Strained structures

The predicted stability of in-adamantane provides a new synthetic challenge, and the possibility to apply such structures to form stable carbocations

Credit: © 2008 ACS

Strained molecules have always fascinated chemists. Some have useful or interesting properties — they can, for example, be used as explosives — others provide a challenge to the prowess of the synthetic chemist or help the understanding of chemical bonding. A relatively rare type of strain is that which arises by the displacement of a hydrogen atom. In/out isomerism refers to structures formed when the hydrogen atoms at the bridgehead of a multicyclic structure are counterintuitively made to point in towards the centre of the molecule rather than out.

Now, Karl Irikura from the National Institute of Standards and Technology in the USA has predicted¹ that one of the smallest structures that could possibly exhibit this type of isomerism will be stable for a short time at low temperature. In-adamantane can be envisaged as the structure formed by inverting one of the methine carbon atoms of adamantane (a saturated tricyclic hydrocarbon with molecular formula C₁₀H₁₆). Importantly, Irikura showed that the structure is both kinetically and thermodynamically stable by calculating the energy barrier to interconversion between the conventional out-isomer and the predicted in-isomer. Irikura predicts that in-adamantane will have a half-life of two days at the temperature of dry ice (195 K), and that it should be isolable in macroscopic quantities because it is not predicted to react with air, water or itself.

Although a half-life of two days at low temperature may not be that useful, a related compound — in-pentadecafluoroadamantane (in-C₁₀F₁₅H) — is predicted to have a half-life of 100 years at room temperature. If such compounds can be synthesized they may also be precursors for non-classical, stable carbocations.