spectroscopy

2. What Does an IR Spectrum Look Like?
• A spectrum is a graph in which the amount of
light absorbed is plotted on the y-axis and
frequency is plotted on the x-axis. An example
is shown below. You can run your finger along
the graph and see whether any light of a
particular frequency is absorbed; if so, you will
see a "peak" at that frequency. If not, you will
see "the baseline" at that frequency.

3. Figure IR1. IR spectrum of benzene. The x-axis labels are, from
right to left, 500, 1000, 1500, 2000, 3000 and 4000 cm-1.

4. In IR spectra (spectra = plural of
spectrum):
• the y axis is usually labeled "transmittance".
Transmittance is the amount of light that passes
through the sample.
• the unit of transmittance is percent (%).
• the x axis is labeled "wavenumbers". Wavenumbers are
proportional to frequency, so the higher the frequency,
the higher the wavenumber.
• the symbol for wavenumber is reciprocal centimeters
(cm-1).
• the x-axis is usually displayed with high wavenumber
on the left and lower wavenumber on the right.

5. • As you run your finger from left to right across
an IR spectrum, you can see whether or not
light is absorbed at particular frequencies.
When the curve dips down, less light is
transmitted. That means light is absorbed. The
dip in the graph is called a peak. Different
bonds absorb different frequencies of light, so
the peaks tell you what kinds of bonds are
present.

6. IR2. Hydrocarbon Spectra
• All organic and biological compounds contain carbon
and hydrogen, usually with various other elements as
well. Hydrocarbons are compounds containing only
carbon and hydrogen, but no other types of atoms.
Since all organic compounds contain carbon and
hydrogen, looking at hydrocarbon spectra will tell us
what peaks are due to the basic C&H part of these
molecules. It is sometimes useful to think of the C&H
part of a molecule as the basic skeleton or scaffolding
used to construct the molecule. The other atoms often
form more interesting and active features, like the
doors, windows and lights on a building.

7. • The simplest hydrocarbons contain only single
bonds between their carbons, and no double or
triple bonds. These hydrocarbons are variously
referred to as saturated hydrocarbons, paraffins
or alkanes. Examples of alkanes include hexane
and nonane. (You can take a look at the Glossary
to see what these names tell you about the
structure.)
•

8. Look at the IR spectrum of hexane. You
should see:
• a set of peaks dipping down from the baseline
at about 2900 cm-1.
• another set of peaks dipping down from the
baseline at about 1400-1500 cm-1.

9. Figure IR2.IR spectrum of hexane.

10. • If you look at an IR spectrum of any other alkane, you will
also see peaks at about 2900 and 1500 cm-1. The IR spectra
of many organic compounds will show these peaks because
the compound may contain paraffinic parts in addition to
parts with other elements in them.
• These two kinds of peaks tell you that C-H bonds are
present.
• Specifically, the bonds involve sp3 or tetrahedral carbons.
• Stretching C-H bonds in alkanes absorb light at around 2900
cm-1.
• Bending H-C-H angles in alkanes absorb light at around
1500 cm-1.

11. IR3. Subtle Points of IR Spectroscopy
• Alkanes show two sets of peaks in the IR
spectrum. Alkanes contain two kinds of bonds:
C-C bonds and C-H bonds. However, these two
facts are not related. The reasons are
explained through bond polarity and
molecular vibrations.

12. Bond polarity can play a role in IR
spectroscopy
• nature rules that only bonds that contain dipoles can
absorb infrared light.
• C-C bonds are usually nonpolar and usually do not
show up as peaks in the IR spectrum.
• C-H bonds are not very polar and do not give rise to
strong peaks in the IR spectrum.
• a whole lot of small C-H peaks can add up together to
look like one big peak. This would happen if a molecule
contained many C-H bonds (a common situation).
•

13. Molecular vibrations play a major role
in IR spectroscopy.
• IR light interacts with vibrating bonds. When light is
absorbed, the bond has a little more energy and vibrates at
a higher frequency.
• a bond does not have an exact, fixed length; it can stretch
and compress. This is called a bond stretching vibration.
• Stretching C-H bonds in alkanes absorb light at around 2900
cm-1.
• bond angles can also bend; for instance, the H-C-H bond
angle can compress and stretch. This is called a bending
vibration.
• Bending H-C-H angles in alkanes absorb light at around
1500 cm-1.

14. • The factors that govern what bonds (and what
vibrations) show up at what frequencies are
easily handled by computational chemistry
software. In fact, prediction of absorption
frequencies in IR spectra can be done using
17th century classical mechanics, specifically
Hooke's Law (devised to explain the vibrational
frequencies of springs). Computation is not the
focus of this chapter but it may help you keep
track of what kinds of vibrations absorb at what
frequencies.

