![Introduction to
Spectroscopy
What is spectroscopy?
One of the frustrations of being a chemist is the fact that
no matter how hard you stare at your test tube or
round-bottomed flask you can’t actually see the individual
molecules you have made! Even though your product
looks the right colour and seems to give sensible results
when you carry out chemical tests, can you be really sure
of its precise structure?
Fortunately, help is at hand. Although you might not be
able to ‘see’ molecules, they do respond when light
energy hits them, and if you can observe that response,
then maybe you can get some information about that
molecule. This is where spectroscopy comes in.
Spectroscopy is the study of the way light
(electromagnetic radiation) and matter interact.
There are a number of different types of spectroscopic
techniques and the basic principle shared by all is to
shine a beam of a particular electromagnetic radiation
onto a sample and observe how it responds to such a
stimulus; allowing scientists to obtain information about
the structure and properties of matter.
What is light?
Light carries energy in the form of tiny particles known as
photons. Each photon has a discrete amount of energy,
called a quantum. Light has wave properties with
characteristic wavelengths and frequency (see the
diagram below).
The energy of the photons is related to the frequency (m)
and wavelength (l) of the light through the two equations:
E = hm and m = c /l
(where h is Planck's constant and c is the speed of light).
Therefore, high energy radiation (light) will have high
frequencies and short wavelengths.
The range of wavelengths and frequencies in light is
known as the electromagnetic spectrum. This spectrum
is divided into various regions extending from very short
wavelength, high energy radiation (including gamma rays
and X-rays) to very long wavelength, low energy radiation
(including microwaves and broadcast radio waves).
The visible region (white light) only makes up a small part
of the electromagnetic spectrum considered to be
380-770 nm. [Note that a nanometre is 10-9 metres].
TH(MET
0-RADIO
103
MICROWAVE
10-2
INFARED
10-5
VISIBLE
5x10-6
ULTRAVIOLET
10-8
X-RAY
10-10
GAMMA RAY
10-12
WAVELENGTH(METRES)
MOLECULES
104 108 1012 1015 1016 1018 1020
FREQUENCY(HZ)
BUILDINGS
HUMANS
BUTTERFLY
PINHEAD
PROTOZOANS
ATOMS
ATOMIC
NUCLEUS
INCREASING WAVELENGTH l INCREASING FREQUENCY m
INCREASING ENERGY E
Copyright © 2009 Royal Society of Chemistry www.rsc.org](https://image.slidesharecdn.com/introductiontospectroscopystudent-141118152209-conversion-gate02/85/Introduction-to-spectroscopy-student-1-320.jpg)
Introduction to spectroscopy student
- 1. Introduction to Spectroscopy What is spectroscopy? One of the frustrations of being a chemist is the fact that no matter how hard you stare at your test tube or round-bottomed flask you can’t actually see the individual molecules you have made! Even though your product looks the right colour and seems to give sensible results when you carry out chemical tests, can you be really sure of its precise structure? Fortunately, help is at hand. Although you might not be able to ‘see’ molecules, they do respond when light energy hits them, and if you can observe that response, then maybe you can get some information about that molecule. This is where spectroscopy comes in. Spectroscopy is the study of the way light (electromagnetic radiation) and matter interact. There are a number of different types of spectroscopic techniques and the basic principle shared by all is to shine a beam of a particular electromagnetic radiation onto a sample and observe how it responds to such a stimulus; allowing scientists to obtain information about the structure and properties of matter. What is light? Light carries energy in the form of tiny particles known as photons. Each photon has a discrete amount of energy, called a quantum. Light has wave properties with characteristic wavelengths and frequency (see the diagram below). The energy of the photons is related to the frequency (m) and wavelength (l) of the light through the two equations: E = hm and m = c /l (where h is Planck's constant and c is the speed of light). Therefore, high energy radiation (light) will have high frequencies and short wavelengths. The range of wavelengths and frequencies in light is known as the electromagnetic spectrum. This spectrum is divided into various regions extending from very short wavelength, high energy radiation (including gamma rays and X-rays) to very long wavelength, low energy radiation (including microwaves and broadcast radio waves). The visible region (white light) only makes up a small part of the electromagnetic spectrum considered to be 380-770 nm. [Note that a nanometre is 10-9 metres]. TH(MET 0-RADIO 103 MICROWAVE 10-2 INFARED 10-5 VISIBLE 5x10-6 ULTRAVIOLET 10-8 X-RAY 10-10 GAMMA RAY 10-12 WAVELENGTH(METRES) MOLECULES 104 108 1012 1015 1016 1018 1020 FREQUENCY(HZ) BUILDINGS HUMANS BUTTERFLY PINHEAD PROTOZOANS ATOMS ATOMIC NUCLEUS INCREASING WAVELENGTH l INCREASING FREQUENCY m INCREASING ENERGY E Copyright © 2009 Royal Society of Chemistry www.rsc.org
