Spin - Spin Coupling => the way in which the spin of one nucleus influences spin of another
Dirac vector model
The electrons in the intervening bonds between the two nuclei transfer spin information from one to another by means of interation between the nuclear and electronic spin.
An electron near the nucleus is assumed to have the lowest energy of interation with the the nucleus when the spin of the electron has its spin direction paired to that of the nucleus.
Types of Spin-Spin Coupling
Dirac model makes it easy to understand why the size of J decreases as the no. of intervening bonds increases.
One-bond couplings, coupling via one bond, occur when a single bond links 2 spin-active nuclei. For a 13C-H bond and other types of bonds where both nuclei have spin (i.e., 1H -1H and 31P -H), the two nuclei prefer to have anti-parallel spins. Therefore, most one-bond couplings have positive J values.
Note : - When 2 spin nuclei are anti-parallel, J is usually positive.
- When 2 spin nuclei are parallel, J is usually negative.
- When 2 spin nuclei are parallel, J is usually negative.
Some one-bond coupling constants
bond
|
coupling constants (Hz)
|
13C -1H
|
110 - 270
sp3 115 - 125 sp2 150 - 170 sp 240 - 270 |
13C -19F
|
- 165 to -370
|
13C -31P
|
48 - 56
|
13C -D
|
20 - 30
|
31P -1H
|
190 - 700
|
Two-bond couplings or geminal coupling, 2J, coupling via 2 bonds, occur in carbon compounds when 2 or more spin-active nuclei are attached to the same carbon atom. The most common type of two-bond couplings, H-C-H, is frequently negative.
The amount of geminal coupling depends on
- H-C-H angle
- hybridization
- substituent effect
- H-C-H angle
- hybridization
- substituent effect
1. H-C-H angle (a)
Geminal coupling constants increase as the a decreases due to the 2 orbitals move closer, and the electron spin correlations become greater.
2. Hybridization
Hybridization is important and can affect geminal coupling. Normally 2J of H-C-H coupling is negative if C is sp3 hybridization but 2J is positive if C is sp2 hybridization.
3. Substituent Effect
The 2J for alkenes may change from positive to negative when they have an electronegative atom attached.
a - substitution
|
more electronegative group
| more positive of J | ||
more electropositive group
| more negative of J | |||
b - substitution
|
more electronegative group
| more negative of J | ||
more electropositive group
| more positive of J | |||
adjacent p-bond
|
hyperconjugative effect
| more negative of J |
Three-bond couplings,3J, coupling via 3 bonds, are also called vicinal coupling.
The spin of one H nucleus in one C - H bond is coupled to the spins of those Hs in adjacent C - H bonds. |
The two possible arrangements of nuclear and electronic spins for 2 coupled protons that are adjacent C atoms are shown below.
Figure a), the spins of H nuclei are paired and the spins of the electrons that are interacting through parallel orbital overlap are also paired. This is expected to represent the lowest energy and have the favored interactions and 3-bond H-C-C-H couplings are expected to be positive.
Vicinal couplings can be affected by the following factors;
- bond length
- dihedral angle
- substituent effect.
- bond length
- dihedral angle
- substituent effect.
1. Dihedral Angle, f : angle between the C-H bonds
The maginitude of the splitting between HA and HB is greatest when f = 0o or 180o and is smallest when f = 90o because the side-side overlap of the two C-H bond orbitals is at a maximum at 0o and 180o, where the C-H bonds are parallel and anti-parallel respectively, and at a maximum at 0o, where the C-H bonds are perpendicular.
Note : From general observation, 3J (180o) is more than 3J (0o) about 2 - 4 Hz.
2. Bond Length
Vicinal coupling decreases as bond length increases.
Vicinal coupling decreases as bond length increases.
3. Substituent Effect The 3J may decrease when they have an electronegative atom attached.
Values of substituted ethanes and ethylenes
3J (Hz)
|
3Jcis (Hz)
|
3Jtrans (Hz)
| |||
Li
|
8.9
|
Li
|
19.3
|
23.9
| |
-SiR3
|
8.0
|
-SiR3
|
14.6
|
20.4
| |
-CN
|
7.6
|
-CH3
|
10.0
|
16.8
| |
-Cl
|
7.2
|
-Cl
|
7.3
|
14.6
| |
-OCH2CH3
|
7.0
|
-F
|
4.7
|
12.8
| |
-O+R2
|
4.7
|
Long-range couplings involve more than 3 bonds. They are found in both saturated and unsaturated systems. Normal range of long-range coupling is 0.1 - 3 Hz.
1. Saturated Systems
4J and 5J are found in systems that bonds exist in the zig-zag arrangement (M or W)
Assignment of stereochemistry in cyclic compounds :
2. Unsaturated Systems
Long-range couplings are common in systems with allylic hydrogens.
polyacetylene
|
long-range coupling (Hz)
|
allene
|
long-range coupling (Hz)
| |
-2.93
|
+3.0
| |||
+2.7
| ||||
+2.2
|
-5.8
| |||
-1.27
|
+2.4
| |||
+1.3
|
+4.58
| |||
+0.4
|
........
CHENNAI, TAMILNADU, INDIA
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