Structure and Spectroscopy of Criegee Intermediates in Gas- and Aqueous Environments

TL;DR

This work addresses how Criegee intermediates, H$_2$COO and syn-CH$_3$CHOO, behave structurally and spectroscopically in gas, ASW, water droplets, and bulk water. It employs two validated energy representations, MS-ARMD and PhysNet, to perform extensive MD simulations across four environments, revealing that droplets allow facile interior-surface exchange while ASW at 50 K restricts diffusion. Infrared spectra show environment-dependent shifts on the order of a few to tens of cm$^{-1}$, consistent with Stark-like effects in interfacial fields around $10^7$ V/cm, with only minor differences between inside and surface positions on droplets. Overall, the results enhance understanding of CI chemistry in atmospheric microenvironments and provide validated computational PES resources and spectral predictions for future studies.

Abstract

The dynamics and spectroscopy of the small (H$_2$COO) and large (CH$_3$CHOO) Criegee intermediates (CIs) in the gas phase, inside/on water droplets, on amorphous solid water (ASW) and in bulk water are investigated using validated energy functions. For both species, facile diffusion between surface and inside positions for water droplets are found whereas on amorphous solid water at low temperatures (50 K) no surface diffusion is observed on the multiple-nanosecond time scale. This is at variance with other species, such as CO or NO on ASW. The infrared spectroscopy of both CIs in contact with an aqueous environment leads to shifts of the spectral features on the order of a few to a few tens of cm$^{-1}$, depending on the vibrational mode considered. This is consistent with Stark-induced spectral shifts for small molecules in protein environments. However, the spectroscopy of both CIs in contact with water droplets does not depend on the positioning relative to the droplet (inside vs. surface).

Figures (11)

• Figure 1: The structures of two Criegee intermediates, H$_2$COO and syn-CH$_3$CHOO, with atoms labelled.
• Figure 2: Radial pair distribution function for H$_2$COO (CI) on the water droplets. The radial distances in panels A to D are the O$_{\rm W}$--C$_{\rm CI}$, O$_{\rm W}$--O$_{\rm W}$, O$_{\rm W}$--CoM$_{\rm CI}$, and CoM$_{\rm droplet}$--CoM$_{\rm CI}$ separations. Averages over 20 independent simulations, each 2 ns in length, are shown for MS-ARMD (red thick lines) and PhysNet (blue thick lines) energy functions for H$_2$COO, respectively. Radial distribution functions for individual trajectories are shown as dotted lines. The two extreme cases, CI located inside the water droplet and adsorbed on its surface are shown in Figure \ref{['sifig:droplet']}.
• Figure 3: Snapshots of the droplet simulations with H$_2$COO (left) and syn-CH$_3$CHOO (right) as the guest molecule.
• Figure 4: Radial pair distribution function for syn-CH$_3$CHOO (CI) on the water droplets. The radial distances in panels A to D are the O$_{\rm W}$--C$_{\rm CI}$, O$_{\rm W}$--O$_{\rm W}$, O$_{\rm W}$--CoM$_{\rm CI}$, and CoM$_{\rm droplet}$--CoM$_{\rm CI}$ separations. Averages over 20 independent simulations, each 2 ns in length, are shown for MS-ARMD (red thick lines) and PhysNet (blue thick lines) energy functions for syn-CH$_3$CHOO, respectively. Radial distribution functions for individual trajectories are shown as dotted lines.
• Figure 5: The CoM of CI sampled from 2 ns MD simulations at 50 K. The water molecules are free to move but only oscillate around their positions at such low temperature. The CIs are free to move but do not diffuse between neighboring wells at the low temperatures on short time scales.