Hydrogenated amorphous silicon nanostructures: novel structure-reactivity relationships for cyclization and ring opening in the gas phase

The effects of the reactive center connectivity and internal rotations on the reactivity of hydrogenated silicon nanostructures toward cyclization and ring opening pathways have been investigated. Rate coefficients for 25 cyclization and ring opening reactions for hydrides containing up to eight silicon atoms have been calculated using G3//B3LYP. The overall reactions exhibit two elementary steps. Overcoming the first barrier results in the formation of a hydrogen-bridged cyclic intermediate from a substituted silylene. Passing over the second barrier converts this intermediate into a cyclic silicon hydride. The rate-determining step varied according to the ring size formed and the temperature. Assuming a rate-determining step, values for the single-event Arrhenius pre-exponential factor, (A) over tilde, and the activation energy, E-a, were calculated from G3//B3LYP rate coefficients corrected for internal rotations, and a group additivity scheme was developed to predict (A) over tilde and E-a. The values predicted by group additivity are more accurate than structure-reactivity relationships currently used in the literature, which rely on a representative (A) over tilde value for each reaction class and the Evans-Polanyi correlation to predict E-a. Internal rotation corrections played a prominent role in cyclization pathways, impacting (A) over tilde values for larger ring formation reactions more strongly than any variations in the connectivity of the reactive center.