Vanadium oxide clusters in substellar atmospheres: A quantum chemical study
arxiv(2024)
摘要
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_2O_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.
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