Note: This is part 3/7 of Marc’s NMR lecture notes
In general:
• chemically equivalent protons have the same
chemical shift!
• chemically non-equivalent protons (often) have
different chemical shifts
Recognizing equivalent
protons with 1H NMR
• thus NMR easily tells number of “different”
types of protons in a molecule…
Examples:
Cl
H3C C-CH3
(a)
H (a)
(b)
H3C CH2-CH2 Br
(a)
(b)
(a)
(a) O
(d)
(b)
(c)
(a)
(d)
(b)
(a)
(e)
(f)
OCH3
(e)
Cl
(c)
O
(c)
(a)
(c)
(d)
(b)
(c)
(d)
two sets of non- three sets of nonfour sets of non- five sets of nonsix sets of nonequivalent protons equivalent protons equivalent protons equivalent protons equivalent protons
Recall: cyclohexane rapidly interconverts
between two chair forms
So far, we have talked about integration and
chemical shift.
The third important information in 1H NMR is:
SPIN-SPIN SPLITTING
0C
At 25
chair flipping occurs faster than “NMR time scale”:
thus Ha and Hb become averaged at 1.36 ppm.
But at -90 0C the conformations are “frozen out”!
thus we see: Hax (1.12 ppm) and Heq (1.60 ppm).
SPIN-SPIN SPLITTING
Often a group of hydrogens will appear as a multiplet
rather than as a single peak.
1,1,2-Trichloroethane
The two signals do not appear as single peaks, but rather
as a “triplet” and a “doublet”.
integral = 2
Multiplets are named as follows:
Singlet
Doublet
Triplet
Quartet
Quintet (5)
Cl H
Sextet (6)
Septet (7)
Octet
Nonet
This happens because of interaction with neighboring
hydrogens and is called
SPIN-SPIN SPLITTING.
H C C Cl
integral = 1
triplet
Cl H
doublet
These “multiplets” are due to
spin-spin splitting and are
predicted by the n+1 rule.
1,1,2-Trichloroethane
integral = 2
Cl H
H C C Cl
n+1
RULE
integral = 1
Cl H
Where do these multiplets come from ?
….. interaction with neighbors
this hydrogen’s peak
is split by its two neighbors
these hydrogens are
split by their single
neighbor
two neighbors
n+1 = 3
triplet
one neighbor
n+1 = 2
doublet
MULTIPLETS
singlet
doublet
triplet
quartet
quintet
sextet
septet
octet
nonet
EXCEPTIONS TO THE N+1 RULE
IMPORTANT !
1)
2)
Protons that are equivalent by symmetry
usually do not split one another
X CH CH Y
X CH2 CH2 Y
no splitting if x=y
no splitting if x=y
Protons in the same group
usually do not split one another
H
C H
H
H
or
C
H
EXCEPTIONS TO THE N+1 RULE
3)
The n+1 rule applies principally to protons in
aliphatic chains or aliphatic rings.
CH3
CH2CH2CH2CH2CH3
or
H
YES
YES
but does not apply (in the simple way shown here)
to protons on double bonds or on benzene rings.
H
CH3
H
H
CH3
NO
NO
COMMON SPLITTING PATTERNS
COMMON ALKYL SPLITTING PATTERNS
ParaPara-disubstituted aromatic rings often are
recognized by a pair of doublets in the aromatic
region! (7(7-8 ppm).
ppm).
Why a pair of doublets?
X CH CH Y
(x=y)
-
O
CH2 CH
+
N
7
6
O
5
(x=y)
Cl
4
3
2
X CH2 CH2 Y
1
(x=y)
0
8.40
8.30
8.20
8.10
8.00
7.90
7.80
7.70
7-8 ppm
Ortho and meta disubstituted aromatic rings
have their own characteristic splitting patterns!
… the chemical shifts vary with the
different substituents though (focus on
splitting)
-
O
F
O
F
+
N
SOME EXAMPLE SPECTRA
WITH SPLITTING
F
O
Br
7
3.5
6
3.0
F
F
5
2.5
F
4
2.0
3
1.5
1.0
2
0.5
1
0.0
0
8.5
8.0
7.5
7.70
7.60
7.50
7.40
7.30
7.20
7.10
7.00
7.60
7.50
7.40
NMR Spectrum of Bromoethane
NMR Spectrum of 2-Nitropropane
H
CH3
C
CH3
+
O N O
-
Br CH2CH3
NMR Spectrum of Acetaldehyde
O
CH3 C
H
INTENSITIES OF
MULTIPLET PEAKS
PASCAL’S TRIANGLE
offset = 2.0 ppm
PASCAL’S TRIANGLE
Intensities of
multiplet peaks
1
The interior
1 1
entries are
the sums of
1 2 1
the two
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
quintet
sextet
50 % of
molecules
octet
opposed to Bo
-1/2
H
HA
H
HA
C
C
C
C
Bo
downfield
neighbor aligned
upfield
neighbor opposed
HOW IT HAPPENS
septet
PROTON HA SPLITS INTO A DOUBLET DUE TO
THE SPIN OF ITS NEIGHBORS
aligned with Bo
+1/2
THE ORIGIN OF
SPIN-SPIN SPLITTING
50 % of
molecules
EXAMPLE: one CH next to another CH
one neighbor
n+1 = 2
doublet
one neighbor
n+1 = 2
doublet
H
H
H
H
C
C
C
C
EXAMPLE: CH2 next to a CH3
EXAMPLE: CH next to a CH2
two neighbors
n+1 = 3
triplet
three neighbors
n+1 = 4
quartet
one neighbor
n+1 = 2
doublet
(note:
error
here)
H
H
C
C
H
H
H
two neighbors
n+1 = 3
triplet
H
H
C
C
H
H
H
THE COUPLING CONSTANT
H H
J
J
THE COUPLING CONSTANT
C C H
J
H H
J
J
J
The coupling constant is the distance J (measured in Hz)
between the peaks in a multiplet.
1H
50 MHz
J = 7.5 Hz
NMR @ 100 MHz
Coupling constants are
constant - they do not
change with strength
NMR instruments
Why buy fancy
and expensive
NMR machines??
7.5 Hz
J = 7.5 Hz
3
2
1
2
1
2
1
100 MHz
J = 7.5 Hz
6
1H
5
4
3
2
1
Spectra are
simplified!
NMR @ 200 MHz
Separation
is larger but
J values are
the same!
6
5
4
3
J = 7.5 Hz
7.5 Hz
Overlapping
multiplets are
Separated!
3
2
1
ppm
200 MHz
J = 7.5 Hz
3
REPRESENTATIVE COUPLING CONSTANTS
6 to 8 Hz
trans
11 to 18 Hz
cis
6 to 15 Hz
Hax
Hax,Hax = 8 to 14
Heq
Heq
Hax
Hax,Heq = 0 to 7
Heq,Heq = 0 to 5
OVERVIEW OF 1H NMR
Information derived from a
1H NMR spectrum!
1. Each different type of hydrogen gives a peak
or group of peaks (multiplet).
2. The chemical shift (in ppm) gives a clue as
to the type of hydrogen generating the peak
(alkane, alkene, benzene, aldehyde, etc.)
3. The integral gives the relative numbers of each
type of hydrogen.
4. Spin-spin splitting gives the number of hydrogens
on adjacent carbons.
5. The coupling constant J often gives useful
information about stereochem (beyond this course!)