- 2. SPECTROSCOPY INTRODUCTION 2 When matter absorbs electromagnetic radiation the change which occurs depends on the type of radiation, and therefore the amount of energy, being absorbed. Absorption of energy causes an electron or molecule to go from an initial energy state (ground state) to a high energy state (excited state) which could take the form of the increased rotation, vibration or electronic excitation. By studying this change in energy state scientists are able to learn more about the physical and chemical properties of the molecules. • Radio waves can cause nuclei in some atoms to change magnetic orientation and this forms the basis of a technique called nuclear magnetic resonance (NMR) spectroscopy. • Molecular rotations are excited by microwaves. • Electrons are promoted to higher orbitals by ultraviolet or visible light. • Vibrations of bonds are excited by infrared radiation. The energy states are said to be quantised because a photon of precise energy and frequency (or wavelength) must be absorbed to excite an electron or molecule from the ground state to a particular excited state. Since molecules have a unique set of energy states that depend on their structure, IR, UV-visible and NMR spectroscopy will provide valuable information about the structure of the molecule. To ‘see’ a molecule we need to use light having a wavelength smaller than the molecule itself (approximately 10–10 m). Such radiation is found in the X-ray region of the electromagnetic spectrum and is used in the field of X-ray crystallography. This technique yields very detailed three-dimensional pictures of molecular structures – the only drawback being that it requires high quality crystals of the compound being studied. Although other spectroscopic techniques do not yield a three-dimensional picture of a molecule they do provide information about its characteristic features and are therefore used routinely in structural analysis. Mass spectrometry is another useful technique used by chemists to help them determine the structure of molecules. Although sometimes referred to as mass spectroscopy it is, by definition, not a spectroscopic technique as it does not make use of electromagnetic radiation. Instead the molecules are ionised using high energy electrons and these molecular ions subsequently undergo fragmentation. The resulting mass spectrum contains the mass of the molecule and its fragments which allows chemists to piece together its structure. In all spectroscopic techniques only very small quantities (milligrams or less) of sample are required, however, in mass spectrometry the sample is destroyed in the fragmentation process whereas the sample can be recovered after using IR, UV-visible and NMR spectroscopy. TECHNIQUE RADIATION WHAT CAN IT SEE? Copyright © 2009 Royal Society of Chemistry www.rsc.org Electrons flipping magnetic spin 10-3 m Nuclear Magnetic Resonance (NMR) spectroscopy Radio waves (10-3 m) How neighbouring atoms of certain nuclei (e.g. 1H, 13C, 19F, 31P) in a molecule are connected together, as well as how many atoms of these type are present in different locations in the molecule. 10-5 m NOTE Molecule vibrations Infra-red spectroscopy Infra-red (10-5 m) The functional groups which are present in a molecule. 10-8 m NOTE Electrons promoted to higher energy state UV-visible spectroscopy Ultra-violet (10-8 m) Conjugated systems (i.e. alternating single and double bonds) in organic molecules as well as the metal-ligand interactions in transition metal complexes. 10-10 m x-ray X-ray crystallography X-rays (10-10 m) How all the atoms in a molecule are connected in a three-dimensional arrangement. Molecules fragment + + + Mass spectrometry Non-spectroscopic technique The mass to charge ratio of the molecular ion (i.e. the molecular weight) and the fragmentation pattern which may be related to the structure of the molecular ion.