NMRNuclearMagneticResonance

Chapter 13

Proton Nuclear Spin States

Two states have the same energy in the absence of a magnetic field

MagneticField

Protons in an external magnetic field

Energy difference between the spin-states depends on the strength of the magnetic field.

NMR Spectrometer

Superconducting NMR Magnet

NMR Sample Tubes

NMR Probe

Sample goes in here

CoilsInsulated tube

NMRSignal

Proton NMR Spectrum of Cl3CH

Magnetic Field

H

Ho

Field here less than Ho

Electrons shield nucleus from external magnetic field.

HC

Magnetic field

E = H

Energy to change spin state proportional to field strength

E =h

Measure field strength in FREQUENCY units

Cycles/second

Tetramethylsilane

TMSReference compound for NMR spectrum.

NMRsignal

Magnetic Field

Cl3CH

TMS

437 Hzat 60x106 Hz

Chemical Shift =Position of Signal - Position of TMS

Spectrometer Frequencyx106

Units = parts per million = ppm

Chemical Shift Scale

=437 Hz - 0 Hz

60x106Hzx 106 = 7.28 ppm

Typical Chemical Shifts

CR H

CO

C H

CCl H

CBr H

CO H

0.9 - 1.8

2.1 - 2.5

3.1 - 4.1

2.7 - 4.1

3.3 - 3.7

C H

CCH

H

CO

H

HOR

Typical Chemical Shifts

2.3 - 2.8

4.5 - 6.5

6.5 - 8.5

9 - 10

0.5 - 5

10 9 8 7 6 5 4 3 2 1 0

TMSRHC-C-HO

C HXC = CH

H

C H

O

Proton Chemical Shifts

ppm down field from TMS

Area proportional to number of protons

How many lines would you expect in the proton NMR spectrum of this compound?

Br2CHCH3

Measure signal from this proton

Field from this proton can be aligned or opposed to applied field

Magnetic Field

Spin Combinations - 2 adjacent protons

-C-CH2-H

1 : 2 : 1

Spin Combinations - 3 Adjacent Protons

-C-CH3

H

1: 3: 3: 1

Adjacent Protons Lines

123

234

n + 1

What is the name of this compound?

Ethyl Bromide

CH3CO

OCH2CH3

ICH2CH2CH3

What is this compound?

CH3COCH2CH3

C6H5COCH3

What is this compound?

What is this compound?

C6H5COOCH2CH3

y

x

Magneticfield changesacross sample