UV/VISIBLE SPECTROPHOTOMETRY Ultraviolet Light and Matter
All matter consists of atoms that are made up of negatively charged electrons that inhabit orbitals that exist in approximately concentric spheres around the nucleus, which is made up of positively charged protons and neutral neutrons. In a neutral atom, there are equal numbers of electrons and protons, so there is no net positive or negative charge. When atoms combine to make molecules (the building blocks of compounds, materials, or substances), they do so by sharing or donating/accepting electrons to form covalent or ionic bonds. The electrons that are shared are those that are farthest from the nucleus, called the valence electrons. Valence electrons in atoms and in molecules can be promoted to a higher energy level by absorbing energy from light or other energy sources. This process is said to be quantized, because the atom or molecule can only absorb the exact amount of energy that corresponds to the dif ference in energy between the occupied and unoccupied energy level. This packet of energy is called a photon. In other words, a molecule will absorb energy and pro mote an electron if it is exposed to a photon of the proper energy. One can visualize these energy levels to be like stairs on a staircase; you can be on one stair or the next stair, but cannot occupy the space between stairs. Figure 5.4 illustrates this. A photon that causes electron promotion in atoms and molecules is in the UV/ visible region of the electromagnetic spectrum. When a substance is exposed to UV/ visible radiation, it will absorb certain photons of particular energy (and thus par ticular frequencies or wavelengths). If the amount of each wavelength of light that is absorbed by a substance throughout the UV/visible region is plotted, a spectrum is generated. See Figure 5.5 for the UV/visible spectrum of heroin. Note that the peaks in the UV/visible spectrum of a typical substance tend to be few in number and quite broad in shape. This is due to the nature of the absorbance of this type of energy—there are not too many electrons that can be promoted in a typical molecule so there are not very many wavelengths where an appreciable number of photons are absorbed. The broadness of the peaks is due to the temperature; at very low temperatures, UV absorptions are narrower. The practical effect of these characteristics of UV/visible spectra is that they are not commonly used for absolute identification of a pure chemical substance. Closely related substances exhibit UV/visible spectra that are practically indistinguishable. For example, morphine and heroin (which is derived from morphine and is similar in structure) have very similar UV/visible spectra. Not every substance will absorb energy in the UV/visible range. Certainly, any substance that appears to the human eye as possessing a color will absorb in this region because the sensation of color is caused by light reflection from a substance that is received by our optic nerves, which in turn, send a signal to the brain that is registered as the quality of color. Many organic substances will also have a UV/visible spectrum because they usually possess a number of conjugated carbon/carbon double bonds. These are alternating single and double (or triple) bonds in the molecule. Any compound that is based on the benzene ring, for example, will absorb strongly in the UV/visible region. It is conceivable that several substances could have the same chromophore and thus the same UV/visible spectrum. This is one reason why UV/visible spectra cannot be used for unequivocal identification of a substance. This can be seen in Figure 5.6, which shows the structures of morphine and diacetylmorphine (heroin). As mentioned above, these two substances have practically indistinguishable UV/visible spectra.
FIGURE 5.4 Electronic energy levels (orbitals) surround the nucleus of an atom. One of their properties is that they are quantized. An electron must reside in an energy level. It cannot be between two of them. In that sense, energy levels are like stairs on a staircase. A person who is climbing a staircase can stop on any stair but cannot stop between stairs.
FIGURE 5.5 The ultraviolet spectrum of heroin. The benzene ring backbone of heroin gives the UV spectrum its characteristic shape. Substituted benzene structures all have their major absorbance near 265 nm.
FIGURE 5.6 Structures of morphine and diacetylmorphine (heroin). The structures of these two substances are very similar, differing only in that the two OH groups on morphine have been replaced by AcO (acetate) groups on heroin. Because of their similarity in structure, their UV spectra are very similar.
