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215-216 HH W13 Notes - Dr. Masato Koreeda Date: January 9, 2013 Topic: _NMR-II___ page 1 of 10. IV. Chemical Shifts - δ unit Each H nucleus in a molecule has a different degree of electron surrounding it. Higher the electron density is found surrounding the 1H nucleus, more the external magnetic energy is needed for the excitation of that 1H nucleus as the electron shields the nucleus. Where each 1H peak appears in the 1H NMR spectrum is the reflection of what kind of a chemical environment each 1H nucleus is in,* thus the name “chemical shift.” *This is manifested in the electron density surrounding each 1H nucleus. NMR spectra are obtained usually in CDCl3 (99.8% D and 0.2% 1H) with (CH3)4Si [tetramethylsilane or TMS] as internal reference in a certain operating magnetic field such as 200 MHz, 400 MHz, etc.
H3C CH3
O (acetone)in CDCl3 on a 200 MHz magnet NMR spectrometer:
TMS
acetoneresidualCHCl3 in CDCl3
lower fieldor down-field
higher fieldor up-field440
Hz"The peak of acetone 1H's appears as one peak at 440 Hz down-filed from TMS on a 200 MHz NMR spectrometer"
H3C CH3
O (acetone)in CDCl3 on a 400 MHz magnet NMR spectrometer:
TMS
acetone
residualCHCl3 in CDCl3
lower fieldor down-field
higher fieldor up-field880 Hz
"The peak of acetone 1H's appears as one peak at 880 Hz down-filed from TMS on a 200 MHz NMR spectrometer"
note: MHz = 106 Hz MHz (mega Hertz); Hz = cps
Since these are too lengthy descriptions of 1H NMR data and the acetone peak appears at a down field position from TMS proportionally to the strength of an operating magnetic field, the following dimension less ppm unit has been introduced. (H3C)2C=O on a 200 MHz NMR spectrometer: 440 Hz / 200 x 106 Hz = 2.2 x 10-6 ≡ 2.2 ppm
(H3C)2C=O on a 400 MHz NMR spectrometer: 880 Hz / 400 x 106 Hz = 2.2 x 10-6 ≡ 2.2 ppm
!!! Now, these are independent of the strength of an operating magnetic field.
215-216 HH W13 Notes - Dr. Masato Koreeda Date: January 9, 2013 Topic: _NMR-II___ page 2 of 10. Conversely, 1 ppm on a 200 MHz NMR spectrometer corresponds to: 1 x 10-6 x 200 x 106 Hz = 200 Hz and 1 ppm on a 400 MHz NMR spectrometer corresponds to: 1 x 10-6 x 400 x 106 Hz = 400 Hz This ppm scale relative to TMS and the increasing value to the lower magnetic field (i.e., to the left from TMS) is called “the δ-scale.” The 1H peaks of most of the organic compounds fall in between δ = 0 ~ 10 ppm (see Table 10.3 on p. 361 of Ege’s book). The Factors That Affect Chemical Shifts (1) Hybridization
C Hsp3
typically δ = 0.5 ~ 1.5 ppm
CHsp2
alkenes
typically δ = 5 ~ 6 ppm
However, -C ≡ C - Hsp
δ = ~ 2.2 ppm!
note:H typically δ = 7 ~ 8 ppm
aromatic H's
(2) Electron Density on each 1H Heffective = H0 (1 – σ) H0 : applied magnetic field σ = shielding constant, 10-2 ~ 10-5 (reflects chemical environments of a specific 1H) • increasing electron density on a 1H → more shielding (larger σ) → more external Ho to reach Heffective → 1H peak at a higher field → a smaller δ value for the peak. Examples: (i) tetramethylsilane (TMS):
H3CCH3
CH3CHH
H
electronegativity: 2.1
e.n.: 2.5e.n.: 1.8
SiConsequently, the electron density of the 1H's of the CH3 increases--> 1H's of the CH become highly shielded --> requires more external magnetic field energy.
