NMR Lecture Notes Part 6

BACKGROUND ON 13C NMR Note: This is part 6/7 of Marc’s NMR lecture notes

12C

is not NMR-active

however…. 13C

CARBON-13 NMR

13C

I= 0

does have spin, I = 1/2 (odd mass)

signals are 6000 times weaker than 1H because: Natural abundance of

13C

is small (1.08% of all C)

PULSED FT-NMR IS REQUIRED (why??) The chemical shift range is larger than for protons 0 - 200 ppm

BACKGROUND ON 13C NMR For a given field strength 13C has its resonance at a different (lower) frequency than 1H.

1H 1.41 T 2.35 T 7.05 T

60 MHz 100 MHz 300 MHz

Divide the hydrogen frequency by ~4 for 13C

13C 1.41 T 2.35 T 7.05 T

15.1 MHz 25.0 MHz 75.0 MHz

MORE BACKGROUND ON 13C NMR Because of low natural abundance there is a low probability of finding two 13C atoms next to each other in a single molecule. 13C

- 13C coupling

NO!

However, 13C does couple to hydrogen atoms (I = 1/2) 13C

- 1H coupling

YES!

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COUPLING TO ATTACHED PROTONS 3 protons

2 protons

H 13

C

1 proton

0 protons

H H

H n+1 = 4

13

C

H

n+1 = 3

13

C

H

n+1 = 2

13

C

n+1 = 1

COUPLING TO ATTACHED PROTONS

Methyl carbon

Methylene carbon

Methine carbon

Quaternary carbon

The effect of attached protons on 13C resonances ( n+1 rule applies ) (J’s are large ~ 100 - 200 Hz)

ETHYL PHENYLACETATE

13C

coupled to the hydrogens

DECOUPLED 13C NMR What could that possibly mean?

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PROTON-DECOUPLED 13C-NMR How? Irradiate the 1H-nuclei while measuring the 13C-nuclei.

RF source 2 “Saturates” the 1H frequencies

1H-13C

13C

Warning! overly-simplified explanation! The hydrogen nuclei are “saturated”: rapid cycling of spin states: +1/2 and -1/2

RF source 1 Pulse tuned to 13C frequencies

signal (FID) measured

When the 13C frequencies are detected, the 13C nuclei see an average coupling (i.e., zero) to the 1H nuclei. 1H nuclei are said to be decoupled from the 13C nuclei Multiplets no longer seen in 13C NMR!

ETHYL PHENYLACETATE Ugly! peaks often overlap!

coupled 13C-NMR

CHEMICAL SHIFTS OF 13C ATOMS

decoupled 13C-NMR

Much easier to interpret

Don’t worry about memorizing, but know the general regions for different types of carbons….

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APPROXIMATE 13C CHEMICAL SHIFT RANGES FOR SELECTED TYPES OF CARBON (ppm (ppm)) R-CH3

8 - 30

C C

65 - 90

R2CH2

15 - 55

C=C

100 - 150

R3CH

20 - 60

C N

110 - 140

C-I

25 - 65

C-Cl

35 - 80

C-N

30 - 65

C-O

40 - 80

150

100

sp3

Saturated carbon no electronegativity effects

O R-C-OR R-C-OH O R-C-NH2 O O R-C-H R-C-R

8 - 30

R-CH2-R

15 - 55

R3CH / R4C

20 - 60 40 - 80 35 - 80

C-Br C

C

C=C

25 - 65

Alkyne carbons - sp

65 - 90 100 - 150

Aromatic ring carbons C=O

185 - 220

RANGE

C-Cl

electronegativity effects

155 - 185 155 - 185

0

C-O

Saturated carbon - sp3

Unsaturated carbon - sp2

O

50 R-CH3

110 - 175

0 - 40

C-Br

200

C=O 200

110 - 175

Acids Amides Esters Anhydrides Aldehydes Ketones 150

100

155 - 185 185 - 220 50

0

Correlation chart for 13C Chemical Shifts (ppm)

1-PROPANOL HO-CH2-CH2-CH3 c

b

a

PROTON DECOUPLED

Sample 13C spectra

200

150

100

50

0

Proton-decoupled 13C spectrum of 1-propanol (22.5 MHz)

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2,2-DIMETHYLBUTANE

CYCLOHEXANOL

BROMOCYCLOHEXANE

CYCLOHEXENE

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CYCLOHEXANONE DEPT 90 and 135: Quick Overview (“distortionless transfer by polarization transfer”)

Normal 13C: shows all C atoms upward! DEPT 90:

shows only CH peaks upward!

DEPT 135: shows CH/CH3 up and CH2 downward!

(note: DEPT eliminates “quaternary” signals which are carbons that lack hydrogens!)

Normal 13C: All carbon upward

Example: ethyl benzene

Example: strychnine

x (TMS)

DEPT 90: only CH upward

Normal 13C: All C upward

DEPT 135: CH and CH3 upward; CH2 downward! x (TMS)

Questions: [1] which peaks are CHs? [2] which peaks are CH2s?

[3] which peaks are CH3s (methyl)? [4] which peaks are quaternaries? (lacking hydrogens)

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Information derived from a 13C NMR spectrum! 1. Each different type of carbon gives a peak!

OVERVIEW OF 13C NMR

2. The chemical shift (in ppm) gives a clue as to the type of carbon generating the peak (alkane, alkene, benzene, carbonyl, etc.) 3. Integrals are not used in 13C NMR. (But sometimes the size of the peak tells how many carbons present) 4. Spin-spin splitting is suppressed by decoupling to give easy-to-interpret spectra. 5. However, DEPT-90 and DEPT-135 can allow assignment of different carbon types.

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