Lecture 3
NMR Spectroscopy:
• Spin-spin Splitting in 1H NMR
• Integration
• Coupling Constants
• 13C NMR
• Sample Preparation for NMR Analysis
Due:
Lecture Problem 1
1H
NMR Spectrum of Ethanol: Spin-Spin Splitting
a
b
c
a
CH3CH2O
H
TMS
a - triplet
b - quartet
c - singlet
b
c
downfield
ppm (d)
upfield
Spin-Spin Splitting
a
b
c
CH3CH2O
H
a - triplet
b - quartet
c - singlet
General rules:
• Neighboring, non-equivalent protons split each
other’s signals
• Equivalent protons do not split each other’s
signals
• Use the n + 1 rule to predict the splitting
pattern of a proton’s signal
n + 1 rule
The signal of a proton with n equivalent neighboring protons is split into a
multiplet of n + 1 peaks.
In ethanol, a neighbors b; they split each other’s peaks.
Note that b neighbors c and no splitting occurs between the two;
b is only affected by a.
In general, protons that reside on heteroatoms (O, N) do not get involved
with spin-spin splitting with neighboring protons. Thus, c appears as a singlet.
Spin-spin Splitting
Spin-Spin Splitting
Determine the splitting patterns for the signals in the 1H NMR spectra of
the following compounds.
OH
O
H
Cl
NH2
CH3
NH2
Complex Spin-Spin Splitting
Consider the 1H NMR spectrum of a substituted alkene:
Spin-Spin Splitting
Spin-Spin Splitting
Determine the splitting patterns for the signals in the 1H NMR spectra of
the following compounds.
O
O
O
H
H3C
Cl
H
Integration
• Area underneath signal; NMR machine will give integrals
• First, gives the relative ratio of different types of protons in compound
• Second, allows determination of actual ratio of different types of protons
1. Measure the length of the integral with a ruler
2. Establish a relative ratio of protons (divide each length by
the lowest number)
Coupling Constants (J)
Protons that split each other’s peaks will have the same coupling constant
or J value.
1H
NMR Spectrum of a
Taxol Derivative
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Taken from Erkan Baloglu’s Masters Thesis
Nuclear Magnetic Resonance
Information Gained:
• Different chemical environments of nuclei being analyzed (1H nuclei):
chemical shift
• The number of different types of H’s: number of signals in spectrum
• The numbers of protons with the same chemical environment:
integration
• The number of protons are bonded to the same carbon: integration
• The number of protons that are adjacent to one another: splitting
patterns
• The exact protons that are adjacent to one another: coupling
constants
13C
NMR Spectroscopy
Information Gained:
• Different chemical environments of carbons in molecule: chemical shift
• The number different types of C’s: number of signals in spectrum
Differences from 1H NMR:
• No splitting of signals (proton-decoupled); thus, only singlets
• No integration
• ppm scale ranges from 0 to 220 ppm
13C
Carbonyl
Carbons
NMR Chemical Shifts
Unsaturated
Carbons
200
C-X
Saturated
Carbons
0
100
downfield
TMS
upfield
ppm
Like with 1H NMR, the more shielded the carbon nuclei, the more upfield
its signal will appear and vice versa.
13C
NMR Spectrum of Chlorohexane
13C
NMR Correlation Chart
NMR Sample Preparation & CDCl3
Sample Prep:
Dissolve ~32 mg of sample in CDCl3 in an NMR tube.
Why use CDCl3?
Deuterated solvents are necessary in NMR because deuterium is NOT NMR active
and will not interfere with your sample’s spectrum.
CDCl3 is 98-99% pure with a trace amount of CHCl3. You will see a small solvent
peak at ~7.26 ppm due to CHCl3 (1H NMR); see a triplet at 77 ppm 13C NMR. This
peak serves as a reference peak; DO NOT
count it as one of your sample’s signals!