Are adamantane crystals covalently bonded?

Question

I'm writing a report and need one thing clarified after reading the Wikipedia article on adamantane:

'All the above methods yield adamantane in the form of polycrystalline powder. Using this powder, single crystals can be grown from the melt, solution or vapor phase (e.g. with the Bridgman–Stockbarger technique). Melt growth result in the worst crystalline quality with a mosaic spread in the X-ray reflection of about 1°. Best crystals are obtained from the liquid phase, but the growth is inpracticably slow – several months for a 5–10 mm crystal. Growth from the vapor phase is a reasonable compromise in terms of speed and quality.[2] Adamantane is sublimated in a quartz tube placed in a furnace, which is equipped with several heaters maintaining a certain temperature gradient (about 10 °C/cm for adamantane) along the tube. Crystallization starts at one end of the tube which is kept near the freezing point of adamantane. Slow cooling of the tube, while maintaining the temperature gradient, gradually shifts the melting zone (rate ~2 mm/hour) producing a single-crystal boule.[16]

Would I be write in thinking that these crystals are not randomly shaped diamond structures you would get if the adamantanes covalently fused with eachother? In other words, are the crystals formed from polycrystalline adamantanes (with different numbers of basic ten carbon adamantanes in one derivative crystal) bonding together without covalent bonds? Or perhaps without many covalent bonds? For example, one C-C bond between two crystals? What other type of bonding could be at work here? Is the macro-crystal structurally weak as a result?

Answer 1

Adamantane is a molecular compound, much like naphthalene, paraffin, and many, many others. Molecular compounds consist of molecules, which stay pretty much the same in the gas and solid states. A molecule has certain molecular formula. Each carbon in a molecule is linked to a certain number of hydrogens and other carbons. There are no dangling covalent bonds to be formed between molecules, let alone between crystals. Consequently, you can't just take some adamantanes and covalently fuse them; some people would discuss diamond crystal structure in such terms, but only in a metaphorical sense.

Adamantane molecules in a crystal (being non-polar molecules without any specific interactions) are held together only by the weak van der Waals forces. Because of that, yes, the crystal is not mechanically strong at all (again, think of paraffin wax).