupling constant J also gives information about the arrangement of the atoms involved.
CHEMICAL SHIFT
DIAMAGNETIC ANISOTROPY
SHIELDING BY VALENCE ELECTRONS
valence electronsshield the nucleusfrom the full effectof the applied field
B induced (opposes Bo)Bo applied
magnetic field lines
Applied magnetic field induces circulationof valenceelectrons - thisgenerates amagnetic fieldthat opposes theapplied field.
Diamagnetic Anisotropy
All different types of protons in a moleculehave a different amounts of shielding.
They all respond differently to the applied magnetic field and appear at different places in the spectrum.
PROTONS DIFFER IN THEIR SHIELDING
UPFIELDDOWNFIELDshielded protons appear here.
deshielded protonsappear here.
SPECTRUM
PEAKS ARE MEASURED RELATIVE TO TMS
TMSshift in Hz
0
Si CH3CH3
CH3
CH3
tetramethylsilane “TMS”
reference compound
n
Rather than measure exact position of a peak, we measure how far downfield it is shifted from TMS.
Highly shieldedprotons appearway upfield.
downfield
chemical shift
= δδδδ = shift in Hzspectrometer frequency in MHz
= ppm
This division gives a number independent of the instrument used.
parts permillion
THE 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 have the same chemical shift (constant value).
NMR Correlation Chart
12 11 10 9 8 7 6 5 4 3 2 1 0
-OH -NH
CH2FCH2ClCH2BrCH2ICH2OCH2NO2
CH2ArCH2NR2CH2SC C-HC=C-CH2CH2-C-
O
C-CH-CC
C-CH2-CC-CH3
RCOOH RCHO C=CH
TMS
HCHCl3 ,
δδδδ (ppm)
DOWNFIELD UPFIELDDESHIELDED SHIELDED
Ranges can be defined for different general types of protons.This chart is general, the next slide is more definite.
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
More shieldedprotons appearupfield
“deshielded”protons appear downfield
deshielding moves protonresonance to lower field
C HClChlorine “deshields” the proton,it takes valence electron density away from carbon, whichin turn takes more density fromhydrogen deshielding the proton. electronegative
element
DESHIELDING BY AN ELECTRONEGATIVE ELEMENT
NMR CHART
δδδδ- δδδδ++++
δδδδ- δδδδ++++
Electronegativity Dependenceof Chemical Shift
Compound CH3X
Element X
Electronegativity of XChemical shift δδδδ
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
Substitution Effects onChemical 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.
ANISOTROPIC 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.
Secondary magnetic fieldgenerated by circulating ππππelectrons deshields aromaticprotons
Circulating ππππ electrons
Ring Current in BenzeneRing Current in Benzene
Bo
Deshielded
H H fields add together
C=CHH
H H
Bo
ANISOTROPIC FIELD IN AN ALKENE
protons aredeshielded
shifteddownfield
secondarymagnetic(anisotropic)field lines
Deshielded
fields add
HYDROGEN BONDING
HYDROGEN 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.
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 EXAMPLES
SPIN-SPIN SPLITTING
Often a group of hydrogens will appear as a multipletrather than as a single peak.
SPIN-SPIN SPLITTING
Multiplets are named as follows:
Singlet QuintetDoublet SeptetTriplet OctetQuartet Nonet
This happens because of interaction with neighboring hydrogens and is called SPIN-SPIN SPLITTING.
C CH
Cl
Cl H
H
Cl
integral = 2
integral = 1
triplet doublet
1,1,2-TrichloroethaneThe two kinds of hydrogens do not appear as single peaks,rather there is a “triplet” and a “doublet”.
The subpeaks are due tospin-spin splitting and are predicted by the n+1 rule.
n + 1 RULE
1,1,2-Trichloroethane
C CH
Cl
Cl H
H
Cl
integral = 2
integral = 1
Where do these multiplets come from ? Ö.. interaction with neighbors
C C
H H
H
C C
H H
H
two neighborsn+1 = 3triplet
one neighborn+1 = 2doublet
singletdoublettripletquartetquintetsextetseptet
MULTIPLETSthis hydrogen’s peakis split by its two neighbors
these hydrogens aresplit by their singleneighbor
EXCEPTIONS TO THE N+1 RULEIMPORTANT !
Protons that are equivalent by symmetryusually do not split one another
CH CHX Y CH2 CH2X Y
no splitting if x=y no splitting if x=y
1)
2) Protons in the same group usually do not split one another
C
H
H
H or C
H
H
moredetaillater
SOME COMMON PATTERNS
SOME COMMON SPLITTING PATTERNS
CH2 CH2X Y
CH CHX Y
CH2 CH
CH3 CH
CH3 CH2
CH3
CHCH3
( x = y )
( x = y )
SOME EXAMPLE SPECTRA WITH SPLITTING
NMR Spectrum of Bromoethane
CH2CH3Br
NMR Spectrum of 2-Nitropropane
CCH3 CH3
N
H
O O+
-
1:6:15:20:16:6:1 in higher multiplets the outer peaksare often nearly lost in the baseline
NMR Spectrum of Acetaldehyde
offset = 2.0 ppm
CCH3
O
H
INTENSITIES OF MULTIPLET PEAKS
PASCAL’S TRIANGLE
1 2 1
PASCALíS TRIANGLE
11 1
1 3 3 11 4 6 4 1
1 5 10 10 5 11 6 15 20 15 6 1
1 7 21 35 35 21 7 1
singlet
doublet
triplet
quartet
quintet
sextet
septet
octet
The interiorentries arethe sums ofthe two numbersimmediatelyabove.
Intensities ofmultiplet peaks
THE COUPLING CONSTANT
J J
J
J J
THE COUPLING CONSTANT
The coupling constant is the distance J (measured in Hz) between the peaks in a multiplet.
J is a measure of the amount of interaction between the two sets of hydrogens creating the multiplet.
C
H
H
C H
H
H
J
NOTATION FOR COUPLING CONSTANTSThe most commonly encountered type of coupling is between hydrogens on adjacent carbon atoms.
C C
HH This is sometimes called vicinal coupling.It is designated 3J since three bondsintervene between the two hydrogens.
Another type of coupling that can also occur in special cases is
C H
H2J or geminal coupling
Geminal coupling does not occur whenthe two hydrogens are equivalent due torotations around the other two bonds.
( most often 2J = 0 )
3J
2J
C C
H H
C CH
H
C CHH
CH
H
6 to 8 Hz
11 to 18 Hz
6 to 15 Hz
0 to 5 Hz
three bond 3J
two bond 2J
three bond 3J
three bond 3J
SOME REPRESENTATIVE COUPLING CONSTANTS
Hax
Hax
Heq
Heq
Hax,Hax = 8 to 14Hax,Heq = 0 to 7Heq,Heq = 0 to 5
three bond 3J
trans
cis
geminal
vicinal
OVERVIEW
TYPES OF INFORMATION FROM THE NMR SPECTRUM
1. Each different type of hydrogen gives a peak or group of peaks (multiplet).
3. The integral gives the relative numbers of each type of hydrogen.
2. The chemical shift (δ, in ppm) gives a clue as to the type of hydrogen generating the peak (alkane, alkene, benzene, aldehyde, etc.)
4. Spin-spin splitting gives the number of hydrogens on adjacent carbons.
5. The coupling constant J also gives information about the arrangement of the atoms involved.
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