13C NMR

< 13C has 1.1 % natural abundance

� 13C NMR is not very sensitive!

< 13C - 13C spin-spin splitting is never observed

almost no molecules have $ 2 13C in them:

for example, the probability of hexane having two adjacent 13C is

0.011 x 0.011 x 0.29 = 3.5 x 10 -6

< 13C NMR is a carbon counter

tells you the number of chemically nonequivalent carbons

carbons appear in a * 0 - 200 ppm region

< 13C NMR spectra normally are not integrated

peak intensities can be used to guess the “type” of carbon:

usually, CH3 > CH2 > CH > C

< DEPT 13C NMR spectrum

tells you the number of chemically nonequivalent CH3 , CH2 , and CH carbons

C do not appear–they must be identifed by comparison with the normal 13C NMR spectrum

ways to present a DEPT 13C NMR spectrum:

Q (13C NMR spectrum) + (DEPT-90) + (DEPT-135)

(all carbons) + (only CH carbons) + (CH and CH3 carbons up, CH2 carbons down)

this is the textbook’s method

Q (13C NMR spectrum) + (CH3 subspectrum) + (CH2 subspectrum) + (CH subspectrum) + (all protonated carbons subspectrum)

Q 13C NMR spectrum with CH3 , CH2 , CH , and C resonances labeled

13C NMR methanol

13C NMR ethanol

13C NMR ethyl acetate

BASIC 13C NMR CHEMICAL SHIFTS

C Type Approximate *, ppm

Alkanes

Methyl 0 - 30

Methylene 15 - 55

Methine 25 - 55

Quaternary 30 - 40

Alkenes

C= C 80 - 145

Alkynes

C/C 70 - 90

Arenes 110 - 170

Benzene 128.7

Alcohols and Ethers

C–O 50 - 90

Amines

C–N 40 - 60

Halogens

C–F 70 - 80

C–Cl 25 - 50

C–Br 10 - 40

C–I -20 - 10

Car bonyls

R2C= O 190 - 220

RXC= O (X = O or N) 150 - 180

13C NMR of a tribromobenzene, C6H3Br3

What’s the structural formula?

13C NMR of 1,3,5-tribromobenzene

13C NMR spectrum of a chloropentane

Use of DEPT 13C NMR to identify chloropentane structural isomers

A C5H11Cl

Use of DEPT 13C NMR to identify chloropentane structural isomers CH2 CH2

CH2

CH3

CH2

A C5H11Cl

Use of DEPT 13C NMR to identify chloropentane structural isomers

CH2

CH

CH3

CH2 CH3

B C5H11Cl

Use of DEPT 13C NMR to identify chloropentane structural isomers

CH2

CH2

CH3

CH3

C C5H11Cl

CH

Use of DEPT 13C NMR to identify chloropentane structural isomers

CH3

CH2

D C5H11Cl

CH3

C

Use of DEPT 13C NMR to identify chloropentane structural isomers

CH3

E C5H11Cl

CH2

C