Molecular-Level Understanding of the Ro-vibrational Spectra of N$_2$O in Gaseous, Supercritical and Liquid SF$_6$ and Xe

TL;DR

This study uses classical molecular dynamics simulations to analyze the vibrational spectra of N$_2$O in SF$_6$ and Xe across different phases, revealing how spectral features evolve with solvent density.

Contribution

It demonstrates that classical MD simulations with accurate potentials can semi-quantitatively describe phase-dependent vibrational spectral changes of N$_2$O.

Findings

Spectral lineshape transitions from P-/R-branches to Q-branch-like with increasing density.

Classical MD accurately captures ultrafast rotational motions in various solvent densities.

Interpretation based on rigid-body rotation theory and phenomenological models.

Abstract

The transition between the gas-, supercritical-, and liquid-phase behaviour is a fascinating topic which still lacks molecular-level understanding. Recent ultrafast two-dimensional infrared spectroscopy experiments suggested that the vibrational spectroscopy of N$_2$O embedded in xenon and SF$_6$ as solvents provides an avenue to characterize the transitions between different phases as the concentration (or density) of the solvent increases. The present work demonstrates that classical molecular dynamics simulations together with accurate interaction potentials allows to (semi-)quantitatively describe the transition in rotational vibrational infrared spectra from the P-/R-branch lineshape for the stretch vibrations of N$_2$O at low solvent densities to the Q-branch-like lineshapes at high densities. The results are interpreted within the classical theory of rigid-body rotation in more/less constraining environments at high/low solvent densities or based on phenomenological models for the orientational relaxation of rotational motion. It is concluded that classical MD simulations provide a powerful approach to characterize and interpret the ultrafast motion of solutes in low to high density solvents at a molecular level.