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Coupling Constants

Coupling Constants. Review of Splitting. Caused by shift due to magnetic fields of adjacent protons We say that these protons are “coupled”. Protons may be coupled to different degrees Coupling constant Typically 7 Hz for adjacent sp 3 carbons Tree diagram. Tree diagram.

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Coupling Constants

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  1. Coupling Constants

  2. Review of Splitting • Caused by shift due to magnetic fields of adjacent protons • We say that these protons are “coupled”

  3. Protons may be coupled to different degrees • Coupling constant • Typically 7 Hz for adjacent sp3 carbons • Tree diagram

  4. Tree diagram • Shortcut: N+1 if all protons coupled with same constant • Look at tree • Coupling constant is 7.1-7.2 Hz

  5. Tree diagram • Draw the tree diagram that shows why signal A is a triplet • What is the coupling constant for signal A

  6. Coupling Constants • Coupling constants are not all 7Hz • In this class, we will need to know J values associated with alkenes

  7. Typical Constants • Use the table to predict typical coupling constants

  8. Example: Cinnamic Acid • In lab, you made transcinnamic acid • Can the trans and cis isomers be differentiated using proton NMR? • Yes—with coupling constants

  9. 400 MHz NMR: Doublet at 6.310ppm and 6.355ppm 400 MHz NMR: Doublet at 5.925ppm and 5.950ppm Calculate the coupling constants for these doublets

  10. Spectra that are not N+1 • Consider the allylic methyl group • Coupled to two protons—but not with the same coupling constant • Not N+1 • Split into a doublet by Ha • That doublet is split into doublet by Hb • It is doublet of doublets

  11. Why not N+1? • Jac = 1.7Hz (typical 0 Hz) • Jbc = 6.9 Hz (typical 4-10 Hz) • If Jac = Jbc = 6.9Hz, what would we observe?

  12. Predict the Splitting • What signals would be observed for proton A? • Proton A is coupled to one proton B (doublet) • Proton A is coupled to three proton C (quartet) • Doublet of quartets with J = 15.6 Hz and J = 1.7Hz

  13. Predict the Splitting • Do the same for proton B • How is signal same/different than proton A signal?

  14. Proton B • Also a doublet of quartets • But coupling constants are 15.6 and 6.9Hz

  15. Exercise • Explain these two observed signals • How would you describe them? • Which proton(s) do they belong to?

  16. Types of problems • Know typical coupling constants • Describe expected signal (dd, dt, etc) • Draw expected proton NMR • Interpret proton NMR given coupling constants

  17. Predict Structure • C4H8O ether • IR: 1650 cm-1

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