CLAYDEN. Organic Chemistry. 2ª edição. Oxford. 2012.
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CLAYDEN. Organic Chemistry. 2ª edição. Oxford. 2012.


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C6H10O2 114.068075 114.1039 358
C6H14N2 114.115693 114.1039 118
C7H14O 114.104457 114.1039 5
C8H18 114.140844 114.1039 369
 \u25cf In the rest of the book, whenever we state that a molecule has a certain atomic composition 
you can assume that it has been determined by high-resolution mass spectrometry on the 
molecular ion.
One thing you may have noticed in the table above is that there are no entries with just one 
nitrogen atom. Two nitrogen atoms, yes; one nitrogen no! This is because any complete mol-
ecule with C, H, O, S, and just one nitrogen in it has an odd molecular weight. This is because C, 
O, S, and N all have even atomic weights\u2014only H has an odd atomic weight. Nitrogen is the 
only element from C, O, S, and N that can form an odd number of bonds (3). Molecules with 
one nitrogen atom must have an odd number of hydrogen atoms and hence an odd molecular 
weight.
 \u25cf Quick nitrogen count (for molecules containing any of the elements C, H, N, O, and S)
Molecules with an odd molecular weight must have an odd number of nitrogen atoms. 
Molecules with even molecular weight must have an even number of nitrogen atoms or 
none at all.
ATOMIC COMPOSIT ION CAN BE DETERMINED BY H IGH-RESOLUT ION MASS SPECTROMETRY 51
2069_Book.indb 51 12/12/2011 8:23:13 PM
Nuclear magnetic resonance
What does it do?
Nuclear magnetic resonance (NMR) allows us to detect atomic nuclei and say what sort of 
environment they are in within the molecule. In a molecule such as propanol, the hydrogen 
atom of the hydroxyl group is clearly different from the hydrogen atoms of its carbon 
skeleton\u2014it can be displaced by sodium metal, for example. NMR (actually 1H, or proton, 
NMR) can easily distinguish between these two sorts of hydrogens by detecting the environ-
ment the hydrogen\u2019s nucleus \ufb01 nds itself in. Moreover, it can also distinguish between all the 
other different sorts of hydrogen atoms present. Likewise, carbon (more precisely 13C) NMR 
can easily distinguish between the three different carbon atoms. NMR is extremely versatile: 
it can even scan living human brains (see picture) but the principle is still the same: being able 
to detect nuclei (and hence atoms) in different environments.
NMR uses a strong magnetic \ufb01 eld
Imagine for a moment that we were able to \u2018switch off\u2019 the earth\u2019s magnetic \ufb01 eld. Navigation 
would be made much harder since all compasses would be useless, with their needles pointing 
randomly in any direction. However, as soon as we switched the magnetic \ufb01 eld back on, they 
would all point north\u2014their lowest energy state. Now if we wanted to force a needle to point 
south we would have to use up energy and, of course, as soon as we let go, the needle would 
return to its lowest energy state, pointing north.
In a similar way, some atomic nuclei act like tiny compass needles when placed in a mag-
netic \ufb01 eld and have different energy levels according to the direction in which they are 
\u2018pointing\u2019. (We will explain how a nucleus can \u2018point\u2019 somewhere in a moment.) A real com-
pass needle can rotate through 360° and have an essentially in\ufb01 nite number of different 
energy levels, all higher in energy than the \u2018ground state\u2019 (pointing north). Fortunately, 
things are simpler with an atomic nucleus: its energy levels are quantized, just like the energy 
levels of an electron, which you will meet in the next chapter, and it can adopt only certain 
speci\ufb01 c energy levels. This is like a compass which points, say, only north or south, or maybe 
only north, south, east, or west, and nothing in between. Just as a compass needle has to be 
made of a magnetic material to feel the effect of the earth\u2019s magnetism, so it is that only cer-
tain nuclei are \u2018magnetic\u2019. Many (including \u2018normal\u2019 carbon-12, 12C) do not interact with a 
magnetic \ufb01 eld at all and cannot be observed in an NMR machine. But, importantly for us in 
this chapter, the minor carbon isotope 13C does display magnetic properties, as does 1H, the 
most abundant atomic nucleus on earth. When a 13C or 1H atom \ufb01 nds itself in a magnetic 
\ufb01 eld, it has two available energy states: it can either align itself with the \ufb01 eld (\u2018north\u2019 you 
could say), which would be the lowest energy state, or against the \ufb01 eld (\u2018south\u2019), which is 
higher in energy.
