Absorption spectroscopy

The majority of physical techniques used to investigate inorganic compounds involve the absorption and sometimes the re-emission of electromagnetic radiation. The frequency of the radiation absorbed provides useful information on the energy levels of an inorganic compound and the intensity of the absorption can often be used to provide quantitative analytical information. Absorption spectroscopy techniques are normally nondestructive as after the measurement the sample can be recovered for further analysis. The spectrum of electromagnetic radiation used in chemistry ranges from the short wavelengths associated with - and X-rays (about 1 nm), to radiowaves with wave lengths of several metres (Fig. 8.8). This spectrum covers the full range of atomic and molecular energies associated with characteristic phenomena such as ionization, vibration, rotation, and nuclear reorientation. Thus, X- and ultraviolet (UV) radiation can be used to determine the electronic structures of atoms and molecules and infrared (IR) radiation can be used to examine their vibrational behaviour. Radiofrequency (RF) radiation, in nuclear magnetic resonance (NMR), can be used to explore the energies associated with reorientations of the nucleus in a magnetic field, and those energies are sensitive to the chemical environment of the nucleus. In general, absorption

Figure 8.8 The electromagnetic spectrum with wavelengths and techniques that make use of the different regions.

spectroscopic methods make use of the absorption of electromagnetic radiation by a molecule or material at a characteristic frequency corresponding to the energy of a transition between the relevant energy levels. The intensity is related to the probability of the transition, which in turn is determined in part by symmetry rules, such as those described in Chapter 6 for vibrational spectroscopy. The various spectroscopic techniques involving electromagnetic radiation have different associated timescales. This variation can influence the structural information that is extracted. When a photon interacts with an atom or molecule we need to consider fac tors such as the lifetime of any excited state and a how a molecule may change during that interval. Table 8.2 summarizes the timescales associated with various spectroscopic techniques discussed in this section. Thus IR spectroscopy takes a much faster snapshot of the molecular structure than NMR, for a molecule may have time to reorientate or change shape in a nanosecond. The temperature at which data are collected should also be taken into account as molecular reorientation rates increase with increasing temperature.

A brief illustration. Iron pentacarbonyl, Fe(CO)₅, illustrates why consideration of such timescales is important when analysing the spectra to obtain structural information. Infrared spectroscopy suggests that Fe(CO)₅ has D3h symmetry with distinct axial and equatorial carbonyl groups whereas NMR suggests that all the carbonyl groups are equivalent.

Table 8.2 Typical timescales of some common characterization methods

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