Nuclear Magnetic Resonance
(NMR) Spectroscopy
Part 2
Proton (1H) NMR
Theory of NMR
• The positively charged nuclei of certain elements (e.g.,
13C and 1H) behave as tiny magnets.
• In the presence of a strong external magnetic field (Bo),
these nuclear magnets align either with ( ) the applied
field or opposed to ( ) the applied field.
Bo
• The latter (opposed) is slightly higher in energy than
aligned with the field.
Energy
DE is very small
Theory of NMR
• The small energy difference between the two alignments
of magnetic spin corresponds to the energy of radio
waves according to Einstein’s equation E=hn.
hn
• Application of just the right radiofrequency (n) causes the
nucleus to “flip” to the higher energy spin state
• Not all nuclei require the same amount of energy for the
quantized spin ‘flip’ to take place.
• The exact amount of energy required depends on the
chemical identity (H, C, or other element) and the
chemical environment of the particular nucleus.
Theory of NMR
• Our department’s NMR spectrometer (in Dobo 245)
has a superconducting magnet with a field strength
of 9.4 Tesla. On this instrument, 1H nuclei absorb
(resonate) near a radiofrequency of 400 MHz;
13C nuclei absorb around 100 MHz.
e• Nuclei are surrounded by electrons.
The strong applied magnetic field (Bo)
induces the electrons to circulate
around the nucleus (left hand rule).
Bo
(9.4 T)
Theory of NMR
• The induced circulation of electrons sets up a secondary
(induced) magnetic field (Bi) that opposes the applied
field (Bo) at the nucleus (right hand rule).
Bi
e-
Bo
• We say that nuclei are shielded from the full applied
magnetic field by the surrounding electrons because the
secondary field diminishes the field at the nuclei.
Theory of NMR
• The electron density surrounding a given nucleus
depends on the electronegativity of the attached atoms.
• The more electronegative the attached atoms, the less
the electron density around the nucleus in question.
• We say that that nucleus is less shielded, or is
deshielded by the electronegative atoms.
• Deshielding effects are generally additive. That is, two
highly electronegative atoms (2 Cl atoms, for example)
would cause more deshielding than only 1 Cl atom.
H
H C H
H

H C Cl
H

H C Cl
H
H
Cl
C and H are deshielded
C and H are more deshielded
Chemical Shift
• We define the relative position of absorption in the NMR
spectrum the chemical shift. It is a unitless number
(actually a ratio, in which the units cancel), but we assign
‘units’ of ppm or  (Greek letter delta) units.
• For 1H, the usual scale of NMR spectra is 0 to 10 (or 12)
ppm (or ).
• The usual 13C scale goes from 0 to about 220 ppm.
• The zero point is defined as the position of absorption of
a standard, tetramethylsilane (TMS):
CH3
• This standard has only one type
CH3 Si CH3
of C and only one type of H.
CH3
Chemical Shifts
Proton Chemical Shift () vs. Electronegativity
CH3 F
H1 Chemical Shift
5
4.5
4
CH3 O
3.5
3
CH3 N
2.5
2
CH3 C
1.5
1 CH3 Si
0.5
0
-0.5
1.5
2
2.5
3
Electronegativity
3.5
4
4.5
Chemical Shifts
• Both 1H and 13C Chemical shifts are related to three
major factors:
– The hybridization (of carbon)
– Presence of electronegative atoms or electron attracting groups
– The degree of substitution (1º, 2º or 3º). These latter effects
are most important in 13C NMR, and in that context are usually
called ‘steric’ effects.
• Now we’ll turn our attention to 1H NMR spectra
(they are more complex, but provide more structural information)
1H
Chemical Shifts
C H
C C
O
C
X
O
O
C
C
OH
11
CH2
C
H
C
H
Aromatic H
O
C C
H
H
12
CH3
CH3
10
9
8
downfield
7
6
5
1H Chemical shift ( )
CH
Ar CH3
TMS
C C H
4
3
upfield
2
1
0
Classification of Protons
• To interpret or predict NMR spectra, one must first be
able to classify proton (or carbon) environments.
