Vanadium oxide clusters in substellar atmospheres: A quantum chemical study

TL;DR

This study uses quantum chemical calculations to determine the structures and thermodynamics of vanadium oxide clusters in substellar atmospheres, revealing their influence on cloud formation and nucleation rates.

Contribution

It reports new low-energy structures and thermodynamic data for vanadium oxide clusters, improving understanding of their role in substellar atmospheric chemistry and cloud nucleation.

Findings

Larger vanadium oxide clusters dominate below ~1000 K.

VO molecules are most abundant between ~1000 K and ~2000 K.

Revised cluster data significantly increase nucleation rate estimates.

Abstract

We aim to understand the formation of cloud condensation nuclei in oxygen-rich substellar atmospheres by calculating fundamental properties of the energetically most favorable vanadium oxide molecules and clusters. A hierarchical optimization approach is applied in order to find the most favorable structures for clusters of (VO)$_{N}$ and (VO$_2$)$_{N}$ for N=1-10, and (V$_2$O$_5$)$_{N}$ for N=1-4 and to calculate their thermodynamical potentials. The candidate geometries are initially optimized applying classical interatomic potentials and then refined at the B3LYP/cc-pVTZ level of theory to obtain accurate zero-point energies and thermochemical quantities. We present previously unreported vanadium oxide cluster structures as lowest-energy isomers. We report revised cluster energies and their thermochemical properties. Chemical equilibrium calculations are used to asses the impact of the updated and newly derived thermodynamic potentials on the gas-phase abundances of vanadium-bearing species. In chemical equilibrium, larger clusters from different stoichiometric families are found to be the most abundant vanadium-bearing species for temperatures below ~1000 K, while molecular VO is the most abundant between ~1000 K and ~2000 K. We determine the nucleation rates of each stoichiometric family for a given (T$_{gas}$, p$_{gas}$) profile of a brown dwarf using classical and non-classical nucleation theory. Small differences in the revised Gibbs free energies of the clusters have a large impact on the abundances of vanadium bearing species in chemical equilibrium at temperatures below ~1000 K, which subsequently has an impact on the nucleation rates of each stoichiometric family. We find that with the revised and more accurate cluster data non-classical nucleation rates are up to 15 orders of magnitude higher than classical nucleation rates.