15. • Hooke's Law states:
• the vibrational frequency is proportional to
the strength of the spring; the stronger the
spring, the higher the frequency.
• the vibrational frequency is inversely
proportional to the masses at the ends of the
spring; the lighter the weights, the higher the
frequency.

16. • Remember, there are two factors here, so you
won't be able to make predictions knowing
only one factor. Some strong bonds may not
absorb at high frequency because they are
between heavy atoms. The information is
presented mostly to help you organize what
bonds absorb at what general frequencies
after you have learned about them.

17. • The reasons explaining why C-H bending
vibrations are at lower frequency than C-H
stretching vibrations are also related to Hooke's
Law. An H-C-H bending vibration involves three
atoms, not just two, so the mass involved is
greater than in a C-H stretch. That means lower
frequency. Also, it turns out that the "stiffness" of
a bond angle (analogous to the strength of a
spring) is less than the "stiffness" of a bond
length; the angle has a little more latitude to
change than does the length. Both factors lead to
a lower bending frequency.

18. Problem IR.1
For each of the following pairs, identify which
bond would show up at a higher wavenumber in
the IR spectrum:
• a) C-H or C-O b) C-O or C=O c) C=N or C=N
• d) N-H or N-O e) a covalent O-H bond or a
hydrogen bond in water

19. IR4. Carbon Carbon Multiple Bonds
• Unsaturated hydrocarbons contain only
carbon and hydrogen, but also have some
multiple bonds between carbons. One type of
unsaturated hydrocarbon is an olefin, also
known as an alkene. Alkenes contain double
bonds between carbons. One example of an
alkene is 1-heptene. It looks similar to hexane,
except for the double bond from the first
carbon to the second.

20. Look at the IR spectrum of 1-heptene.
You should see:
• a set of peaks dipping down from the baseline
at about 2900 cm-1.
• another set of peaks dipping down from the
baseline at about 1500 cm-1.

21. Figure IR3. IR spectrum of 1-heptene.

22. • So far, these peaks are the same as the ones
seen for hexane. We can assign them as the C-
H stretching and bending frequencies,
respectively.
• Looking further, you will also see:
• a small peak around 3100 cm-1.
• a small peak near 1650 cm-1.
• medium peaks near 800 and 1000 cm-1.

23. • The peak at 3100 cm-1 hardly seems different
from the C-H stretch seen before. It is also a C-
H stretch, but from a different type of carbon.
This stretch involves the sp2 or trigonal planar
carbon of the double bond, whereas the peak
at 2900 involves an sp3 or tetrahedral carbon.

24. • The peak at 1650 cm-1 can be identified via
computational methods as arising from a
carbon-carbon double bond stretch. It is a
weak stretch because this bond is not very
polar. Sometimes it is obscured by other,
larger peaks.

25. • The larger peaks near 800 and 1000 cm-1 are
bending vibrations. They are due to a C=C-H
bond angle that bends out of the plane of the
double bond (remember that the carbons on
either end of the double bond are trigonal
planar). They are called oop bends. Oop bends
are often prominent in alkenes and are easier
to spot than an sp2 C-H stretching mode or a
C=C stretching mode.

26. Problem IR.2.
• Given this information about the infrared
spectra of alkenes, which bond do you think is
stronger, an sp2 or an sp3 C-H bond?
•

27. Problem IR.3.
• What do you think would happen to the peak
due to carbon-carbon double bond stretching
if an electronegative atom were nearby in the
molecule?

28. Problem IR.4.
• Oop bends can be diagnostic of the position
and geometry of double bonds.
• Compare the oop bending modes or peaks
seen in 1-heptene to those in Z-2-octene, aka
cis-2-octene (in Z-2-octene, the double bond
adopts a curled shape with alkyl substituents
coming from the same side of the double
bond).

29. Figure IR4. IR spectrum of Z-2-octene.

30. • 2. Also compare it to E-2-octene, aka trans-2-
octene (in which the double bond has a zig-
zag shape, with alkyl substituents coming from
opposite sides).

31. Figure IR5. IR spectrum of E-2octene.

32. • c) Make predictions about the oop bending
modes in 1-octene, Z-2-hexene and E-2-
hexene.

33. Problem IR.5.
• Why do you think an sp3 CH2 bending mode
occurs around 1500 cm-1 but a C=CH oop
bending mode occurs around 800-1000 cm-1 ?

34. Problem IR.6.
• In the IR spectrum of 1-octyne, new peaks
appear at 3300 and 2100 cm-1.
•

35. Figure IR6. IR spectrum of octyne.

36. • By comparison with the other hydrocarbons,
can you identify the peak at 3300 cm-1?
• The peak at 2100 cm-1 is due to a carbon-
carbon bond. Which one? Compare this peak
to the one seen from a carbon-carbon bond in
1-hexene and explain the differences