Stability and partial oligomerization of naphthalene under high pressure at room temperature

Polymerization Mechanism of Nitrogen-Containing Heteroaromatic Compound Under High-Pressure and High-Temperature Conditions

Hydrogenated carbon nitride is synthesized by polymerization of 1,5-naphthyridine, a nitrogen-containing heteroaromatic compound, under high-pressure and high-temperature conditions. The polymerization progressed significantly at temperatures above 573 K at 0.5 GPa and above 623 K at 1.5 GPa. The reaction temperature was relatively lower than that observed for pure naphthalene, suggesting that the reaction temperature is considerably lowered when nitrogen atoms exist in the aromatic ring structure. The polymerization reaction largely progresses without significant change in the N/C ratio. Three types of dimerization are identified; naphthylation, exact dimerization, and dimerization with hydrogenation as determined from the gas chromatograph-mass spectrometry analysis of soluble products. Infrared spectra suggest that hydrogenation products were likely to be formed with sp³ carbon and NH bonding. Solid-state ¹³C nuclear magnetic resonance reveals that the sp³/sp² ratio is 0.14 in both the insoluble solids synthesized at 0.5 and 1.5 GPa. Not only the dimers but also soluble heavier oligomers and insoluble polymers formed through more extensive polymerization. The major reaction mechanism of 1,5-Nap was common to both the 0.5 and 1.5 GPa experiments, although the required reaction temperature increased with increasing pressure and aromatic rings preferentially remained at the higher pressure.

Pressure-Induced Polymerization of Polycyclic Arene-Perfluoroarene Cocrystals: Single Crystal X-ray Diffraction Studies, Reaction Kinetics, and Design of Columnar Hydrofluorocarbons

Pressure-induced polymerization of aromatic compounds leads to novel materials containing sp³ carbon-bonded networks. The choice of the molecular species and the control of their arrangement in the crystal structures via intermolecular interactions, such as the arene-perfluoroarene interaction, can enable the design of target polymers. We have investigated the crystal structure compression and pressure-induced polymerization reaction kinetics of two polycyclic 1:1 arene-perfluoroarene cocrystals, naphthalene/octafluoronaphthalene (NOFN) and anthracene/octafluoronaphthalene (AOFN), up to 25 and 30 GPa, respectively, using single-crystal synchrotron X-ray diffraction, infrared spectroscopy, and theoretical computations based on density-functional theory. Our study shows the remarkable pressure stability of the parallel arene-perfluoroarene π-stacking arrangement and a reduction of the interplanar π-stacking separations by ca. 19-22% before the critical reaction distance is reached. A further strong, discontinuous, and irreversible reduction along the stacking direction at 20 GPa in NOFN (18.8%) and 25 GPa in AOFN (8.7%) indicates the pressure-induced breakdown of π-stacking by formation of σ-bonded polymers. The association of the structural distortion with the occurrence of a chemical reaction is confirmed by a high-pressure kinetic study using infrared spectroscopy, indicating one-dimensional polymer growth. Structural predictions for the fully polymerized high-pressure phases consisting of highly ordered rods of hydrofluorocarbons are presented based on theoretical computations, which are in excellent agreement with the experimentally determined unit-cell parameters. We show that the polymerization takes place along the arene-perfluoroarene π-stacking direction and that the lateral extension of the columns depends on the extension of the arene and perfluoroarene molecules.

Evidence for Orientational Order in Nanothreads Derived from Thiophene

Nanothreads are one-dimensional sp³ hydrocarbons that pack within pseudohexagonal crystalline lattices. They are believed to lack long-range order along the thread axis and also lack interthread registry. Here we investigate the phase behavior of thiophene up to 35 GPa and establish a pressure-induced phase transition sequence that mirrors previous observations in low-temperature studies. Slow compression to 35 GPa results in the formation of a recoverable saturated product with a 2D monoclinic diffraction pattern along (0001) that agrees closely with atomistic simulations for single crystals of thiophene-derived nanothreads. Paradoxically, this lower-symmetry packing signals a higher degree of structural order since it must arise from constituents with a consistent azimuthal orientation about their shared axis. The simplicity of thiophene reaction pathways (with only four carbon atoms per ring) apparently yields the first nanothreads with orientational order, a striking outcome considering that a single point defect in a 1D system can disrupt long-range structural order.

Decomposition and oligomerization of 2,3-naphthyridine under high-pressure and high-temperature conditions

The chemical reaction of 2,3-naphthyridine, a nitrogen-containing aromatic compound, was investigated at pressures ranging from 0.5 to 1.5 GPa and temperatures from 473 to 573 K. A distinct decrease in the amount of residual 2,3-naphthyridine was observed in the samples recovered after reaction at ˃523 K at 0.5 and 1.0 GPa, and ˃548 K at 1.5 GPa. The formation of o-xylene and o-tolunitrile accompanied a decreasing N/C ratio of the reaction products, indicating decomposition of the aromatic ring and release of nitrogen. Precise analysis of the reaction products indicated the oligomerization of decomposed products with the residual 2,3-naphthyridine to form larger molecules up to 7mers. Nitrogen in the aromatic ring accelerated reactions to decompose the molecule and to oligomerize at lower temperatures than those typically reported for aromatic hydrocarbon oligomerization. The major reaction mechanism was similar between 0.5 and 1.5 GPa, although larger products preferentially formed in the samples at higher pressure.

Hydrogen and Hydrocarbon Gases, Polycyclic Aromatic Hydrocarbons, and Amorphous Carbon Produced by Multiple Shock Compression of Liquid Benzene up to 27.4 GPa

Phase diagrams of benzene have been reported on the basis of data mainly obtained from static compression at various pressure-temperature, P-T, conditions. However, there are few data in the high-pressure and high temperature-region of the phase diagram. To understand the physical and chemical behavior of benzene in that region, multiple shock compression of benzene was evaluated by a recovery experimental system that directly analyzed the shocked samples. The shocked samples were composed of the remaining benzene, gases (H₂, CH₄, C₂H₄, C₂H₆, C₃H₆, and C₃H₈), polycyclic aromatic hydrocarbons with molecular weights from 128 (naphthalene) to 300 (coronene), and amorphous carbon. The abundances of these chemical species varied according to the P-T conditions induced by shock compression. Samples in the lower-pressure and lower-temperature region of the a-C:H phase in the phase diagram contained a significant amount of benzene as well as amorphous carbon. In the higher-pressure and higher-temperature region of the a-C:H phase, benzene was mostly converted into amorphous carbon (H/C = 0.2), H₂, and CH₄. Therefore, the amorphous carbon in the present study was produced by a different pathway than that in previous studies that have detected hydrogenated amorphous carbon (H/C = 1). For earth sciences, the present study can provide basic information on the delivery to the early earth of extraterrestrial organic materials related to the origin of life.