NMR - The
Chemical Shift
3-1
FACTORS THAT INFLUENCE PROTON SHIFTS
Local Fields
• Shielding by e- that surround the resonating nuclei arise from local fields
• They are a simple function of e- density affected by induction, resonance
and hybridization effects
• The magnetic field at the nucleus is altered from B0 to a quantity B0(1-s)
where s is called the shielding
CHEM 430 – NMR Spectroscopy
2
NMR - The
Chemical Shift
3-1
FACTORS THAT INFLUENCE PROTON SHIFTS
Local Fields
Electronegativity effects
• A nearby e- withdrawing atom or group with decrease e- density
moving the observed resonance downfield to higher n
• Conversely, a nearby e- donating atom or group will increase e- density
moving the observed resonance upfield to lower n
OH
CH3
B0(1 – s)
sOH < sCH3
DE = hn
B0
CHEM 430 – NMR Spectroscopy
3
3
NMR - The
Chemical Shift
3-1
FACTORS THAT INFLUENCE PROTON SHIFTS
Local Fields
B0(1 – s)
sO < sC
DE = hn
B0
CHEM 430 – NMR Spectroscopy
4
4
NMR - The
Chemical Shift
3-1
FACTORS THAT INFLUENCE PROTON SHIFTS
Local Fields
Electronegativity effects
The trend follows Pauling electronegativity (EN) scale:
CH3F
CH3O-
CH3Cl
CH3Br
CH3I
CH4
(CH3)4Si
CH3Li
EN
4.0
3.5
3.1
2.8
2.5
2.1
1.8
1.0
d of H
4.26
3.40
3.05
2.68
2.16
0.23
0.0
-0.4
CHEM 430 – NMR Spectroscopy
5
5
NMR - The
Chemical Shift
3-1
FACTORS THAT INFLUENCE PROTON SHIFTS
Local Fields
Electronegativity effects
• The effect is cumulative:
d of H
CH3Cl
CH2Cl2
CHCl3
3.05
5.30
7.27
• The effect drops sharply with distance:
d of H
-CH2Br
-CH2CH2Br
-CH2CH2CH2Br
3.30
1.69
1.25
CHEM 430 – NMR Spectroscopy
6
6
NMR - The
Chemical Shift
3-1
FACTORS THAT INFLUENCE PROTON SHIFTS
Local Fields
Electronegativity effects
• The effect is a useful tool in quickly deducing simple aliphatic chains:
CHEM 430 – NMR Spectroscopy
7
7
NMR - The
Chemical Shift
3-1
FACTORS THAT INFLUENCE PROTON SHIFTS
Local Fields
Resonance effects
• Donation or withdrawal of e- through resonance will have shielding or
deshielding effects, respectively:
resonance donation
shielding effect
resonance withdrawal
deshielding effect
CHEM 430 – NMR Spectroscopy
8
8
NMR - The
Chemical Shift
3-1
FACTORS THAT INFLUENCE PROTON SHIFTS
Local Fields
Hybridization effects
• Hybridization of the carbon atom influences e- density
• As proportion of s-character increases (sp3  sp2  sp) bonding
electrons move toward C and away from hydrogen - deshielding
• This effect however is secondary to the influence of the p-cloud of
electrons from the unhybridized p-orbitals as we will see
CHEM 430 – NMR Spectroscopy
9
9
NMR - The
Chemical Shift
3-1
FACTORS THAT INFLUENCE PROTON SHIFTS
Nonlocal Fields
• Purely magnetic effects from a neighboring group can influence nuclear
shielding
• As we saw earlier the combined effects of local and nonlocal fields:
• Nonlocal fields have a major influence on chemical shift only if the group
has a non-spherical shape
CHEM 430 – NMR Spectroscopy
10
10
NMR - The
Chemical Shift
3-1
FACTORS THAT INFLUENCE PROTON SHIFTS
Nonlocal Fields
• The electrons in a spherical substituent also precess in the applied field,
creating a non-local field
• As the molecule tumbles the lines of magnetic force will remain lined up
with the applied field, but the position of the attached nuclei will change
• The effect cancels itself out leaving only the effect on the local field by the
substituent
CHEM 430 – NMR Spectroscopy
11
11
NMR - The
Chemical Shift
3-1
FACTORS THAT INFLUENCE PROTON SHIFTS
Nonlocal Fields
• The electrons in a non-spherical substituent also precess in the applied
field, creating a non-local field
• In benzene, the 6-p-orbitals overlap to allow full circulation of electrons;
as these electrons circulate in the applied magnetic field they oppose the
applied magnetic field at the center:
B0
CHEM 430 – NMR Spectroscopy
12
12
NMR - The
Chemical Shift
3-1
FACTORS THAT INFLUENCE PROTON SHIFTS
Nonlocal Fields
• At the ring periphery, the effect is opposite and the protons are
deshielded :
CHEM 430 – NMR Spectroscopy
13
13
NMR - The
Chemical Shift
3-1
FACTORS THAT INFLUENCE PROTON SHIFTS
Nonlocal Fields
• The benzene system is not spherical, but an oblate ellipsoid
• As the molecule tumbles in in solution there is either the effect of
shielding inside the ring or deshielding outside the ring if the ring is
perpendicular to the applied field
OR
• No effect if the ring is parallel to the applied field
CHEM 430 – NMR Spectroscopy
