Transcript Document

This week in Iverson’s organic chemistry….
Using Nuclear Magnetic Resonance
Spectroscopy to Deduce Structure
ROTD: Understanding splitting in greater detail.
And, who am I?
Jonathan L. Sessler
The Splitting Problem: What is Going On?
4.07
4.08
3.5
1.24
3.0
2.0
2.5
1.5
2.0
1.26
1.22
1.5
1.0
4.10
4.05
1.0
0.5
0.5
0.0
0.0
4.20
4.10
4.00
1.30
1.20
1.10
3.90
9
2.00
8
7
H3C
O
CH3
6
5
3
2
1.24
O
4
4.07
4.08
4.10
1.26
1.22
4.05
1
0
4.0
3.5
3.0
2.5
2.0
1.5
1.0
Origins of Signal Splitting
• When the chemical shift of one nucleus is
influenced by the spin of another, the two are
said to be coupled
• Consider nonequivalent hydrogens Ha and Hb
on adjacent carbons
– the chemical shift of Ha is influenced by whether
the spin of Hb is aligned with or against the applied
field
Y
Ha
Hb
C
C
X
Origins of Signal Splitting
H0
Hb
Magnetic field of H b
subtracts from the applied
field; H b signal appears at
a higher applied field
Hb
Magnetic field of H b adds
to the applied field; H a
signal appears at a lower
applied field
Ha
Remember…it is the NET field that counts
One neighbor (n=1): splits into 2 (1+1=2) or is a doublet
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
1. THEORY: When there are two sets of adjacent H atoms, the
number of peaks multiply. For example, a CH2 group (Hb)
with a CH2 group on either side could show 3 x 3 = 9
splittings! You can say this group is a "triplet of triplets”.
2. PRACTICE: For alkyl groups in molecules with FREELY
ROTATING SINGLE BONDS, complex splittings simplify because
coupling constants ("J") are all about the same. In practice, if there are
n adjacent H atoms, equivalent or not, you will see n+1 peaks. This is
an approximation, but almost always true on spectra taken with all but
the most sophisticated NMR spectrometers.
2. PRACTICE: For alkyl groups in molecules with FREELY
ROTATING SINGLE BONDS, complex splittings simplify because
coupling constants ("J") are all about the same. In practice, if there are
n adjacent H atoms, equivalent or not, you will see n+1 peaks. This is
an approximation, but almost always true on spectra taken with all but
the most sophisticated NMR spectrometers.
Two neighbors (n=2): splits into 3 (2+1=3) or is a triplet
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
2. PRACTICE: For alkyl groups in molecules with FREELY
ROTATING SINGLE BONDS, complex splittings simplify because
coupling constants ("J") are all about the same. In practice, if there are
n adjacent H atoms, equivalent or not, you will see n+1 peaks. This is
an approximation, but almost always true on spectra taken with all but
the most sophisticated NMR spectrometers.
Three neighbors (n=3): splits into 4 (3+1=4) or is a quartet
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Ethyl Fragment
The Splitting Problem: What is Going On?
4.07
4.08
3.5
1.24
3.0
2.0
2.5
1.5
2.0
1.26
1.22
1.5
1.0
4.10
4.05
1.0
0.5
0.5
0.0
0.0
4.20
4.10
4.00
1.30
1.20
1.10
3.90
9
2.00
8
7
H3C
O
CH3
6
5
3
2
1.24
O
4
4.07
4.08
4.10
1.26
1.22
4.05
1
0
4.0
3.5
3.0
2.5
2.0
1.5
1.0
Let’s Compare and Contrast
2. PRACTICE: For alkyl groups in molecules with FREELY
ROTATING SINGLE BONDS, complex splittings simplify because
coupling constants ("J") are all about the same. In practice, if there are
n adjacent H atoms, equivalent or not, you will see n+1 peaks. This is
an approximation, but almost always true on spectra taken with all but
the most sophisticated NMR spectrometers.
2. PRACTICE: For alkyl groups in molecules with FREELY
ROTATING SINGLE BONDS, complex splittings simplify because
coupling constants ("J") are all about the same. In practice, if there are
n adjacent H atoms, equivalent or not, you will see n+1 peaks. This is
an approximation, but almost always true on spectra taken with all but
the most sophisticated NMR spectrometers.
Common Splitting Patterns
CH3
X
3H singlet (methyl)
CH3
H3C C X
9H singlet (t-butyl)
CH3
CH3CH2
X
2H quartet and 3H triplet (ethyl)
CH3
H C X
CH3
6H doublet and 1H septet (isopropyl)
Let’s See an Example
3. For alkenes or ring structures such as cyclopropanes the splitting
does not simplify (no bond rotation) and you see full multiplicative
splitting ("doublet of doublets", etc.)
First, let’s take a look at simple cis and trans alkenes:
Let’s start with cis (J = 5-9 Hz)
First, let’s take a look at simple cis and trans alkenes:
And trans (J = 9-15 Hz) BIGGER THAN cis
3. For alkenes or ring structures such as cyclopropanes the splitting
does not simplify (no bond rotation) and you see full multiplicative
splitting ("doublet of doublets", etc.)
3.1. Non-equivalent H atoms on the same C atom can split each other
(called geminal coupling), for example on alkenes. This coupling can
have very small coupling constants, so it can be difficult to see.
Net Result:
Alkene NMR spectra can get complicated in a hurry!
For the advanced student, see if you can figure this guy out.
3. For alkenes or ring structures such as cyclopropanes the splitting
does not simplify (no bond rotation) and you see full multiplicative
splitting ("doublet of doublets", etc.)
4. Key Idea: Chemical shift tells you what functional groups
are present, integration tells you how many equivalent H atoms
there are and splitting patterns tell you how the atoms are
connected to each other.
OTHER IMPORTANT FACTS ABOUT NMR SPECTROSCOPY
5. Deuterium atoms do not show up in 1H-NMR spectra, so
deuerated solvents are used to dissolve NMR samples.
6. The H atoms of relatively acidic functional groups (alcohols,
carboxylic acids, amines) exchange rapidly, so they often do not split
adjacent protons, and they can be replaced (signal disappears) with
deuterium by adding a drop of D2O to the NMR sample.
7. H-bonding changes the location of a signal for H-bonding groups
in a concentration dependent manner explaining why -OH and -NH2
group signals can vary so much in location.
OH Signal
OH Signal
There is no visible OH signal!
(Due to exchange with the D’s of D2O)
A More Complicated “Real World” Example
A More Complicated “Real World” Example - Part 2
D2O added
OTHER IMPORTANT FACTS ABOUT NMR SPECTROSCOPY
9. The splitting of a -CH2- group adjacent to a chiral center will be
"messed up", that is split into many peaks. This is useful for
identifying chiral centers in molecules.
10. 13C NMR tells you how many different types of carbon atoms
are in a molecule. Because 13C atoms are so rare, you don't find
two in the same molecule and there is no splitting (12C nuclei do
NOT have spin quantum numbers of 1/2 so they do not split a 13C
signal). Integration is NOT accurate with 13C NMR. We will not
use 13C NMR in this class, but you will see it in lab.
Okay; that’s it for today!