Download H local

NMR spectra of some simple molecules
Effect of spinning: averaging field inhomogeneity (nmr1.pdf pg 2)
Ho
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
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
Intensity
Increasing frequency
0
10
9
8
7
6
5
4
3
2
1
0
ppm
Typical chemical shifts for protons: 0 –10 ppm
In a 300 MHz instrument, differences in  range about 3000
Hz (3000 Hz shifts relative to a total of 300*106 cycles /sec)
Intensity
aromatic
CH
-CH=
CH2
CH3
0
10
9
8
7
6
5
4
3
2
1
0
ppm
Typical chemical shifts for protons: 0 –10 ppm
>C=C<
CR4
>C=O
aromatic
Intensity
CHR3
R2CH2
CH3
0
200
160
120
80
ppm
Typical chemical shifts for 13C: 0 to 220 ppm
40
0
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
Hext
Hlocal
lower frequency, lower energy, nucleus is shielded, upfield shift
Heff > than Hext  must increase for resonance
higher frequency, higher energy, nucleus is
deshielded, downfield shift
Hext
Hlocal
Field sweep instruments: At 600 MHz
ω = constant; Hext swept from
“140000 to 146000 gauss”
Hext
Hlocal
Heff < than Hext  must decrease for resonance
lower frequency, lower energy, nucleus is shielded, upfield shift
Then resonance would occur at a lower value of Hext
nucleus is deshielded, downfield shift
Hext
Hlocal
Sigma bonds
electron
cloud
Field due to
circulating e-
Hexternal field
nucleus
All protons have the same precessional frequency in a vacuum
Field felt by the nucleus Heff = Hext - Hlocal
For resonance either Hext must be increased or  decreased
relative to the situation where Hlocal = 0
π bonds in acetylenes
Hext
Hlocal
H
H
O
shielding cone
π bonds in alkenes
Hlocal
and aldehydes
Hext
deshielding region
π bonds in aromatic
compounds
Hext
Hlocal
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
H
 -3.0
H
 0.3
H
CH2
H
H
H
H
 9.3
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
For positive J
+J/4
A
+J/4
A
-3J/4
+J/4
J =0
No H – H interaction
H – H interaction
A2 Case
For negative J
-J/4
A
+3J/4
-J/4
A
-J/4
No H – H interaction
H – H interaction
J =0
AX; X > A
J=0
A
Relative ordering of energy levels
without AX interactions
X
X
Energy
A
A
Both opposed to
magnetic field
X
AX; X > A
+J/4
X +J/2
A + J/2
-J/4
Relative ordering of energy levels
with AX interactions
X -J/2
-J/4
A – J/2
+J/4
Both opposed to
magnetic field
A
X
For positive J
In the absence of coupling, ie J = 0
Intensity
In the presence of coupling, ie J ≠ 0
J
X
0
10
9
8
7
A
6
5
ppm
4
3
2
J
1
0
AX; X > A
-J/4
A – J/2
+J/4
X -J/2
Relative ordering of energy levels
with AX interactions
X +J/2
+J/4
Both opposed to
magnetic field
A
X
A + J/2
-J/4
For negative J
Intensity
X
A
J
J
0
10
9
8
7
6
5
ppm
4
3
2
1
0
A2X X > A
No AX interaction, JAA ≠ 0
A2
X
A2X X > A
A +J/2
X+J/2
0
A -J/2
X
No AX interaction
X
X-J/2
A -J/2
0
A+J/2
A2
X
For positive JAX
X
A2X X > A
A +J/2
A+J/2
0
A+J/2
A -J/2
AX interaction
Note that the A
transitions are twice as
intense
A -J/2
0
A+J/2
A2
X
A-J/2
A-J/2
J=0
For positive J
No A2X coupling
A2X coupling
X
A
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
H
OH
C
CH3
CH3
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?
O
CH3
C
CH2 CH3
3
3
2
CH3
CH3
CH2
O
O
CH2 O
C
C
C
O
CH2
CH3
O
CH2
CH3
H
CH3
CH3
CH2
O
O
CH2 O
C
C
CH2
CH3
C
geminal
2J
vicinal
3J
4J
5J
Magnitude of the Vicinal Coupling Constant J
Karplus Equation
3J
CHCH
H
H
= 10 cos2(φ) where φ is the dihedral angle
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
H5
H3
H2
60 MHz, 600 Mz
H4
H1
H1= H2 = H3  1.0 J12 = -10; J13 = -10; J23 = -10
H4 = H5 =  1.5 J14 = 7; J 15 = 7; J4,5 = -12
Effect of Magnetic field strength on 1H NMR Spectra
H1
Raccoon
CN
60 MHz, 600 Mz
H2
H3
H1=  8.0 J12 = 8; J13 = 17; J23 = -6
H2 =  8.6 J
H3 =  8.9