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, 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 2.6 A) 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.

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 (a-C:H). 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 S1 excited state (1B2u). From this comparison we conclude that pressure induces a mixing between the ground and the S1 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.

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The chemical transformation of crystalline benzene into an amorphous solid (a-C:H) 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.

Using an improved DFT calculation of the vibrational frequencies based on the B3-LYP functional and the 6-31G* basis set, the infrared, Raman, neutron inelastic and luminescence spectra of C 60 are rediscussed, and a revised assignment of all the silent modes is obtained and compared with the most recent assignments.