Introduction to Spectroscopy
Objectives of the Unit
Understand general concepts of Spectroscopy
Be familiar with the common forms of spectroscopy used in molecular structure
determination
Be able to correctly interpret IR, 13C NMR and MS spectra
Be able to use IR, 13C NMR and MS spectra to solve the structures of simple organic
molecules
Origins of spectroscopy
Spectrum
Scope
The word spectrum originates with Newton's dispersion of white light into a rainbow of colours with
a prism. The spectrum of white light is the range of colours that make it up. Each individual colour
from the prism cannot be further separated into other colours. Each part of the spectrum has a
characteristic frequency, wavelength and quantum energy. The spectrum is a continuum or range of
frequency.
Old definitions from a technical Dictionary:
A spectroscope is a device used to observe a spectrum without the ability to record it.
A spectrograph is an instrument used to photographically record a spectrum.
A spectrogram is a photograph of a spectrum
A spectrometer measures refractive indices
A spectrophotometer is a spectroscope combined with a photometer and can measure
quantitatively the relative intensity in different parts of a spectrum.
These definitions are outdated as they refer primarily to visible light and older methods of recording
it.
All modern spectrometers ‘’look” at a spectrum and record the intensity of irradiation throughout
the whole spectrum. The resulting 2 dimensional curve of wavelength or frequency VS intensity is
now called a spectrum
What is a spectrum?
' the colours of light separated by a prism'
The visible spectrum is a small part of a much larger spectrum of electromagnetic radiation. This
radiation can be thought of as transverse waves in perpendicular electric and magnetic fields:
The Electromagnetic Spectrum
g-rays
X-rays
10-11
UV
Infrared
Visible light
10-7
10-9
10-5
Micro
waves
Radio Waves
10-1
10-3
101
103
l(m)
1Å
frequency
(Hz)
1mm
1nm
109
wave #
(cm-1)
103
106
108
106
102
10
1011
1013
1015
1017
1019
1km
1mm 1cm
10-1
10
107
109
104
102
10
10-2
10-4
10-6
10-3
10-2
10-4
J mol -1
eV
Nuclear
transistions
Inner
electron
transitions
1 J mol -1
= 2.5053 x 10 9 Hz
= 8.3567 x 10 -2 cm-1
= 1.0360 x 10 -5 eV
Outer
electron
transitions
Molecular
vibrations
Molecular
rotations
10-8
Nuclear magnetic
resonance
105
Electromagnetic waves travel through transparent media (including a vacuum) at the speed of light
2.998x108 m s-1 (slightly slower through denser media). A wave passing a point in space will cause
an oscillation of the electric and magnetic field. The frequency of the oscillation is related to the
wavelength and speed of light.
c  
where:
c


=
=
speed of light
frequency
(m s-1)
(Hz)
=
wavelength
(m)
In some regions of the electromagnetic spectrum it is convenient to use wave numbers, that is, the
number of wavelengths in 1 cm. The units of wave number are cm-1 and are sometimes referred to as
reciprocal centimetres.
Physicists near the beginning of this century began to treat electromagnetic radiation as a stream of
particles. Just as matter, which seems continuous at our scale is actually made of particles at the
atomic scale; so electromagnetic energy is continuous and wave-like at long wavelengths and particle
like at short wavelengths. The dual wave/particle nature of electromagnetic radiation is one of the
fundamental concepts of modern physics and the foundation of quantum theory. Since
electromagnetic radiation comes in discrete particles, each must have a characteristic energy related
to the frequency of the radiation.
  h
where:

h

=
=
=
energy
Plank's constant
frequency
(Joules)
6.625x10-34 (J sec)
(Hz) (sec-1)
One can measure a spectrum in terms of wavelength, wave number, frequency, and energy. All of
these units are interchangeable and all used in various forms of spectroscopy.
Characteristics of a Spectrum:
Intensity, Absorbency, and Abundance: (the Y axis):
The Y axis in spectra measures the intensity of the radiation at a particular spot in the spectrum. In
more wave-like regions of the electromagnetic spectrum this intensity may be power (energy per
second). In more particle like regions photons may be counted directly. The energy may be emitted
by the sample (an emission spectrum) or absorbed by the sample from a radiation source (an
absorbance spectrum). In absorbance mode the transmitted beam is often compared to that present
in the absence of the sample. The relative intensity is plotted on the Y axis as percent transmittance
or absorbance. The advantage of plotting relative intensity is that the radiation source need not have
constant intensity across the spectrum.
Resolution:
The resolution is a measure of the ability to discriminate close signals in the spectrum. There are a
number of different ways of measuring resolution. Commonly it is defined as the width of a peak at
one half of its maximum intensity. It must be remembered that in some cases the width of a peak is
the natural shape of the transition, and is not a good measurement of the resolution of the spectrum.
Interaction of Energy with Matter
i)
Classical Model Balls on Springs
The oscillating electrical or magnetic field associated with electromagnetic radiation can interact with
electric or magnetic moments in molecules. For example carbon monoxide is a diatomic molecule
comprised of an oxygen and a carbon atom bonded together. Since the two atoms are not the same,
the electrons are attracted to the more electronegative atom (oxygen) creating a dipole. The bond
between the two atoms is not rigid, it can be thought of as a spring relaxed at the average inter
atomic distance, compressed when the two atoms are brought closer together, and stretched when
they are pulled apart. Such an arrangement (two balls on a spring) will oscillate at a natural
frequency that can be described by classical physics:
If this system is irradiated with electromagnetic radiation at the natural vibrational frequency, the
balls will oscillate absorbing the energy. Conversely, if the system is excited it will vibrate at the
natural frequency emitting the corresponding electromagnetic radiation. In general, the classical
approach attempts to model reality with a single construction (balls on a spring).
ii)
Quantum Mechanics Model
Some forms of energy/matter interactions are poorly explained by classical mechanics (i.e. electronic
transitions). Classical mechanics views time, matter and energy as continuous variables. This view
was modified early on at the microscopic level by the realisation that matter is made of discrete
packets (atoms and subatomic particles). Time and matter are also discontinuous at the microscopic
level; quantum mechanics is a modification of classical mechanics that successfully deals with the
discontinuous nature of the microscopic scale. One of the fundamental outcomes of a quantum
mechanical treatment of molecular energies is that the energies, be they rotational, vibrational
nuclear or electronic, are quantized. This means that emissions and absorbances can only occur at
specific energies, and thus specific wavelengths of electromagnetic radiation. The quantum
mechanical approach uses populations of constructions to model reality.
E1
Absorption
Emission
E0
The frequency of electromagnetic radiation absorbed or emitted is given by:
n=
( E1 - E 0 )
h
For most transitions the energy difference is sufficient that almost all of the population is in the lower
energy state and the higher energy state can be considered unoccupied at thermal equilibrium.