J/A+A/668/A35 TiO2 nanoclusters fundamental properties (Sindel+, 2022)
Revisiting fundamental properties of TiO2 nanoclusters as condensation seeds
in astrophysical environments.
Sindel J.P., Gobrecht D., Helling C., Decin L.
<Astron. Astrophys. 668, A35 (2022)>
=2022A&A...668A..35S 2022A&A...668A..35S (SIMBAD/NED BibCode)
ADC_Keywords: Atomic physics
Keywords: molecular data - astrochemistry -
planets and satellites: atmospheres -
planets and satellites: gaseous planets - molecular processes
Abstract:
The formation of inorganic cloud particles takes place in several
atmospheric environments, including those of warm, hot, rocky, and
gaseous exoplanets, brown dwarfs, and asymptotic giant branch stars.
The cloud particle formation needs to be triggered by the in situ
formation of condensation seeds since it cannot be reasonably assumed
that such condensation seeds preexist in these chemically complex
gas-phase environments.
We aim to develop a method for calculating the thermochemical
properties of clusters as key inputs for modelling the formation of
condensation nuclei in gases of changing chemical composition. TiO2
is used as benchmark species for cluster sizes N=1-15.
We created a total of 90000 candidate (TiO2)N geometries for cluster
sizes N=3-15. We employed a hierarchical optimisation approach,
consisting of a force-field description, density-functional based
tight-binding, and all-electron density-functional theory (DFT) to
obtain accurate zero-point energies and thermochemical properties for
the clusters.
In 129 combinations of functionals and basis sets, we find that
B3LYP/cc-pVTZ, including Grimme's empirical dispersion, performs
most accurately with respect to experimentally derived thermochemical
properties of the TiO2 molecule. We present a hitherto unreported
global minimum candidate for size N=13. The DFT-derived thermochemical
cluster data are used to evaluate the nucleation rates for a given
temperature-pressure profile of a model hot-Jupiter atmosphere. We
find that with the updated and refined cluster data, nucleation
becomes unfeasible at slightly lower temperatures, raising the lower
boundary for seed formation in the atmosphere.
The approach presented in this paper allows finding stable isomers for
small (TiO2)N clusters. The choice of the functional and basis set
for the all-electron DFT calculations has a measurable impact on the
resulting surface tension and nucleation rate, and the updated
thermochemical data are recommended for future considerations.
Description:
In the corresponding paper, we explored the geometries and
thermochemical properties of small (TiO2)N molecular clusters. This
was done through a hierarchical approach, with the final results being
calculated with quantum- chemical hybrid DFT methods at the
B3LYP/cc-pVTZ level of theory including empirical dispersion. The
results from this calculations, which are cartesian coordinates and
thermochemical properties from these calculations were used to
calculate nucleation rates of these species in the atmospheres of hot
Jupiter type exoplanets. We found that with updated and more accurate
cluster data, nucleation becomes inefficient at higher altitudes and
lower temperatures when compared to previous data.
There are 246 data entries in this archive, consisting of two sub-sets
of 123 entries each. Each data-entry corresponds to a (TiO2)N
molecular cluster of size N=1-15. The cluster coordinates are
optimised using GAUSSIAN software at the B3LYP/cc-pVTZ level of
theory, including empirical dispersion. For all sizes N>1 multiple
isomers have been calculated, with their rank I given in the
description. Rank 1 therefore denotes the most stable isomer of each
size. In the first subset, for each cluster its final cartesian
coordinates (x,y,z) of each atom in the cluster are given. In the
second subset, the frequency calculation results have been used in
combination with a rapidly rotating harmonic oscillator (RRHO)
approach to calculate thermochemical data for the corresponding
cluster. For each temperature T the entropy S, difference to reference
enthalpy H-Ho, free enthalpy of formation ΔH and Gibbs free
energy of formation ΔG are given in the table.
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
list.dat 44 246 List of entries
coords.dat 38 3627 Coordinates (for the 246 entries)
thermo.dat 102 7011 Thermochemical data (for the 246 entries)
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Byte-by-byte Description of file: list.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 2 I2 --- Rank Rank
4- 5 I2 --- Cl Molecular cluster identification within the rank
7- 44 A38 --- Note Explanation of the (Rank, Cl) code
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Byte-by-byte Description of file: coords.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 2 I2 --- Rank Rank
4- 5 I2 --- Cl Molecular cluster identification within the rank
7- 8 A2 --- Atom Atomic symbol of the atom
10- 18 F9.6 0.1nm x x coordinate of the atom (Angstroem)
20- 28 F9.6 0.1nm y y coordinate of the atom (Angstroem)
30- 38 F9.6 0.1nm z z coordinate of the atom (Angstroem)
--------------------------------------------------------------------------------
Byte-by-byte Description of file: thermo.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 2 I2 --- Rank Rank
4- 5 I2 --- Cl Molecular cluster identification within the rank
7- 13 F7.2 K T Temperature T
15- 33 F19.14 J/mol/K S Entropy S
35- 56 F22.17 kJ/mol H-Ho Enthalpy difference to reference temperature
(298.15)
58- 78 F21.14 kJ/mol DfH Enthalpy of formation
80-102 F23.16 kJ/mol DfG Gibbs free energy of formation
--------------------------------------------------------------------------------
Acknowledgements:
Jan Philip Sindel, JanPhilip.Sindel(at)oeaw.ac.at
(End) Patricia Vannier [CDS] 14-Sep-2022