Silane-initiated nucleation in chemically active plasmas: Validation of density functionals, mechanisms, and pressure-dependent variational transition state calculations

The growth of anionic silicon hydride clusters is a critically important process in nanodusty plasmas. In the current study, we focus on the formation of homologs of silylene (Si_n+1H_2n+2^-, n = 3, 4) and silyl (Si_nH_2n+1^-, n = 4, 5) anions via anion-neutral reaction pathways. Species like silyl or silylene anions and their related elementary reactions, which are involved in the formation of silicon hydride clusters, were not used in developing exchange-correlation (xc) density functionals (i.e., they were not included in the training set of semiempirical density functionals); therefore, we explored the accuracy of various widely used xc density functionals based on reaction energies and barrier heights. Among the 21 density functionals we tested, M06-2X has the best performance for a hybrid functional, and MN15-L has the best performance for a local functional. Thermal rate constants of the elementary reactions involved in the reaction mechanism are calculated using M06-2X and multistructural canonical variational transition state theory with the small-curvature tunneling approximation (MS-CVT/SCT). The pressure dependence of unimolecular isomerization reactions is treated with system-specific quantum RRK theory (SS-QRRK) and the Lindemann-Hinshelwood mechanism.