L6 - H NMR 2

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objectives

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

The molecule contains four methyl groups attached to a silicon atom in a tetrahedral arrangement.
All the hydrogen atoms are chemically equivalent.

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

Imagine you had an opinion on something. If nobody influenced you, your opinion would be the same. However if another person had a view on the topic, they would either agree or disagree with you. Their ideas would either enhance what you thought or diminish it. There would be two possibilities of equal chance.
If there were two people offering views they could either be both for it (1 possibility) , both against (1 possibility) or one could be in favour and the other against (2 possibilities). There would be three possibilities of relative chance 1:2:1

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?

TWO SIGNALS

Quartet and triplet :- ratio of peak areas = 3 : 2

Carbons 1 & 4 are the similar and so are carbons 2 & 3 so there are only two different chemical environments.

The signal for H’s on carbon 2 is a quartet - you ignore the two neighbours on carbon 3 because they are chemically identical.

TWO SIGNALS

both singlets :- ratio of peak areas = 2 : 1

Hydrogens on OH groups only give singlets. The signal for H’s on each carbon are not split, because

- H’s on the neighbouring carbon are chemically

identical... and

- H’s on adjacent OH groups do not couple.

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.