Microwave spectroscopy: Experimental techniques (Chapter 14)

For more than 15–20 years now, the field of microwave (MW) spectroscopy is progressing impressively: As will be seen in the subsequent chapter, this is partially by virtue of the experimental developments that combine jet-expansion sources with specific means of sample preparation for new chemical systems. But even more important, progress is due to advancements in experimental equipment, namely the rise of very sensitive timedomain techniques with high-resolution. Although structural studies continue to be a strength of rotational spectroscopy, many of the problems that became amenable for investigation in the recent years involve processes of intramolecular dynamics, such as conformational, tautomeric equilibria, and other large-amplitude motions as well as intermolecular vibrational energy redistribution and isomerization. Numerous interesting systems such as molecules with multiple internal motions, larger complexes, aggregates, biomolecules, and transient species can almost routinely be treated now. The studies also cover different kinds of intermolecular interactions, extending from hydrogen bonding and van der Waals interactions to the effects observed in quantum solvation. The development of pulsed excitation multiresonance techniques and the advent of real-time broadband microwave excitation and detection are impressively widening the capabilities of rotational spectroscopy to characterize the structure and dynamics of larger molecular species.