The electromagnetic spectrum

The following is a simplified schematic representation of an absorption spectrometer  

The types of spectroscopy that are of most use to oiganic chemists employ infrared light or radiation in the radio region. Light in the infrared region has energy that 

Types of absorption spectra line spectrum and broadband spectrum. 

The absorption of radiation depends on the structure of the organic compound and the wavelength of the radiation. Different wavelengths of radiation affect organic compounds in different ways. The absorption of radiation increases the energy of the organic molecule. This can lead to (i) excitation of electrons from one molecular orbital to another in UV spectroscopy (ii) increased molecular motions (e.g. vibrations) 

Type of spectroscopy Radiation source Energy range (kJ mol Type of transition 

The wavelength, A, of electromagnetic radiation is the distance between any two peaks or troughs of the wave. 

The energy, E associated with electromagnetic radiation is quantized. The relationship is given by 

The energy of electrom netic radiation is also directly proportional to the quantity 1/A. This quantity is known as the wavenumber. 

The regions of the spectrum used in organic chemistry. Usually, the wavelength or the reciprocal of the wavelength, the wavenumber, is used to identify absorptions of organic molecules. The visible spectrum is only a tiny sliver of the entire electromagnetic spectrum. 

The portion of the spectrum where no absorption occurs is the base line. This horizontal line may be located at the top or bottom of a graph. Absorption then is recorded as a peak down from the base line. In an infrared spectrum (a), the base line is at top of the spectrum. In an NMR spectrum (b), the base line is at the bottom of the spectrum. 

The electromagnetic spectrum - units. The wavelengths of electromagnetic radiation of interest vary from metres for the radiofrequency range to about 10 10 m for X-rays. A wave has associated with it both wavelength, X, and frequency, v, which are related by the equation  

The specific regions and the phenomena they produce are correlated with the wavelength and the frequency in Table 3.1. 

Spectral region Wavelength Frequency in wavenumbers (cm-1) Special phenomena 

The limitations on the extent of the various regions given above are, of course, arbitrary. 

There are two main types of molecular vibrations stretching and bending. A stretching vibration is a vibration along a bond axis such that the distance between the two atoms is decreased or increased. A bending vibration involves a change in bond angles. 

Next on the electromagnetic spectrum, with longer wavelengths than ganunarays, are X-rays, familiar to us from their medical use. X-rays pass through many substances that 

A To produce a medical X-ray, the patient is exposed to short-wavelength electromagnetic radiation that can pass through the skin to create an image of bones and internal organs. 

Beyond visible light lies infrared (IR) radiation. The heat you feel when you place your hand near a hot object is infrared radiation. All warm objects, including human bodies, emit infrared light. Although infrared hght is invisible to our eyes, infrared sensors can detect it and are often employed in night vision technology to help people see in the dark. 

Another side effect of exposing healthy cells to radiation is that they too may become cancerous. If a treatment for cancer may cause cancer, why do we continue to use it In radiation therapy, as in most other disease therapies, there is an associated risk. We take risks all the time, many of them for lesso reasons. For example, every time we fly in an airplane or drive in a car, we risk injury or even death. Why Because we perceive the benefit—the convenience of being able to travel a significant distance in a short time—to be worth the relatively small risk. The situation is similar in cancer therapy, or any other medical therapy for that matter. 

The benefit of cancer therapy (possibly curing a cancer that might othawise kill you) is worth the risk (a slight increase in the chance of developing a future cancer). 

Fragmentation of halogenoalkanes (R—X) usually leads to cleavage of the weak carbon-halogen bond to form a carbocation (R ) and a halogen radical (or atom, X ). 

Fragmentation of alcohols (ROH), ethers (ROR) or amines (e.g. RNH2) usually leads to cleavage of the C—C bond next to the heteroatom to generate a resonance-stabilised carbocation. This is called a-cleavage. 

