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

Abstract

Octadecane (C18H38) is an aliphatic hydrocarbon that is abundant in carbonaceous chondritic meteorites. It is debated whether these hydrocarbons found in the meteorite are pristine or are a result of subsequent modifications as these meteorites are delivered to the Earth. It is well-known that meteorites are often subjected to extreme pressures and temperatures upon entering the Earth’s atmosphere. To explore the behavior of octadecane at high pressures, that is, how its molecular structure responds to compression, we use a diamond anvil cell in conjunction with Raman spectroscopy. We find that at room temperatures, upon compression to ∼5 GPa, a linear-chain octadecane molecule transforms into a bent-chain configuration. Similar transitions from linear to a bent configuration in other hydrocarbons have been documented. We find a linear correlation between the transition pressure from linear to bent configuration, and the chain length of the alkane, that is, longer chain lengths, is likely to be less stable in the linear configuration under compression. These kinks in the bent-chain configuration are likely sites for the dissociation of the longer chain hydrocarbons to smaller hydrocarbons. The octadecane sample examined in this study did not undergo any additional transition to the highest pressure (∼18 GPa) explored in this study.

Behavior of Long-Chain Hydrocarbons at High Pressures and Temperatures

“…The ambient condition Raman spectra of C 18 and C 23 can be characterized by vibrations related to the (1) oscillation of the carbon atoms in the hydrocarbon chain simulating an accordion motion known as longitudinal accordion mode (LAM), (2) C–C stretching (ν) mode, (3) H–C–H twisting (τ) mode, (4) H–C–H bending (δ) modes, and (5) C–H stretching (ν) modes (Figure ). , The observed modes in the wavenumber region till 600 cm –1 are due to LAM’s. The fundamental LAM, that is, m = 1, is the lowest energy mode and observed at wavenumber ∼131 cm –1 for C 18 and ∼103 cm –1 for C 23 .…”

“…In a series of recent static high-pressure Raman spectroscopic studies on C 18 and C 23 conducted at room temperatures, the appearance of multiple new low wavenumber modes close to the fundamental LAM in the pressure regime of 0.3–3 GPa , was observed. Complementary first-principles simulations on linear and bent long-chain alkanes provided valuable insights into pressure-induced low-energy modes. , The high-pressure experimental study and complementary Raman spectra based on first-principles provided clues to how linear chains, C 18 and C 23 , transformed to a bent geometry at the transition pressure, P tr . , These recent studies showed that the transition pressure, P tr , varied inversely with the chain length of alkane, that is, longer alkanes transformed from linear to bent configurations at lower pressures than the relatively shorter alkanes. The transition was always accompanied by the appearance of new low-energy modes in both high-pressure experiments and complementary first-principles simulations.…”

“…Previous studies have indicated that a single chain of alkane, when suspended in a vacuum at low temperatures, exists in a linear extended conformation. , As the chain length increases to n ∼ 17–18, the weak interaction between segments of the chain introduces a hairpin bend in the alkane . An alkane with a bent configuration is generally characterized by the appearance of new modes close to the fundamental LAM. ,, Furthermore, the fundamental LAM frequency of alkane chains is approximately inversely proportional to the chain length, n , which indicates that the new modes are modified LAM frequency due to the formation of shorter chain segments (e.g., i and j are the length of chain segments such that i + j = n ). The length of each segment (i.e., i and j ) can vary as the bend may be introduced on compression at different points in the alkane chain.…”

“…Similarly, high-temperature studies often provide constraints on (∂ν/∂ T ) P , ( P = 1 bar). Indeed, recent studies on two alkanes (C 18 and C 23 ) provided valuable constraints on (∂ν/∂ P ) 22 °C (Tables and ). , In this present study, we aim to understand the behavior of the ν bent mode, which is not present at ambient pressures and thus pose a challenge as we are unable to constrain (∂ν/∂ T ) 1 bar . However, we are able to provide constraints on (∂ν/∂ T ) P max , where P max is the maximum thermal pressure explored in our study using the formalism where ν P max,22 °C = ν P ,22 °C + (∂ν/∂ P ) 22 °C ( P max – 1 bar).…”

“…Previous studies have revealed that over geological time scales, liquid hydrocarbons progressively decompose to lighter hydrocarbons and ultimately to methane in the temperature region of 150–200 °C. − Solid alkanes like octadecane and nonadecane found in the Paris meteorite undergo decomposition to diamond agglomerates and hydrogen at temperatures of more than 2700 °C and pressures in between 10 and 20 GPa . Recent studies on heavier alkanes have revealed compression-induced bending of linear alkane chains, which led to the speculation of dissociation of the chains at the low-energy bent region if sufficient thermal energy is provided. , However, the actual dissociation process is not well understood and will likely depend on the actual pressure ( P )–temperature ( T ) path followed. Furthermore, the behavior of the long-chain aliphatic hydrocarbon at moderate temperatures and pressures remains relatively unknown.…”