14
14
NMR - The
Chemical Shift
3-1
FACTORS THAT INFLUENCE PROTON SHIFTS
Nonlocal Fields
• Groups that have appreciably different currents induced by B0 resulting
from different orientations in space are said to have diamagnetic
anisotropy
• Just as the local effect can result in shielding (electron donation) or
deshielding (electron withdrawal) the nonlocal effect can result in either
permutation
• Regions of shielding are indicated by (+) and deshielding (-)
CHEM 430 – NMR Spectroscopy
15
15
NMR - The
Chemical Shift
3-1
FACTORS THAT INFLUENCE PROTON SHIFTS
Nonlocal Fields
• The effect was modeled quantitatively by McConnell
• The following equation relates shielding to the influence of a magnetic
dipole on the point in space where a proton(nuclei) resides:
sA – shielding for a proton at (r, q)
cL and cT are the diamagnetic
susceptibilities of the group
longitudinal and transverse to B0
• At q = 54o 44’ the expression (3cos2q – 1) goes to zero
• On either side of this ‘magic angle’ sA changes sign
CHEM 430 – NMR Spectroscopy
16
16
NMR - The
Chemical Shift
3-1
FACTORS THAT INFLUENCE PROTON SHIFTS
Nonlocal Fields
• The protons of benzene reside on the periphery of the ring within the
deshielding cone
• Molecules have been constructed for the purpose of confirming the
shielding effect predicted by the model:
H2
C
-0.5
2.0 d
CH2
-1.0 d
H
9.3 H
CHEM 430 – NMR Spectroscopy
17
-3.0
17
FACTORS THAT INFLUENCE PROTON SHIFTS
Nonlocal Fields
• 4n + 2 p-electrons (aromatic) result in the diamagnetic circulation of e-s
• Pople demonstrated that 4n p-systems have the opposite or paramagnetic
circulation:
5.2
10.3
CHEM 430 – NMR Spectroscopy
18
18
NMR - The
Chemical Shift
3-1
FACTORS THAT INFLUENCE PROTON SHIFTS
Nonlocal Fields
• For a prolate ellipsoid it is sometimes not as clear as to which
arrangement has the stronger induced current
• The acetylene system provides a simple example
CHEM 430 – NMR Spectroscopy
19
19
NMR - The
Chemical Shift
3-1
FACTORS THAT INFLUENCE PROTON SHIFTS
Nonlocal Fields
• We say the shift for the terminal acetylene proton experiences magnetic
anisotropy. Usual shifts for this H are d 1.8-3.0. For reference sp3 ethane
is at 0.86 and sp2 ethene at 5.28
CHEM 430 – NMR Spectroscopy
20
20
NMR - The
Chemical Shift
3-1
FACTORS THAT INFLUENCE PROTON SHIFTS
Nonlocal Fields
• Alkanes do not possess the same degree of electron circulation as alkynes
but do exert nonlocal fields on adjacent nuclei
• The C—C s-bond shields a proton on its side more than its end
CHEM 430 – NMR Spectroscopy
21
21
NMR - The
Chemical Shift
3-1
FACTORS THAT INFLUENCE PROTON SHIFTS
Nonlocal Fields
• The result of which is a deshielding of equatorial protons in
conformationally locked systems:
• Even simple alkane systems show anisotropy:
CHEM 430 – NMR Spectroscopy
22
22
NMR - The
Chemical Shift
3-1
FACTORS THAT INFLUENCE PROTON SHIFTS
Nonlocal Fields
• The result of which is a deshielding of equatorial protons in
conformationally locked systems:
• Even simple alkane systems show anisotropy:
CHEM 430 – NMR Spectroscopy
23
23
NMR - The
Chemical Shift
3-1
FACTORS THAT INFLUENCE PROTON SHIFTS
Nonlocal Fields
• The highly shielded position of cyclopropane resonances may be
attributed either to an aromatic-like ring current or to the anisotropy of
the bond that is opposite to a group in the three- membered ring:
• The effect is much larger than the indicated 1.2 ppm , because the
cyclopropane carbon orbital to hydrogen ( compared with the orbital in
cyclohexane) deshields the proton.
• A cyclopropane ring also can shield more distant hydrogens:
Heq 1.2 less than Hax
CHEM 430 – NMR Spectroscopy
24
24
NMR - The
Chemical Shift
3-1
FACTORS THAT INFLUENCE PROTON SHIFTS
Nonlocal Fields
• Most common single bonds (C-N, C-O) have shielding properties that
parallel those of the bond, although the geometry is more complex than
that for the C-C bond.
• Lone electron pairs can have a special effect:
• In N-methylpiperidine the axial lone pair shields the vicinal Hax by an n 
s* interaction without any effect on Heq. As a result, Hax increases to about
1.0 ppm or more in similar systems.
CHEM 430 – NMR Spectroscopy
25
25
NMR - The
Chemical Shift
3-1
FACTORS THAT INFLUENCE PROTON SHIFTS
Nonlocal Fields
• The anisotropy of double bonds is more difficult to assess, because they
have three nonequivalent axes (the McConnell equation, with only two
axes, does not apply).
