Low-Frequency (Gigahertz to Terahertz) Depolarized Raman Scattering Off n-Alkanes, Cycloalkanes, and Six-Membered Rings: A Physical Interpretation.

Andrew J Farrell,Klaas Wynne,Mario González-Jiménez,Gopakumar Ramakrishnan

doi:10.1021/acs.jpcb.0c03769

Abstract

Molecular liquids have long been known to undergo various distinct intermolecular motions, from fast librations and cage-rattling oscillations to slow orientational and translational diffusion. However, their resultant gigahertz to terahertz spectra are far from simple, appearing as broad shapeless bands that span many orders of magnitude of frequency, making meaningful interpretation troublesome. Ad hoc spectral line shape fitting has become a notoriously fine art in the field; a unified approach to handling such spectra is long overdue. Here we apply ultrafast optical Kerr-effect (OKE) spectroscopy to study the intermolecular dynamics of room-temperature n-alkanes, cycloalkanes, and six-carbon rings, as well as liquid methane and propane. This work provides stress tests and converges upon an experimentally robust model across simple molecular series and range of temperatures, providing a blueprint for the interpretation of the dynamics of van der Waals liquids. This will enable the interpretation of low-frequency spectra of more complex liquids.

Highlights

Intermolecular interactions facilitate energy transfer and are crucial in driving chemical reactions in the condensed phase, making the investigation of the nature of these interactions key to our understanding of chemical reactivity.[1,2] Intermolecular dynamics consist of a broad range of interactions on many different time scales, typically anything ∼1 ps or slower (≤1 THz)
Similar to the librational modes, collision-induced contributions at high frequencies are more localized “cage-rattling” motions, becoming progressively more diffusive at lower frequencies. This line shape division is applied in dielectric relaxation spectroscopy (DRS), where orientational and translational diffusions are known as α- and β-relaxation respectively, and cage-rattling is referred to as a “fast-β” process.[9−11]
The Brillouin-zone edge transverse and longitudinal acoustic (TA and LA) phonon frequencies of the plastic phase of solid methane were used to fit the optical Kerr-effect (OKE) spectrum of liquid methane.[59]. This is unreasonable in the liquid phase and unsurprisingly produces a poor fit, but it illustrates the potential origin of the rattling dynamics described by the fast-β mode in the disordered liquid

Summary

Introduction

Intermolecular interactions facilitate energy transfer and are crucial in driving chemical reactions in the condensed phase, making the investigation of the nature of these interactions key to our understanding of chemical reactivity.[1,2] Intermolecular dynamics consist of a broad range of interactions on many different time scales, typically anything ∼1 ps or slower (≤1 THz). In the 1970s, Bucaro and Litovitz suggested that motions at these frequencies could be approximated as pairwise collisions causing molecular frame distortions.[7,8] Similar to the librational modes, collision-induced contributions at high frequencies are more localized “cage-rattling” motions, becoming progressively more diffusive at lower frequencies. This line shape division is applied in dielectric relaxation spectroscopy (DRS), where orientational and translational diffusions are known as α- and β-relaxation respectively, and cage-rattling is referred to as a “fast-β” process (see Figure 1).[9−11]

Methods

Results

Conclusion