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The room-temperature pressure-induced reaction of nitromethane has been studied by means of infrared spectroscopy in conjunction with ab initio molecular dynamics simulations. The evolution of the IR spectrum during the reaction has been monitored at 32.2 and 35.5 GPa performing the measurements in a diamond anvil cell. The simulations allowed the characterization of the onset of the high-pressure reaction, showing that its mechanism has a complex bimolecular character and involves the formation of the aci-ion of nitromethane. The growth of a three-dimensional disordered polymer has been evidenced both in the experiments and in the simulations. On decompression of the sample, after the reaction, a continuous evolution of the product is observed with a decomposition into smaller molecules. This behavior has been confirmed by the simulations and represents an important novelty in the scene of the known high-pressure reactions of molecular systems. The major reaction product on decompression is N-methylformamide, the smallest molecule containing the peptide bond. The high-pressure reaction of crystalline nitromethane under irradiation at 458 nm was also experimentally studied. The reaction threshold pressure is significantly lowered by the electronic excitation through two-photon absorption, and methanol, not detected in the purely pressure-induced reaction, is formed. The presence of ammonium carbonate is also observed. [ABSTRACT FROM AUTHOR]

The ambient temperature photoinduced reactivity of mixtures containing water and some of the simplest model hydrocarbons has been studied in a diamond anvil cell below 1 GPa. The near-UV 350 nm emission of an Ar ion laser has been employed to photodissociate water molecules through two-photon absorption processes. The hydroxyl radicals generated through this process are able to trigger a chemical reaction in the mixtures containing ethane and acetylene, which are otherwise stable under the same P−T−hν conditions, whereas the contribution of water has no effect or is very limited in the case of the ethylene and propene mixtures, respectively. The reaction evolution and the reaction products were characterized by using FTIR spectroscopy. The formation of fluorescent products limits or prevents, as in the case of acetylene, the characterization by Raman spectroscopy. Particularly relevant is the in situ efficient sequestration of the CO2formed during the reaction, through the formation of a clathrate hydrate, in the mixtures where water is largely in excess. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Vibrational and electronic spectra of crystalline indole were measured up to 25.5 GPa at room temperature in a diamond anvil cell. In particular, Fourier transform infrared (FTIR) spectra in the mid-infrared region and two-photon excitation profiles and fluorescence spectra in the region of the HOMO−LUMO transitions were obtained. The analysis of the FTIR spectra revealed a large red-shift of the N−H stretching mode with increasing pressure, indicating the strengthening of the H-bond between the NH group and the π electron density of nearest neighbor molecules. The frequencies of four vibronic bands belonging to the 1Laand 1Lbsystems were obtained as a function of pressure. Comparison with literature data shows that the crystal acts as a highly polar environment with regard to the position of the 1Lborigin and of the fluorescence maximum, which are largely red-shifted with respect to the gas phase or to solutions in apolar solvents. A large, and increasing with pressure, frequency difference between the 1Lborigin and the blue edge of the fluorescence spectrum suggests that the emitting state is 1La, that is known to be more stabilized than 1Lbby dipolar relaxation. Crystalline indole was found to be very stable with respect to pressure-induced reactivity. Only traces of a reaction product, containing saturated C−H bonds, are detected after a full compression−decompression cycle. In addition, differently from many unsaturated compounds at high pressure, irradiation with light matching a two-photon absorption for a HOMO−LUMO transition has no enhancing effect on reactivity. The chemical stability of indole at high pressure is ascribed to the crystal structure, where nearest neighbor molecules, formig H-bonds, are not in a favorable position to react, while reaction between equivalent molecules, for which a superposition of the π electron clouds would be possible, is hindered by H-bonded molecules. Consistently, no excimer emission was observed except at the cell opening at the end of the compression−decompression run. Extremely limited chemical reactivity and excimer formation likely occur at crystal defects, evidencing the strict connection between the two phenomena. [ABSTRACT FROM AUTHOR]