Therefore, (H3C)4Si H’s appear at a significantly higher field than H’s in most of the organic compounds; one of the reasons why TMS is used as the internal reference compound.
(ii) Halomethanes [H3C-X]
HH
H
electronegativity: 4.0
e.n.: 2.5
C F
δ 4.30
HH
H
e.n.: 3.0
e.n.: 2.5
C Cl
δ 3.05
HH
He.n.: 2.5
C Br
δ 2.70e.n.: 2.8
HH
He.n.: 2.5
C I
δ 2.10e.n.: 2.5
least shielded;most deshielded
215-216 HH W13 Notes - Dr. Masato Koreeda Date: January 9, 2013 Topic: _NMR-II___ page 3 of 10. (iii) H3C---OR H3C---NR2 H3C---SR δ 3.5 e.n.: 3.5 δ 2.5-3.0 e.n.: 3.0 δ 2.2-2.5 e.n.: 2.5 (iv) alkenes vs α,β-unsaturated ketones
H
H
H
HO
δ 5.22
5.5
6.5 H
HO
H
HO
+ HO
+
--
Hdeshielded
α
β
(3) Magnetic Anisotropy ----- A through-space effect (i) Aromatic Ring-Current Effect
See: Günther, H. NMR Spectroscopy; Wiley: New York, 1998; p. 85. For a recent dispute on the origin of the aromatic ring-current effect, see: Wannere, C. S.; von Ragué Schleyer, P. Org. Lett. 2003, 5, 605.
H
aromatic H'sH
Hδ 5.22
H
HH
HH
7.22
|Δδ| = ca. 2 ppm
H
H
HH
H
H
Induced magnetic field
Induced magnetic fieldHo
These aromatic delocalized 6 π electrons start circulatinginto one direction in an external magnetic field (Ho). Thiscirculation of the 6 π-electrons results in the induction of a secondary magnetic field following the Fleming's rule.
flow of the 6 π-electrons in the ring
induced magneticfield opposed tothe applied magneticfield
induced magnetic fieldreinforcing the appliedmagnetic field
Ho
This type of differenct effect in magnetic properties at a different point in space is referred to as "magnetic anisotropy."
H
circulatingπ-electrons
215-216 HH W13 Notes - Dr. Masato Koreeda Date: January 9, 2013 Topic: _NMR-II___ page 4 of 10. (3) (i) Aromatic Ring-Current Effect (continued)
H
ca. 55 °H's located inside this imaginarycone requires more external magnetic field power for their excitation --> high- field shifted; smaller δ values
H's located outside the cone, including the H's on the benzene ring, requires less externalmagnetic field power (i.e., energy) for their excitation --> down-field shift; larger δ values
CH2
CH2
CH2
H2C
H2CH2C
H2C
H2C
CH2H2C
shieldingzone
deshielding zone
δ 2.63
1.55
1.08
0.70
0.51
Example:
Strongly deshielded
Strongly shielded by the ringcurrent effect of the benzene6π-electrons
The closer the H is to the center of the benzene ring, the stronger the extent of the ring current effect is.
(ii) Acetylene groups.
C C HR
R
H
In a magnetic field, theseπ-electrons induce a secondarymagnetic field
C C HR
shielding zonedeshielding zone
As this H is quite close to the center of the C ≡ C bond,an extremely strong shielding effect is observed for this H.
δ 2.2
V. Integration The peak intensity is proportional to the number of H’s belonging to each peak in the case of 1H NMR.
H3COCH2OCH3
TMS
3.55.0 0 ppm
12 36
These integrated areas are expressed in arbitrary numbers. Only their numerical ratio is important.
chemical shifts
integration of each of the peaks
These 12 and 36 represent the integrated areas of each peak and their ratios correspond to the ratios of the number of H’s belonging to each peak, i.e., 12 : 36 = 1 : 3. This could mean 1 H and 3 H’s, 2H’s and 6 H’s, or 3 H’s and 9 H’s, etc.
