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1 H NMR Interpretation

1 H NMR Interpretation. Number of Signals (Resonances) Position of Signals (Chemical Shift) Peak Integrations Splitting Patterns (Multiplicity) Exchangeable Protons. Number of Signals – Simple Cases. Number of Signals – Complex Cases. 11 signals?. 6 signals?. 90 MHz spectrum:.

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1 H NMR Interpretation

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  1. 1H NMR Interpretation • Number of Signals (Resonances) • Position of Signals (Chemical Shift) • Peak Integrations • Splitting Patterns (Multiplicity) • Exchangeable Protons

  2. Number of Signals – Simple Cases

  3. Number of Signals – Complex Cases 11 signals? 6 signals? 90 MHz spectrum: 400 MHz spectrum

  4. Effect of Magnetic Field Strength on Spectrum Quality 60 MHz 300 MHz ppm = Hz/MHz MHz = Hz/ppm So the instrument frequency tells you how many Hz there are in 1 ppm

  5. Chemical Shift Proton NMR shifts normally range from 0 ppm (TMS) to 10 ppm Inductive effects explain a lot: CHCl3 in CDCl3 = 7.26 ppm H2O in CDCl3 = 1.55 ppm

  6. Chemical ShiftAnisotropic Effects – Aromatic Compounds

  7. Anisotropic Effects – Alkenes and Alkynes

  8. Positions of Signals – Characteristic Shifts

  9. Peak Integrations • The area under an NMR signal (the integration) is proportional to the number of absorbing protons. • The area ratios tell us the ratios of the protons causing the peak. • Strategy - find a peak that you can assign unambiguously and set its integration at the appropriate number of Hs.

  10. Splitting Patterns • Consider the spectrum below:

  11. Peak intensities in a multiplet • Doublet – The two spin states of the proton causing splitting are nearly equally populated (because the energy difference is so small). Therefore a doublet has a peak ratio of 1:1. • Triplet - For the two Hs causing splitting there are two different ways to align one proton with B0 and one proton against B0 (ab and ab) the middle peak of the triplet is twice as intense as the two outer peaks and the peak ratio is 1:2:1.

  12. Quartet (H signal split by 3 equivalent protons, e.g. an H on a carbon next t o a CH3)

  13. Pascal’s triangle

  14. Rules for predicting splitting patterns If Ha and Hb are not equivalent, splitting is observed when: “vicinal” “germinal” • The n+1 rule - A set of n protons splits the signal of a nearby proton into n + 1 peaks. • Equivalent protons do not split each other’s signals. • Hs bonded to O or N usually do not show splitting

  15. The coupling constant, J, is measured in Hz and has to be the same forward and reverse!

  16. Exchangeable Protons From http://web.mnstate.edu/marasing/CHEM429_2008/Documents/1_H_NMR_Interpretation.pdf

  17. Solving an NMR unknown • With a given formula and NMR spectra, you can determine a molecule’s structure by a 4-step process • Calculate the degree of unsaturation or hydrogen deficiency index (HDI). • Consider the number of C- 13 NMR signals as an indication of symmetry in the molecule • Analyze the proton NMR shifts, integrations and multiplicities draw out the molecular fragments that match. • Assemble the fragments into a complete structure like puzzle pieces. Klein, Organic Chemistry 2e

  18. C11H14O

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