H
C
C
C
O
H
H
H
H H
H H
1H NMR distinguishes
the coloured hydrogens
13C NMR distinguishes
the boxed carbons
 \u25a0 When NMR is used medically 
it is usually called magnetic 
resonance imaging (MRI) for 
fear of alarming patients wary of 
all things nuclear.
CHAPTER 3   DETERMINING ORGANIC STRUCTURES52
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The property of a nucleus that allows magnetic interactions, i.e. the property possessed by 
13C and 1H but not by 12C, is spin. If you conceive of a 13C and 1H nucleus spinning, you can 
see how the nucleus can point in one direction\u2014it is the axis of the spin that is aligned with 
or against the \ufb01 eld.
Let\u2019s return to the compass for a moment. If you want to move a compass needle away from 
pointing north, you have to push it\u2014and expend energy as you do so. If you put the compass 
next to a bar magnet, the attraction towards the magnet is much greater than the attraction 
towards the north pole, and the needle now points at the magnet. You also have to push much 
harder if you want to move the needle. Exactly how hard it is to turn the compass needle 
depends on how strong the magnetic \ufb01 eld is and also on how well the needle is magnetized\u2014if 
it is only weakly magnetized, it is much easier to turn it round and if it isn\u2019t magnetized at all, 
it is free to rotate.
Likewise, for a nucleus in a magnetic \ufb01 eld, the difference in energy between the nuclear spin 
aligned with and against the applied \ufb01 eld depends on:
\u2022 how strong the magnetic \ufb01 eld is, and
\u2022 the magnetic properties of the nucleus itself.
The stronger the magnetic \ufb01 eld, the greater the energy difference between the two 
alignments of the nucleus. Now there is an unfortunate thing about NMR: the energy dif-
ference between the nuclear spin being aligned with the magnetic \ufb01 eld and against it is 
really very small\u2014so small that we need a very, very strong magnetic \ufb01 eld to see any differ-
ence at all.
NMR also uses radio waves
A 1H or 13C nucleus in a magnetic \ufb01 eld can have two energy levels, and energy is needed to \ufb02 ip 
the nucleus from the more stable state to the less stable state. But since the amount of energy 
needed is so small, it can be provided by low-energy electromagnetic radiation of radio-wave 
frequency. Radio waves \ufb02 ip the nucleus from the lower energy state to the higher state. Turn 
off the radio pulse and the nucleus returns to the lower energy state. When it does so, the 
energy comes out again, and this (a tiny pulse of radio frequency electromagnetic radiation) 
is what we detect.
We can now sum up how an NMR machine works.
 1. The sample of the unknown compound is dissolved in a suitable solvent, placed in a 
narrow tube, and put inside a very strong electromagnet. To even out imperfections in 
 \u25a0 This picture shows a typical 
NMR instrument. The fat cylinder 
is the supercooled magnet. The 
device hanging over it is an 
automatic sample changer and 
the console in the foreground 
controls the machine.
Nuclear spin is quantized and 
has the symbol I. The exact 
number of different energy levels 
a nucleus can adopt is determined 
by the value of I of the particular 
isotope. The nuclear spin I can 
have various values such as 0, 1/2, 
1, 3/2... and the number of energy 
levels is given by 2I + 1. Some 
examples are 1H, I = 1/2; 2H (= 
D), I = 1; 11B, I = 5/2; 12C, I = 0.
NMR machines contain very 
strong electromagnets. The 
earth\u2019s magnetic \ufb01 eld has a \ufb01 eld 
strength of between 30 and 60 
microtesla. A typical magnet used 
in an NMR machine has a \ufb01 eld 
strength