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NMR spectra of some simple molecules
Effect of spinning: averaging field inhomogeneity (nmr1.pdf pg 2)
Because the protons have a magnetic field associated with them, the field changes as across the nmr tube. Diffusion tends to offset this field gradient
Ho
Chemical Shifts
Heff = The magnetic field felt at the proton
Heff = Hext + Hlocal ; Heff : magnetic field felt by the nuclei
Hext : external magnetic field
Hlocal: local field induced by the external field
Hlocal: Electrons in a chemical bond are considered to be in motion and are charged. This induces a local magnetic field which can shield (oppose) or deshield (enhance) the magnetic field experienced by the nucleus. Since the precessional frequency of the nucleus is governed by Heff, changes in this field as a result of local fields caused by bonding electrons, the resonance frequency of magnetically and chemically non-equivalent nuclei differ resulting in slightly different values of . This is the origin of the chemical shift. The local magnetic field is induced by the external field and is directly proportional to the external field
Hlocal : the effect of the external magnetic field on the bonding electrons depends on electron density and molecular structure.
Hlocal is directly proportional to Hext
Remember H is a vector. This property has both magnitude and direction
ppm
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Inte
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y
0
Typical chemical shifts for protons: 0 –10 ppmIn a 300 MHz instrument, differences in range about 3000 Hz (3000 Hz shifts relative to a total of 300*106 cycles /sec)
Increasing frequency
ppm
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Inte
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0
Typical chemical shifts for protons: 0 –10 ppm
CH
CH2
CH3
-CH=
aromatic
ppm
04080120160200
Intensity
0
Typical chemical shifts for 13C: 0 to 220 ppm
CR4
CHR3
R2CH2
CH3
aromatic
>C=C<
>C=O
Common terms used in NMR (terms originating from use of CW instruments)
Shielded: the induced local field opposes the external field
Deshielded: the induced local field field augments the external field
Upfield shift: shift toward lower frequency; higher magnetic field, lower energy
Downfield shift: shift toward higher frequency; lower magnetic field
higher energy
Frequency sweep instruments:
Hext = constant; swept 10 ppm
Heff < than Hext must decrease for resonance
lower frequency, lower energy, nucleus is shielded, upfield shift
Hext Hlocal
Hext Hlocal
Heff > than Hext must increase for resonancehigher frequency, higher energy, nucleus is deshielded, downfield shift
Field sweep instruments: At 600 MHzω = constant; Hext swept from
“140000 to 146000 gauss”
Heff < than Hext must decrease for resonance
lower frequency, lower energy, nucleus is shielded, upfield shift
Hext Hlocal
Then resonance would occur at a lower value of Hext
nucleus is deshielded, downfield shift
Hext Hlocal
nucleuselectron cloud
Field due to circulating e-
Hexternal field
Field felt by the nucleus Heff = Hext - Hlocal
For resonance either Hext must be increased or decreased
relative to the situation where Hlocal = 0
All protons have the same precessional frequency in a vacuum
Sigma bonds
H
H
π bonds in acetylenesHext
Hlocal
π bonds in alkenes
and aldehydes
Hext
O
shielding cone
deshielding regionHlocal
H
Field felt by the nucleus Heff = Hext + Hlocal
For resonance either Hext must be decreased or increased
relative to the situation where Hlocal = 0
Hext
Hlocal
π bonds in aromatic compounds
H
HH
-3.0
9.3
CH2
HH
H H 0.3
Hext
An Example of A Simple Spectrum
Area: 9:1:2
Other Factors Influencing Hlocal
Hlocal is influenced by all local fields; the field effect of the bonding electrons results in the chemical shift, a relatively small perturbation Hlocal is induced by the external field and depends on its magnitude
What about the field effects of the local protons?
Suppose we have two identical protons attached to the same carbon.
What are the possible spin states of this system and how do they effect the local magnetic field?
