5.3 Spin-Spin Splitting J-Coupling

© Copyright Hans J. Reich 2010 All Rights Reserved University of Wisconsin 5.3 Spin-Spin Splitting J-Coupling H Hα C → → The actual local magnet...
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© Copyright Hans J. Reich 2010 All Rights Reserved University of Wisconsin

5.3 Spin-Spin Splitting J-Coupling

H

Hα C





The actual local magnetic field encountered by a nucleus is affected by neighboring magnetic nuclei (spin-spin splitting) as a result of perturbation of the electron distribution. The principal magnetic nuclei are other protons, the 100% abundant spin ½ nuclei 19F and ~31P, and some spin 1 or greater (quadrupolar) nuclei such as 14N, 2H, 11B, and 12B. Although Br, Cl, and I all have isotopes with spin >½, coupling is not seen because of relaxation effects. This will be discussed in more detail in Section 7. H

C

Hβ C

Note: vertical scale of the couplings is grossly exaggerated

C

s

s

E

d

d

t

s

d

dd dd

Two equal couplings.

t

q

Two different couplings.

Sign of Coupling Constants Coupling constants can be either positive or negative. These are defined as shown below: JAX < 0

JAX > 0 A=β

X=α

X=β X=α

X=β

Coupling constants are positive if the spin state in which A and X have opposite spins (αβ) is lower in energy than the one in which they have the same spin (ββ). The signs of couplings shows some consistency. • 1JC-H and most other one-bond couplings are positive. • 2JH-H in sp3 CH2 groups are almost all negative, some others are positive.

A=α

X=β

X=α

X=β

X=β

X=α

X=β

X=α X=α

• 3JH-H is always positive.

A-signal d

d

For first order patterns the signs of the couplings have no effect on the appearance of the spectrum, and so cannot be determined by observation. However, decoupling experiments (spin tickling) can provide the relative signs. For second-order patterns (e.g. ABX or AA'BB'), the relative signs of coupling constants often have dramatic effects on the appearance of the spectrum, and relative signs can be determined by proper analysis of the multiplets.

5-HMR-3.1

Two Different Couplings to one Proton Consider the NMR spectrum of 3,4-dichlorobenzoyl chloride below. R-19M C7H3Cl3O 270 MHz 1H NMR Spectrum (CDCl3)

Cl

O

Cl 40 30 20 10 0

8.2

9

8

8.1

7

6

8.0

7.9 ppm

5

Cl

Hz

7.8

4

7.7

3

7.6

7.5

2

1

0

ppm The proton-proton couplings in benzene are typically 7-9 Hz for Jortho, 2-3 Hz for Jmeta and