N 20��: Year of Chemistry

dent’s standard text on the theory of chemical bonding was Charles Coul­ son’s Valence (1952). Absent from it was this theoretical chemist’s real view on the sticks that generations of students have drawn to link atoms into molecules. “A chemical bond is not a real thing: it does not exist: no one has ever seen it, no one ever can. It is a figment of our own imagination,” he later wrote. There is a good reason for postponing this awkward truth. The bond is literally the glue that makes the entire discipline cohere; to consider it an objective reality is necessary for any kind of discourse on chemistry. The discipline is in fact riddled with such conven­ ient (and contested) fictions, such as electro­ negativity, oxidation state, tautomerism and acidity. Disputes about the correct description of bonding have ruffled chemists’ feathers since the concept of molecular structure first emerged in the mid­nineteenth century. Now they are proliferating, as new theoreti­ cal and experimental techniques present fresh ways to probe and quantify chemi­ cal bonds. Traditional measures such as crystallographic interatomic distances and dissociation energies have been supple­ mented by spectroscopic techniques for determining vibrational frequencies, meth­ ods such as nuclear magnetic resonance to measure shifts in the electronic environment of atoms and their magnetic interactions, measurements of force constants (bond stiffness) and a host of quantum­chemical tools for calculating such aspects as electron distributions or localization. An indication of the shifting ground is the recent decision to redefine the nature of the hydrogen bond in the light of experimental results that alter the traditional electrostatic description. The nature of the chemical bond is now further complicated by the introduction of the dynamical dimension. Molecules have traditionally been regarded, if not as static, then as having clear architectural frame­ works that are merely shaken and rotated by thermal motions. The bonds get stretched and bent, but they still have an equilibrium length and strength that seems to justify their being pictured as lines and stalks. Now, thanks to ultrafast spectroscopies, such time­ average values may not always accurately characterize either structure or reactivity. What is measured in a bond depends not just on how but on when it is measured. Some chemists argue that as a result of this, the very existence (or not) of a bond depends on how the problem is probed. Others are committed to absolute criteria. This differ­ ence of opinion goes to the heart of what chemistry is about: can all be reduced to quantum physics, or are fuzzy rules of thumb essential? More pressingly, the issue of how best to describe a chemical bonding pattern has tangible implications for a wide range of problems in chemistry, from molecules in which atoms are coerced out of their Beyond the bond