HYDROGEN BONDING DESHIELDS PROTONS The chemical shift depends on how much hydrogen bonding is taking place.
R O
H
H O R
H
O R
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 O-H electron density around the proton - it is deshielded and shifted d downfield fi ld iin the th NMR spectrum. t
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SOME MORE EXTREME EXAMPLES O
H
O C R
R C O
H
Carboxylic acids have strong hydrogen bonding - they form dimers.
O With carboxylic acids the O O-H H absorptions are found between 10 and 12 ppm very far downfield.
H3C O
O
H O
In methyl salicylate, salicylate which has strong internal hydrogen bonding, the NMR absortion for O-H is at about 14 ppm, way way downfield way, downfield. Notice that a 6-membered ring is formed.
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1.6 1. 6 Integration
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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
Integration = determination of the area under a peak The area under a peak is proportional to the number of protons that generate the peak. ITS Chemistry
Benzyl Acetate
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The integral line rises an amount proportional to the number of H in each peak
METHOD 1 integral line iintegral t l line
Prof. Dr. Taslim Ersam
55 : 22 : 33 =
5:2:3
simplest ratio of the heights
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Benzyl Acetate (FT (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 per H Integrals are good to about 10% accuracy.
Modern instruments report the integral as a number.
Prof. Dr. Taslim Ersam
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1.7. SpinSpin-Spin Splitting
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SPIN--SPIN SPLITTING SPIN Often a group of hydrogens will appear as a multiplet rather than as a single peak. Multiplets are named as follows: Singlet Si l t Doublet Triplet Quartet
Quintet Q i t t Septet Octet Nonet
This happens because of interaction with neighboring y g and is called SPIN-SPIN SPLITTING. hydrogens ITS Chemistry
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1,1,21 1 2-Trichloroethane 1,1,2 T i hl th integral = 2
Cl H H C C Cl integral = 1
triplet
Cl H
doublet
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n+1
RULE
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this hydrogen’s hydrogen s peak is split by its two neighbors
these hydrogens are split by their single neighbor
H
H
H
H
C
C
C
C
H two neighbors n+1 = 3 triplet p
H one neighbor n+1 = 2 doublet
MULTIPLETS
singlet doublet triplet quartet quintet sextet septet
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Some Common Patterns
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SOME COMMON SPLITTING PATTERNS X CH CH Y (x=y)
CH-CH2
X CH2 CH2 Y (x=y)
CH-CH3
-CH2-CH3
CH3 CH CH 3
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tert-butyl group
CH3 H3C
C
Cl
9 equivalent protons = singlet
CH3
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EXCEPTIONS TO THE N+1 RULE IMPORTANT ! 1)
Protons P t that th t are equivalent i l t by b symmetry t usually do not split one another X CH2 CH2 Y
X CH CH Y no splitting if x=y 2)
no splitting if x=y
Protons in the same group usually y do not split p one another H C H H
H or
C H
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SOME EXAMPLE SPECTRA WITH SPLITTING
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NMR Spectrum of Bromoethane
Br CH2CH3
Prof. Dr. Taslim Ersam
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NMR Spectrum p of 2-Nitropropane p p
H CH3
C
CH3
+ N O O-
1:6:15:20:16:6:1 Prof. Dr. Taslim Ersam
in higher multiplets the outer peaks are often nearly lost in the baseline
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NMR Spectrum of Acetaldehyde
O CH3 C
H
offset = 2.0 ppm
Prof. Dr. Taslim Ersam
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The propyl group CH3-CH2-CH2-X Can you predict the splitting patterns for this compound?