The pressure induced reactivity of carbon monoxide was investigated in a wide temperature range (100−400 K) completely avoiding any irradiation of the sample with visible or higher frequency light. FTIR spectroscopy was employed to monitor the reaction and infrared sensors for measuring the pressure. With this approach we have been able to separate the effects of the three variables (P, T and hν) that establish the conditions for the occurrence of the chemical reaction. A new instability boundary, not affected by the photoactivation of the reaction, is provided. The reaction has been studied in three different crystal phases (ϵ, δ, and β), but the small differences in the reaction products are ascribable to the temperature changes rather than to the crystalline arrangement. For T< 300 K the analysis of the IR spectra reveals the formation of an extended amorphous material formed, according to the vibrational assignment and to the kinetic data, by polycarbonyl linear chains containing a large amount of anhydride groups. For T≥ 300 K the formation of carbon dioxide and epoxy rings, and the simultaneous decrease of carbonyl species, let suppose a decarboxylation of the extended solid product. Once exposed to the atmosphere, the reaction product readily and irreversibly reacts with water giving rise to carboxylic groups. [ABSTRACT FROM AUTHOR]

Angle dispersion X-ray diffraction (AXDX) experiments on nitromethane single crystals and powder were performed at room temperature as a function of pressure up to 19.0 and 27.3 GPa, respectively, in a membrane diamond anvil cell (MDAC). The atomic positions were refined at 1.1, 3.2, 7.6, 11.0, and 15.0 GPa using the single-crystal data, while the equation of state (EOS) was extended up to 27.3 GPa, which is close to the nitromethane decomposition threshold pressure at room temperature in static conditions. The crystal structure was found to be orthorhombic, space group P212121, with four molecules per unit cell, up to the highest pressure. In contrast, the molecular geometry undergoes an important change consisting of a gradual blocking of the methyl group libration about the C−N bond axis, starting just above the melting pressure and completed only between 7.6 and 11.0 GPa. Above this pressure, the orientation of the methyl group is quasi-eclipsed with respect to the NO bonds. This conformation allows the buildup of networks of strong intermolecular O···H−C interactions mainly in the bcand acplanes, stabilizing the crystal structure. This structural evolution determines important modifications in the IR and Raman spectra, occurring around 10 GPa. Present measurements of the Raman and IR vibrational spectra as a function of pressure at different temperatures evidence the existence of a kinetic barrier for this internal rearrangement. [ABSTRACT FROM AUTHOR]

The high-pressure reactivity of isoprene has been studied at room temperature up to 2.6 GPa by using the diamond anvil cell technique in combination with Fourier transform infrared spectroscopy. Both dimerization and polymerization reactions take place above 1.1 GPa. At this pressure, the two processes are well separated in time, the dimerization being the only one occurring in the first 150 h. Both processes simultaneously occur as the pressure increases. The reaction product is composed of a volatile fraction, identified as sylvestrene, and a transparent rubberlike solid formed by cis-1,4- and 3,4-polyisoprene. The activation volume of the dimerization reaction has been obtained from the kinetic data. The photoinduced reaction, studied at room temperature for two different pressures, takes place through a two-photon absorption process, and the threshold pressure is lowered to 0.5 GPa. At this pressure, both the dimerization and polymerization processes occur, but the dimerization is not as selective as in the purely pressure-induced reaction. 4-Ethenyl-2,4-dimethylcyclohexene is obtained in addition to sylvestrene. By increasing the pressure, the photoinduced reaction becomes more selective, and the monomer is quantitatively transformed into the same polymer obtained in the purely pressure-induced reaction. [ABSTRACT FROM AUTHOR]

The room temperature photoinduced reactivity of liquid ethanol has been studied as a function of pressure up to 1.5 GPa by means of a diamond anvil cell. Exploiting the dissociative character of the lowest electronic excited states, reached through two-photon absorption of near-UV photons (350 nm), irreversible reactive processes have been triggered in the pure system. The active species are radicals forming along two main dissociation channels involving the split of C−O and O−H bonds. The characterization of the reaction products has been performed by in situ FTIR and Raman spectroscopy. At pressures of a few megapascals, molecular hydrogen is the main reaction product, an important issue in the framework of environmentally friendly synthesis of this energetic vector. In the gigapascal range, the main products are ethane, 2-butanol, 2,3-butanediol, 1,1-diethoxyethane, and some carbonylic compounds. The relative amount of these species changes with pressure reflecting the nature of the radicals formed in the photodissociation process. As the pressure increases, the processes requiring a greater molecularity are favored, whereas those requiring internal rearrangements are inhibited. Disproportion products like CH4, H2O, and CO2increase when the amount of ethanol decreases due to the reaction, becoming the main products only when ethanol is exhausted. [ABSTRACT FROM AUTHOR]