bond lengths are 1.54 Å, almost identical to that of diamond. The carbon–hydrogen distances are 1.112 Å. At ambient conditions, adamantane crystallizes in a face-centered cubic structure (space group Fm3m, a = 9.426 ± 0.008 Å, four molecules in the unit cell) containing orientationally disordered adamantane molecules. This structure transforms into an ordered, primitive, 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 sites 1 and 2 (see picture below). The 1H and 13C NMR chemical shifts are respectively 1.873 and 1.756 ppm and are 28.46 and 37.85 ppm. The simplicity of these spectra is consistent with high molecular symmetry. Mass spectra of adamantane and its derivatives are rather characteristic. The main peak at m/z = 136 corresponds to the ion. Its fragmentation results in weaker signals as m/z = 93, 80, 79, 67, 41 and 39. The infrared absorption spectrum of adamantane is relatively simple because of the high symmetry of the molecule. The main absorption bands and their assignment are given in the table: * Legends correspond to types of oscillations: δ – deformation, ν – stretching, ρ and ω – out of plane deformation vibrations of CH2 groups. Optical activity Adamantane derivatives with different substituents at every nodal carbon sites are