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in functional groups such as OH, C=O,
NH2, and NO2.
NMR requires electromagnetic waves in the radio-wave region of the spectrum to make nuclei
flip from one state to another. The amount of energy needed for stretching and bending individual
bonds, while still very small, corresponds to rather shorter wavelengths. These wavelengths lie in the
infrared, that is, heat radiation just to the long wavelength side of visible light. When the carbon
skeleton of a molecule vibrates, all the bonds stretch and relax in combination and these absorptions
are unhelpful. However some bonds stretch essentially independently of the rest of the molecule.
This occurs if the bond is either:
• much stronger or weaker than others nearby, or
• between atoms that are much heavier or lighter than their neighbours
Indeed, the relationship between the frequency of the bond vibration, the mass of the atoms, and the
strength of the bond is essentially the same as Hooke’s law for a simple harmonic oscillator.
The equation shows that the frequency of the vibration ν is proportional to the (root of) a force
constant f—more or less the bond strength—and inversely proportional to the (root of) a reduced
mass µ, that is, the product of the masses of the two atoms forming the bond divided by their sum.
Stronger bonds vibrate faster and so do lighter atoms. You may at first think that stronger bonds
ought to vibrate more slowly, but a moment’s reflection will convince you of the truth: which
stretches and contracts faster, a tight steel spring or a slack steel spring?
Infrared spectra are simple absorption spectra. The sample is exposed to infrared radiation and
the wavelength scanned across the spectrum. Whenever energy corresponding to a specific wave-
length is absorbed, the intensity of the radiation reaching a detector momentarily decreases, and this
is recorded in the spectrum. Infrared spectra are usually recorded using a frequency measurement
called wavenumber (cm–1) which is the inverse of the true wavelength λ in centimetres to give con-
venient numbers (500–4000 cm–1). Higher numbers are to the left of the spectrum because it is real-
ly wavelength that is being scanned.
Infrared spectra 65
m1 m1
m1 m1
bond vibration in the infrared
Hooke’s law describes the movement
of two masses attached to a spring.
You may have met it if you have studied
physics. You need not be concerned
here with its derivation, just the result.
π µ
= 1
2 c
µ =
m m
m m
1 2
1 2
We need to use another equation here:
The energy, E, required to excite a bond vibration can be expressed as the inverse of a wavelength
λ or as a frequency ν. Wavelength and frequency are just two ways of measuring the same thing.
More energy is needed to stretch a strong bond and you can see from this equation that larger E
means higher wavenumbers (cm–1) or smaller wavelength (cm).
To run the spectrum, the sample is either dissolved in a solvent such as CHCl3 (chloroform) that
has few IR absorptions, pressed into a transparent disc with powdered solid KBr, or ground into an
oily slurry called a mull with a hydrocarbon oil called ‘Nujol’. Solutions in CHCl3 cannot be used for
looking at the regions of C–Cl bond stretching nor can Nujol mulls be used for the region of C–H
stretching. Neither of these is a great disadvantage, especially as nearly all organic compounds have
some C–H bonds anyway.
We shall now examine the relationship between bond stretching and frequency in more detail.
Hooke’s law told us to expect frequency to depend on both mass and bond strengths, and we can
illustrate this double dependence with a series of bonds of various elements to carbon.
Values chiefly affected by mass of atoms: (lighter atom, higher frequency)
C–H C–D C–O C–Cl
3000 cm–1 2200 cm–1 1100 cm–1 700 cm–1
Values chiefly affected by bond strength (stronger bond, higher frequency)
C≡≡O C=O C–O
2143 cm–1 1715 cm–1 1100 cm–1
Just because they were first recorded in this way, infrared spectra have the baseline at the top and
peaks going downwards. You might say that they are plotted upside down and back to front. At least
you are now accustomed to the horizontal scale running backwards as that happens in NMR spectra
too. A new feature is the change in scale at 2000 cm–1 so that the right-hand half of the spectrum is
more detailed than the left-hand half. A typical spectrum looks like this.
66 3 . Determining organic structures
h is Planck’s constant and c the
velocity of light.
E h h
c= =ν
You should always check the way
the spectrum was run before
making any deductions!
There are four important regions of the infrared spectrum
You will see at once that the infrared spectrum contains many lines, particularly at the right-hand
(lower frequency) end; hence the larger scale at this end. Many of these lines result from several
bonds vibrating together and it is actually the left-hand half of the spectrum that is more useful.
The first region, from about 4000 to about 2500 cm–1 is the region for C–H, N–H, and O–H bond
stretching. Most of the atoms in an organic molecule (C, N, O, for example) are about the same
weight. Hydrogen is an order of magnitude lighter than any of these and so it dominates the stretch-
ing frequency by the large effect it has on the reduced mass. The reduced mass of a C–C bond is (12 ×
12)/(12 + 12), i.e. 144/24 = 6.0. If we change one of these atoms for H, the reduced mass changes to
(12 × 1)/ (12 + 1), i.e. 12/13 = 0.92, but, if we change it instead for F, the reduced mass changes to (12
× 19)/ (12 + 19), i.e. 228/31 = 7.35. There is a small change when we increase the mass to 19 (F), but
an enormous change when we decrease it to 1 (H).
Even the strongest bonds—triple bonds such as C≡C or C≡N—absorb at slightly lower frequencies
than bonds to hydrogen: these are in the next region from about 2500 to 2000 cm–1. This and the other
two regions of the spectrum follow in logical order of bond strength as the reduced masses are all about
the same: double bonds such as C=C and C=O from about 1900–1500 cm–1 and single bonds at the right-
hand end of the spectrum. These regions are summarized in this chart, which you should memorize.
Looking back at the typical spectrum, we see peaks in the X–H region at about 2950 cm–1 which are
the C–H stretches of the CH3 and CH2 groups. The one rather weak peak in the triple bond region
(2270 cm–1) is of course the C≡N group and the strong peak at about 1670 cm–1 belongs to the C=O
group. We shall explain soon why some IR peaks are stronger than others. The rest of the spectrum is
in the single bond region. This region is not normally interpreted in detail but is characteristic of the
compound as a whole rather in the way that a fingerprint is characteristic of an individual human
Infrared spectra 67
IR spectra are plotted ‘upside
down’ because they record
transmission (the amount of light
reaching the detector) rather than
(spectrum taken as a Nujol mull)
frequency / cm–1
The concept of reduced mass was
introduced on p. 000.
Remember. Hooke’s law says that
frequency depends on both mass and a
force constant (bond strength).
•The regions of the infrared spectrum
C Cl
bonds to
4000 3000 2000 1500 1000
frequency scale in wavenumbers (cm-1)
increasing energy required to vibrate bond
note change in scale
being—and, similarly, it cannot be ‘interpreted’. It is indeed called the fingerprint region. The useful
information from this spectrum is the presence of the CN and C=O groups and the exact position of
the C=O absorption.
The X–H region distinguishes C–H, N–H, and O–H bonds
The reduced masses of the C–H, N–H, and O–H combinations are all about the same. Any difference
between the positions of the IR bands