It’s hard to know if anyone cares about this any more except the original protagonists, most of whom are long gone (RIP H. C. Brown), but a huge controversy raged for decades on whether a nonclassical cation (e.g. not representable by a Lewis structure) existed in the fused ring system of norbornane and its derivatives. Science vol. 341 pp. 62 – 64 ’13 contains a truly definitive answer (hopefully) along with a lot of historical background should you be interested. An Xray crystallographic structure of a norbornyl cation (complexed with a Al2Br7- anion) at 40 Kelvin shows symmetrical disposition of the 3 carbons of the nonclassical cation. It was tricky, because the cation is so symmetric that it rotates within crystals at higher temperatures. The bond lengths between the 3 carbons are 1.78 to 1.83 Angstroms — far longer than the classic length of 1.54 Angstroms of a C – C single bond.
The controversy was still exercising physical organic chemists decades later — here are some notes from an earlier post when I was going through Anslyn and Dougherty’s book. “661 -> 664 — Fascinating to see the ‘answer’ to the questions about classical and nonclassical norbornyl cations that so exercised Schleyer and many other chemists back in the late 50′s and early 60′s. Stable ion media and solid state NMR (both unknown at the time) did the trick. Not finding distinct NMR structures at 5 degrees KELVIN is good enough evidence (for me) that nonclasical norbornyl cations exist. Also if the barrier to interconversion of 2 norbornyl cations is under .2 kiloCalories/mole, this makes the controversy essentially irrelevant at room temperature and above, where the average thermal energy/molecule is .6 kiloCalories/mole. Even if two forms actually exist, it’s got to be like keto-enol tautomerism. A fascinating conclusion to the controversy.”
Historical note. Schleyer was my undergraduate advisor back in the day in ’58. He is now 83 and is the last chemist standing. Bravo ! ! !
It will be interesting to see if letters to Science disputing this work emerge in the coming weeks.