13.18 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

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

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

CHEMICAL SHIFTS OF 13 C ATOMS

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 /

Some useful information! sp 3 carbons are upfield (shielded), 15 to 75 When the carbon has electronegative groups, appear further downfield, 50 to 75 sp 2 carbons appear down field, 120 to 170 C=O carbons appear furthest to the left, 170 to 220ppm

SPECTRA

WWU Chemistry Toluene

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

Cyclohexanol

Cyclohexanone

a a b b c c 1,2-Dichlorobenzene

1,3-Dichlolrobenzene

1,4-Dichlorobenzene

Some Unknowns! The next three slides are carbon-13 spectra for: Cyclohexanol Cyclohexanone 2-Methylcyclopentanone Assign them! Hit the enter key to see the answer.

2-methylcyclopentanone

cyclohexanone

cyclohexanol

The next slide shows some structures. Predict the number of carbon peaks you would observe in the 13 C NMR. Hit the enter key for answers.

6 peaks 8 peaks 4 peaks 3 peaks 7 peaks 10 peaks

The following four slides show carbon spectra for four isomers. They all have the formula C 7 H 14 O. They all show strong peaks at 1715 cm -1 in the infrared spectrum. Determine the structures for each. Possible answers: 4-heptanone 2-heptanone 2,4-dimethyl-3-pentanone 5-methyl-2-hexanone

C 7 H 14 O Compound 1

C 7 H 14 O Compound 2

C 7 H 14 O Compound 3

C 7 H 14 O Compound 4

Answers to the previous four spectral questions: 1) 2,4-dimethyl-3-pentanone 2) 4-heptanone 3) 2-heptanone 4) 5-methyl-2-pentanone