Notice the use of single-headed eurly arrows (Section 4.1) 

Ultraviolet (UV), infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy involve the interaction of molecules with electromagnetic radiation. When 

In a vacuum all electromagnetic radiation travels at the same speed, the speed of light c, and may be characterized by its wavelength X, in air or vacuum, or by its wavenumber v or frequency v, both conventionally in a vacuum, where 

Radio TV Microwave Millimetre wave Mid-infrared Near- infrared Visible Near- ultraviolet Far- ultraviolet X-ray Y-ray 

The biological effects of EMR are best ascribed to four regions of the spectrum  

Ionizing. This is the high frequency area of the spectrum where chemical bonds are broken and direct damage to cellular destruction occurs. Examples are gamma radiation and x-rays. 

Microwave and radiofrequency (RF) The spectral region where cellular heating is induced. Examples are microwaves and radio transmissions. 

Power frequency. The spectral region where energies are sufficiently low so that cellular heating is not readily induced. The most common and most studied example is that of electrical power line generated waves. 

Spectroscopic processes rely on the fact that electromagnetic radiation (EMR) interacts with atoms and molecules in discrete ways to produce characteristic absorption or emission profiles. This is examined in more detail in Section 2.2. Before we can look into the origin of spectra, we have to look at some of the properties of EMR. 

Our ability to perceive colour depends on many factors. However, the interaction mechanism of EMR with matter is of major importance. These optical processes will be discussed in more detail in the next section. From a visual detection point of view, our ability to perceive different colours is dependent on the optical process involved, for example, if the light is absorbed or reflected by the observed object. 

The wavelength, X, of EMR can be expressed as a function of its frequency, v, and the speed of light, c, by the following simple equation  

However, EMR behaves as a particle and as a wave (the dual nature of light) and the wavelength of such a particle, a photon, is related to energy by the equation 

The visible region of the electromagnetic spectrum constitutes but a tiny part, as can be seen from Fig. 3. 

A corresponding experiment on the violet end of the spectrum was performed by Johaim Wilhelm Ritter (1776—1810), a German chemist and physicist. Ritter placed silver chloride crystals, that were known to darken on exposure to light, in 

Hard Soft Vacuum Near ible Near Mid Far Sub- mm- Micro- Radio- 

On 22 February, I also encountered rays alongside violet in the colour spectrum of colours—outside it—by means of horn silver. They reduce even more strongly than violet light itself and the field of these rays is very wide. 

Other regions of the spectrum were gradually discovered and characterized so that now, in Fig. 2.4, we can see the entire electromagnetic spectrum laid out. 

If we can t see the regions of the spectmm beyond the visible, why are they important As we can see from the diagram, the other regions can be very useful. They include microwaves and radio waves, without which modem life would be inconceivable. They also include the high energy regions, which can be very 

FIGURE 14.1 Light acts as a wave, with a wavelength A and a frequency v. In vacuum, all light has the same velocity, 2.9979 X 10 m/s. This value is given the symbol c. 

Light travels at different speeds in different media (like air or water), but because gases are so dispersed the speed of light through air is usually treated as the same as in vacuum. This assumption is not valid for condensed phases like water, glass, or any other transparent medium. 

The quantum theory of light, discussed in Chapter 9, provides a relationship between the energy of light and its frequency. Recall that light of a particular frequency v comes in bundles of energy (which we call photons) having a certain, specific amount given by the formula 

Equations 14.4 and 14.5 allow us to convert between energy, wavelength, and frequency of light. 

It is typical to divide the possible values of wavelength/frequency/energy of light into various regions. Table 14.1 lists the approximate frequencies, wavelengths, and 

Ultraviolet (UV) spectroscopy, covered in Chapter 15, observes electronic transitions and provides information on the electronic bonding in the sample. 

These spectroscopic techniques are complementary, and they are most powerful when used together. In many cases, an unknown compound cannot be completely identified from one spectrum without additional information, yet the structure can be determined with confidence using two or more different types of spectra In Chapter 13, we consider how clues from different types of spectroscopy are combined to provide a reliable structure. 