• Protons situated over double bonds are, in general, more shielded than
those in the plane both for alkenes and for carbonyl groups
• The position of the methylene protons in norbornene may be explained in
this fashion since the syn and endo protons, respectively, are shielded with
respect to the anti and exo protons.
CHEM 430 – NMR Spectroscopy
26
26
NMR - The
Chemical Shift
3-1
FACTORS THAT INFLUENCE PROTON SHIFTS
Nonlocal Fields
• The highly deshielded position of aldehydes ( ca. 9.8) is attributed to a
combination of a strong polar effect and the diamagnetic anisotropy of
the carbonyl group.
CHEM 430 – NMR Spectroscopy
27
27
NMR - The
Chemical Shift
3-1
FACTORS THAT INFLUENCE PROTON SHIFTS
Nonlocal Fields
• The nonspherical array of lone pairs of e- may exhibit diamagnetic
anisotropy, although, alternatively, the effect may be considered a
perturbation of local currents.
• A proton that is H-bonded to a lone pair is invariably deshielded.
• For example: the -OH proton in ethanol
• CCl4 resonates at 0.7 (dilute no H-bonding)
• CD3CD2OH resonates at 5.3 (H-bonding)
• Carboxylic protons resonate at extremely high frequency (11– 14),
because every proton is H-bonded within a dimer or higher aggregate.
CHEM 430 – NMR Spectroscopy
28
28
NMR - The
Chemical Shift
FACTORS THAT INFLUENCE PROTON SHIFTS
3-1
Nonlocal Fields
• Lone- pair anisotropy also has been invoked to explain trends in ethyl
groups :
For XCH2CH3:
X
d -CH2-
d –CH3
F
4.36
1.24
Cl
3.47
1.33
Br
3.37
1.65
I
3.16
1.86
• The resonance position of –CH2- attached to X is explained by the polar
effect
• The trend for the more distant –CH3 group is opposite; as X increases
size, the lone pair moves closer to the –CH3 group and deshields it.
CHEM 430 – NMR Spectroscopy
29
29
NMR - The
Chemical Shift
3-1
FACTORS THAT INFLUENCE PROTON SHIFTS
Summary
• Functional group effects on proton chemical shifts are explained largely by
two general effects.:
1. Local Effects: Electron withdrawal or donation by induction
(including hybridization) or by resonance alters the electron density
and hence the local field around the resonating proton.
• Higher electron density shields the proton (lower n, upfield)
• Low electron density deshields the proton (higher n, downfield)
2. Nonlocal Effects: Diamagnetic anisotropy of nonspherical
substituents is largely responsible for the proton resonance positions
of aromatics, acetylenes, aldehydes, cyclopropanes, cyclohexanes,
alkenes, and hydrogen- bonded species.
CHEM 430 – NMR Spectroscopy
30
30
NMR - The
Chemical Shift
3-2
PROTON CHEMICAL SHIFTS AND STRUCTURE
3-2a Saturated Aliphatics
Alkanes.
CHEM 430 – NMR Spectroscopy
31
31
PROTON CHEMICAL SHIFTS AND STRUCTURE
3-2a Saturated Aliphatics
Functionalized Alkanes.
Oxygen:
Nitrogen:
Sulfur and the Halides:
CHEM 430 – NMR Spectroscopy
32
32
NMR - The
Chemical Shift
3-2
PROTON CHEMICAL SHIFTS AND STRUCTURE
3-2a Saturated Aliphatics
Functionalized Alkanes.
CHEM 430 – NMR Spectroscopy
33
33
PROTON CHEMICAL SHIFTS AND STRUCTURE
3-2a Saturated Aliphatics
Functionalized Alkanes.
• After years of collective observation of 1H (and 13C) NMR it is possible to
predict chemical shift to a fair precision using Shoolery Tables
• These tables use a base value for 1H (and 13C) chemical shift to which are
added adjustment increments for each group on the carbon atom
d = 0.23 + DX + DY + DZ
H
X C H
H
methyl
H
X C Y
H
methylene
H
X C Z
Y
methine
• The tables work well for methyl and methylene but diverge greatly with
methine due to the increased interaction between effects
CHEM 430 – NMR Spectroscopy
34
34
NMR - The
Chemical Shift
3-2
PROTON CHEMICAL SHIFTS AND STRUCTURE
3-2a Saturated Aliphatics
Functionalized Alkanes. Example Shoolery Table - Methylene
X or Y
Increment
X or Y
Increment
-H
0.34
-OC(=O)OR
3.01
-CH3
0.68
-OC(=O)Ph
3.27
-C—C
1.32
-C(=O)R
1.50
-CC-
1.44
-C(=O)Ph
1.90
-Ph
1.83
-C(=O)OR
1.46
-CF2-
1.12
-C(=O)NR2 or H2
1.47
-CF3
1.14
-CN
1.59
-F
3.30
-NR2 or H2
1.57
-Cl
2.53
-NHPh
2.04
-Br
2.33
-NHC(=O)R
2.27
-I
2.19
-N3
1.97
-OH
2.56
-NO2
3.36
-OR
2.36
-SR or H
1.64
-OPh
2.94
-OSO2R
3.13
CHEM 430 – NMR Spectroscopy
35
35
NMR - The
Chemical Shift
3-2
PROTON CHEMICAL SHIFTS AND STRUCTURE
3-2a Saturated Aliphatics
Functionalized Alkanes. Example Shoolery Table - Methine
X ,Y or Z
Increment
X, Y or Z
Increment
-F
1.59
-OC(=O)OR
0.47
-Cl
1.56
-C(=O)R
0.47
-Br
1.53
-C(=O)Ph
1.22
-NO2
1.84
-CN
0.66
-NR2 or H2
0.64
-C(=O)NH2
0.60
-NH3+
1.34
-SR or H
0.61
-NHC(=O)R
1.80
-OSO2R
0.94
-OH
1.14
-CC-
0.79
-OR
1.14
-C=C
0.46
-C(=O)OR
2.07
-Ph
0.99
-OPh
1.79
CHEM 430 – NMR Spectroscopy
36
36
PROTON CHEMICAL SHIFTS AND STRUCTURE
3-2a Saturated Aliphatics
Functionalized Alkanes.
