Beyond the Molecule: Intermolecular Forces from Gas Liquefaction to X-H⋅⋅⋅π Hydrogen Bonds.

Interest toward molecule-molecule interactions developed during the first half of the 19th century with studies on gas liquefaction. In 1869, Andrews carried out the first accurate study on the effects of temperature and pressure on the behaviour of real gases, and in 1873, van der Waals formulated an equation capable of accounting for critical phenomena of each individual gas The nature of the intermolecular forces responsible for aggregation of gases was investigated in the early 20th century by Keesom and Debye (electrostatic interactions between dipoles) and by London (interaction between instantaneous dipoles, studied by quantum mechanics). The hydrogen bond theory, a particular case of dipolar interaction, originated in the 1920s in Berkeley in the institute directed by G. N. Lewis. Later it was made clear that hydrogen bonding is responsible for the aggregation of most biological systems, from proteins to nucleic acids. This Perspective describes the most significant steps through which the science of intermolecular interactions has progressed over the last two centuries, revisiting classical experiments and theoretical formulations, which led to the complex state of art of today.