NMR Spectroscopy

NMR spectroscopy gives information on the type of hydrogen atoms in a molecule. Specifically, -CH3, -CH2, CH2OH etc,

The location of peaks and the height of the peaks indicate what type of hydrogen atoms and the number of hydrogen atoms, are present respectively.

How NMR works

The nuclei of hydrogen atoms possess spin that exists in two different states of equal energy.

When a radio frequency with a varying magnetic field is applied to a molecule, the hydrogen’s spin state align themselves either with or against the magnetic field.

This alignment of the hydrogen absorbs energy. This energy comes from the absorption of radiation from the radio frequency being applied.

This absorption, or chemical shift, makes a spectrum.

Why are there different absorption peaks? Each hydrogen does not absorb the same amount of

radiation because they do not all have the same bond strength between itself and the carbon to which it is bonded. Bond strength is affected by surrounding bonds.

Examples: CH3CH2CH3 There would be two peaks, or chemical shifts on its NMR

spectrum indicating two different types of C-H bonds, CH3- and CH2- .

The CH3- peak would be higher because there 6 of these H’s. There are only 2 of the other.

NMR spectroscopy

Type of proton Chemical shift/ppm R-CH3 0.9 R2-CH2 1.3 R3CH 2.0 CH3COR(ester) 2.0 R-C-CH3 (Ketone) 2.1 R-OH 4.5 R-C-H (aldehyde) 9.7

Example: CH3CHCl2 There are 2 peaks, or chemical shifts,

indicating 2 types of Hydrogen C-H bonds: CH3-, andCHCl2,

The 2 peaks on this spectrum would not be similar in height, with the CH3- peak being higher with 3H’s the CHCl2 has only one H.

NMR spectrum of 1,1dichloroethane

How many peaks would there be in propane’s NMR spectrum?

CH3CH2CH3

NMR spectrum for propane

CH3

• C• CH2

Summary ChartMass spectroscopy

Infrared

spectroscopy

NMR

spectroscopy

Determines molar mass and branches

Determines functional groups

Determines the types of hydrogens present

Uses electromag.

Uses infrared radiation

Uses radiowaves

Chart has:

Y=%abundance

X=mass

Chart has:

Y=%transmissi

X=wavelength

Chart has:

Y=#of protons

X=chem.shift/

Summary continued

No single test is adequate in determining the structure of a molecule. The tests are effective when combined with each other.

The best way to identify an unknown molecule is to compare its tests results with a known molecule.