Chapter 1 Structure and Bonding

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Chapter 10 NMR in Practice
I.
Chemical Equivalence
A.
Using Molecular Symmetry
1) Chemically equivalent protons have identical chemical shifts
2) It is not always easy to see if protons are equivalent
3) If protons are related by a mirror plane, they are equivalent
4) If protons are related by a rotational axis, they are equivalent
5) Rotational axis = line of molecular rotation producing identical structures
B.
Conformational Changes can cause equivalence
1) Chloroethane has 2 1H NMR peaks, not 3
a) Conformational rotation averages out gauche and anti positions
b) Must be fast on the NMR timescale (Lifetime < 1 s)
c) Cool down solution to –180 oC to see all three peaks
2) Cyclohexane has only one 1H NMR peak, not 2
a) Axial and equatorial protons have different chemical environments
b) Ring flip conformational change is fast on NMR timescale, averages
out the axial and equatorial protons to one peak
c) Cool to –90 oC to see both peaks
II.
Integration
A.
The number of protons responsible for a peak determines the peak size
1) Integration = finding the area under a 1H NMR peak
2) The NMR computer will trace a line above each peak that has its length
proportional to the area under the peak
3) We measure the length of each line (area under each peak) and compare
them with each other
4) There can only be integer numbers of protons, so we normalize to whole #
B.
We can use chemical shift and integration to assign structure
1) Dichlorination of propane:
2) CH3CH2CHCl2
1H
2H
3H
3) ClCH2CH2CH2Cl
4H
2H
4) CH3CHClCH2Cl
1H 2H
3H
III. Spin-Spin Splitting
A.
Neighboring protons effect each other
1) Protons are tiny magnets, with a and b spin states in a magnetic field
2) Proton on the same carbon or on adjacent carbons influence the total
magnetic field felt by their neighbor, just as e- magnetic field does
3) Simple case of one proton on each adjacent carbon
4)
5)
6)
7)
Ha
C
Hb
Ha
Hb
C
C
B.
This phenomenon is called Spin-Spin Splitting
Single peak is split into a Multiplet by spin-spin splitting
Singlet, Doublet, Triplet, Quartet, etc… to Multiplet
Coupling Constant = J (Hz) = how far apart the multiplet peaks are
Spin-spin splitting is only seen for:
a) Geminal Protons = chemically inequivalent protons on the same
carbon (J up to 18 Hz)
b) Vicinal Protons = chemically inequivalent protons on adjacent
carbons (J = 6-8 Hz)
c) Both protons are always split by same J. If Ha splits Hb, then Hb
splits Ha by the same amount
d) Chemically equivalent protons do not split each other
Local Field Effects are Additive
1) What if a proton has multiple neighboring protons?
Triplet, 1:2:1, 3H,
1.5 ppm
C
Hc
Hd
C
C
Hc
Hd
Quartet, 1:3:3:1, 2H,
1.3 ppm
Hd
2)
N + 1 Rule = N equivalent nuclei split a neighboring proton resonance
into N + 1 peaks
a) CH—CH3 doublet(3H) and quartet(1H)
b) CH2—CH3 triplet(3H) and quartet(2H)
3)
Pascal’s Triangle Predicts relative intensities of the peaks in a multiplet
4)
Example: Bromoethane
5)
Example: 2-Iodopropane
6)
Example: 1,1-dichloro-2,2-diethoxyethane
D.
Complications to Splitting Patterns
1) First Order Spectrum = Dd >> J (easy to see all splitting)
2) Non-First Order Spectrum = Dd = J (all smashed together)
3) You can go to higher magnetic field to spread it back out
90 MHz
500 MHz
4)
Many close d peaks in same molecule = won’t see all of the splitting
Long Alkyl Chains are Notorious for this
5)
We must take into account all neighbors, when predicting splitting
a) Predict splitting based on one type of neighbor
b) Apply splitting by other type of neighbor to each of the split peaks
from the first type of neighbor
6)
Sometimes the spectrum will appear First Order, but isn’t (J’s are same)
7)
Fast Proton Exchange “Decouples” some protons
a) RCH2OH protons exchange quickly with protic solvents
b) See an average RCH2O—H peak
c) Happens fast on the NMR time scale
d) See no coupling to the CH2 group, unless you cool solution down
CH3OH
E.
NMR Solvents
1) You usually take NMR spectra of a sample dissolved in a solvent
2) The fast tumbling of molecules in solution is best for NMR
3) Solvent is at much greater concentration than sample, so you would see
only the solvent protons in your spectrum
4) We use Deuterated solvents (2H) because they have the same chemical
properties (solubilities) but have shifts outside the 1H range.
5) You can’t get rid of all 1H, so you usually still see a small solvent peaks
IV. Carbon-13 NMR
A.
Abundance of 13C effects its use in NMR
1) 98.9% of carbon is 12C, 1.11% is 13C
2) Much weaker signal for carbon NMR, we must take many scans, ( time)
3) 13C-13C next to each other is statistically unlikely; no C-C splitting
4) Carbon peaks are split by the 1H’s attached to them
a) Useful to tell us how many H’s are attached (triplit = CH2)
b) Usually Decouple the protons with a broad constant proton pulse,
which keeps protons a/b flipping and gives no splitting
c) Decoupling produces all singlets in the 13C NMR spectrum
B.
Chemical Shift in Carbon-13 NMR
1) Carbon resonances occur over a large range 0-250 ppm (TMS = 0 ppm)
2) This is very useful, because often proton NMR is smashed together
3) Every chemically inequivalent carbon gives a unique singlet
a) Alkyl groups: 5-50 ppm
b) Alkyl Halides: 25-50
c) Alcohols/Ethers: 50-90
d) Alkenes: 100-150
e) Carbonyl: 170-210
Bromoethane Carbon-13 NMR
C.
DEPT (Distortionless Enhanced Polarization Transfer)
1) 13C NMR Experiment that tells you how many H’s are attached to each C
All C’s
CH only
+CH3 and –CH2 only
D.
Using 13C NMR to assign structure
10.1
73.2
CH3CH2CHCl2
34.9
22.4
49.5
CH3CHClCH2Cl
55.8
42.2
42.2
ClCH2CH2CH2Cl
35.6
COSY 2D NMR Spectrum
HETCOR 2D NMR Spectrum

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