• After years of collective observation of 1H (and 13C) NMR it is possible to
predict chemical shift to a fair precision using Shoolery Tables
• These tables use a base value for 1H (and 13C) chemical shift to which are
added adjustment increments for each group on the carbon atom
d = 0.23 + DX + DY + DZ
H
X C H
H
methyl
H
X C Y
H
methylene
H
X C Z
Y
methine
• The tables work well for methyl and methylene but diverge greatly with
methine due to the increased interaction between effects
CHEM 430 – NMR Spectroscopy
37
37
NMR - The
Chemical Shift
3-2
PROTON CHEMICAL SHIFTS AND STRUCTURE
3-2b Unsaturated Aliphatics
Alkynes.
•Anisotropy of C≡C results in a low frequency for terminal-H (1.8 – 3.0)
Alkenes.
•Anisotropy of C=C results in a higher frequency for alkenylic (vinyl)-H
•Range is very large (4.5 – 7.7) and highly subject to other groups on C=C
•Reference value used for ethene is 5.28
CHEM 430 – NMR Spectroscopy
38
38
NMR - The
Chemical Shift
3-2
PROTON CHEMICAL SHIFTS AND STRUCTURE
3-2b Unsaturated Aliphatics
Alkenes.
• Conjugation usually increases the observed frequency
• Angle strain increases s-character and therefore moves the resonance to
higher frequency
• The presence of a C=O group w/d electrons by both resonance and
induction:
CHEM 430 – NMR Spectroscopy
39
39
NMR - The
Chemical Shift
3-2
PROTON CHEMICAL SHIFTS AND STRUCTURE
3-2b Unsaturated Aliphatics
Alkenes.
CHEM 430 – NMR Spectroscopy
40
40
NMR - The
Chemical Shift
3-2
PROTON CHEMICAL SHIFTS AND STRUCTURE
3-2b Unsaturated Aliphatics
Alkenes.
• These effects were quantified by Tobey and of Pascual, Meier, and Simon,
• Using an empirical approach they tabulated a series of geminal, cis and
trans substituents relative to the proton being observed:
d = 5.28 + Zgem + Zcis + Ztrans
• Although the parameters incorporate inductive and resonance effects,
steric effects can cause deviations from observed positions.
CHEM 430 – NMR Spectroscopy
41
41
NMR - The
Chemical Shift
3-2
PROTON CHEMICAL SHIFTS AND STRUCTURE
3-2b Unsaturated Aliphatics
Alkenes.
CHEM 430 – NMR Spectroscopy
42
42
NMR - The
Chemical Shift
3-2
PROTON CHEMICAL SHIFTS AND STRUCTURE
3-2c Aromatics
•Diamagnetic anisotropy from aromatic ring current shifts the protons to high
frequency – benzene is used as a reference (7.27)
•Rings with only aliphatic substituents tend to bunch the resonances about this
frequency (toluene, ~ 7.2)
•Conjugating substituents tend to spread the aromatic resonances based on
contributing resonance structures and inductive effects:
CHEM 430 – NMR Spectroscopy
43
43
NMR - The
Chemical Shift
3-2
PROTON CHEMICAL SHIFTS AND STRUCTURE
3-2c Aromatics
• The a-protons in aromatic heterocycles are shifted due to the polar effect
of the heteroatom:
• As with the alkane and alkene systems a systematic observation has been
made of aromatic H-resonances. They can be predicted from the
following:
d = 7.27 + SSi
CHEM 430 – NMR Spectroscopy
44
44
NMR - The
Chemical Shift
3-2
PROTON CHEMICAL SHIFTS AND STRUCTURE
3-2c Aromatics
CHEM 430 – NMR Spectroscopy
45
45
NMR - The
Chemical Shift
3-2
PROTON CHEMICAL SHIFTS AND STRUCTURE
3-2c Aromatics
Aromatics:
CHEM 430 – NMR Spectroscopy
46
46
NMR - The
Chemical Shift
3-2
PROTON CHEMICAL SHIFTS AND STRUCTURE
3-2c Aromatics
Heteroaromatics:
CHEM 430 – NMR Spectroscopy
47
47
NMR - The
Chemical Shift
3-2
PROTON CHEMICAL SHIFTS AND STRUCTURE
3-2d Protons on Oxygen and Nitrogen
•Chemical shifts of protons attached to highly electronegative atoms such as O
or N are influenced strongly by acidity, basicity, and hydrogen-bonding
Protons on Oxygen
• For –OH, minute amounts of acidic or basic impurities can bring about
rapid exchange
• They are averaged with other exchangeable protons, either in the same
molecule or in other molecules, including the solvent.
