High‐pressure Raman study of liquid and crystalline CH3F up to 12 GPa

A small-volume PVTX system for broadband spectroscopic calibration of downhole optical sensors

An instrument is presented that is capable of measuring the optical spectrum (long-wave ultraviolet through short-wave mid-infrared) of fluids under a range of temperature and pressure conditions from ambient pressure up to 138 MPa (20 000 psi) and 422 K (300 °F) using ∼5 ml of fluid. Temperature, pressure, and density are measured in situ in real-time, and composition is varied by adding volatile and nonvolatile components. The stability and accuracy of the conditions are reported for pure ethane, and the effects of temperature and pressure on characteristic regions of the optical spectrum of ethane are illustrated after correction for temperature and pressure effects on the optical cell path length, as well as normalization to the measured density. Molar absorption coefficients and integrated molar absorption coefficients for several vibrational combination bands are presented.

High-pressure vibrational and polymorphic response of 1,1-diamino-2,2-dinitroethene single crystals: Raman spectroscopy

Raman spectroscopy was used to examine the vibrational and polymorphic behavior of 1,1-diamino-2,2-dinitroethene (FOX-7) to elucidate its structural and chemical stability under high pressure. Measurements were performed on single crystals compressed in a diamond anvil cell, and data were obtained over the entire frequency range of FOX-7 Raman activity. Several new features were observed with increase of pressure: (i) new vibrational peaks and discontinuity in the shifts of the peaks at 2 and 4.5 GPa, (ii) apparent coupling or mixing of several modes, and (iii) changes in the NH2 stretching spectral shape and modes shift. The spectral changes at 2 GPa, in contrast to previous reports, involved only a few peaks and likely resulted from a small molecular transformation. In contrast, changes at 4.5 GPa involved most of the modes, and the pressure for the onset and completion of the changes depended on the pressure medium. A large pressure hysteresis regarding the changes at 4.5 GPa implies a reconstructive transformation. We suggest that this transformation reflects a change in the balance between interlayer (van der Waals) and in-layer (H-bonding) interactions. Despite these transformations, further compression to 40 GPa and subsequent release of pressure did not cause any irreversible changes. This finding implies that FOX-7 has remarkable chemical stability under high pressures. The observed coupling between the various modes with increasing pressure was analyzed within the Fermi resonance model. The potential implication of the coupling of modes for shock insensitivity of FOX-7 is briefly discussed.

Pressure tuning of the Fermi resonance in liquid methanol: Implications for the analysis of high-pressure vibrational spectroscopy experiments

It has been argued that pressure tuning allows for unambiguous assignment of the nonperturbed bands involved in the Fermi coupling of molecular systems in the condensed phase. Here we study the pressure evolution of the Fermi resonance occurring in liquid methanol between the symmetric methyl-stretch fundamental and the methyl-bending overtones. Our analysis is based on Raman experiments in both stretching and bending fundamental regions, which are used to evaluate the effect of pressure on accidental degeneracies occurring in the vibrational spectra of liquid methanol. We emphasize that the difference in frequency of the Fermi doublet constitutes the governing quantity to determine the condition at which the exact degeneracy of the unperturbed modes occurs. Analysis based on the intensity ratio of the Fermi doublet must be disregarded. We confirm the necessity of measuring the full vibrational spectrum under pressure in order to obtain the Fermi coupling parameters unambiguously and to give a correct assignment of the bands involved in the resonance phenomenon. We also analyze the possible occurrence of several simultaneous resonance effects using a multilevel perturbation model. This model provides an appropriate description of the frequencies observed in the experiments over the whole pressure range if we consider that the main resonance occurs between nu3 and 2nu10, in contrast to previous assignments. Our global analysis leads to some general rules concerning measurement and interpretation of high-pressure vibrational spectroscopy experiments.