We report a detailed experimental characterization of the infrared absorption properties of the high pressure, red phase of oxygen. Spectra are measured in the far infrared, down to $100{\mathrm{cm}}^{-1\mathrm{}}$, in the fundamental vibration and in the overtone regions up to 63 GPa. A new, strong peak is observed at about $300{\mathrm{cm}}^{-1\mathrm{}}$. Samples less than 0.5 μm thick were produced to measure the strong vibron mode avoiding saturation. We find that the diatomic oxygen molecule is not a good basis to describe the vibrational spectrum of the $\epsilon$ phase. The spectra can be interpreted in a straightforward way on the basis of an association among the oxygen molecules leading to the formation of ${\mathrm{O}}_4$ units.

The purpose of this tutorial review is to illustrate the effects that the application of high pressures can have on chemical reactions involving highly compressible molecular materials. The essentials of the high-pressure technology (generation and in situ control of high pressures) are described with particular attention to the versatile diamond anvil cell (DAC) apparatus. The general effects of pressure on chemical equilibrium, reaction rate and reaction mechanism are discussed. The motivation for application of high-pressure methods (in the 1–300 MPa range) to chemical synthesis and in biochemistry are illustrated focusing the attention on environmental effects and with an excursus on developing biotechnological applications. The peculiarities and the unexpected outcomes of chemical reactions occurring at very high pressures (≥300 MPa) are discussed considering the extraordinary results obtained in polymerization and amorphization of simple molecules and of unsaturated hydrocarbons. The possible connection of the high temperature–high pressure thresholds for chemical reactions with microscopic counterparts (intermolecular distances, molecular orientations) is also discussed.

The chemical transformation of benzene under pressure is investigated, at room temperature and at 100 K, by means of infrared spectroscopy. Pressurization-decompression cycles in the 0–50 GPa pressure range have been performed to achieve the complete transformation of the monomer. The yellow-brownish recovered sample has been identified as an amorphous hydrogenated carbon $\mathrm{(a-}\mathrm{C:H}\mathrm{).}$ A correlation has been established between the pressure behavior of the frequencies of both Raman and infrared internal modes, and the corresponding vibrational energies in the $S_1$ excited state ${(}^1{B}_{\text{2u}}\mathrm{).}$ From this comparison we conclude that pressure induces a mixing between the ground and the $S_1$ electronic states. The increased ring flexibility enhances the interactions among nearest-neighbor molecules inducing the formation of a network of interconnected benzene units where the aromatic character is lost. The bond breaking mainly occurs during the decompression cycle favored by the density decrease. Radical species form in this stage and rapidly propagate to give the denser a-C:H final product.

The high-pressure structural properties of benzene have been investigated up to the occurrence of the chemical reaction by means of x-ray diffraction and infrared absorption techniques. The infrared spectra show that sample annealing above 500 K produces pure phase-II crystals. X-ray diffraction patterns collected at 540 K on these crystals allowed the equation of state of benzene to be obtained. These results indicate the stability of phase II up to the pressure where benzene reacts and no evidence of the III and ${\mathrm{III}}^{\prime }$ crystal structures is gained. On these bases the existing phase diagram of benzene is discussed and a simplified one is proposed.