Microwave Spectroscopy of Large Molecules and Molecular Complexes

Abstract The development of Fourier transform microwave (FTMW) spectroscopy and — most notably — its combination with supersonic‐jet expansion techniques allow for challenging investigations of larger and larger interesting molecular systems. Large isolated molecules studied by microwave (MW) spectroscopy include long carbon chains of astronomical interest, organic molecules containing two or more aromatic rings, small biomolecules, and transition metal complexes. Often, these systems are characterized by large‐amplitude motions and multifolded conformational and tautomeric equilibria. Rotational spectra have been reported for many large molecular complexes, such as adducts of large organic and biological molecules with rare gases, water (or other small proton donors/acceptors), and oligomers and hetero adducts of large molecules. Some of these complexes have interaction energies that are in the intermediate regime between bonding and nonbonding (charge‐transfer complexes), and some of them, involving chiral molecules, act as a guide to understanding molecular recognition. In addition, the rotational spectra of several carbonyl sulfide (OCS)He n species have been measured (up to n = 39), providing useful data to model superfluidity. The potentiality of MW spectroscopy to obtain spectroscopic and chemical information on these systems is demonstrated by numerous investigations. Many interesting examples, most of them achieved in the last two decades, have been compiled here.