• Only a single resonance is observed for all the exchangeable pro-tons
at a weighted-average position and no coupling is observed.
• The resonance varies from sharp to slightly broadened, depending on
the exchange rate.
CHEM 430 – NMR Spectroscopy
48
48
NMR - The
Chemical Shift
3-2
PROTON CHEMICAL SHIFTS AND STRUCTURE
3-2d Protons on Oxygen and Nitrogen
•-OH may be determined by exchange experiments described earlier (shaking
the NMR sample with D2O)
• At infinite dilution in (no H-bonding), the OH resonance of alcohols may be
found at about 0.5.
•Under more normal conditions of 5% to 20% solutions, hydrogen bonding
results in resonances in the 2– 4 range.
•More acidic phenols (ArOH) have resonances downfield at 4– 8. Interaction
with an ortho group shifts this to 10 or higher.
CHEM 430 – NMR Spectroscopy
49
49
NMR - The
Chemical Shift
3-2
PROTON CHEMICAL SHIFTS AND STRUCTURE
3-2d Protons on Oxygen and Nitrogen
•Most carboxylic acids exist as H-bonded dimers or oligomers, even in dilute
solution.
•Hydrogen bonding coupled with the strong deshielding of the carboxlate group
places the acid protons resonate far downfield (11– 14)
•Likewise, other highly H-bonded acidic protons also may be found in this
range, such as sulfonic, phosphonic and enolic protons
CHEM 430 – NMR Spectroscopy
50
50
NMR - The
Chemical Shift
3-2
PROTON CHEMICAL SHIFTS AND STRUCTURE
3-2d Protons on Oxygen and Nitrogen
Protons on nitrogen
•Similar properties to -OH
•Lower electronegativity of N results upfield shifts than those OH protons:
• 0.5– 3.5 for aliphatic amines, - NH2
• 3– 5 for aromatic amines ( anilines), Ar-NH2
• 4– 8 for amides, pyrroles, and indoles
• 6– 8.5 for ammonium salts, -NH3+ (amino acid protons can exchange
rapidly with solvent or other exchangeable protons to achieve an
averaged position)
•The most common nuclide 14N is quadrupolar and possesses unity spin –
(could split the resonance of attached protons into a 1: 1: 1 triplet) - rapid
relaxation of quadrupolar 14N averages the spin states usually appearing as a
broadened resonance
•Broadening may render NH resonances almost invisible.
CHEM 430 – NMR Spectroscopy
51
51
NMR - The
Chemical Shift
3-2
PROTON CHEMICAL SHIFTS AND STRUCTURE
3-2d Protons on Oxygen and Nitrogen
CHEM 430 – NMR Spectroscopy
52
52
NMR - The
Chemical Shift
3-4
PROTON CHEMICAL SHIFTS AND STRUCTURE
3-4 Factors That Influence Carbon Shifts
• Carbon is the defining element in organic compounds, but its major
nuclide 12C has a spin of zero.
• Advent of pulsed FT-methods allowed practical examination of the lowabundance nuclide 13C ( 1.11%)
• The low probability of having two adjacent nuclei (0.01%) in a single
molecule removes complications from carbon–carbon couplings.
• When C-H couplings are removed by decoupling techniques, the spectrum
is essentially free of all spin–spin coupling, and one singlet arises for each
distinct type of carbon.
• Integration is rarely done as 13C has a much larger range of relaxation
times than 1H and because the decoupling field perturbs intensities
CHEM 430 – NMR Spectroscopy
53
53
NMR - The
Chemical Shift
3-4
PROTON CHEMICAL SHIFTS AND STRUCTURE
3-4 Factors That Influence Carbon Shifts
• Analysis therefore is simpler for 13C than for 1H spectra – but…
• Diamagnetic shielding (sd) which is responsible for 1H chemical shifts, is
caused by circulation of the electron cloud about the nucleus – 1Hs are
surrounded solely by s electrons ( lacking angular momentum) and
consequently exhibit only diamagnetic shielding
• In carbon 2p electrons have angular momentum that can hinder free
circulation - hindrance to free electron circulation creates an additional
mechanism called paramagnetic shielding (sp)
• Because sp serves to reduce sd the two mechanisms have opposite signs
•
13C
nuclei (and almost all other nuclides as well) additionally are
surrounded by p electrons and exhibit both forms of shielding.
CHEM 430 – NMR Spectroscopy
54
54
NMR - The
Chemical Shift
3-4
PROTON CHEMICAL SHIFTS AND STRUCTURE
3-4 Factors That Influence Carbon Shifts
•The paramagnetic component can be quite large!