Nomenclature used to describe spin-spin coupling
First Order Spectra: Chemical shift difference ∆ > 10 J
AX ; A2X; A3X; AMX; A3MX; A3M2X; …
J is a measure of the effective magnetic field of neighboring protons. The effect is generally considered to be transmitted through chemical bonds and not through space
Non-first Order Spectra: Chemical shift ∆ < 10 J
AB ; A2B; A3B; ABC; A3CB; A3B2X; A3B2C …
A2 Case, J = 0 H-C-C-C-C-H
Energy or H
Remember: Ne/Ng = e-H/RT 1
A2 Case H-C-H
+J/4
+J/4
+J/4
-3J/4
A
A
No H – H interaction H – H interaction
For positive J
J = 0
A2 Case
-J/4
-J/4
-J/4
+3J/4A
A
No H – H interaction H – H interaction
For negative J
J =0
AX; X > A
A X
Relative ordering of energy levels without AX interactions
Both opposed to magnetic field
Energy
A
J = 0
A
X
X
AX; X > A
A X
Relative ordering of energy levels with AX interactions
Both opposed to magnetic field
A + J/2
A – J/2
X +J/2
X -J/2
+J/4
+J/4
-J/4
-J/4
For positive J
ppm
012345678910
Inte
nsit
y
0X A
JJ
In the absence of coupling, ie J = 0
In the presence of coupling, ie J ≠ 0
AX; X > A
A X
Relative ordering of energy levels with AX interactions
Both opposed to magnetic field
A + J/2
A – J/2
X +J/2X -J/2
+J/4
+J/4
-J/4
-J/4For negative J
ppm
012345678910
Inte
nsit
y
0
X A
J J
A2X X > A
No AX interaction, JAA ≠ 0
A2 X
A2X X > A
No AX interaction
A2 X
X
A+J/2
A -J/2
0
A +J/2
A -J/2
0
X
X-J/2
X+J/2
For positive JAX
X
A2X X > A
AX interaction
A2 X
A+J/2
A -J/20
A +J/2
A -J/2
0
For positive JJ = 0
A+J/2
A+J/2
A-J/2
A-J/2
Note that the A transitions are twice as intense
X
No A2X coupling
A
A2X coupling
The 2nS +1 Rule
The number of lines observed for a particular nucleus as a result of n “identical” neighbors is 2nS + 1 where S is the spin of the neighboring nucleus. For most nucleus, S = ½, the relationship simplifies to n+1 lines
“identical” in this context refers to nuclei that have the same or very similar coupling constants to the nucleus being observed.
number of “identical neighbors” multiplicity of nucleus observed
1 2 (1:1)
2 3 (1:2:1)
3 4 (1:3:3:1)
4 5 (1:4:6:4:1)
5 6 (1:5:10:10:5:1)
Examples of First Order Spectra
C
CH3 CH3
OHH
CH3CH2OH
What information do you get out of a 1H NMR spectrum?
Chemical Shift?
An indication of the type of proton and its environment
Multiplicity?
An indication of the number of nearest neighbors and their proximity
Area?
A measure of the relative number of hydrogen nuclei in the molecule
The compound has a IR frequency of 1720 cm-1 and a molecular formula of C4H8O. What is its structure?
3
2
3
CCH3
O
CH2 CH3
OC CH2
O
CCOCH2
CH2
H
O
CH3
CH3 CH3
OC CH2
O
CCOCH2
CH2
CH2
O
CH3
CH3
CH3 CH3
geminal 2J
vicinal 3J
4J
5J
Magnitude of the Vicinal Coupling Constant J
Karplus Equation
3JCHCH = 10 cos2(φ) where φ is the dihedral angle
HH
Summary of the Field Dependence of and J
is the local field that is induced by the magnitude of the external field, Ho. is therefore chemical shift dependent.
J is dependent on the magnetic moment of the proton and is
therefore independent of the external field, Ho.
Effect of Magnetic field strength on 1H NMR Spectra
Raccoon
60 MHz, 600 Mz
H1= H2 = H3 1.0 J12 = -10; J13 = -10; J23 = -10
H4 = H5 = 1.5 J14 = 7; J 15 = 7; J4,5 = -12
H1H4
H5 H3
H2
Effect of Magnetic field strength on 1H NMR Spectra
Raccoon
60 MHz, 600 Mz
H1= 8.0 J12 = 8; J13 = 17; J23 = -6
H2 = 8.6 J
H3 = 8.9
CN
H1
H3
H2