L1 - HL NMR

objectives

Understandings

• In a high resolution 1H NMR spectrum, single peaks present in low resolution can split into further clusters of peaks.

Applications and Skills

• Explanation of the use of tetramethylsilane (TMS) as the reference standard.

Guidance

• Students should be able to interpret the following from 1H NMR spectra: number of peaks, area under each peak, chemical shift and splitting patterns. Treatment of spin-spin coupling constants will not be assessed but students should be familiar with singlets, doublets, triplets and quartets.
• High resolution 1H NMR should be covered.

Tetramethyl Silane - the NMR Reference Signal

• non-toxic liquid - SAFE TO USE
• inert - DOESN’T REACT WITH COMPOUND BEING ANALYSED
• has a low boiling point - CAN BE DISTILLED OFF AND USED AGAIN
• all the hydrogen atoms are chemically equivalent - PRODUCES A SINGLE PEAK
• twelve hydrogens so it produces an intense peak - DON’T NEED TO USE MUCH
• signal is outside the range shown by most protons - WON’T OBSCURE MAIN SIGNALS
• given the chemical shift of d = 0
• the position of all other signals is measured relative to TMS

Chemical Shift

High Resolution NMR

• low resolution nmr gives 1 peak for each environmentally different group of protons
• high resolution gives more complex signals - doublets, triplets, quartets, multiplets
• the signal produced indicates the number of protons on adjacent carbon atoms

• The splitting pattern depends on the number of hydrogen atoms on adjacent atoms

Multiplicity - (Spin-Spin Coupling)

• low resolution nmr gives 1 peak for each environmentally different group of protons
• high resolution gives more complex signals - doublets, triplets, quartets, multiplets
• the signal produced indicates the number of protons on adjacent carbon atoms

• Splitting patterns are worked out by considering the effect adjacent, chemically different protons have on another signal in a given environment.
• The spin of the proton producing the signal is affected by each of the two forms of the adjacent proton.
• One orientation augments/enhances its field and the other opposes/reduces it. This is done by calculating the various possible combinations of alignment of adjacent protons.

HOWEVER Signals for the H in an O-H bond are not affected by hydrogens on adjacent atoms so are not split

An Analogy for Spin-Spin Coupling

O-H bonds and splitting patterns

• The signal due to the hydroxyl (OH) hydrogen is a singlet ... there is no splitting
• H’s on OH groups do not couple with adjacent hydrogen atoms
• Arises because the H on the OH, rapidly exchanges with protons on other molecules (such as water or acids) and is not attached to any particular oxygen long enough to register a splitting signal.

When is a hydrogen chemically different?

An nmr spectrum provides several types of information :-

• number of signal groups tells you the number of different proton environments
• chemical shift the general environment of the protons
• peak area (integration) the number of protons in each environment
• multiplicity how many protons are on adjacent atoms

In many cases this information is sufficient to deduce the structure of an organic molecule but other forms of spectroscopy are used in conjunction with nmr.

HOW TO WORK OUT AN NMR SPECTRUM

1. Get the formula of the compound

2. Draw out the structure

3. Go to each atom in turn and ask the ‘census’ questions

4. Work out what the spectrum would look like ... signals due to H’s nearer electronegative atoms (Cl,Br,O) are shifted downfield to higher d values

Interpreting Spectra - 1-bromopropane

The signal is shifted furthest away (downfield) from TMS as the hydrogen atoms are nearest the electronegative bromine atom.

summary notes

determine the spectra below