•Chemical-shift range for 1H is only a few ppm, while paramagnetic
shifts for other nuclei can extend over a range of hundreds or even
thousands of ppm.
•Qualitatively, angular momentum can arise from excited electronic
states and from bonding. The effects are larger when electron density
about the nucleus increases.
•These three considerations were gathered by Ramsay, Karplus, and
Pople into the simple empirical relationship…
CHEM 430 – NMR Spectroscopy
55
55
NMR - The
Chemical Shift
3-4
PROTON CHEMICAL SHIFTS AND STRUCTURE
3-4 Factors That Influence Carbon Shifts
• DE = average energy of excitation of certain electronic transitions, such as
the n  p* transition for many 13C and 15N nuclei.
• The radial term includes the average distance r from the nucleus of the 2p
electrons - this term serves as a measure of electron density.
• SQij represents p-bonding to carbon. The negative sign in the equation
indicates that paramagnetic shielding is in the opposite direction from sd.
• Structural changes can affect all three components of the equation.
CHEM 430 – NMR Spectroscopy
56
56
NMR - The
Chemical Shift
3-4
PROTON CHEMICAL SHIFTS AND STRUCTURE
3-4 Factors That Influence Carbon Shifts
• The quantity DE represents the weighted- average energy difference
between the ground and certain excited states.
• Because of symmetry considerations, the p  p* transition is often
excluded
• Low-lying excited states (with small DE) make the largest contribution,
since DE appears in the denominator.
CHEM 430 – NMR Spectroscopy
57
57
NMR - The
Chemical Shift
3-4
PROTON CHEMICAL SHIFTS AND STRUCTURE
3-4 Factors That Influence Carbon Shifts
•Saturated molecules, such as alkanes, typically have no low-lying excited states
( and hence possess a large DE), so that sp is small and alkane carbon
resonances are found upfield
*Note that paramagnetic shielding causes shifts to high frequency, whereas
diamagnetic shielding causes shifts to low frequency
•On the other hand carbonyl carbons have a low-lying excited state involving
the movement of electrons from the oxygen lone pair to the antibonding p
orbital that generates a paramagnetic current.
•This n  p* transition causes the large shift to high frequency that
characterizes carbonyl groups— up to 220 ppm
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3-4 Factors That Influence Carbon Shifts
• The radial term is responsible for effects related to electron density that
parallel polar effects on proton chemical shifts.
• sP is larger when the p-electrons are closer to the nucleus. Thus,
substituents that donate or withdraw electrons influence the
paramagnetic shift.
• e- donation increases repulsion between electrons, which can be
relieved by an increase in r, sP decreases, causing an upfield shift
• e-withdrawal permits electrons to move closer to the nucleus,
increasing sP ,causing an upfield shift
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3-4 Factors That Influence Carbon Shifts
• SQij is related to charge densities and bond orders and can be considered
a measure of multiple bonding. The greater the degree of multiple bonding, the greater the downfield shift
• This term provides a rationale for the series:
CH3CH3 ( 6) < H2C=CH2 (123) < H2C=C=CH2 ( 214)
• Arene shifts are similar to those of alkenes (benzene, 129) - The effects of
diamagnetic anisotropy on carbon chemical shifts are similar in
magnitude to the effects on protons, but are small in relation to the range
of carbon shifts.
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3-4 Factors That Influence Carbon Shifts
•Alkynes do not follow this pattern, but are at an intermediate position (72 for
acetylene), because their linear structure has zero angular momentum about
the axis.
•There are exceptions, the most prominent being the effect of heavy atoms. The
series ( CH3Br (10), CH2Br2 (22), CHBr3 (12), and CBr4 (-29) defies any
explanation based on electronegativity, unlike the analogous series given before
for chlorine.
•This so- called heavy- atom effect has been attributed to a new source of
angular momentum from spin–orbit coupling.
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3-5 Carbon Chemical Shifts and Structure
• You should know the most basic of shift tables:
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3-5 Carbon Chemical Shifts and Structure
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3-5 Carbon Chemical Shifts and Structure
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3-5a Saturated Aliphatics
• Replacement of H on carbon (a) or an adjoining carbon (b) will shift the
resonance by 9 ppm
• The replacement at the g-position however shifts -2.5 ppm
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3-5a Saturated Aliphatics
• For this reason 13C shifts lend themselves readily to empirical analysis:
• Each Ai or substituent parameter is added to -2.5 ppm (shift for CH4) for
a maximum set of five carbons:
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3-5a Saturated Aliphatics
• In general –CH3 resonances appear at 5-20, -CH2- at 15-35 and –CH- at
25-45
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3-5a Saturated Aliphatics
• Some complications: Corrections must be applied if branching is present
• The 13C methyl is corrected for the presence of an adjacent 3o carbon by
adding -1.1 and for an adjacent 4o carbon by adding -3.4.
• The 13C methylene has corrections of and -2.5 and -7.2, respectively, for
adjacent 3o and 4o carbons.
• The 13C methine has respective corrections of -3.7, -9.5 and -1.5 for
adjacent 2o, 3o, and 4o carbons.
• 4o carbons have corrections of -1.5 and -8.4 for adjacent 1o and 2o
carbons. Corrections for adjacent tertiary and quaternary carbons
undoubtedly are significant, but are not known accurately.
