Absorption of Energy by Molecules

The spectra of atoms are often called line spectra, for it will be recalled that they consist of a relatively small number of discrete wavelengths, represented as thin lines on the traditional spectrograms. On the other hand, the spectra of molecules are termed band spectra, for they consist 

The energies necessary for electronic excitation of molecules are comparable to those necessary for the excitation of the valence electrons of atoms—that is, 1.5 to 8.0 electron volts. From the Planck relationship (p. 6), the student may verify (Exercise 10) the relationship between the energy change AE associated with a process and the wavelength, X, of the resulting photon  

In view of the different ways in which a molecule may absorb quantized energy, spectroscopists often describe the energy levels of molecules using quantum numbers (only very roughly analogous to the quantum 

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Chapter 16 dealt with the spectrometric determination of substances in solution, that is, the absorption of energy by molecules, either organic or inorganic. This chapter deals with the spectroscopy of atoms. Since atoms are the simplest and purest form of matter and cannot rotate or vibrate as a molecule does, only electronic transitions within the atom can take place when energy is absorbed. Because the transitions are discrete (quantized), line spectra are observed. There are various ways to obtain free atoms (atomic vapor) and to measure the absorption or emission of radiation by these. 

As diverse as these techniques are all of them are based on the absorption of energy by a molecule and all measure how a molecule responds to that absorption In describing these techniques our emphasis will be on then application to structure determination We 11 start with a brief discussion of electromagnetic radiation which is the source of the energy that a molecule absorbs m NMR IR and UV VIS spectroscopy... 

The preceding calculation of the thermal energy balance of a planet neglected any absorption of radiation by molecules within the atmosphere. Radiation trapping in the infrared by molecules such as CO2 and H20 provides an additional mechanism for raising the surface temperature - the greenhouse effect. The local temperature of a planet can then be enhanced over its black body temperature by the atmosphere. 

What about molecules and complex ions Absorption of light by molecules and complex ions results in the promotion of electrons to higher energy states in the same way as in atoms. However, it is more complicated because molecules and complex ions have energy states that atoms do not. 

Nuclear magnetic resonance (NMR) spectroscopy this involves the absorption of energy by specific atomic nuclei in magnetic fields and is probably the most powerful tool available for the structural determination of molecules (Chapter 29). 

In order to better understand the emission and absorption of light by molecules, it is necessary to look at the quantum-mechanical concept of the nature of tight In this concept, light is considered to be a beam of photons whose energies are quantized. Detailed description of quantum mechanics and spectroscopy is beyond the scope of this book. Here will only be presented the major conclusions necessary for better understanding of light. 

We interpret the first delta function as representing the absorption of energy by the acceptor molecule, while the second delta function represents the emission of energy by the donor. 

The same equation also applies to the absorption of light by atoms. The frequency of the light absorbed in the transition from a lower state to an upper state is equal to the difference in energy of the upper state and the lower state divided by Planck s constant. The equation also applies to the emission and absorption of light by molecules and more complex systems. 

In this chapter we have introduced some of the basic concepts of photochemistry. We have shown some of the possible effects resulting from absorption of photons by molecules. While photochemistry is a mature science there are stiU constant new and exciting developments, both recently found and waiting to be discovered. It remains a very active area of research in both fundamental science and across diverse areas of applications from medicine to sustainable energy. The following chapters explore these in more depth. 

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