Molecular spectroscopy involves the interaction of electromagnetic radiation with the molecules of the material in question. Before considering the spectroscopy itself, we will look at the electromagnetic spectrum and briefly explain the nature of light.
Many things in nature wiggle back and forth. We call a wiggle in time a vibration. We call a wiggle in time and space a wave. Some examples are the vibrations of a guitar string, a mass attached to a spring, the swinging of a pendulum, and atoms in molecules.
The to-and-fro motion of a pendulum bob along a single plane in a small arc is called simple harmonic motion. If a pendulum bob leaks sand while undergoing simple harmonic motion, it will trace and retrace a short straight line. If we were to swing a pendulum above a conveyor belt that moves in a perpendicular direction to the plane of the swinging pendulum, the trace it now makes is called a sine curve. This curve is a pictorial representation of a wave.
The points of maximum displacement above the equilibrium position are called crests and the points of maximum displacement below the equilibrium position are called troughs. The amplitude is equal to the maximum displacement of the pendulum from its position of rest. The distance from the top of one crest to the top of the next one is equal to the wavelength. How often a vibration occurs is described by its frequency. The frequency of a vibrating object specifies the number of to-and-fro vibrations it makes in a given time (usually seconds). The source of all periodic waves is something that vibrates. The frequency of the vibrating source and the frequency of the wave it produces are the same.
If the end of a stick is moved back and forth in still water waves are produced on the water surface. Similarly if an electrically charged rod is shaken to and fro in empty space electromagnetic waves are produced. This is because the moving charge is actually an electric current, and a magnetic field surrounds an electric current. In accordance with Faraday’s law a changing magnetic field induces a changing electric field. The changing electric field, in accordance with Maxwell’s counterpart to Faraday’s law, will induce a changing magnetic field. The vibrating electric and magnetic fields regenerate each other to make up the electromagnetic wave which moves outward from the vibrating charge. For most purposes, light can be thought of as a traveling electrical field alternating from positive to negative values. An electric field is measured by the force it exerts on a charge.
In a vacuum, all electromagnetic waves move at the same speed and differ from one another in their frequency (() and wavelength ((). The frequency of the electromagnetic wave as it vibrates through space is identical to the frequency of the oscillating electric charge generating it. The classification of electromagnetic waves according to frequency is the electromagnetic spectrum. The spectrum is broken into arbitrary regions for classification. Visible light makes up a very small portion of electromagnetic spectrum. The highest frequency visible light appears violet. Higher frequencies cannot be detected by the human eyes and are referred to as the ultraviolet. Frequencies higher than the ultraviolet extend into the x-ray and gamma ray region. Below the lowest frequency of visible light is the infrared radiation, often called heat waves. Still lower are microwaves and radiowaves.
Some useful relationships between the speed of electromagnetic radiation (c), wavelength ((), frequency ((), and the energy per photon (E) are given below. Planck’s constant (h= 6.63 x 10 -34 joule·seconds) allows us to calculate the energy if the frequency or wavelength is known. Notice that energy is directly proportional to frequency and inversely proportional to wavelength.
E=h( E=hc/( c=((
Molecular spectroscopy involves the absorption of electromagnetic radiation by the material whose molecular structure we are attempting to determine. The relationship that describes the amount of radiation absorbed is Beer’s law. The absorption (A) is proportional to the inherent absorbing ability of the substance ((; molar absorptivity or extinction coefficient), the concentration of the absorbing compound (c) and the distance the radiation is traveling through the sample (l).