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3-5a Cyclic Alkanes.
• For small cycloalkanes: -3.8 , cyclopropane has the most upfield resonance of
hydrocarbons. Cyclobutane and cyclopentane are higher.
• Larger cycloalkanes generally resonate within 2 ppm of cyclohexane, at
27.7.
• The fixed stereochemistry of cyclohexane requires a set of empirical
parameters that depend on the axial or equatorial nature of the substituents,
as well as on the distance from the resonating carbon.
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3-5a Cyclic Alkanes.
• The following table lists parameters for methyl substitution are added to the
value for cyclohexane ( 27.7).
• The substituent parameter for g -axial methyl is large and negative ,
reflecting the pure gauche stereochemistry between the perturbing and
resonating carbons.
• A g - equatorial group represents g - anti effect and has little perturbation
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3-5a Cyclic Alkanes.
• Corrections again are needed for branching.
• Geminal methyls: a = -3.4, b = -1.2
• Example: the calculated resonance for C2 of 1,1,3- trimethylcyclohexane :
27.7 + 5.2 + (8.9 * 2) - 1.2 = 49.5 ( observed 49.9)
• Other examples:
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3-5a Functionalized Alkanes.
• The replacement of a hydrogen atom on carbon with a heteroatom or an
unsaturated group usually results in downfield shifts due to polar effects
on the radial term.
• The effect parallels the same structural change on 1H chemical shifts but
arises from a different mechanism.
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3-5a Functionalized Alkanes.
Alkyl Halides.
•Strongly EWGs have large positive effects. In the halogen series:
CH3F 75.4 CH3Cl 25.1 CH3Br 10.2 CH3I -20.6
•Multiple substitution results in larger effects— 77.7 for CHCl3
•Recall that the effect of heavy atoms such as iodine or bromine is influenced
by a spin– orbit mechanism and hence may not follow the simple order of
electronegativity.
•The general range for the halogen effect in hydrocarbons extends from the
values given above for the simple systems to about a 25- ppm downfield shift
for CH2X and CHX systems, since the a and b effects of the unspecified
hydrocarbon pieces contribute to the downfield shift
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3-5a Functionalized Alkanes.
Alcohols and Ethers:
•The range for HO- substituted carbons is 49– 75. (MeOH at 49.2)
•The range for RO-substituted carbons is 59– 80. (CH3OCH3 at 59.5)
•The ether range is translated a few ppm downfield from alcohols, because
each ether must have one additional b-effect with respect to the analogous
alcohol.
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3-5a Functionalized Alkanes.
Amines:
• The lower EN of nitrogen moves the amine range somewhat upfield
(CH3NH2 at 28.3), the range for amines extending some 30 ppm higher
• The amine range is larger than the alcohol range because nitrogen can
carry up to three substituents, with more a and b effects possible
Other EN groups:
• CH3SCH3 at 19.5, CH3CN at 1.8, and CH3NO2 at 62.6, with the respective
ranges for thioalkoxy, cyano, and nitro substitution extending some 25
ppm downfield
• The anomalous low- frequency position for -CN substitution is related to
the cylindrical shape of the group and its reduced angular momentum.
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3-5a Functionalized Alkanes.
Attached C=C or C=O
•An attached double bond has only a small effect on a methyl group.
•For example, the position for the methyls of trans-2-butene is 17.3, and that
for the methyl of toluene is 21.3.
•The range for carbons on double bonds is about 15– 40.
•Methyls on carbonyl groups are at a slightly downfield: 30.8 for acetone and
31.2 for acetaldehyde, with a range of about 30– 45.
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3-5a Functionalized Alkanes.
• Introducing heteroatoms or unsaturation into alkane chains requires
completely new sets of empirical parameters that depend on the
substituent, on its distance from the resonating carbon ( a, b, g), and on
whether the substituent is terminal or internal:
• These parameters on the next slide represent the effect on a resonating
carbon of replacing a hydrogen atom with X:
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3-5a Functionalized Alkanes.
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3-5a Functionalized Alkanes.
• With the exception of cyano-, acetyleno-, and the heavy atom iodine, the
a-effects are determined largely by the ENof the substituent.
• The b-effects are all positive and generally of similar magnitude ( 6 to 11
ppm) and that the g-effects are all negative and generally of similar
magnitude (-2 to -5 ppm).
• Although the details are not entirely understood, it is clear that simple
polar considerations do not dominate the b- and g-effects.
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3-5a Functionalized Alkanes.
Examples:
• To use the substituent parameters given in the table, one adds the
appropriate values to the chemical shift of the 13C in the unsubstituted
hydrocarbon analogue (already having the C-effects):
• In the first example 16 is the base value for the 1-carbon in propane and
27 is the base value for the carbons in cyclopentane
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3-5b Unsaturated Compounds.
• The effects of diamagnetic anisotropy on a carbon and a proton have
similar magnitudes, but the much larger paramagnetic shielding renders
the phenomenon relatively unimportant for carbon.
• Thus, benzene (128.4) and the alkenic carbon of cyclohexene (127.3)
have almost identical carbon resonance positions, in contrast to the
situation with their protons.
• The full range of alkene and aromatic resonances is about 100– 170.
