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CARBON-13 NMR

CARBON-13 NMR. SALIENT FACTS ABOUT 13 C NMR. 12 C is not NMR-active I = 0. however…. 13 C does have spin, I = 1/2 (odd mass). 13 C signals are 6000 times weaker than 1 H because:. 1. Natural abundance of 13 C is small (1.08% of all C). 2. Magnetic moment of 13 C is small.

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CARBON-13 NMR

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  1. CARBON-13 NMR

  2. SALIENT FACTS ABOUT 13C NMR 12C is not NMR-active I = 0 however…. 13C does have spin, I = 1/2 (odd mass) 13C signals are 6000 times weaker than 1H because: 1. Natural abundance of 13C is small (1.08% of all C) 2. Magnetic moment of 13C is small PULSED FT-NMR IS REQUIRED The chemical shift range is larger than for protons 0 - 200 ppm

  3. SALIENT FACTS ABOUT 13C NMR For a given field strength 13C has its resonance at a different (lower) frequency than 1H. Divide the hydrogen frequency by 4 (approximately) 1H for carbon-13 1.41 T 60 MHz 2.35 T 100 MHz 13C 7.05 T 300 MHz 1.41 T 15.1 MHz 2.35 T 25.0 MHz 7.05 T 75.0 MHz

  4. SALIENT FACTS ABOUT 13C NMR (cont) Because of its low natural abundance (0.0108) there is a low probability of finding two 13C atoms next to each other in a single molecule. not probable 13C - 13C coupling NO! Spectra are determined by many molecules contributing to the spectrum, each having only one 13C atom. However, 13C does couple to hydrogen atoms (I = 1/2) very common 13C - 1H coupling YES!

  5. COUPLING TO ATTACHED PROTONS

  6. 13 13 13 13 C C C C COUPLING TO ATTACHED PROTONS 3 protons 2 protons 1 proton 0 protons H H H H H H n+1 = 4 n+1 = 3 n+1 = 2 n+1 = 1 Methyl carbon Methylene carbon Methine carbon Quaternary carbon The effect of attached protons on 13C resonances ( n+1 rule applies ) (J’s are large ~ 100 - 200 Hz)

  7. 13C coupled to the hydrogens ETHYL PHENYLACETATE

  8. DECOUPLED SPECTRA

  9. DECOUPLING THE PROTON SPINS PROTON-DECOUPLED SPECTRA A common method used in determining a carbon-13 NMR spectrum is to irradiate all of the hydrogen nuclei in the molecule at the same time the carbon resonances are being measured. This requires a second radiofrequency (RF) source (the decoupler) tuned to the frequency of the hydrogen nuclei, while the primary RF source is tuned to the 13C frequency. RF source 1 RF source 2 1H-13C pulse tuned to carbon-13 “the decoupler” continuously saturates hydrogens 13C signal (FID) measured

  10. In this method the hydrogen nuclei are “saturated”, a situation where there are as many downward as there are upward transitions, all occuring rapidly. During the time the carbon-13 spectrum is being determined, the hydrogen nuclei cycle rapidly between their two spin states (+1/2and -1/2) and the carbon nuclei see an average coupling (i.e., zero) to the hydrogens. The hydrogens are said to be decoupled from the carbon-13 nuclei. You no longer see multiplets for the 13C resonances. Each carbon gives a singlet, and the spectrum is easier to interpret.

  11. 13C coupled to the hydrogens 13C decoupled from the hydrogens ETHYL PHENYLACETATE in some cases the peaks of the multiplets will overlap this is an easier spectrum to interpret

  12. d d q s s t t d SOME INSTRUMENTS SHOW THE MULTIPLICITIES OF THE PEAKS ON THE DECOUPLED SPECTRA s = singlet t = triplet d = doublet q = quartet CODE : This method gives the best of both worlds.

  13. CHEMICAL SHIFTS OF 13C ATOMS

  14. APPROXIMATE 13C CHEMICAL SHIFT RANGES FOR SELECTED TYPES OF CARBON (ppm) R-CH3 8 - 30 C C 65 - 90 R2CH2 15 - 55 C=C 100 - 150 R3CH 20 - 60 C N 110 - 140 110 - 175 C-I 0 - 40 C-Br 25 - 65 O O C-Cl 35 - 80 R-C-OR R-C-OH 155 - 185 O C-N 30 - 65 R-C-NH2 155 - 185 O O C-O 40 - 80 R-C-H R-C-R 185 - 220

  15. 200 150 100 50 0 200 150 100 50 0 RANGE R-CH3 8 - 30 R-CH2-R Saturated carbon - sp3 15 - 55 no electronegativity effects 20 - 60 R3CH R4C / 40 - 80 C-O Saturated carbon - sp3 35 - 80 C-Cl electronegativity effects C-Br 25 - 65 Alkyne carbons - sp 65 - 90 C C Unsaturated carbon - sp2 C=C 100 - 150 Aromatic ring carbons 110 - 175 Acids Amides Esters Anhydrides 155 - 185 C=O Aldehydes Ketones 185 - 220 C=O Correlation chart for 13C Chemical Shifts (ppm)

  16. nitriles acid anhydrides acid chlorides amides esters carboxylic acids aldehydes a,b-unsaturated ketones ketones 220 200 180 160 140 120 100 ppm 13C Correlation Chart for Carbonyl and Nitrile Functional Groups

  17. SPECTRA

  18. 1-PROPANOL HO-CH2-CH2-CH3 c b a PROTON DECOUPLED 200 150 100 50 0 Proton-decoupled 13C spectrum of 1-propanol (22.5 MHz)

  19. 2,2-DIMETHYLBUTANE

  20. BROMOCYCLOHEXANE

  21. CYCLOHEXANOL

  22. TOLUENE

  23. CYCLOHEXENE

  24. CYCLOHEXANONE

  25. b c a a c b 1,2-DICHLOROBENZENE

  26. 1,3-DICHLOROBENZENE solvent d b a c d a

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