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Ebook Introduction to spectroscopy (4th edition): Part 2

(BQ) Part 2 book "Introduction to spectroscopy" has contents: Ultraviolet spectroscopy, mass spectrometry, combined structure problems, nuclear magnetic resonance spectroscopy. | 14782_07_Ch7_p381-417.pp2.qxd 2/2/08 1:22 AM Page 381 C H A P T E R 7 ULTRAVIOLET SPECTROSCOPY M ost organic molecules and functional groups are transparent in the portions of the electromagnetic spectrum that we call the ultraviolet (UV) and visible (VIS) regions—that is, the regions where wavelengths range from 190 nm to 800 nm. Consequently, absorption spectroscopy is of limited utility in this range of wavelengths. However, in some cases we can derive useful information from these regions of the spectrum. That information, when combined with the detail provided by infrared and nuclear magnetic resonance (NMR) spectra, can lead to valuable structural proposals. 7.1 THE NATURE OF ELECTRONIC EXCITATIONS When continuous radiation passes through a transparent material, a portion of the radiation may be absorbed. If that occurs, the residual radiation, when it is passed through a prism, yields a spectrum with gaps in it, called an absorption spectrum. As a result of energy absorption, atoms or molecules pass from a state of low energy (the initial, or ground state) to a state of higher energy (the excited state). Figure 7.1 depicts this excitation process, which is quantized. The electromagnetic radiation that is absorbed has energy exactly equal to the energy difference between the excited and ground states. In the case of ultraviolet and visible spectroscopy, the transitions that result in the absorption of electromagnetic radiation in this region of the spectrum are transitions between electronic energy levels. As a molecule absorbs energy, an electron is promoted from an occupied orbital to an unoccupied orbital of greater potential energy. Generally, the most probable transition is from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The energy differences between electronic levels in most molecules vary from 125 to 650 kJ/mole (kilojoules per mole). For most molecules, the lowest-energy occupied .