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13.18 Carbon13.18 Carbon--13 NMR13 NMR 12C is not NMR-active I = 0 however…. 13C does have spin, I = 1/2 (odd mass) 1. Natural abundance of 13C is small (1.08% of all C) 2. Magnetic moment of 13C is small 13C signals are 6000 times weaker than 1H because: SALIENT FACTS ABOUT SALIENT FACTS ABOUT 1313C NMRC NMR PULSED FT-NMR IS REQUIRED The chemical shift range is larger than for protons 0 - 220 ppm COUPLING TO ATTACHED PROTONS The effect of attached protons on 13C resonances n+1 = 4 n+1 = 3 n+1 = 2 n+1 = 1 C 13 3 protons 2 protons 1 proton 0 protons H H H C 13 H H C 13 H C 13 Methyl carbon Methylene carbon Methine carbon Quaternary carbon ( n+1 rule applies ) COUPLING TO ATTACHED PROTONSCOUPLING TO ATTACHED PROTONS (J’s are large ~ 100 - 200 Hz) ETHYL PHENYLACETATEETHYL PHENYLACETATE 13C coupled to the hydrogens DECOUPLED SPECTRA ETHYL PHENYLACETATEETHYL PHENYLACETATE 13C coupled to the protons 13C decoupled from the protons in some cases the peaks of the multiplets will overlap this is an easier spectrum to interpret CHEMICAL SHIFTS OF 13C ATOMS Aldehydes Ketones Acids Amides Esters Anhydrides Aromatic ring carbons Unsaturated carbon - sp2 Alkyne carbons - sp Saturated carbon - sp3 electronegativity effects Saturated carbon - sp3 no electronegativity effects C=O C=O C=C C C 200 150 100 50 0 200 150 100 50 0 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 13C Chemical Shifts (ppm) C-O C-Cl C-Br R3CH R4C R-CH2-R R-CH3 RANGE / SPECTRA WWU Chemistry Toluene CH3 Proton-decoupled 13C spectrum of 1-propanol (22.5 MHz) 200 150 100 50 0 11--PropanolPropanol PROTON DECOUPLED HO-CH2-CH2-CH3 c b a CyclohexanolCyclohexanol CyclohexanoneCyclohexanone Cl Cl a a b b c c 1,21,2--DichlorobenzeneDichlorobenzene 1,31,3--DichlolrobenzeneDichlolrobenzene Cl Cl 1,4-Dichlorobenzene Cl Cl
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