Chapter 13 NMR Spectroscopy

NMR - Nuclear Magnetic Resonance

NMR is a form of spectroscopy that uses an instrument with a powerful magnet to analyze organic compounds.

Invented by physicists (1950’s), then used by chemists (1960’s).

Why is it called NMR?Nuclear Magnetic ResonanceNuclear – because it looks at the nucleus of an atom,

most commonly a hydrogen atom. A hydrogen atom nucleus consists of one proton with a

+1 charge and “spin” of ½. It acts like a tiny bar magnet.

generatesmagnetic

field

No external magnetic field applied to sample

Random orientation of nuclear spins

Sample placed in an external magnetic field

NMR – Effect of Magnetic Field

NMR: Absorption of Energy

Initial State – nucleus at low energy level

NMR: Information Obtained from a Spectrum

An NMR Spectrum will generally provide three types of information:Chemical Shift – indicates the electronic environment of

the nucleus (shielded or deshielded)Integration – gives the relative number of nuclei producing

a given signalSpin-Spin Coupling – describes the connectivity

1H NMR Spectrum – H2O

A sample of water is placed in an NMR instrument, and a proton spectrum is recorded (scanned from left to right).

When does nucleus absorb energy?

3.

2, External Field (Ho)from magnet

NMR: Simple 1H NMR Spectrum Showing Chemical Shift

Two types of protons (a CH2 and a CH3) give two separate signals at two different chemical shifts.

NMR: Chemical Shift Practice

Assign the four groups shown to the four NMR singals, based on each element’s electronegativity.

Group

-O-CH3

-Si-CH3

-C-CH3

Cl3C-H

EN

NMR: Chemical Shift Regions

Chemical shift measured in ppm.

Chemical shift zero is set to TMS (tetramethylsilane),a reference compound

NMR: Chemical Shift Regions

Alkane Region (high electron density):

Heteroatom Region:

Double Bond Region:

NMR: Chemical Equivalence and Number of Signals

How many signals will the following compounds show in their 1H NMR Spectrum? (Hint: check for symmetry)

OMeBr

O

Cl

Cl

NH2ClH

Cl

ClH

H

HNH2

Cl

HH

NMR: Chemical Equivalence and Number of Signals

How many signals should appear in the proton NMR spectrum for these compounds?

In theory:

O

octane

Signals actually resolved:

NMR: Overlapping Proton Signals

Protons b, c, and d are all nearly the same, and their signals are not resolved in this spectrum.

Review: How Many NMR Signals?

How many signals will the following compounds show in their 1H NMR Spectrum? (Hint: check for symmetry)

ClH

H HHH

H

H CH3

H

H

H

H H

CH3CH2Cl

CC

No rotation aboutdouble bonds

NMR: Chloroethane

CC

ClH

H

HH

H

Fast rotation around single bonds gives an “averaged” spectrum for the three methyl hydrogens.

NMR: A Second Proton Spectrum

Note: the signal for the nine methyl H’s is larger than the CH2 signal

NMR: Integration Indicates Relative Number of Nuclei

The height of the integration line (“integral”) gives you the relative number of nuclei producing each signal.

NMR: Splitting into a Doublet

Note that the signal at 1.6 ppm for the methyl group is split into two peaks.Remember that this is one signal, composed of two separate peaks.

doublet

NMR: Signal Splitting, n+1 Rule• A signal is often split into multiple peaks due to

interactions with protons on carbons next door. Called spin-spin splitting

• The splitting is into one more peak than the number of H’s on adjacent carbons (“n+1 rule”)

• Splitting of a signal can give doublets (two peaks), triplets (three peaks), quartets (4 peaks), ect.

• The relative intensities given by Pascal’s Triangle:doublet 1 : 1triplet 1 : 2 : 1quartet 1 : 3 : 3 : 1pentet: 1 : 4 : 6 : 4 : 1

NMR: Signal Splitting, n+1 Rulen+1 Rule: A signal in the proton NMR spectrum will be split into n+1 peaks, where n is the number of protons on adjacent carbons.

Example: CH3-CH2-Br

For the Methyl Group – There are two protons ‘next door’ (n=2), so the methyl signal will be split into three peaks (2+1), which is called a triplet.

For the -CH2- Group: Three protons next door means the CH2 signal will be split into 4 (3+1) peaks, called a quartet.

1H NMR Spectrum for Bromoethane

integration:2 H 3 H

Note the expansions printed above

NMR: Signal Splitting, n+1 Rule

NMR: Signal Splitting, n+1 Rule

H

NMR: Origin of Spin-Spin Splitting

Net result:

NMR: Doublets and Triplets

Doublet: the one proton next doorcan be either up or down (α or β)

Triplet: for the two protons next door,there are four combinations possible:

α α α β β β β α

NMR: Signal Splitting, n+1 Rule

Using the n+1 rule, predict the 1H NMR spectrum of 2-iodopropane.Give splitting pattern, integration, and approximate chemical shift.

IHI

CH3H3CC

Note that the methyl groups are equivalent, so they will give one signal in the NMR spectrum.

NMR: Using the n+1 Rule

NMR: Spectrum of 2-iodopropane

Seven line pattern

doublet

NMR: Rules for Spin-Spin Splitting• The signal of a proton with n equivalent neighboring H’s is split

into n + 1 peaks

• Protons farther than two carbon atoms apart do not split each other

• Equivalent protons do not split each other

Common 1H NMR Patterns 1. triplet (3H) + quartet (2H)

2. doublet (1H) + doublet (1H)

3. large singlet (9H)

4. singlet 3.5 ppm (3H)

5. large double (6H) + muliplet (1H)

6. singlet 2.1 ppm (3H)

Common 1H NMR Patterns 7. multiplet ~7.2 ppm (5H)

8. multiplet ~7.2 ppm (4H)

9. broad singlet, variable chemical shift