• Easiest to classify are those that are unrelated, or
different. Replacement of each of those one at a time
with some group (G) in separate models creates
constitutional isomers.
G
CH3CH2CH2CH3:
CH3CH2CHCH3
G
CH3CH2CH2CH2
These protons have different chemical shifts. This
classification is usually the most obvious.
Classification of Protons
• Homotopic hydrogens are those that upon replacement
one at a time with some group (G) in separate models
creates identical structures.
G
CH3CH2CH2CH3:
CH3CH2CH2C
H
H
H
H
CH3CH2CH2C
G
H
CH3CH2CH2C
H
G
Homotopic protons have the same chemical shifts. We
sometimes call them identical. Methyl hydrogens will always be
in this category (because of free rotation around the bond to the
methyl carbon). Molecular symmetry can also make protons
homotopic.
Classification of Protons
• If replacement of one hydrogen at a time in separate
models creates enantiomers, the hydrogens are
enantiotopic.
G
CH3CH2CH2CH3:
H
H
C
CH3CH2
G
C
CH3
CH3CH2
CH3
Enantiotopic protons have the same chemical shifts.
Classification of Protons
• If replacement of hydrogens in separate models creates
diastereomers, the hydrogens are diastereotopic.
H
CH3
H
C
H
C
CH3
H Br
:
CH3
G
C
G
C
CH3
H Br
CH3
H
C
C
CH3
H Br
Diastereotopic protons have different chemical shifts. Usually,
in order to have diastereotopic protons, there has to be a
stereocenter somewhere in the molecule. However, cis-trans
alkene stereoisomers may also have diastereotopic protons.
1H
NMR Problems
• How many unique proton environments are there in:
CH3CH2Br
2 environments
CH3CH2Br
CH3
CH3
CH3OCH2CH2CHCH3
CH3
5 environments
CH3OCH2CH2CHCH3
CH3
CH3
H
H
C
C
C
C
H
C
C
H
4 environments
H
1H
NMR Problems
CH3
CH3
CH3
H
CH3
C C
C C
CH3
CH3
H
4 environments
CH3CH2
CH2CH3
CH3CH2
C C
C C
H
CH2CH3
H
H
H
3 environments
Symmetry Simplifies Spectra!!!
O
CH3CCH3
OCH3
O
CH3COCH3
CH3
Spin-spin splitting (Coupling)
• Proton NMR spectra are not typically as simple as CMR
(13C NMR) spectra, which usually give a single peak for
each different carbon atom in the structure.
• Proton NMR spectra are often much more complex.
• Because of its nuclear spin, each proton exerts a slight
effect on the localized magnetic field experienced by its
neighboring proton(s).
• The spin state ( or ) of any one proton is independent
of any other proton.
• The energies of protons of different spin states are so
nearly equal that there is close to a 50:50 chance for
each proton to be up (or down).
Spin-spin splitting (Coupling)
• The spin states of the neighboring protons (those on the
adjacent carbon) exert a small influence on the magnetic
field, and therefore on the chemical shift of a given
proton.
• The result is that proton signals in the NMR spectrum
are typically split into multiplets. This phenomenon is
called coupling; the consequence is signal splitting.
• The type of multiplet (doublet, triplet, quartet, etc.)
depends on the number of protons on the next carbon.
The n+1 rule
• The multiplicity of a proton or a group of protons is given
by the n+1 rule, where n = the number of protons on the
adjacent (adjoining) carbon atom (or atoms)
n
1
2
3
4
5
6
n+1 multiplet name (abbrev)
2
doublet
(d)
3
triplet
(t)
4
quartet
(q)
5
quintet/pentet
6
sextet
7
septet/heptet
-
intensity pattern
1:1
1:2:1
1:3:3:1
1:4:6:4:1
1 : 5:10 :10: 5 : 1
1 : 6:15 :20:15:6:1
Multiplets
• Consider the ethyl group in chloroethane CH3CH2Cl.
