High-pressure behavior of a linear chain alkane, tricosane

Abstract

Exploring the behavior of hydrocarbon under pressure is important for understanding its role in planetary sciences and also for exploring novel organic chemistry. In this study, we explored the high-pressure behavior of a linear-chain hydrocarbon, tricosane (C23H48), using Raman spectroscopy. We compressed tricosane up to 23 GPa and did not find any evidence for pressure-induced amorphization within the conditions explored in this study. Upon compression, we observe new modes in the low energy region 100–300 cm−1. In order to understand the appearance of these new modes at high pressures, we used complementary ab initio calculations and explored the effect of chain configurations (linear and bent) on the predicted Raman spectra. We find that these new modes observed at higher pressures are better explained by bent configuration of tricosane chains. Thus, based on high-pressure Raman spectra, it is very likely that a linear chain of tricosane is bent under pressure, i.e., it undergoes a pressure-induced trans-gauche transformation. It is also likely that such bent regions (i.e., kinks) will act as sites along which large chain hydrocarbons could dissociate into smaller chain lengths at extreme conditions relevant to the interiors of Jovian planets.

Spectroscopic Investigation of the High-Pressure Behavior of Aliphatic Hydrocarbon: Implications for Planetary Processes

“…At high pressures, although, a transition to a hairpin/folded state appears unlikely based on a comparison between the observed and calculated spectra (Figure ). Our results on octadecane are similar to a recent study on tricosane (C 23 H 48 ), which also reveal pressure-induced new low wavenumber modes, albeit at a lower pressure of 0.3 GPa . The appearance of the new modes was attributed to bent-chain configurations .…”

“…Aside from the appearance of the low wavenumber modes around 4–5 GPa, no other distinct changes in the spectral pattern were observed that would be indicative of a transformation/dissociation to a different compound. Similar phase stability as a function of pressure was observed for tricosane to 23 GPa . The phase stability of C 18 and C 23 over a large range of pressure indicates that the aliphatic hydrocarbons, likely, would have survived the impact experienced by the interplanetary dust particles or microsized meteorites during the exogenous delivery of organic matter in the late heavy bombardment period (4.5–3.8 Ga).…”

“…In the case of octadecane, the fundamental LAM ( m = 1) is observed at 131 cm –1 , and the overtones at 357 cm –1 ( m = 3) and 490 cm –1 ( m = 5) (Table ). The LAMs of the hydrocarbon chains are inversely related to the size lengths of the chain and have been discussed extensively in an earlier report . In addition to LAMs, a weak peak at 514 cm –1 is also observed (Figure ).…”

“…Long-chain alkanes (C 18 and C 19 ) are known to undergo complete dissociation at high pressure and temperature into nanodiamonds; however, the exact dissociation mechanisms are not well understood. The probable mechanism of dissociation will likely depend on the actual P-T path followed . While this present study cannot confirm the dissociation of long-chain alkanes, it certainly provides valuable insights into the breakdown mechanism.…”

“…For instance, the smaller-sized IDPs and micrometeorites experience temperatures over 500 °C during atmospheric entry and passage. − It is speculated that extreme conditions experienced by the meteorites, comets, and IDPs may likely result in extensive modification of its original constituent organic matter. To better understand the high-pressure and high-temperature behavior of hydrocarbons, static and shock high-pressure high-temperature experiments have been conducted on hydrocarbons and they indicate indeed that these hydrocarbons undergo significant modifications ,,− (Supporting Information Table ST1). Upon compression to ∼2 GPa and at temperatures of ∼1000–1500 K, methane (CH 4 ) polymerizes to longer-chain alkanes and also forms graphite (C) and molecular hydrogen (H 2 ) .…”