The Advanced Chemistry Series: Spectroscopy and the Electromagnetic Spectrum
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Interaction of electromagnetic radiation with matter. Infrared spectroscopy. Symmetry of molecules, group theory and its applications in vibrational spectroscopy. Group frequencies and assignments of infrared band. Sampling techniques and applications. Applications of infrared spectroscopy in basic and industrial research.
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Modern data analytical methods for infrared spectroscopy. This book is the latest addition to the Comprehensive Analytical Chemistry series. The chapters are designed to give the reader not only the understanding of the basics of infrared spectroscopy but also to give ideas on how to apply the technique in these different fields. Since spectroscopy is the study of the interaction of electromagnetic radiation with matter, the first two chapters deal with the characteristics, properties and absorption of electromagnetic radiation.
Chapter 3 provides the basis for vibrations in molecules from a classic mechanical point of view. Absorption of infrared radiation by a vibration in a molecule depends on the symmetry of the molecule and the symmetry of the vibrations. However, these symmetry aspects are not usually treated in textbooks on infrared spectroscopy.
Therefore, Chapter 4 deals with the symmetry aspects of molecules and illustrates how the reader can determine the vibrations that are infrared active. The amplitude is related to the intensity of the wave, which for light is the brightness, and for sound is the loudness. Thus, for electromagnetic radiation in a vacuum:. Wavelength and frequency are inversely proportional: As the wavelength increases, the frequency decreases. The inverse proportionality is illustrated in Figure 2. This figure also shows the electromagnetic spectrum , the range of all types of electromagnetic radiation.
Each of the various colors of visible light has specific frequencies and wavelengths associated with them, and you can see that visible light makes up only a small portion of the electromagnetic spectrum. Because the technologies developed to work in various parts of the electromagnetic spectrum are different, for reasons of convenience and historical legacies, different units are typically used for different parts of the spectrum. For example, radio waves are usually specified as frequencies typically in units of MHz , while the visible region is usually specified in wavelengths typically in units of nm or angstroms.
What is the frequency of this light? Since c is expressed in meters per second, we must also convert nm to meters. What is the wavelength in meters of these radio waves? Many valuable technologies operate in the radio 3 kHz GHz frequency region of the electromagnetic spectrum. At the low frequency low energy, long wavelength end of this region are AM amplitude modulation radio signals kHz that can travel long distances.
FM frequency modulation radio signals are used at higher frequencies In AM radio, the information is transmitted by varying the amplitude of the wave Figure 4.
In FM radio, by contrast, the amplitude is constant and the instantaneous frequency varies. Other technologies also operate in the radio-wave portion of the electromagnetic spectrum. The frequencies associated with these applications are convenient because such waves tend not to be absorbed much by common building materials. One particularly characteristic phenomenon of waves results when two or more waves come into contact: They interfere with each other. Figure 5 shows the interference patterns that arise when light passes through narrow slits closely spaced about a wavelength apart.
The Electromagnetic Spectrum
The fringe patterns produced depend on the wavelength, with the fringes being more closely spaced for shorter wavelength light passing through a given set of slits. When the light passes through the two slits, each slit effectively acts as a new source, resulting in two closely spaced waves coming into contact at the detector the camera in this case. The dark regions in Figure 5 correspond to regions where the peaks for the wave from one slit happen to coincide with the troughs for the wave from the other slit destructive interference , while the brightest regions correspond to the regions where the peaks for the two waves or their two troughs happen to coincide constructive interference.
Likewise, when two stones are tossed close together into a pond, interference patterns are visible in the interactions between the waves produced by the stones. Such interference patterns cannot be explained by particles moving according to the laws of classical mechanics.
Because the wavelengths of X-rays , picometers [pm] are comparable to the size of atoms, X-rays can be used to determine the structure of molecules. When a beam of X-rays is passed through molecules packed together in a crystal, the X-rays collide with the electrons and scatter. Constructive and destructive interference of these scattered X-rays creates a specific diffraction pattern.
Calculating backward from this pattern, the positions of each of the atoms in the molecule can be determined very precisely. One of the pioneers who helped create this technology was Dorothy Crowfoot Hodgkin. She was born in Cairo, Egypt, in , where her British parents were studying archeology.
Even as a young girl, she was fascinated with minerals and crystals. When she was a student at Oxford University, she began researching how X-ray crystallography could be used to determine the structure of biomolecules. She invented new techniques that allowed her and her students to determine the structures of vitamin B 12 , penicillin, and many other important molecules.
Diabetes, a disease that affects million people worldwide, involves the hormone insulin. Hodgkin began studying the structure of insulin in , but it required several decades of advances in the field before she finally reported the structure in Understanding the structure has led to better understanding of the disease and treatment options. Not all waves are travelling waves. Standing waves also known as stationary waves remain constrained within some region of space.
As we shall see, standing waves play an important role in our understanding of the electronic structure of atoms and molecules. The simplest example of a standing wave is a one-dimensional wave associated with a vibrating string that is held fixed at its two end points. Figure 6 shows the four lowest-energy standing waves the fundamental wave and the lowest three harmonics for a vibrating string at a particular amplitude.
The motion of string segments in a direction perpendicular to the string length generates the waves and so the amplitude of the waves is visible as the maximum displacement of the curves seen in Figure 6. The key observation from the figure is that only those waves having an integer number, n, of half-wavelengths between the end points can form.
A system with fixed end points such as this restricts the number and type of the possible waveforms. This is an example of quantization , in which only discrete values from a more general set of continuous values of some property are observed. Another important observation is that the harmonic waves those waves displaying more than one-half wavelength all have one or more points between the two end points that are not in motion.