215-216 HH W13 Notes - Dr. Masato Koreeda Date: January 9, 2013 Topic: _NMR-II___ page 5 of 10. Integration (continued). • If there is a 1 : 1 mixture (on a molar basis) of acetone and chlorform [CHCl3], the integrated ratio of each of the two peaks should be 1 : 6.
TMS
2.27.26 0 ppm
2 12
chemical shiftsintegration of each of the peaks
acetone [C3H6O]chloroform[CHCl3]
Conversely, if the 1H NMR spectrum of a mixture of acetone and chloroform shows a 1 : 1 intensity by integration, what is the molar ratio of these two solvents in the solution?
TMS
2.27.26 0 ppm
3.5 3.5
chemical shiftsintegration value of each of the peaks
acetone [C3H6O]chloroform[CHCl3] integration
trace
VI. Spin-Spin Couplings [Nuclear Spin-Nuclear Spin Interactions]-Through-Bond Interactions Acyclic Systems
Cl C C BrCl Br
H H δ 4.5δ 5.8
3-bond interaction
This 3-bond interaction is often referredto as a "vicinal coupling" and is expressedas
3J1,2 = 7.0 Hzbetween H-1 and H-2
1 2
through 3 bonds(i.e., H1-C1, C1-C2, and C2-H2)
The two spin states of H1 interact, through bonds, with the two spin states of H2, called "coupling," resulting in the formation of the spectral pattern consisting of a pair of doublets.
These H's are called a 3-bond neighbor to each other..
4.55.8 δ (ppm)
3J1,2
215-216 HH W13 Notes - Dr. Masato Koreeda Date: January 9, 2013 Topic: _NMR-II___ page 6 of 10. V. Spin-Spin Coupling (continued)
3-Bond Neighbor Analysis:
Cl H
H H
ClH
has 2 3-bond neighbors (i.e., neighboring H's)
have 4 3-bond neighbors
have 2 3-bond neighborsH
H
============================================================ ethyl acetate [H3C-CH2-O-C(=O)-CH3 ]
HC
CO CH3
HH
H H O
δ (ppm)
2 chemicallyequiv. H's;have 3 3-bond neighbors
3 chemically equivalent H's;have 2 3-bond neighbors
01.02.04.17.2
CHCl3TMS
triplet
quartet-CH2-
-CH3
4.0 6.1 6.02 : 3 : 3
integration values
1H NMR spectrum of ethyl acetate
integration orintensity ratio
singlet O CH3
O
For the CH3 H's: there are two 3-bond neighbors.
After the interaction with thefirst 3-bond neighbor(assuming J = 6 Hz)
After the interation with thesecond 3-bond neighbor -each of the above doublet peaks splits to a doublet
6 Hz
These middle two peaks overlap.
6 Hz6 Hz 6 Hz6 Hz
Overall,
peak intensity ratio1 : 2 : 1
215-216 HH W13 Notes - Dr. Masato Koreeda Date: January 9, 2013 Topic: _NMR-II___ page 7 of 10. V. Spin-Spin Coupling (continued)
6 Hz 6 Hz6 Hz
6 Hz 6 Hz6 Hz
Now, the interation with thethird 3-bond neighbor - each of the 1 : 2 : 1 triplet peaks splits to a doublet
Overall, the CH2 peaksshow up as a 1 : 3 : 3 : 1quartet.
For the CH2 H's: there are three 3-bond neighbors.
After the interaction with thefirst 3-bond neighbor(assuming J = 6 Hz)
After the interation with thesecond 3-bond neighbor -each of the above doublet peaks splits to a doublet
6 Hz
These middle two peaks overlap.
6 Hz6 Hz 6 Hz6 Hz
Thus, becoming a tripletwith a 1 : 2 : 1 intensity ratio.
These two peaks overlap.
These two peaks overlap.
