Some molecules of great theoretical interest are so unstable that they cannot be isolated under normal, everyday conditions. Two molecules that fall into this category are cyclobutadiene and benzyne. Even though chemists can obtain convincing evidence for their existence, there is nothing like having the pure materials in a bottle so that their properties can be studied directly.

Cyclobutadiene is particularly important because of its central place in the theory of aromaticity. According to this theory, benzene is unusually stable because of its continuous system of six pi-electrons?this unusual stability is termed aromaticity. On the other hand, cyclobutadiene is particularly unstable?its unusual instability is termed antiaromaticity. Similarly, benzyne is predicted to be particularly unstable because the normal linear geometry at the carbons involved in a triple bond is constrained to a bent geometry in benzyne, at a great energetic cost.

These compounds, when formed, do not normally survive because their molecules react with one another to form products of considerably greater stability. Both compounds had been studied previously at very low temperature–for example, cyclobutadiene has been examined in an argon matrix. Trapping such molecules at low temperature cuts down on random diffusion so that the molecules find each other more slowly. In effect, the argon matrix serves as a “chemical quicksand” to keep the molecules apart.

Chemists cannot put these very unstable molecules into glass bottles; but, in 1991, Donald Cram, a Nobel prize-winning chemist from UCLA, figured out a way to put them into molecular bottles–one molecule per “bottle.” He demonstrated that he could trap individual cyclobutadiene molecules within bottle-like molecules called hemicarcerands. (1) The way this is done is to trap a relatively stable chemical precursor within the chemical bottle and then form the cyclobutadiene molecule by a photochemical reaction–a reaction induced by light. Spectra of incarcerated cyclobutadiene could be determined using this technique.

More recently, a similar technique has been applied to the production of incarcerated benzyne by Ralf Warmuth, a chemist at UCLA working in the research group of Prof. Cram. (2) The structure of the hemicarcerand is shown below; the “Guest” is the entrapped benzyne.

Heating the incarcerand to 145°C in the presence of the benzyne precursor benzocyclobutenedione (B) creates thermal motions that in effect widen the “portals” of the incarcerand, thus admitting molecules of B.

Irradiating B at wavelengths >400 nm produced incarcerated benzocyclopropenone (P). Benzocyclopropenone itself is a very unstable and reactive molecule; yet, in the hemicarcerand, P could be studied at room temperature. Finally, irradiation of P at a wavelength of 280 nm (at 77 K) formed benzyne:

Warmuth compared the carbon magnetic resonance (CMR) spectrum of incarcerated benzyne with the CMR determined at 20 K in an argon matrix and found, after correcting for the effects of the hemicarcerand, that the two spectra were nearly identical.

One of the interesting things that can be deduced from CMR spectra is the bond order. Although benzyne is conventionally shown with a single triple bond, the CMR spectra suggested that the structure of benzyne is perhaps better represented as a resonance hybrid?a “blend” of the following two structures?in which structure 2 is more important.

Noting that this result was not supported by various levels of theoretical calculations on benzyne, the author pointed out that this conclusion is based on the comparison of the benzyne spectrum with that of unstrained models ( i.e., linear alkynes); the absence of suitable model compounds might explain the difference.

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