Other Magnetic Nuclei than 1H2H (Deuterium): I = 1; simplifies proton spectrum as H-D coupling is smallX-CH2-CH2-CH2-COY X-CH2-CH2-CD2-COYtriplet, quintet, triplet triplet, slightly broad triplet

31P: I = 1/2 (100% natural abundance)large coupling constants P-H = 200-700 Hz

19F: I = 1/2 (100% natural abundance)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)

13C: I = 1/2 (1.1% Natural abundance)C-H coupling (about 100-200 Hz) is not seen unless enriched with 13C

Fluoroacetone, CH3COCH2F

19F,H coupling (I = ½)

2J 4J

13C-NMR Spectroscopy

12C not magnetically active but13C has I = ½; its natural abundance is 1.1%;

The sensitivity of 13C is only 1/5700 of 1H; this sensitivity problem is overcome with Fourier Transform (FT) NMR instrumentation (1970’s);

Usually, the peak splitting due to couplings with protons are removed by broadband decoupling in a double resonance experiment;broadband decoupling can also enhance the 13C signal intensity caused by the Nuclear Overhauser Effect (NOE)

13C chemical shifts are reported relative to TMS;

300 MHz for 1H-NMR = 75.5 MHz for 13C-NMR;

10 mg in 0.5 mL of solvent in 5 mm tube

Double Resonance: Spin-Spin Decoupling

CH2

CH2

OHH3C

triplet - sextet - triplet

irradiate

CH2

CH2

OHH3C

triplet - quartet

CH2

CH2

OHH3C

irradiateCH2

CH2

OHH3C

singlet - singlet

CH2

CH2

OHH3C

irradiateCH2

CH2

OHH3C

triplet - triplet

Protons can be readily decoupled if they are about 100 Hz apart

13C-NMR of diethylphthalate

proton coupled

13C{1H} NMR of diethylphthalate

proton decoupled

why low intensity?

13C{1H}-NMR of diethylphthalate

Proton decoupled10-s delay

Peak Intensities in 13C-NMR

• the relaxation times in 13C-NMR vary over a wide range so peak areas do not integrate for the correct number of nuclei;

• long delays would work but the time required is prohibitive;

• NOE response is not uniform for all C atom environments;

• C atoms without protons attached give low intensity;

• substitution of D for H results in decreased intensity;

• deuterium has I = 1 so 13C is split into 3 lines ratio 1:1:1 when coupled to one deuterium (possible spin states for D are -1, 0, +1)

• thus CDCl3 exhibits a 1:1:1 triplet in 13C-NMR

Chemical Shifts in 13C-NMR

Cl

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

1 2

4 4

3 6

4 6

Number of different aromatic 13C resonances in substituted benzene molecules

Cl Cl

Br Cl

Cl

BrCl Cl

Br Cl

Cl

Cl

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

13C shifts of functional groups

Well defined for acyclic, saturated hydrocarbons

= -2.5 + ∑Aini

Methane = -2.1replacement of H by C (CH3, CH2, CH, C) causes a +9.1 shift in the -position, +9.4 in the -position, and -2.5 in the -position.

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

Calculation of 13C shifts

Calculation of 13C shifts

Well defined for acyclic, saturated hydrocarbons

= -2.5 + ∑Aini

t-butyl alcohol

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