The wavelength and frequency, which are inversely proportional, are related by the equation 

Electromagnetic waves travel as photons, which are massless packets of energy. The energy of a photon is proportional to its frequency and inversely proportional to its wavelength. A photon of frequency v (or wavelength A) has an energy given by 

Gamma X-rays Ultraviolet Infrared Microwave Radio and TV Waves 

FIGURE 7.4 The electromagnetic spectrum. The exponential numbers across the top are approximate wavelength values. Energy increases right to left. 

FIGURE 7.5 The visible region of the electromagnetic spectrum. (From Kenkel, ]., Kelter, R, and Hage, D., Chemistry An Industry-Based Introduction with CD-ROM, CRC Press, Boca Raton, FL, 2001. With permission.) 

FIGURE 7.6 A representation of the absorption of visible light by a sheet of paper, resulting in the paper having a red color. The following abbreviations are used vi = violet, bl = blue, gr = green, yl = yellow, or = orange, and re = red. 

Cosmic rays 1022 3x1011 9.5x108 4.1x107 3x10-14 3x10-4 A 

Gamma TiQrs Nuclear transitions 1019 3x107 9.5x105 4.1x104 3x10-10 3 A 

Soft X-rays Inner electron transitions 1017 1x105 9.5x103 4.1x102 3x10-8 300 A 

Vacuum uv Valance elecbmi transitions 1.5x1015 5x104 143 6.2 2x10-7 200 nm 

Nearir Vibrational ir overtone and combinatiem region 1.2x1014 4x103 12 0.52 2.5x10-6 2.5 pm 

Type of radiation Frequency range (Hz) Wavelength range Type of transition 

Near-infrared 10 Mxl0 2.5 pm-750 nm Outer electron molecular 

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Buildings Humans Honey Bee Pinpoint Protozoans Molecules Atoms Atomic Nuclei 

Loss of vaiency eiectrons Core-electron transitions 

Wavelength, or frequency, v, are, in principle, equally valid as energy units for characterizing these radiations, and indeed are the preferred units in other parts of the electromagnetic spectrum. Relationships between these quantities for all electromagnetic radiation are  

We have already seen that gamma emissions are the result of transitions between the excited states of nuclei. As the whole technique of gamma spectrometry rests on (a) the uniqueness of gamma energies in the characterization of radioactive species, and (b) the high precision with which such energies can be measured, it is of interest to consider briefly some relevant properties of the excited states. 

Some types of dishes contain substances that absorb microwave radiation, but most do not. 

The longest wavelengths of light are radio waves, which are used to transmit the signals used by AM and FM radio, cellular telephones, television, and other forms of communication. 

CAN YOU ANSWER THIS W/iy would visible light not work to destroy cancerous tumors  

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In the electromagnetic spectrum, the ultra violet region is between that of X-rays and visible light. This corresponds to the energies hv ot one hundred to a few tens of electron-volts (wavelengths from 180 to 400 nm). 

Spectroscopy, or the study of the interaction of light with matter, has become one of the major tools of the natural and physical sciences during this century. As the wavelength of the radiation is varied across the electromagnetic spectrum, characteristic properties of atoms, molecules, liquids and solids are probed. In the... 

MW frequency of 10 Hz. There are various considerations that influence the choice of the radiation frequency. Higher frequencies, which require higher magnetic fields, give inlierently greater sensitivity by virtue of a more favourable Boltzmaim factor (see equation (b 1.15.11)). However, several factors place limits on the frequency employed, so that frequencies in the MW region of the electromagnetic spectrum remain favoured. One limitation is the sample size at frequencies around 40 GHz the dimensions of a typical... 

The positions of hnes or bands in the electromagnetic spectrum may be expressed either as wave lengths (X) or as frequencies (v). The units employed in the measurement of wave lengths are ... 

The wave lengths of the various parts of the electromagnetic spectrum of immediate interest are ... 

