1. CARBON-13 NMR

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

3. SALIENT FACTS ABOUT 13 C NMR For a given field strength 13C has its resonance at a different (lower) frequency than 1 H. 1H1H 13 C 1.41 T 60 MHz 2.35 T 100 MHz 7.05 T 300 MHz 1.41 T 15.1 MHz 2.35 T 25.0 MHz 7.05 T 75.0 MHz Divide the hydrogen frequency by 4 (approximately) for carbon-13

4. Because of its low natural abundance (0.0108) there is a low probability of finding two 13 C atoms next to each other in a single molecule. However, 13 C does couple to hydrogen atoms (I = 1/2 ) 13 C - 13 CcouplingNO! 13 C - 1 HcouplingYES! Spectra are determined by many molecules contributing to the spectrum, each having only one 13 C atom. SALIENT FACTS ABOUT 13 C NMR (cont) not probable very common

5. COUPLING TO ATTACHED PROTONS

6. The effect of attached protons on 13 C resonances n+1 = 4n+1 = 3n+1 = 2n+1 = 1 C 13 3 protons2 protons1 proton0 protons H H H C 13 H H C H C Methyl carbon Methylene carbon Methine carbon Quaternary carbon ( n+1 rule applies ) COUPLING TO ATTACHED PROTONS (J’s are large ~ 100 - 200 Hz)

7. ETHYL PHENYLACETATE 13 C coupled to the hydrogens

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 13 C frequency. 1 H- 13 C RF source 2 RF source 1 continuously saturates hydrogens pulse tuned to carbon-13 13 C signal (FID) measured “the decoupler”

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 (+ 1/2 and - 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 13 C resonances. Each carbon gives a singlet, and the spectrum is easier to interpret.

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

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

13. CHEMICAL SHIFTS OF 13 C ATOMS

14. R-CH 3 8 - 30 R 2 CH 2 15 - 55 R 3 CH20 - 60 C-I 0 - 40 C-Br25 - 65 C-N30 - 65 C-Cl35 - 80 C-O40 - 80 C C 65 - 90 C=C 100 - 150 C N 110 - 140 110 - 175 R-C-OR O R-C-OH O 155 - 185 R-C-NH 2 O 155 - 185 R-C-H O R-C-R O 185 - 220 APPROXIMATE 13 C CHEMICAL SHIFT RANGES FOR SELECTED TYPES OF CARBON (ppm) SELECTED TYPES OF CARBON (ppm)

15. Aldehydes Ketones Acids Amides Esters Anhydrides Aromatic ring carbons Unsaturated carbon - sp 2 Alkyne carbons - sp Saturated carbon - sp 3 electronegativity effects Saturated carbon - sp 3 no electronegativity effects C=O C=C C 200150100500 200150100500 8 - 30 15 - 55 20 - 60 40 - 80 35 - 80 25 - 65 65 - 90 100 - 150 110 - 175 155 - 185 185 - 220 Correlation chart for 13 C Chemical Shifts (ppm) C-O C-Cl C-Br R 3 CHR4CR4C R-CH 2 -R R-CH 3 RANGE /

16. nitriles acid anhydrides acid chlorides amides esters carboxylic acids aldehydes ,  -unsaturated ketones ketones 220200180160140120100ppm 13 C Correlation Chart for Carbonyl and Nitrile Functional Groups

17. SPECTRA

18. Proton-decoupled 13C spectrum of 1-propanol (22.5 MHz) 200150100500 1-PROPANOL PROTON DECOUPLED HO-CH 2 -CH 2 -CH 3 cba

19. 2,2-DIMETHYLBUTANE

20. BROMOCYCLOHEXANE

21. CYCLOHEXANOL

22. TOLUENE

23. CYCLOHEXENE

24. CYCLOHEXANONE

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

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