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INTENSITIES OF MULTIPLET PEAKS
PASCAL’S TRIANGLE
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PASCAL’S TRIANGLE
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Intensities of multiplet peaks
1 The interior entries are 1 1 the sums of the two 1 2 1 numbers immediately 1 3 3 1 above. 1 4 6 4 1 1 5 10 10 5 1 1 6 15 20 15 6 1 1 7 21 35 35 21 7 1
singlet doublet triplet quartet t t quintet q sextet septet octet ITS Chemistry
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THE ORIGIN OF SPIN--SPIN SPLITTING SPIN HOW IT HAPPENS
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THE CHEMICAL SHIFT OF PROTON HA IS AFFECTED BY THE SPIN OF ITS NEIGHBORS aligned g with Bo +1/2 50 % of molecules
opposed pp to Bo -1/2
H
HA
H
HA
C
C
C
C
50 % of molecules
Bo downfield d fi ld neighbor aligned
upfield fi ld neighbor opposed
At any given time about half of the molecules in solution will have spin +1/2 and the other half will have spin -1/2.
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SPIN ARRANGEMENTS one neighbor g n+1 = 2 doublet
one neighbor g n+1 = 2 doublet
H
H
H
H
C
C
C
C
yellow spins blue spins The resonance positions (splitting) of a given hydrogen is affected by the possible spins of its neighbor.
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SPIN ARRANGEMENTS two neighbors g n+1 = 3 triplet
one neighbor g n+1 = 2 doublet
H
H
H
H
C
C
C
C
H
H methine spins
methylene spins
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SPIN ARRANGEMENTS three neighbors g n+1 = 4 quartet
H
H
C
C
H
H
H
two neighbors g n+1 = 3 triplet
H
H
C
C
H
H
H
methylene spins methyl spins
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2. The Coupling Constant
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THE COUPLING CONSTANT J J
H H
J
C C H H H
J
J
J
The coupling constant is the distance J (measured in Hz) between the peaks in a multiplet. multiplet J is a measure of the amount of interaction between the two sets of hydrogens creating the multiplet. ITS Chemistry
100 MHz
6
5
75H 7.5 Hz
J = 7.5 Hz
4
200 MHz
3
2
1
400 Hz
Separation is larger The shift Th hift is i dependant on the field
200 Hz 100 Hz
Coupling constants are constant - they y do not change at different field strengths
3
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FIELD COMPARISON
200 Hz 7 5 Hz 7.5
J = 7.5 Hz
2
1
ppm ITS Chemistry
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50 MHz Why buy a higher field instrument?
Spectra are simplified!
J = 7.5 Hz
3
2
1
2
1
2
1
100 MHz J = 7.5 Hz
Overlapping multiplets are t d separated.
3 200 MHz
Second-order effects are minimized.
J=7 7.5 5H Hz
3
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NOTATION FOR COUPLING CONSTANTS The most commonly encountered type of coupling is between hydrogens on adjacent carbon atoms. 3J
H H C C
This is sometimes called vicinal coupling. 3J since three bonds It is designated g intervene between the two hydrogens.
Another type of coupling that can also occur in special cases is 2J or geminal coupling H ( most often 2J = 0 ) C H Geminal coupling does not occur when 2J the two hydrogens are equivalent due to rotations around the other two bonds. ITS Chemistry
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LONG RANGE COUPLINGS Couplings larger than 2J or 3J also exist, but operate y in special p situations,, especially p y in unsaturated only systems. Couplings larger than 3J (e.g., 4J, 5J, etc) are usually called “long-range coupling.”
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SOME REPRESENTATIVE COUPLING CONSTANTS
H H vicinal
C C
6 to 8 Hz
three bond
3J
11 to 18 Hz
three bond
3J
6 to 15 Hz
three bond
3J
0 to 5 Hz
two bond
H C C
trans cis geminal
H H
H C C
H
C H
2J
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H
H
cis
6 to 12 Hz
trans
4 to 8 Hz
three bond
3J
4 to 10 Hz
three bond
3J
0 to 3 Hz
four bond
4J
0 to 3 Hz
four bond
4J
H C C H
H C C C H H C C C H
Couplings that occur at distances greater than three bonds are called long-range couplings and they are usually small (
