introduction to c-13 nmr

• The 13C nucleus is present in only 1.08% natural abundance. Therefore, acquisition of a spectrum usually takes much longer than in 1H NMR. • The magnetogyric ratio of the 13C nucleus is about 1/4 that of the 1H nucleus. Therefore, the resonance frequency in 13C NMR is much lower than in 1H NMR. (75 MHz for 13C as opposed to 300 MHz for 1H 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 1H 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 13C 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.

Introduction to C-13 NMR

http://www.chemistry.ccsu.edu/glagovich/teaching/316/nmr/images/fig15.gif

Carbon-13 Chemical Shift Table

CC triple bonds

Alkane: 2-methylpentane

Alcohol: 2-hexanol

OH

Br

Alkyl Halide: 3-bromopentane

Alkene: 1-hexene

Aromatic Ring: eugenol

HO

O

Carboxylic Acid: pentanoic acid

CO2H

Ester: ethyl valerate

O

O

Amide: pentanamide

O

NH2

Ketone: 3-methyl-2-pentanone

O

Aldehyde: 2-methylpentanal

O

H

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

CH2CH3CH3CH2 OH

Enantiotopic vs Diastereotopic CH3’s

*

O

O

CH3

enantiotopic

*

*

* *

diastereotopic

OH

Determine the number of signals in the proton-decoupledC-13 NMR spectrum of each of the following compounds:

H3C

CH3

O

CH3

OH

CH3

NH

OCH3OCCH3

O

HO

OH

H3C CH3

CH3

ppm Carbon #139.07 1 131.62 2 124.07 3 123.71 4 59.16 5 39.64 6 26.51 7 25.66 8 17.66 9 16.24 10

8

9

Carbon-13 NMR Spectrum of Geraniol

T1 and NOE Effects in C-13 NMRBecause of unequal T1 and NOE effects, peaks heights vary widely in C-13 NMR.This is why C-13 spectra are normally not integrated.

Carbon T1 (sec) NOE

CH3 16 0.61

1 89 0.56

2 24 1.6

3 24 1.7

4 17 1.6

CH31

2

34

1

2

3

4

CH3

Carbon-13 Proton-Coupled Patterns

http://www.chemistry.ccsu.edu/glagovich/teaching/316/nmr/13ccoupled.html

Carbon-13 Proton-Coupled Spectrum of Ethyl Phenylacetate

Typical coupling constants for 13C-1H one-bond couplings are between 100 to 250 Hz.

http://www.chemistry.ccsu.edu/glagovich/teaching/316/nmr/13ccoupled.html

O

O

Difficult to interpret

C=O

DEPT Spectra

normal C-13 spectrum

DEPT-45

DEPT-90

DEPT-135

C

CH CH2 CH3

Quaternary carbons (C) do not show up in DEPT.

O

O

Simulated DEPT Spectra of Ethyl Phenylacetate

Normal C-13 spectrum

DEPT-45

DEPT-90

DEPT-135O

O

DEPT Spectra of Codeine

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

www.lasalle.edu/~price/DEPT%20and%20COSY%20Spectra.ppt

DEPT Spectra of Ipsenol

Normal C-13 spectrum

CDCl3

DEPT-135

DEPT-90

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:

H3C

CH3

O

CH3

OH

CH3

NH

OCH3OCCH3

O

HO

OH

H3C CH3

CH3