CARBON-13 NMR

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 ppm

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

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

COUPLING TO ATTACHED PROTONS

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 ~ Hz)

ETHYL PHENYLACETATE 13 C coupled to the hydrogens

DECOUPLED SPECTRA

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 C signal (FID) measured “the decoupler”

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.

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

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.

CHEMICAL SHIFTS OF 13 C ATOMS

R-CH R 2 CH R 3 CH C-I C-Br C-N C-Cl C-O C C C=C C N R-C-OR O R-C-OH O R-C-NH 2 O R-C-H O R-C-R O APPROXIMATE 13 C CHEMICAL SHIFT RANGES FOR SELECTED TYPES OF CARBON (ppm) SELECTED TYPES OF CARBON (ppm)

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 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 /

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

SPECTRA

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

2,2-DIMETHYLBUTANE

BROMOCYCLOHEXANE

CYCLOHEXANOL

TOLUENE

CYCLOHEXENE

CYCLOHEXANONE

a a b b c c 1,2-DICHLOROBENZENE

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