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ In general, in an sp3 acyclic system:
If an H has an n-number of 3-bond neighbor H’s, the NMR signal of the former splits into an (n + 1)-number of peaks. This is due to the fact that all of the vicinal couplings (i.e., 3J) are virtually identical in a freely rotating acyclic system.
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ For example,
CCH
H C
H H
H H
HO C
HH
Hhas 6 3-bond neighbors
have no 3-bond neighbors
have 1 3-bond neighbor
(6 + 1) = 7 peaks: septet
(0 + 1) = 1 peak: singlet
(1 + 1) = 2 peaks: doublet
215-216 HH W13 Notes - Dr. Masato Koreeda Date: January 9, 2013 Topic: _NMR-II___ page 8 of 10.
V. Spin-Spin Coupling (continued)
“A 1H-1H spin-spin coupling exists between any 2- and 3-bond neighboring H’s. However, the only couplings that can be observed in the spectrum are those between chemically non-equivalent H’s.”
215-216 HH W13 Notes - Dr. Masato Koreeda Date: January 8, 2013 Topic: _NMR-II___ page 9 of 10. V. Spin-Spin Coupling (continued) (6) vinyl acetate
HB HX
O
OHA
CH3δ 2.15 (singlet)
4.88 (doublet of doublets)
4.57 (doublet of doublets)7.27 (doublet of doublets)
H H
O
OH
CH3
H H
O
OH
CH3
These are high-field shifted due to the following resonance contribution:
The coupling constants among these three H's are:3JAX = 14.2 Hz (trans-vicinal); 3JBX = 6.5 Hz (cis-vicinal); 2JAB = 1.2 Hz (geminal)
down-field shifted due to the strong inductive effect by the oxygen atom
For HX:
Upon interaction with HA
Then, upon interactionwith HB, each of the doubletsplits into a doublet
3JAX
14.2 Hz
6.5 Hz
6.5 Hz
3JBX3JBX
"a doublet of doublets"
Unlike those in an acyclic system, these middle two peaks do not overlap, thus not producing a 1 : 2 : 1 triplet pattern.
3JAX3JBX≠
The situation is the same for both HA and HB. Therefore, HA and HB each shows a doublet of doublets.
215-216 HH W13 Notes - Dr. Masato Koreeda Date: January 9, 2013 Topic: _NMR-II___ page 10 of 10.
V. Spin-Spin Coupling (continued) (7) Aromatic Hydrogens: typically 3J ~ 8Hz (ortho); 4J ~ <1 Hz (meta); 5J ~ 0 Hz (para)
HB HA
HAHB
Cl H
O
H H
HH
Two groups of chemically equivalent aromatic H's: each shows up as a doublet as a result of 3JAB ~ 8 Hzwith a 2 H-intensity.
(a) para-Disubstituted Benzene
quartet (3 3-bond neighbors) at 2.8 ppm
triplet (2 3-bond neighbors) at 1.2 ppm
(c) Long-range couplings
HA
Br
HB
CH3HC
NO2
singlet at 2.5 ppm
HA
Br
HB
CH3HC
NO2
~ 0 Hz
8.6 Hz
2.2 Hz
3J (ortho coupling)
4J (meta coupling)
5J (para coupling)
doublet at7.40 ppm
a doublet of doublets at 8.04 ppm
doublet at 8.38 ppm
(b) Three isomers of nitroaniline
NO2
HA
HC
NH2HD
HB
7.48 (s)6.95 (d)
7.55 (d)HA
NO2
HC
NH2HD
HB 7.36 (t)HB
HANH2
HA
HBNO28.08 (d)
ortho meta para
6.67 (t)
7.34 (t)
6.82 ppm (d)
6.61 (d)
7.93 (d)
Chemical shifts of each of these aromatic H's are explainable in terms of resonance contributions of the NH2 and NO2 groups.
NH H NH H
HNH H
H
N HH
H
H
HH
H most e-density increase
highest field-shift (smaller δ ppm)
some e-density increase
high-field shift
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