Hydrogenation of C$_{24}$ carbon clusters : structural diversity and energetic properties

TL;DR

This study explores the structural diversity and energetic properties of hydrogenated C24 carbon clusters using genetic algorithms and DFTB, revealing how hydrogenation affects stability, structure, and ionization potential with astrophysical implications.

Contribution

It provides a comprehensive analysis of the structural and energetic evolution of C24Hn clusters across different hydrogenation levels using advanced computational methods.

Findings

Flake structures are most abundant and lowest in energy.

Hydrogenation increases spherical isomers and decreases cage populations.

Ionization potentials decrease with higher hydrogenation ratios.

Abstract

This work aims at exploring the potential energy surfaces of C$_{24}$H$_n${n=0,6,12,18,24} up to 20-25\,eV using the genetic algorithm in combination with the density functional based tight binding (DFTB) potential. The structural diversity of the non fragmented structures was analysed using order parameters which were chosen as the number of 5 or 6 carbon rings and the asphericity constant $\beta$. The most abundant and lowest energy population was found to correspond to a flakes population, constituted of isomers of variable shapes possessing a large number of 5 or 6-carbon rings. This population is characterized by a larger number of spherical isomers when $n_H/n_C$ increases. Simultaneously, the fraction of the pretzels population constituted of spherical isomers possessing fewer 5 or 6 carbon ring cycles increases. For all hydrogenation rates, the fraction of cages population remains extremely minor while the branched population is the highest energy population for all $n_H/n_C$ ratios. For all C$_{24}$H$_n${n=0,6,12,18,24} clusters, a detailed study of the evolution of the carbon ring size distribution as a function of energy clearly shows that the stability is correlated to the number of 6-carbon rings. A similar study for hybridization $sp^n$ (n=1-3) shows that the number of $sp^1$ carbon atoms increases with energy while globally the number of $sp^3$ carbon atoms increases with $n_H/n_C$. The average values of the ionization potentials of all populations were found to decrease when $n_H/n_C$ increases, ranging from 7.9\,eV down to 6.4\,eV that we correlated to carbon atoms hybridization $sp^n$ (n=1-3). These results are of astrophysical interest as they should be taken into account in astrophysical models especially regarding the role of carbonaceous species in the gas ionization.