The electromagnetic spectrum showing the colors of the visible spectrum. 

The determination of an analyte s concentration based on its absorption of ultraviolet or visible radiation is one of the most frequently encountered quantitative analytical methods. One reason for its popularity is that many organic and inorganic compounds have strong absorption bands in the UV/Vis region of the electromagnetic spectrum. In addition, analytes that do not absorb UV/Vis radiation, or that absorb such radiation only weakly, frequently can be chemically coupled to a species that does. For example, nonabsorbing solutions of Pb + can be reacted with dithizone to form the red Pb-dithizonate complex. An additional advantage to UV/Vis absorption is that in most cases it is relatively easy to adjust experimental and instrumental conditions so that Beer s law is obeyed. 

The so-called peak power delivered by a pulsed laser is often far greater than that for a continuous one. Whereas many substances absorb radiation in the ultraviolet and infrared regions of the electromagnetic spectrum, relatively few substances are colored. Therefore, a laser that emits only visible light will not be as generally useful as one that emits in the ultraviolet or infrared ends of the spectrum. Further, witli a visible-band laser, colored substances absorb more or less energy depending on the color. Thus two identical polymer samples, one dyed red and one blue, would desorb and ionize with very different efficiencies. 

A dye molecule has one or more absorption bands in the visible region of the electromagnetic spectrum (approximately 350-700 nm). After absorbing photons, the electronically excited molecules transfer to a more stable (triplet) state, which eventually emits photons (fluoresces) at a longer wavelength (composing three-level system.) The delay allows an inverted population to build up. Sometimes there are more than three levels. For example, the europium complex (Figure 18.15) has a four-level system. 

Neodymium and YAG Lasers. The principle of neodymium and YAG lasers is very similar to that of the ruby laser. Neodymium ions (Nd +) are used in place of Cr + and are often distributed in glass rather than in alumina. The light from the neodymium laser has a wavelength of 1060 nm (1.06 xm) it emits in the infrared region of the electromagnetic spectrum. Yttrium (Y) ions in alumina (A) compose a form of the naturally occurring garnet (G), hence the name, YAG laser. Like the ruby laser, the Nd and YAG lasers operate from three- and four-level excited-state processes. 

The electromagnetic spectrum measures the absorption of radiation energy as a function of the frequency of the radiation. The loss spectrum measures the absorption of mechanical energy as a function of the frequency of the stress-strain oscillation. 

In the electromagnetic spectrum, the energy absorbed makes up the difference between two allowed energy states in the absorber. In the loss spectrum the frequency absorbed closely matches the frequency of dissipative modes of molecular motion in the sample. 

The electromagnetic spectrum is a quantum effect and the width of a spectral feature is traceable to the Heisenberg uncertainty principle. The mechanical spectrum is a classical resonance effect and the width of a feature indicates a range of closely related r values for the model elements. 

Theoretical analysis of certain features in the electromagnetic spectrum yields basic molecular parameters such as bond lengths and bond stiffness. We shall see presently that the mechanical spectra can be related to molecular parameters and not just modelistic characteristics as we have used until now. 

The scattering of visible light by polymer solutions is our primary interest in this chapter. However, since is a function of the ratio R/X, as we saw in the last section, the phenomena we discuss are applicable to the entire range of the electromagnetic spectrum. Accordingly, a general review of the properties of this radiation and its interactions with matter is worthwhile before a specific consideration of scattering. 

Table 3.1 summarizes the details of typical sources, absorption cells, dispersing elements and detectors used in different regions of the electromagnetic spectrum. 

Assuming reasonable bond lengths, estimate the frequency of the J= 15 — 14 transition in the linear molecule N=C—(C=C)6—H. In which region of the electromagnetic spectrum does it lie ... 

Question. Calculate, to three significant figures, the wavelength of the first member of each of the series in the spectrum of atomic hydrogen with the quantum number (see Section f.2) n" = 90 and 166. In which region of the electromagnetic spectrum do these transitions appear ... 

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