Behavior of Long-Chain Hydrocarbons at High Pressures and Temperatures

Abstract

Although long-chain aliphatic hydrocarbons are documented in meteorites, their origin is poorly understood. A key question is whether they are pristine or a byproduct of terrestrial alteration? To understand if these long-chain hydrocarbons are indigenous, it will be important to explore their thermodynamic and mechanical stability at conditions experienced by extraterrestrial objects during atmospheric entry and passage. Extreme pressures and temperatures experienced by meteorites are likely to alter the molecular organization of these long-chain hydrocarbons. These structural changes associated with extreme conditions are often documented via laboratory-based Raman spectroscopic measurements. So far, Raman spectroscopic measurements have investigated the effect of static compression on the aliphatic hydrocarbons. The effect of temperature on the structural changes remains poorly explored. To bridge this gap, in this study, we have explored the behavior of two aliphatic hydrocarbons at simultaneously high pressures and temperatures. We have used a resistively heated diamond anvil cell. On compression to moderate pressures, the appearance of new vibrational modes in the low-energy region confirms prior studies and is related to the bending of the linear chains. Upon heating to ∼220 °C, we note that the new low-energy mode undergoes softening. The mode softening is likely due to the combination of unbending of the alkane chain and mode anharmonicity.

Conformational Energy Benchmark for Longer n-Alkane Chains

“…Since then, a number of experimental and theoretical studies have been carried out with the aim of determining conformation energies, i.e., the energy difference between two different conformers, of n -alkanes as accurately as possible. Shorter n -alkanes are typical subjects of experimental studies investigating conformational enthalpies or low-energy conformers. , Also theoretical investigations of torsional and conformational energies feature mainly short alkane chains. − For longer alkane chains ( n > 10), previous theoretical studies focus mainly on the lowest lying conformer, − investigating the balance between repulsive hydrogen contacts and attractive London dispersion to predict the change from the linear zigzag conformation to a hairpin like, closed conformer.…”

Performance of Localized-Orbital Coupled-Cluster Approaches for the Conformational Energies of Longer n-Alkane Chains

“…Since the pioneering work of Pitzer, the existence of different conformers of n -alkane is well known. Over the years, conformational enthalpies and low-lying conformers of shorter unbranched alkane chains have been studied experimentally. − On the other hand, theoretically, the torsional space and equilibrium conformational energies of short n -alkanes have also been explored theoretically. − Previous theoretical studies − on the longer n -alkane chains ( n > 10) often investigated a handful of the lowest energy conformers to find the balance between repulsive hydrogen contacts and attractive London dispersion and finally predict the energy gap between the linear zigzag and hairpin-like conformers.…”

Pressure and Composition Effects on a Common Nanoparticle Ligand–Solvent Pair

“…Due to their industrial importance and astrophysical interest, organic small molecules and their behavior under pressure have been the subject of much research. − High pressure allows the effect of intermolecular interactions on conformational and phase changes as well as high-pressure reactions including polymerization to be probed in the absence of thermal effects. In these studies, Raman spectroscopy is often used to monitor conformation changes under pressure. − In linear alkanes, with increasing pressure, crystallization is followed by conformational changes and an order–disorder transition that can be followed by changes in intensities of the C–C and C–H stretches associated with the various gauche and trans conformers the molecule .…”