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Little Interactions Mean a Lot

Noncovalent bonds are weaklings compared to familiar chemical reactions, but they add up to strongly influence the shape and behavior of molecules.

Roald Hoffmann

Is the Straight and Narrow Preferred?

Small energies play out in other interesting ways in some of the simplest organic molecules: the unbranched long-chain hydrocarbons, such as the liquid to waxy “normal” alkanes, which are written chemically as CH3(CH2)nCH3. The molecules in gasoline belong to this family with n = 6 or so; increasing n from six leads to diesel fuel, jet fuel, oil, and lubricants. When alkanes become solid, which happens for still higher values of n, there isn’t much use for them, except as candle wax. The unbranched, or anti conformation, is energetically preferred, as shown for C18H38 below.

One wants to know the preferred shape of a molecule to understand its chemical personality—how it behaves, and how it reacts. A conformation is a geometry of a molecule that differs from another one by rotation around only single covalent C–C bonds. At each interior point in the hydrocarbon chain, for every four carbons, a choice is available between an extended conformation, which chemists refer to as all-anti, and two mirror-image curled-up forms, known as gauche conformations. The all-anti and one of the two gauche conformations are shown below for lighter fluid, or n-butane, C4H10.

The energy difference between the two conformations is truly minuscule. The gauche geometry is merely about one kilocalorie per mole in energy above the global minimum of the anti form; this is just 1/40th of the energy of a photon that is absorbed by a chlorophyll molecule in photosynthesis.

The two “conformers” are related by 120 degrees of rotation around the central C–C bond. Here’s another small energy at work, now controlling the geometry of a molecule. That rotation faces a small barrier; the reasons for it are still rousing my fellow theoreticians to much debate. But there is little uncertainty about its value; it is around 3 kilocalories per mole. Because there are two gauche forms, these conformations are favored by entropy. So, only as temperature approaches zero kelvin would one have the frozen-in anti conformation of n-butane as the only form present. At room temperature in a liquid or gas there is, however, enough energy from molecular collisions to have a significant number of n-butane molecules in the gauche conformation.

Suppose we have two conformations of a longer-chain, normal alkane, say C18H38. The all-anti, extended one is shown above, and a kinked form is shown at right. The turn in the middle of the kinked chain is achieved by incorporating two gauche “turns” (play with a model, and you will see it can be done).

That kinking costs energy, but only a wee amount—each gauche turn has an associated penalty of about 1 kilocalorie per mole. What is accomplished by the kink is visually apparent in the depiction below; the second part of the chain is brought near the first. In that conformation, or actually in a family of conformations looking roughly like that, there is a new source of stabilization that is unavailable in the extended, all-anti geometry: attractive dispersion forces between the two parts of the chain. The more chance for hydrogen atoms to come close to each other, the more stable that arrangement, so the folded molecule is more stable.

Small as they individually are, dispersion interactions add up. For an isolated molecule, for some n in CH3(CH2)n CH3 for example, the energy lowering (stabilization) in dispersion interaction will win out over the increase in energy (destabilization) in kinking, the latter accomplished by two or more gauche conformations along the chain. The molecule that I thought would be “straight” when I forgot to consider dispersion forces actually prefers energetically to be kinked.

To my knowledge, Jonathan Goodman first wrote down this idea in 1997. Theory in his and others’ hands confirmed the notion: For CH3(CH2)nCH3, the crossover from extended to kinked (and eventually curled in a more complex way) comes at around n = 16, or 18 carbon atoms. Remarkably, the experimental proof for this hypothesis has recently come forward, in work by N. O. B. Lüttschwager and colleagues.

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