Nmr spin spin coupling
- 1. 1 Nuclear Magnetic Resonance Spectroscopy • Consider the spectrum below: 1Spin-Spin Splitting/ coupling.
- 2. Chapter 13 2
- 4. Spin-Spin Splitting in 1H NMR Spectra Peaks are often split into multiple peaks due to magnetic interactions between nonequivalent protons on adjacent carbons, The process is called spin-spin splitting The splitting is into one more peak than the number of H’s on the adjacent carbon(s), This is the “n+1 rule” The relative intensities are in proportion of a binomial distribution given by Pascal’s Triangle The set of peaks is a multiplet (2 = doublet, 3 = triplet, 4 = quartet, 5=pentet, 6=hextet, 7=heptet…..) 1 1 1 1 2 1 1 3 3 1 1 4 6 4 1 1 5 10 10 5 1 1 6 15 20 15 6 1 singlet doublet triplet quartet pentet hextet heptet
- 5. Rules for Spin-Spin Splitting Equivalent protons do not split each other Protons that are farther than two carbon atoms apart do not split each other
- 6. 6 1H NMR—Spin-Spin Splitting Splitting is not generally observed between protons separated by more than three bonds. If Ha and Hb are not equivalent, splitting is observed when:
- 7. 7 • Spin-spin splitting occurs only between nonequivalent protons on the same carbon or adjacent carbons. Spin-Spin Splitting Let us consider how the doublet due to the CH2 group on BrCH2CHBr2 occurs: • When placed in an applied field, (B0), the adjacent proton (CHBr2) can be aligned with () or against () B0. • Thus, the absorbing CH2 protons feel two slightly different magnetic fields—one slightly larger than B0, and one slightly smaller than B0. • Since the absorbing protons feel two different magnetic fields, they absorb at two different frequencies in the NMR spectrum, thus splitting a single absorption into a doublet, where the two peaks of the doublet have equal intensity.
- 8. 8 The Origin of 1H NMR—Spin-Spin Splitting The frequency difference, measured in Hz, between two peaks of the doublet is called the coupling constant, J. J
- 9. 9 Spin-Spin Splitting Let us now consider how a triplet arises: • When placed in an applied magnetic field (B0), the adjacent protons Ha and Hb can each be aligned with () or against () B0. • Thus, the absorbing proton feels three slightly different magnetic fields—one slightly larger than B0(ab). one slightly smaller than B0(ab) and one the same strength as B0 (ab).
- 10. 10 Spin-Spin Splitting • Because the absorbing proton feels three different magnetic fields, it absorbs at three different frequencies in the NMR spectrum, thus splitting a single absorption into a triplet. • Because there are two different ways to align one proton with B0, and one proton against B0—that is, ab and ab—the middle peak of the triplet is twice as intense as the two outer peaks, making the ratio of the areas under the three peaks 1:2:1. • Two adjacent protons split an NMR signal into a triplet. • When two protons split each other, they are said to be coupled. • The spacing between peaks in a split NMR signal, measured by the J value, is equal for coupled protons.
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