Microwave spectroscopy: Molecular systems (Chapter 15)

The development of Fourier transform microwave (FTMW) spectroscopy and - most notably - its combination with supersonic-jet expansion techniques allows for challenging investigations of chemically and physically interesting molecular systems. These are, among others, in situ prepared molecular species such as radicals, ions, and other transient species, generated by combining an electrical discharge, laser ablation, or laser photolysis, and highly dynamical systems including weakly bound molecular complexes. From these latter systems, information on nonbonding intermolecular interactions and on the internal dynamics is easily obtained. Many chemical problems, difficult to unravel with other techniques, were solved only by rotational spectroscopy experiments on supersonic jets. The widely unexplored interactions in the intermediate regime between bonding and nonbonding, molecular recognition, molecular aggregation, and many more are investigated with these techniques without having to rely on ab initio calculations – their support is of great help though, both in guiding spectroscopic searches and in interpreting the spectra. The laboratory preparation and investigation of a large number of new transient species generated data on existing chemical systems, many of them being either of important astrochemical interest or crucial in the understanding of (photo-)chemical processes in the atmosphere. Because of the emerging new techniques, the conformational equilibria within amino acids and other biomolecules were precisely described. The potential of MW spectroscopy to obtain this information is demonstrated by numerous investigations. Many interesting examples, most of them achieved in the last 15–20 years, are compiled here.