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
ppm ()
CH3CH2OH
TMS
a - tripletb - quartetc - singlet
a ab
b
c
c
downfield upfield
Spin-Spin Splitting
CH3CH2OH
a - tripletb - quartetc - singlet
a b c 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 ruleThe signal of a proton with n equivalent neighboring protons is split into amultiplet 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 involvedwith 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 ofthe following compounds.
OH
Cl
O
H
NH2NH2
CH3
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 ofthe following compounds.
O
O
O
H3C
Cl
HH
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 ruler2. 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 constantor J value.
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
1H NMR Spectrum of a Taxol Derivative
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 NMR Chemical Shifts
CarbonylCarbons
UnsaturatedCarbons C-X
SaturatedCarbons TMS
200 100 0
ppm
Like with 1H NMR, the more shielded the carbon nuclei, the more upfieldits signal will appear and vice versa.
downfield upfield
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 activeand will not interfere with your sample’s spectrum.
CDCl3 is 98-99% pure with a trace amount of CHCl3. You will see a small solventpeak 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 NOTcount it as one of your sample’s signals!