• The methyl protons experience a magnetic field that is
somewhat influenced by the chlorine on the adjacent
carbon, but is also affected slightly by the nuclear spin
states of the adjacent methylene (CH2) protons.
• The two CH2 protons can have the following
possible combination of spins:
magnetic field
two spin up (1 way)
one up and one down (2)
two spin down (1)
1 : 2 : 1 .
• This results in a 1:2:1 triplet for the methyl group
Multiplets
• The magnetic field experienced by the CH2 protons in
chloroethane (CH3CH2Cl) is mainly influenced by the
electronegative chlorine.
• However, it is slightly perturbed by the spin states of the
three methyl (CH3) protons on the adjoining carbon
• They have four possible combinations of spins:
Three spin up (1 way)
Two up and one down (3)
Two down and one up (3)
Three spin down (1)
1 : 3 : 3 : 1
• As a result, the CH2 group appears as a 1:3:3:1 quartet.
Spectrum of chloroethane
• Putting the multiplets together gives
the predicted spectrum.
CH
• The pattern of a downfield quartet
CH
and an upfield triplet is typical of
the presence of an ethyl group
in the molecular structure.
• Note that the triplet is larger than
the quartet. That is because
4
3
2
there are 3 protons giving rise
to the triplet, and only 2 protons
CH3CH2Cl
giving rise to the quartet.
• The integrated signal areas are in a 3:2 ratio.
3
2
TMS
1
0
1H
NMR Problems
• Predict the splitting patterns (multiplets) for each proton
environment in the following:
singlet
singlet
CH3CHBr2
doublet
doublet
CH3OCH2CH2Br
triplet
triplet
triplet
ClCH2CH2CH2Cl
triplet
quartet
O CH3
CH3CH2COCHCH3
quartet
quintet
septet
The Integral
• Integration is performed to determine the relative number
of protons in a given environment.
• The number is set at 1, 2 or 3 for a given peak, then the
areas of the other signals are reported relative to that
one.
• The integral should be rounded to the nearest whole
number; after all, there is either 1, or 2, or 3 protons in a
certain environment, never a decimal fraction.
• Our spectrometer prints the integral below the spectrum
written sideways and in red.
CH3
CH3
O
CH3COCH2CH3
OCH2
CH3
(2H)
(3H) (3H)
CH3
OCH2
CH3 CH3
O
CH3CH2COCH2CH3
OCH2
CH2
CH3 CH3
OCH2
CH2
.
ethylC
butanoate
6H12O2
O
CH3
CH3CH2CH2COCH2CH3
CH2
OCH2
CH2
CH3
CH3
CH3
O
CH3CCH2CH3
CH2
CH3
CH3
CH2
CH3
H
H
O
H
H
CH3C
H
H
H
H
CH3
H
C
H
H
H
C
C
C
H
H
C
C
H
H
CH3
CH3CH2CH2OH
CH2
CH2
O
OH
CH3
CH2
CH2
O
OH
CH3
CH2
CH3CH2CH2CH2OH
CH2
O
CH2
OH
CH3
CH2
O
CH2
OH
CH2
CH2
and CH3
CH2
CH3CH2CH2CH2CH2OH
CH2
O
CH2
OH
CH2
CH2
and
CH2
O
CH2
OH
CH3
CH3
CH3
H
H
H
H
H
H
H H other Hs
CH3
HH
H
other Hs
CH3
CH3CH2CH2Br
CH2
CH2
CH3
CH2
CH2
2-methyl-1-hexene
CH3
CH2
CH3
CH2
C
CH3
CH2CH2CH2CH3
CH2
CH2
CH2
CH3
1-octene
CH2CH2CH2CH2
C8H16
CHCH2CH2CH2CH2CH2CH3
CH2
H
H
H
C10H14 (sec-butylbenzene)
H
Aromatic Hs
CH3
CHCH2CH3
H
H
H