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3-5b Unsaturated Compounds.
Alkenes
•Alkenic carbons that bear no substituents resonate at 104– 115 for
hydrocarbons (isobutylene [ (CH3)2C=CH2] at 107.7)
•Alkenic carbons that have one substituent resonate in the range 120– 140
(trans-2-butene at 123.3)
•Finally, disubstituted alkenic carbons resonate the most downfield at 140–
165 (isobutylene [ (CH3)2C=CH2] at 146.4)
•Polar substituents on double bonds, especially those in conjugation, can
alter the resonance position appreciably. Unsaturated ketones, such as 3- 33
and 3- 34, have lower- frequency a resonances and higher- frequency b
resonances. The effect is d (“ CRR ¿ ) d d d (“ CHR) ( CH3) 2C“ CH2 d d
(“ CH2).
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3-5b Unsaturated Compounds.
Alkenes
• Polar substituents on double bonds, especially those in conjugation, can
alter the resonance position appreciably; e- donation or w/d alters the
radial term through resonance.
• Unsaturated ketones have more upfield a resonances and more downfield
b resonances.
• The effect is reduced in acyclic molecules
• Electron donation in enol ethers reverses the trend:
CH2=CHOCH3 (a = 153.2, b = 84.2)
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3-5b Unsaturated Compounds.
Alkenes
• Alkene chemical shifts may be estimated from substituent parameters
added to the shift for ethene ( 123.3).
• For a, b and g carbons on the same end of the double bond as the
resonating carbon, respective increments of 10.6, 7.2, and -1.5 are added.
• For a, b and g carbons on the opposite end of the double bond from the
resonating carbon, respec-tive increments of -7.9, -1.8 and -1.5 are added.
• An increment of -1.1 is added if any two substituents are cis to each other.
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3-5b Unsaturated Compounds.
Alkenes
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3-5b Unsaturated Compounds.
Alkynes and Nitriles
•Terminal alkyne carbon generally resonates in the narrow range 67– 70.
•An alkyne carbon that carries a carbon substituent resonates at a slightly
higher frequency ( 74– 85), because of a and b effects from the R group.
•Effects of conjugating, polar substituents expand the range to 20– 90.
•Nitriles resonate in the range 117– 130 (CH3C≡N at 116.9). The n p *
transition pushes the range downfield.
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3-5b Unsaturated Compounds.
Aromatics
•Alkyl substitution has its major effect on the ipso carbon.
•Because this carbon has no attached proton, its relaxation time is much
longer than those of the other carbons, and its intensity is usually lower.
•Conjugating substituents like –NO2 have strong perturbations on the
aromatic resonance positions, as the result of a combination of traditional a,
b and g effects and changes in e- density through delocalization
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3-5b Unsaturated Compounds.
Aromatics
• Heteroaromatics display a similar interplay of effects:
• Aromatic resonances may be calculated empirically by adding increments
to the benzene chemical shift (128.4) for each substituent that is ipso,
ortho, meta, or para to the resonating carbon as seen on the following
table:
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3-5b Unsaturated Compounds.
Aromatics
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3-5b Unsaturated Compounds.
Aromatics
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3-5b Unsaturated Compounds.
Aromatics
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3-5c Carbonyl Groups
General
•Carbonyl groups have no direct representation in 1H NMR spectra, so
NMR provides unique information for their analysis.
13C
•The entire carbonyl chemical shift range, 160– 220, is well removed to high
frequency from that of almost all other functional groups, on account of the
effect of the n  p* transition on the magnitude of the paramagnetic shift.
•Like aromatic ipso carbons and nitriles, carbonyl carbons other than those
in aldehydes carry no attached protons and hence relax more slowly and
tend to have low intensities.
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3-5c Carbonyl Groups
Aldehydes and Ketones
•Aldehydes resonate toward the middle of the carbonyl range, at about 190–
205 (acetaldehyde at 199.6)
•Unsaturated aldehydes, in which the carbonyl group is conjugated with a
double bond or phenyl ring, are shifted upfield (benzaldehyde at 192.4)
•The a, b and g effects of substituents on ketones add to the carbonyl chemical shift and hence are found on the downfield end of the carbonyl range
from195– 220 (acetone at 205.1 and cyclohexanone at 208.8)
•Again, adjacent unsaturation shifts the resonances to lower frequency.
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3-5c Carbonyl Groups
Carboxylic Acid Derivatives
•Carboxylic derivatives fall into the range 155– 185.
•The resonances for the series carboxylate , carboxyl , and ester often are
well defined:
NaOAc (181.5), HOAc(177.3), and CH3OAc(170.7)
•The range for esters is about 165– 175, and that for acids is d 170– 185.
•Acid chlorides are at slightly upfield at 160– 170, (168.6 for AcCl) .
•Anhydrides have a similar range: 165– 175 (167.7 for Ac2O).
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3-5c Carbonyl Groups
Carboxylic Acid Derivatives
•Lactones overlap the ester range, with the six- membered lactone at 176.5.
•Amides have a similar range: 160– 175 (172.7 for AcNH2) .
•Oximes have a larger range, from 145– 165.
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3-5c Carbonyl Groups
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3-5c Carbonyl Groups
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