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C-13 NMR Spectroscopy

Learn about the theory and principles behind 13C NMR spectroscopy, including nuclear magnet behavior and chemical shifts. Discover the uses of 13C NMR in determining the number of carbon atoms and the electronic environment in a molecule. Explore how 13C NMR can verify reaction products and identify different carbon resonances.

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C-13 NMR Spectroscopy

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  1. C-13 NMR Spectroscopy Manolito G. Ybañez Jr. BSCT 2013

  2. Theory of NMR • The positively charged nuclei of certain elements (e.g., 13C and 1H) behave as tiny magnets. • In the presence of a strong external magnetic field (Bo), these nuclear magnets align either with ( ) the applied field or opposed to ( ) the applied field.

  3. Theory of NMR • The latter (opposed) is slightly higher in energy than aligned with the field. • The small energy difference between the two alignments of magnetic spin corresponds to the energy of radio waves according to Einstein’s equation E=hn. DE is very small

  4. Theory of NMR • Application of just the right radiofrequency (n) causes the nucleus to “flip” to the higher energy spin state • Not all nuclei require the same amount of energy for the quantized spin ‘flip’ to take place. • The exact amount of energy required depends on the chemical identity (H, C, or other element) and the chemical environment of the particular nucleus.

  5. Theory of NMR • The induced circulation of electrons sets up a secondary (induced) magnetic field (Bi) that opposes the applied field (Bo) at the nucleus (right hand rule). • We say that nuclei are shielded from the full applied magnetic field by the surrounding electrons because the secondary field diminishes the field at the nuclei.

  6. Theory of NMR • The electron density surrounding a given nucleus depends on the electronegativity of the attached atoms. • The more electronegative the attached atoms, the less the electron density around the nucleus in question. • We say that that nucleus is less shielded, or is deshielded by the electronegative atoms.

  7. Theory of NMR • Deshielding effects are generally additive. That is, two highly electronegative atoms (2 Cl atoms, for example) would cause more deshielding than only 1 Cl atom. C and H are deshielded C and H are more deshielded

  8. Chemical Shift • We call the relative position of absorption in the NMR spectrum (which is related to the amount of deshielding) the chemical shift. It is a unitless number (actually a ratio, in which the units cancel), but we assign ‘units’ of ppm or d (Greek letter delta) units. • For 1H, the usual scale of NMR spectra is 0 to 10 (or 12) ppm (or d).

  9. Chemical Shift • The usual 13C scale goes from 0 to about 220 ppm. • The zero point is defined as the position of absorption of a standard, tetramethylsilane (TMS): • This standard has only one type of C and only one type of H.

  10. Chemical Shifts

  11. CMR Spectra • Each unique C in a structure gives a single peak in the spectrum; there is rarely any overlap. • The intensity (size) of each peak is NOT directly related to the number of that type of carbon. Other factors contribute to the size of a peak: • Carbon chemical shifts are usually reported as downfield from the carbon signal of tetramethylsilane (TMS).

  12. 13C Chemical Shifts

  13. Predicting 13C Spectra • Problem 1: Predict the number of carbon resonance lines in the 13C spectra of the following (= # unique Cs): 4 lines plane of symmetry

  14. Predicting 13C Spectra • Predict the number of carbon resonance lines in the 13C spectra of the major product of the following reaction: 7 lines 5 lines plane of symmetry

  15. Predicting 13C Spectra

  16. CMR Spectra • Each unique C in a structure gives a single peak in the spectrum; there is rarely any overlap. • The intensity (size) of each peak is NOT directly related to the number of that type of carbon. Other factors contribute to the size of a peak: • Carbon chemical shifts are usually reported as downfield from the carbon signal of tetramethylsilane (TMS).

  17. Uses of 13C NMR Spectroscopy 13C NMR spectroscopy provides information about: • The number of nonequivalent carbons atoms in a molecule • The electronic environment of each carbon • How many protons are bonded to each carbon

  18. Uses of 13C NMR Spectroscopy 13C NMR spectroscopy can verify that E2 elimination of an alkyl halide gives the more substituted alkene (Zaitsev’s rule) • 1-Methylcyclohexene has five sp3-carbon resonances in the 20 to 50 d range and two sp2-carbon resonances in the 100 to 150 d range • Methylenecyclohexene, due to symmetry, has only three sp3-carbon resonance peaks and two sp2-carbon resonance peaks

  19. Uses of 13C NMR Spectroscopy The 13C NMR spectrum of the E2 reaction product from the treatment of 1-chloro-1-methylcyclohexane with a base. The product is clearly identified as 1-methylcyclohexene.

  20. Thank you!!!

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