Chapter 4 - Environmental Dependence of Vibrational Spectra

Abstract

This chapter examines the dependence of vibrational spectra on the environment of the molecule. In the gaseous state, the molecules can be considered to be noninteracting and can rotate freely with a characteristic angular momentum. Coupling of the vibrational transition with the rotational degrees of freedom of the molecule occurs resulting in band structure. This band structure is highly dependent upon the molecular symmetry. In the liquid state, molecules may have any orientation relative to the spectrometer. Here, hydrogen bonding can have a dramatic effect on the IR and Raman spectra. In the solid state, the crystallinity of the molecule is important, and the nature of the sample preparation techniques employed can also be important. In general, crystalline solids can be frozen into one or more configurations fixed in a lattice resulting in sharper bands. Both IR and Raman spectroscopies are very sensitive to the molecular interactions involved in hydrogen bonding. Intramolecular hydrogen bonding can have a dramatic effect on the IR and Raman spectra and typically favors formation of five- and six-membered rings. Conjugated intramolecular hydrogen bonding systems in particular can result in very strong hydrogen bonds. Fermi resonance involves a fundamental that interacts with a combination or overtone band resulting in two relatively strong bands, where only one nearly coincident fundamental is expected. Because the frequencies of the fundamentals themselves are typically environmentally sensitive, changes in solvents or crystalline state can affect the observed Fermi resonance band intensities.