Hydrogenation of C₂₄ Carbon Clusters: Structural Diversity and Energetic Properties

This work aims at exploring the potential energy surfaces of C₂₄H using the genetic algorithm in combination with the density functional based tight binding potential. The structural diversity was analyzed using order parameters, in particular the sum of the numbers of 5- and 6-carbon rings R_5/6. The most abundant and lowest energy population was designated as the flake population (isomers of variable shapes, large R_5/6 values), characterized by an increasing number of spherical isomers when n_H/n_C increases. Simultaneously, the fraction of the pretzel population (spherical isomers, smaller R_5/6 values) increases. The fraction of the cage population (largest R_5/6 values) remains extremely minor while the branched population (smallest R_5/6 values) remains the highest energy population for all n_H/n_C ratios. For all C₂₄H clusters, the evolution of the carbon ring size distribution with energy clearly shows the correlation between the stability and the number of 6-carbon rings. The average values of the ionization potentials of all populations were found to decrease when n_H/n_C increases, ranging from 7.9 down to 6.4 eV. This trend was correlated to geometric and electronic factors, in particular to carbon hybridization. These results are of astrophysical interest, especially regarding the role of carbon species in the gas ionization.