These special points are nodes. The energies of the standing waves with a given amplitude in a vibrating string increase with the number of half-wavelengths n. Since the number of nodes is n — 1, the energy can also be said to depend on the number of nodes, generally increasing as the number of nodes increases. An example of two-dimensional standing waves is shown in Figure 7 , which shows the vibrational patterns on a flat surface.
Although the vibrational amplitudes cannot be seen like they could in the vibrating string, the nodes have been made visible by sprinkling the drum surface with a powder that collects on the areas of the surface that have minimal displacement. For one-dimensional standing waves, the nodes were points on the line, but for two-dimensional standing waves, the nodes are lines on the surface for three-dimensional standing waves, the nodes are two-dimensional surfaces within the three-dimensional volume.
Because of the circular symmetry of the drum surface, its boundary conditions the drum surface being tightly constrained to the circumference of the drum result in two types of nodes: radial nodes that sweep out all angles at constant radii and, thus, are seen as circles about the center, and angular nodes that sweep out all radii at constant angles and, thus, are seen as lines passing through the center. The upper left image in Figure 7 shows two radial nodes, while the image in the lower right shows the vibrational pattern associated with three radial nodes and two angular nodes.
You can watch the formation of various radial nodes here as singer Imogen Heap projects her voice across a kettle drum. The last few decades of the nineteenth century witnessed intense research activity in commercializing newly discovered electric lighting.
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This required obtaining a better understanding of the distributions of light emitted from various sources being considered. Artificial lighting is usually designed to mimic natural sunlight within the limitations of the underlying technology. Such lighting consists of a range of broadly distributed frequencies that form a continuous spectrum.
Figure 8 shows the wavelength distribution for sunlight. The most intense radiation is in the visible region, with the intensity dropping off rapidly for shorter wavelength ultraviolet UV light, and more slowly for longer wavelength infrared IR light. The blackbody spectrum matches the solar spectrum quite well. A blackbody is a convenient, ideal emitter that approximates the behavior of many materials when heated. A good approximation of a blackbody that can be used to observe blackbody radiation is a metal oven that can be heated to very high temperatures. The oven has a small hole allowing for the light being emitted within the oven to be observed with a spectrometer so that the wavelengths and their intensities can be measured.
Figure 9 shows the resulting curves for some representative temperatures. Each distribution depends only on a single parameter: the temperature. This common observation was at the heart of the first paradox that showed the fundamental limitations of classical physics that we will examine. Physicists derived mathematical expressions for the blackbody curves using well-accepted concepts from the theories of classical mechanics and classical electromagnetism.
The theoretical expressions as functions of temperature fit the observed experimental blackbody curves well at longer wavelengths, but showed significant discrepancies at shorter wavelengths. Not only did the theoretical curves not show a peak, they absurdly showed the intensity becoming infinitely large as the wavelength became smaller, which would imply that everyday objects at room temperature should be emitting large amounts of UV light. Finally, around , Max Planck derived a theoretical expression for blackbody radiation that fit the experimental observations exactly within experimental error.
Planck developed his theoretical treatment by extending the earlier work that had been based on the premise that the atoms composing the oven vibrated at increasing frequencies or decreasing wavelengths as the temperature increased, with these vibrations being the source of the emitted electromagnetic radiation.
But where the earlier treatments had allowed the vibrating atoms to have any energy values obtained from a continuous set of energies perfectly reasonable, according to classical physics , Planck found that by restricting the vibrational energies to discrete values for each frequency, he could derive an expression for blackbody radiation that correctly had the intensity dropping rapidly for the short wavelengths in the UV region.
Although Planck was pleased he had resolved the blackbody radiation paradox, he was disturbed that to do so, he needed to assume the vibrating atoms required quantized energies, which he was unable to explain. If the atom gains energy the electron passes from a lower energy level to a higher energy level, energy is absorbed that means a specific wave length is absorbed. Consequently, a dark line will appear in the spectrum. This dark line constitutes the absorption spectrum.
If the atom loses energy, the electron passes from higher to a lower energy level, energy is released and a spectral line of specific wavelength is emitted. These colour are so continuous that each of them merges into the next. Hence the spectrum is called as continuous spectrum.
If this light is resolved by a spectroscope, It is found that some isolated colored lines are obtained on a photographic plate separated from each other by dark spaces. This spectrum is called line spectrum. Each line in the spectrum corresponds to a particular wavelength. Each element gives its own characteristic spectrum. Absorption spectrum : This kind of spectrum has dark band of lines when electromagnetic radiation is passed through a material which absorbs it.
The bands corresponding to the energy absorbed by the material appears dark in the spectrum. Emission spectrum : This kind of spectrum is observed when the energy absorbed by a material is emitted. When energy is absorbed by a material, its electrons get excited to a higher energy state. When the excited electrons return to their ground state they emit radiation. If an electric discharge is passed through hydrogen gas taken in a discharge tube under low pressure, and the emitted radiation is analyzed with the help of spectrograph, it is found to consist of a series of sharp lines in the UV, visible and IR regions.
This series of lines is known as line or atomic spectrum of hydrogen. The lines in the visible region can be directly seen on the photographic film. Each line of the spectrum corresponds to a light of definite wavelength. The entire spectrum consists of six series of lines, each series, known after their discoverer as the Balmer, Paschen, Lyman, Brackett, Pfund and Humphrey series.
The wavelength of all these series can be expressed by a single formula. Infra — red. Reference books of Physical Chemistry.