1

Magnetic Nuclei other than 1H2H (Deuterium): I = 1H,D-Exchange might be used to simplify 1H-NMR spectra since H-D couplings are generally small;

-CH2 - CH2 - CH2 - CO - -CH2 - CH2 - CD2 - CO -triplet of triplets slightly broadened triplet

31P: I = 1/2 (100% natural abundance)Chemical shift range between 270 to -480 ppm;Large P-H (1J) coupling constants between 200-700 Hz;

19F: I = 1/2 (100% natural abundance)Chemical shift range between 276 to -280 ppm;coupling constants F-CH = 50-100 Hz;

29Si: I = 1/2 (100% Natural abundance)Si-CH coupling constant is about 6 Hz; only low intensity (satellites)(Si-H is about 215 Hz)13C: I = 1/2 (1.1% Natural abundance)C-H coupling (about 100-200 Hz) is not resolved unless the molecule is enriched with 13C

2

3

Fluoroacetone, CH3COCH2F19F,H coupling (I = ½)

2J 4J

4

13C-NMR Spectroscopy

Some Facts:• 12C is not magnetically active but 13C is (I = ½); its natural

abundance is 1.1%;• The low abundance of 13C causes a sensitivity problem (only

1/5700 of 1H), which has been overcome with the development of Fourier Transform (FT) NMR instrumentation in the 1970’s; However, higher concentrations are usually used for solution NMR (10 mg in 0.5 mL of solvent for a 5 mm tube);

• 13C chemical shifts are reported relative to TMS;• 300 MHz for 1H-NMR equals 75.5 MHz for 13C-NMR;• Peak splittings due to couplings with protons are usually

removed by broadband decoupling in a double resonanceexperiment;

• Broadband decoupling can also enhance the 13C signal intensity caused by the Nuclear Overhauser Effect (NOE);

• The range of chemical shift values is much wider than for 1H (typically between 0-200 ppm); Therefore, chemical shift is of high analytical value in 13C-NMR;

5

Peak Intensities in 13C-NMR

• Relaxation times in 13C-NMR vary over a wide range so that peak areas do not integrate for the correct number of nuclei; Long delays between each acquisition would resolve this problem but the required measurement time is prohibitive;

• NOE response is not uniform for all C atom environments; Carbon atoms without protons attached to them have low intensities because of the missing NOE and for other reasons;

• Substitution of H by D results in decreased intensity of the 13C signal;

• Deuterium has I = 1 so that a 13C signal is split into 3 lines ratio 1:1:1 when coupled to one deuterium (possible spin states of D are -1, 0, +1; CDCl3 exhibits a 1:1:1 triplet in 13C-NMR!)

6

Double Resonance: Spin-Spin Decoupling

CH2

CH2

OHH3C

triplet - sextet - triplet

irradiate

CH2

CH2

OHH3C

triplet - quartet

CH2

CH2

OHH3Cirradiate

CH2

CH2

OHH3C

singlet - singlet

CH2

CH2

OHH3Cirradiate

CH2

CH2

OHH3C

triplet - triplet

Protons can be readily decoupled individually if their deferens in resonance frequency (chemical shift) is ≥100 Hz;

7

13C-NMR of diethylphthalate

proton coupled

why triplet?

q of t(q not resolved)

t of q(t not resolved)

8

13C{1H} NMR of diethylphthalate

proton decoupled

why low intensity?

9

13C{1H}-NMR of diethylphthalate

proton decoupledwith 10 seconds delay

10

Chemical Shifts in 13C-NMR

Cl

Carbon chemical shifts parallel (generally) proton shifts but with a much broader range

64

63

44

21

Number of different aromatic 13C resonances in substituted benzene molecules

Cl Cl

Br Cl

Cl

BrCl Cl

Br Cl

Cl

Cl

11

Diamagnetic shielding (electrons in s- and p-orbitals) and paramagnetic shielding (electrons in p-orbitals with angular momentum) contribute to the shift of C-atoms.

Group Specific Chemical Shifts in 13C-NMR

12

Well defined for acyclic, saturated hydrocarbons

δ = -2.5 + ∑Aini

δMethane = -2.5 ppmreplacement of H by C (CH3, CH2, CH, C) causes a shift of +9.1 ppm in the α-position, +9.4 ppm in the β-position, and -2.5 ppm in the γ-position;

Replacement of hydrogen causes a relative constant shift that depends primarily on the electronegativity of X.

Calculation of 13C shifts

corrections for branching

13

XAll tables from “Silverstein & Webster”

X

αβ

γ

αβ

γ

Functional group X attached to internal carbon

Functional group X attached to terminal carbon

14

15

t-butyl alcohol

16

2,2,4-trimethyl-1,3-pentanediol

17

CH3

CH3H3C

The equation for alkanes is based on a weighted average for open chain conformers. A complete new set of parameters is necessary for cyclic alkanes as the average conformation is different.

Cyclopropane = -2.6, cyclobutane = 23.3, and cyclohexane = 27.7;all other rings have 27.7 ± 2 ppm

β-ax = 5.2

β-eq = 8.9

β-gem = -1.2; extra correction factor for branching at β-C (2 methyl groups);

27.7 + 5.2 + (2 x 8.9) + -1.2 = 49.5 (obs. 49.9)

Cycloalkanes

18

AlkenesChemical shift ranges of alkenecarbon atoms strongly depend on their degree of substitutionC=CH2 = 104-115 ppm; C=CHR = 120-140 ppm;C=CR2 = 140-165 ppm;

α,β,γ represent substituents on the same end of the double bond while α , β’, γ’ are on the far side.

Example:

19

Starting with benzene = 128.7

20

Comprehensive Table of aromatic 13C-NMR chemical shift increments

21

OH

Cl Cl

δ = 16 + 31 – 4 = 43 (obs. 42)

δ = 27 + 8 = 35 (obs. 34)

O

128.4149.8

Starting with ethene = 123.3

NO2

Starting with benzene = 128.7 or 128.5 (depending on table)

148.3123.4

129.5

134.7

Example Calculations (Predictions)

δ1 = -2.5+9.1+9.4+(-2.5x2)+0.3 = 11.3[1α,1β,2γ,1δ]δ2 = -2.5+(9.1x2)+(9.4x2)+(-2.5)+(-2.5) = 29.5[2α,2β,1γ,2º(3º)]δ3 = -2.5+(9.1x3)+(9.4x2)+(-3.7x2) = 36.2[3α,2β,3º(2º)]

1 2 3

4

-M effect

O153.2

84.2+M effect

22

CH, CH3

CH2

CH

Distortionless Enhancement by Polarization Transfer (DEPT)

α-TerpineneConventional 13C-NMR