Spin - Spin Coupling

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.

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

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.

1. Dihedral Angle, f : angle between the C-H bonds

           The maginitude of the splitting between H_A and H_B is greatest when f = 0^o or 180^o and is smallest when f = 90^o because the side-side overlap of the two C-H bond orbitals is at a maximum at 0^o and 180^o, where the C-H bonds are parallel and anti-parallel respectively, and at a maximum at 0^o, where the C-H bonds are perpendicular.

Note : From general observation, ³J (180^o) is more than ³J (0^o) about 2 - 4 Hz.

2. Bond Length
           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

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