Raman Spectroscopy Study on Chemical Transformations of Propane at High Temperatures and High Pressures

Formation and Stability of Dense Methane-Hydrogen Compounds

Through a series of x-ray diffraction, optical spectroscopy diamond anvil cell experiments, combined with density functional theory calculations, we explore the dense CH_{4}-H_{2} system. We find that pressures as low as 4.8 GPa can stabilize CH_{4}(H_{2})_{2} and (CH_{4})_{2}H_{2}, with the latter exhibiting extreme hardening of the intramolecular vibrational mode of H_{2} units within the structure. On further compression, a unique structural composition, (CH_{4})_{3}(H_{2})_{25}, emerges. This novel structure holds a vast amount of molecular hydrogen and represents the first compound to surpass 50 wt % H_{2}. These compounds, stabilized by nuclear quantum effects, persist over a broad pressure regime, exceeding 160 GPa.

Impact on the Raman spectra of liquids when a polarized light source is used

The polarization state of the excitation light used in two Raman systems was controlled to study its effect in the unpolarized Raman spectra of unstructured samples. Both systems work in different regions of the electromagnetic spectrum (NIR and visible). Four polarization states (linear, linear at 45° and 90°, and circular) were used to excite liquid samples (ethanol, acetone, and their mixture). The results show that the Raman peaks intensities' ratio varies according to the polarization state of the excitation light. Peaks related to functional groups and C-H stretching modes increase their intensity when circular polarization (CP) is applied. The latter may help to study liquid mixtures with low concentrations. Different polarizing light states give a more detailed spectroscopic analysis since it gathers more structural information of the samples tested in this work with an undefined structure.

Raman analysis of intermediate phases during melting and freezing in long-chain n-alkanes

Structural changes of the n-alkanes (n = 18, 20, 21, 22) have been investigated by Raman spectroscopy during the melting from solid phase to the intermediate transition phase to the molten phase. With increasing temperature docosane (C₂₂) undergoes an orthorhombic-hexagonal (rotator, R) phase follow by an alpha (α) phase. In addition, the α phase as an intermediate transition between the solid and liquid phases is observed in heneicosane (C₂₁). For eicosane (C₂₀) and octadecane (C₁₈) the transition from solid to liquid phase is almost instantaneous unlike docosane (C₂₂) and heneicosane (C₂₁) where the intermediate transitions are observed. During freezing from the liquid phase, the even n-alkanes show the rotator phase (R) a few degrees lower than melting and just before the crystalline solid phase. The intermediate phase transition temperature can be spectrally determined with the 1100-1350 cm^-1 Raman region of the CH₂ band.