Pure adamantane is a colorless crystalline solid with a characteristic camphor smell, It is practically insoluble in water, but readily soluble in nonpolar organic solvents. Adamantane has an unusually high melting point for a hydrocarbon. At 270 ¡ãC, its melting point is much higher than other hydrocarbons with the same molecular weight, such as camphene (45 ¡ãC), limonene (¨C74 ¡ãC), ocimene (50 ¡ãC), terpinene (60 ¡ãC) or twistane (164 ¡ãC), or than a linear C10H22 hydrocarbon decane (¨C28 ¡ãC). However, adamantane slowly sublimates even at room temperature. Adamantane can distill with water vapor.

Structure

Adamantane molecule consists of three condensed cyclohexane rings fused in the chair conformation. The molecular parameters were deduced by electron diffraction and X-ray crystallography. The carbon-carbon bond length is 1.54 Å and is almost identical to that of diamond, and the carbon-hydrogen distance is 1.112 Å.

At ambient conditions, adamantane crystallizes in a face-centered cubic structure (space group Fm3m, a = 9.426 ¡À 0.008 Å, four molecules in the unit cell) containing orientationally disordered adamantane molecules. This structure transforms into an ordered body-centered tetragonal phase (a = 6.641 Å, c = 8.875 Å) with two molecules per cell either upon cooling to 208 K or pressurizing to above 0.5 GPa.

This phase transition is of the first order; it is accompanied by an anomaly in the heat capacity, elastic and other properties. In particular, whereas adamantane molecules freely rotate in the cubic phase, they are frozen in the tetragonal one; the density increases stepwise from 1.08 to 1.18 g/cm3 and the entropy changes by a significant amount of 1594 J/(mol¡¤K).

Hardness

Elastic constants of adamantane were measured using large (centimeter sized) single crystals and the ultrasonic echo technique. The principal value of the elasticity tensor, C11, was deduced as 7.52, 8.20 and 6.17 GPa for the <110>, <111> and <100> crystalline directions. For comparison, the corresponding values for crystalline diamond are 1161, 1174 and 1123 GPa. The arrangement of carbon atoms is the same in adamantane and diamond.However, in the adamantane solid, molecules do not form a covalent lattice as in diamond, but interact through weak Van der Waals forces. As a result, adamantane crystals are very soft and plastic.

Spectroscopy

The nuclear magnetic resonance (NMR) spectrum of adamantane consists of two poorly resolved signals, which correspond to the inequivalent sites 1 and 2 (see picture below). Their positions are 1.873 ppm and 1.756 ppm for adamantane in CDCl3 and 1H NMR, and are 28.46 ppm and 37.85 ppm for 13C NMR. The simplicity of the NMR spectrum is a good monitor of the purity of adamantane ¨C most derivatives have lower molecular symmetry and therefore more complex spectra.