NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY… or NMR for short

13C – nmr 13C ~ 1.1% of carbons

1) number of signals: how many different types of carbons

2) splitting: number of hydrogens on the carbon

3) chemical shift: hybridization of carbon sp, sp2, sp3

4) chemical shift: environment

Nuclear Magnetic Resonance (nmr)

-the nuclei of some atoms spin: 1H, 13C, 19F, …

-the nuclei of many atoms do not spin: 2H, 12C, 16O, …

-moving charged particles generate a magnetic field ()

-when placed between the poles of a powerful magnet, spinning nuclei will align with or against the applied field creating an energy difference. Using a fixed radio frequency, the magnetic field is changed until the ΔE = EEM. When the energies match, the nuclei can change spin states (resonate) and give off a magnetic signal.

ΔE

S

A spinning nucleus with it's magnetic field aligned with the magnetic field of a magnet

- spin state,favorable,lower energy

N

S

N

N

S - spin state,unfavorable,higher energy

A spinning nucleus with it's magnetic field aligned against the magnetic field of a magnet

S

N

magnetic field = 14,092 gauss

for 1H v = 60,000,000 Hz (60 MHz)

nmr spectrum

inte

nsit

y

10 9 8 7 6 5 4 3 2 1 0

chemical shift (ppm)

magnetic field

THEORY OF NMR The small energy difference between the two

alignments of magnetic spin corresponds to the energy of radio waves according to Einstein’s equation E=h.

Application of just the right radiofrequency (causes the nucleus to “flip” to the higher energy spin state

Not all nuclei require the same amount of energy for the quantized spin ‘flip’ to take place.

The exact amount of energy required depends on the chemical identity (H, C, or other element) and the chemical environment of the particular nucleus.

h

THEORY OF NMR The electron density surrounding a given nucleus

depends on the electronegativity of the attached atoms.

The more electronegative the attached atoms, the less the electron density around the nucleus in question.

We say that that nucleus is less shielded, or is deshielded by the electronegative atoms.

Deshielding effects are generally additive. That is, two highly electronegative atoms (2 Cl atoms, for example) would cause more deshielding than only 1 Cl atom.

C

H

HH

H

C

H

ClH

H

C

H

ClH

Cl

C and H are deshielded C and H are more deshielded

CHEMICAL SHIFT We call the relative position of absorption in

the NMR spectrum (which is related to the amount of deshielding) the chemical shift. It is a unitless number (actually a ratio, in which the units cancel), but we assign ‘units’ of ppm or (Greek letter delta) units.

For 1H, the usual scale of NMR spectra is 0 to 10 (or 12) ppm (or ).

The usual 13C scale goes from 0 to about 220 ppm.

The zero point is defined as the position of absorption of a standard, tetramethylsilane (TMS):

This standard has only one type of C and only one type of H.

Si

CH3

CH3

CH3

CH3

CMR SPECTRA Each unique C in a structure gives a single

peak in the spectrum; there is rarely any overlap. The carbon spectrum spans over 200 ppm; chemical

shifts only 0.001 ppm apart can be distinguished; this allows for over 2x105 possible chemical shifts for carbon.

The intensity (size) of each peak is NOT directly related to the number of that type of carbon. Other factors contribute to the size of a peak: Peaks from carbon atoms that have attached

hydrogen atoms are bigger than those that don’t have hydrogens attached.

Carbon chemical shifts are usually reported as downfield from the carbon signal of tetramethylsilane (TMS).

13C CHEMICAL SHIFTS

downfield upfield

20406080100120140160180200220 0

CH3

CH2

CH

C X (halogen)

C N

C O

C C

C N

C CC O

13C Chemical shift ()

TMSAromatic C

PREDICTING 13C SPECTRA Problem 13.6 Predict the number of carbon

resonance lines in the 13C spectra of the following (= # unique Cs):

4 lines

plane of symmetry

CH3

C C

CC

C

CH3CH3

O

CH3

CH3

O

CH3

CH3

CC

cCH3

O

CH3 5 lines

5 lines CH3

C

CH3

CCH3

H

PREDICTING 13C SPECTRA

Predicte the number of carbon resonance lines in the 13C spectra of the major product of the following reaction:

7 lines

5 lines

plane of symmetry

CH3

C

cc

C

CC

CH3 CH2

C

Cc

C

CC

CH2CH2

CH3 CH2ClCH3

orKOH

ethanol, heat ???

PREDICTING 13C SPECTRA

Predicte the number of carbon resonance lines in the 13C spectra of the major product of the following reaction:

7 lines

5 lines

plane of symmetry

CH3

C

cc

C

CC

CH3 CH2

C

Cc

C

CC

CH2CH2

CH3 CH2ClCH3

orKOH

ethanol, heat ???

CH3

CDCl3 (solvent)

CH3CCH3

O

C

O

OCH3

CDCl3 (solvent)

CH3

CH3COCH3

O

C

O

CDCl3 (solvent)

CH3

CH3CH3COCH2CH3

O

C

O

OCH2

CH2

CH3CH3

CDCl3 (solvent)

CH3

CH3CH2COCH2CH3

O

C

O

OCH2

C

O

CH2CH2 CH3

CH3CCH2CH3

O

CH3

CDCl3 (solvent)

CH2 Br

CDCl3 (solvent)

CH3

CH3CH2CH2Br

CH2

CDCl3 (solvent)

CH2 OH

CH3

CH3CH2CH2CH2OH

CH2

CH2

CDCl3 (solvent)

CH2

CH3

CH3CH2CH2CH2CH2OHCH2

CH2

CH2 OH

2-bromobutane

a c d b CH3CH2CHCH3

Br

13C-nmr