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Summary
The concept of a molecule as a unit of bound atoms can be traced to Robert Boyle's 1661 treatise “The Sceptical Chymist” (1). Chemists often depict the strong covalent bonds in molecules formed through electronic interactions of atoms by sticks or springs. By contrast, much weaker attractive forces between molecules in liquids and solids, such as van der Waals forces, are typically unspecific and nondirectional and cannot be represented by sticks or springs. An exception is the hydrogen bond (H-bond) (2), which can create relatively strong directional interactions between molecules when the atoms that carry opposite partial charges attract each other. Discrimination between very strong H-bonds and covalent bonds can become somewhat arbitrary. On page 160 of this issue, Dereka et al. (3) study what happens if the strength of an intramolecular hydrogen bond becomes comparable to the strength of the intermolecular covalent bonds, blurring the concept of what a “molecule” is.
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