Mid‐Infrared Spectroscopy of the Condensed Phase

Abstract This cle introduces some of the main issues that affect the mid‐infrared study of condensed phase samples. It serves as a brief introduction to many of the articles on mid‐infrared spectroscopy that follow in this Handbook. A brief discussion is given of the differences between condensed‐phase (liquid and solid) and gas‐phase absorption band shapes. In the liquid‐state, with some compounds tautomerism can occur such that there is an equilibrium between two or more structural isomers, each of which will have a distinctive spectrum. Many compounds (organic, inorganic, polymeric) are polymorphic, that is they may crystallize into two or more different form. As a consequence of different packing in the crystals each form will have a unique mid‐infrared spectrum, although in some instance the distinctions may only be slight. Mid‐infrared spectroscopy is perhaps the foremost informative technique for studying hydrogen bonding. Enantiomers have indistinguishable mid‐infrared spectra, unless interrogated by the special technique of vibrational circular dichroism (VCD). Many solid materials contain water, either as an integral part of a structure or absorbed within such as a thermoplastic polymer; examples are given of different absorption band profiles in two inorganic systems. Crystal field and factor group splitting of fundamental bands pertaining to crystal structures is discussed, as is the spectra of the disordered state and the influence of defect‐induced structures and spectral changes that occur during the transition in solids from the crystalline through the glassy to the amorphous state. The influences that a sample's bulk physical form and its physical environment may impose on a mid‐infrared spectrum are also introduced. The effects of temperature, pressure, stress and strain on bandwidths and position are briefly considered. For powders, particle size and packing density influences are outlined; the effects of polished and matte surfaces are discussed. For samples such as oriented polymer film or birefringent crystal, alignment of the sample axes with respect to the geometric axes of the spectrometer may have an effect on the relative intensities of bands observed in the recorded spectrum. Their anisotropy may be probed using linearly polarized infrared radiation. Continuous, non‐scattering samples that are uniformly thick may induce multiple passing of the infrared beam within the sample, such that superimposed on a transmission spectrum of the sample one will observe a sinusoidal interference fringe pattern.