Introduction to C-13 NMR • The 13 C nucleus is present in only 1.08% natural abundance. Therefore, acquisition of a spectrum usually takes much longer than in 1 H NMR.

• The magnetogyric ratio of the 13 C nucleus is about 1/4 that of the 1 H nucleus.

Therefore, the resonance frequency in 13 C NMR is much lower than in 1 H NMR. (75 MHz for 13 C as opposed to 300 MHz for 1 H in a 7.04 Tesla field).

• At these lower frequencies, the excess population of nuclei in the lower spin state is reduced, which, in turn, reduces the sensitivity of NMR detection.

• Unlike 1 H NMR, the area of a peak is not proportional to the number of carbons giving rise to the signal. Therefore, integrations are usually not done.

• Each unique carbon in a molecule gives rise to a 13 C NMR signal. Therefore, if there are fewer signals in the spectrum than carbon atoms in the compound, the molecule must possess symmetry.

• When running a spectrum, the protons are usually decoupled from their respective carbons to give a singlet for each carbon atom. This is called a proton-decoupled spectrum.

Carbon-13 Chemical Shift Table C  C triple bonds http://www.chemistry.ccsu.edu/glagovich/teaching/316/nmr/images/fig15.gif

Alkane: 2-methylpentane

Alcohol: 2-hexanol OH

Alkyl Halide: 3-bromopentane Br

Alkene: 1-hexene

O HO Aromatic Ring: eugenol

Carboxylic Acid: pentanoic acid 2 H

Ester: ethyl valerate O O

Amide: pentanamide O NH 2

Ketone: 3-methyl-2-pentanone O

Aldehyde: 2-methylpentanal O H

Symmetry in C-13 NMR Each unique carbon in a molecule gives rise to a 13 C NMR signal. Therefore, if there are fewer signals in the spectrum than carbon atoms in the compound, the molecule must possess symmetry. Examples: CH 3 CH 2 CH 2 CH 3 OH

Enantiotopic vs Diastereotopic CH 3 ’s enantiotopic O O CH 3 diastereotopic OH * * * * *

Determine the number of signals in the proton-decoupled C-13 NMR spectrum of each of the following compounds: H 3 C CH 3 O CH 3 HO N H O OCCH 3 OCH 3 OH CH 3 CH 3 OH H 3 C CH 3

Carbon-13 NMR Spectrum of Geraniol

ppm

139.07 131.62 124.07 123.71 59.16 39.64 26.51 25.66 17.66 16.24

Carbon #

1 2 3 4 5 6 7 8 9 10 9 8

T

1 and NOE Effects in C-13 NMR Because of unequal

T

1 and NOE effects, peaks heights vary widely in C-13 NMR.

This is why C-13 spectra are normally not integrated.

CH 3 1 2 3 4 Carbon CH 3 1 2 3 4

T

1 (sec) 16 89 24 24 17 NOE 0.61

0.56

1.6

1.7

1.6

1 2 3 4 CH 3

Carbon-13 Proton-Coupled Patterns http://www.chemistry.ccsu.edu/glagovich/teaching/316/nmr/13ccoupled.html

Carbon-13 Proton-Coupled Spectrum of Ethyl Phenylacetate Difficult to interpret O O C=O Typical coupling constants for 13 C 1 H one bond couplings are between 100 to 250 Hz. http://www.chemistry.ccsu.edu/glagovich/teaching/316/nmr/13ccoupled.html

DEPT Spectra DEPT-135 DEPT-90 DEPT-45 C normal C-13 spectrum CH CH 2 CH 3 Quaternary carbons (C) do not show up in DEPT.

Simulated DEPT Spectra of Ethyl Phenylacetate O O DEPT-135 DEPT-90 O O DEPT-45 Normal C-13 spectrum

DEPT Spectra of Codeine

Predict the normal C-13, DEPT-90, and DEPT-135 spectra of ipsenol, whose structure appears below.

DEPT Spectra of Ipsenol DEPT-90 DEPT-135 CDCl 3 Normal C-13 spectrum www.lasalle.edu/~price/

DEPT

%20and%20COSY%20

Spectra

.ppt

Determine the number and appearance of the signals in the DEPT-45, DEPT 90, and DEPT 135 NMR spectrum of each of the following compounds: H 3 C CH 3 O CH 3 HO N H O OCCH 3 OCH 3 OH CH 3 CH 3 OH H 3 C CH 3