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The reactivity induced in acetylene, benzene and phenylacetylene crystals by the application of high external pressure is discussed. Acetylene, the simplest model system involving the CC triple bond, reacts above 3.0 GPa in the orthorhombic crystal phase. The molecular arrangement in this phase determines the final conformation of the recovered polymer. The role of laser irradiation in branching the polyenic chains, through the dissociation of the double bonds, is revealed. Benzene, the model aromatic molecule, starts to react in the solid state above 23 GPa. A progressive distortion of the rings is observed on increasing pressure up to 50 GPa but the transformation becomes complete, close to ambient pressure, only during the decompression run. An amorphous hydrogenated carbon compound (a-C:H) is formed in this reaction. Laser irradiation with suitable wavelengths reduces the reaction pressure threshold revealing the role of the S1 excited state in driving the high-pressure reaction. Finally, phenylacetylene was taken into account since it is the simplest molecule involving both the triple bond and the aromatic ring and its reactivity could be possibly interpreted according to the previous model systems. Phenylacetylene starts to react in the solid state above 8 GPa. The reaction mainly involves the triple bond of the ethynyl fragment forming polyphenylacetylenic chains. On the basis of the infrared spectra a possible polymer structure is proposed. Copyright © 2003 John Wiley & Sons, Ltd.

In this work we present an infrared study of the high pressure phases of benzene crystal up to 25 GPa. The far infrared portion of the spectrum has been studied for the first time above 4 GPa. From the pressure behavior of the frequencies and the integrated peak absorptions we are able to confirm, beside the transition between phases II-III at 4.8 GPa, the transition between phases III-IIIЈ at 11.2 GPa. The spectral pattern changes very smoothly among the different phases and the number of bands is always consistent with a crystal structure having two molecules per cell and the inversion center as a symmetry element. At 17.4 GPa a structural change is suggested from the slope decrease of the frequency evolution with pressure of almost all the crystal modes. Particularly important are the changes of the doubly degenerate 18 bending mode, which could be strongly involved in the irreversible transformation occurring at higher pressure.

The infrared spectrum of the ⑀ phase of solid oxygen has been studied between room temperature and 20 K as a function of pressure up to 63 GPa. Besides the strong absorption in the fundamental O 2 vibron mode and the broad doublet in the overtone region, another peak is detected in the far infrared region. The analysis of the overtone bands allows the determination of the density of states of the O 2 vibron region which consists of two separated energy regions, including one the infrared and the other the Raman bands observed in the 1500-1650 cm Ϫ1 range. This result, consistent with the analysis of the other Raman and infrared bands at lower frequency, is interpreted on the basis of a crystal composed by a molecular unit formed by four oxygen atoms. This hypothesis explains the strong infrared absorption which is in contrast with the model of a crystal composed by diatomic oxygen molecules. Very thin crystalline slabs (р0.4 m) allowed to measure the intensity of the strong infrared absorption at 1500-1550 cm Ϫ1 . The measurement of the Raman spectrum as a function of the incident power and of the laser excitation frequency shows how the intensity and the frequency of the Raman lines are affected by the experimental conditions. Finally, a simple chain model provides indirect proof of our assignment of the low-frequency infrared mode and allows to rule out an association in polymeric units formed by more than four atoms even at pressures close to the insulator-metal transition.

Success in designing and tailoring solid-state reactions depends on the knowledge of the mechanisms regulating the reactivity at the microscopic level. In spite of several attempts to rationalize the reactivity of crystals 1-5 , the question of the existence of a critical distance for a reaction to occur remains unsolved. In this framework, the role of lattice phonons, which continuously tune the relative distance and orientation of the molecules, is still not fully understood. Here, we show that at the onset of the transformation of crystalline benzene to an amorphous hydrogenated carbon the intermolecular C-C distance is always the same (about 2.6Å) once collective motions are taken into account, and it is independent of the pressure and temperature conditions. This conclusion is supported by first-principles molecular-dynamics simulations. This is a clear demonstration of the role of lattice phonons in driving the reactivity in the crystalline phase by fine-tuning of the nearest-neighbour distances. The knowledge of the critical C-C distance can be crucial in planning solid-state reactions at moderate pressure.

T he possibility to synthesise materials of technological interest by using only physical methods is an appealing deal in modern chemistry. Stars, planets and meteorites are natural laboratories where pressure and temperature can reach ultra-high values, thus allowing chemical processes not feasible by standard synthetic methods. From this point of view, pressure represents a powerful method to force atoms, ions or molecules to interact strongly, realizing high, repulsive energy configurations. This is of particular importance for small molecular compounds, because the high compressibility of these systems determines a wealth of changes occurring on compression, ranging from simple phase changes to insulator-to-metal transitions and also to chemical transformations 1,2 .

The chemical transformation of crystalline benzene into an amorphous solid ͑a-C:H͒ was induced at high pressure by employing laser light of suitable wavelengths. The reaction was forced to occur at 16 GPa, well below the pressure value (23 GPa) where the reaction normally occurs. Different laser sources were used to tune the pumping wavelength into the red wing of the first excited singlet state S 1 ͑ 1 B 2u ͒ absorption edge. Here the benzene ring is distorted, presenting a greater flexibility which makes the molecule unstable at high pressure. The selective pumping of the S 1 level, in addition to structural considerations, was of paramount importance to clarify the mechanism of the reaction.