Section 2: Spectroscopic and Database Information7: Molecular Beam and Spectroscopic Techniques: Towards Fundamental Understanding of Intermolecular Interactions/Bonds

This chapter focuses on microwave and infrared spectroscopic investigations on molecular complexes formed in a supersonic beam, typically at 3 K. These complexes are bound by intermolecular forces that were historically classified as ‘van der Waals forces’ and ‘hydrogen bonding’. As these complexes are investigated at very low T and P, isolated from solvent or lattice effects, intermolecular interactions can be accurately probed. For this same reason, what is learned from molecular complexes in the gas phase may not be directly relevant to the condensed phase, a solution or a crystal. However, comparison of the structure of molecular complexes with that found in the condensed phase has helped in enhancing our fundamental understanding of intermolecular forces. We discuss two specific examples, the phenylacetylene–water complex and 1,2-ethanediol or ethylene glycol, and show how the combination of various spectroscopic and theoretical techniques have been applied over the last decade to unravel the intricacies of inter/intramolecular hydrogen bonds. Intermolecular bonds, involving other elements in the periodic table, in particular, halogen and carbon, are discussed as well. Recent spectroscopic confirmation of a pnicogen bond and nπ* interaction are also pointed out.