Probing selectivity of bond cleavage and formation during pulsed laser processing of organic liquids

Reactions of alkyl radicals have been studied in radiolysis experiments for decades, and have more recently been investigated for their contributions to synthesis of nanomaterials by pulsed laser processing in organic liquids. This work probes the selectivity of specific radical formation pathways during pulsed laser processing in organic liquid through investigation of the branched alkane 2,3-dimethylbutane, which only has two distinct C-C and C-H bonds. Molecular iodine is added as a selective alkyl radical scavenger due to its diffusion-limited reaction rate that yields stable alkyl iodide products. We find that although laser processing and radiolysis produce the same isomers of alkanes (C₉H₂₀, C₁₁H₂₄, and C₁₂H₂₆) and alkyl iodides (C₃H₇I, C₅H₁₁I, and C₆H₁₃I), three significant differences in the relative yields of specific isomers are observed. First, somewhat stronger selectivity towards cleavage of the central C-C bond over cleavage of a peripheral C-C bond is observed for laser processing. Second, laser processing yields lower selectivity towards H atom loss from a central tertiary carbon atom compared to H atom loss from a peripheral primary carbon. Third, laser processing produces lower yields of alkanes requiring quaternary C-C bond formation than expected based on the relative yields of alkyl iodide isomers, and therefore the precursor radicals. These results reveal that laser processing generates a distinct nonequilibrium reaction environment compared to radiolysis, which can inform design of organic liquid environments for nanomaterial synthesis by laser processing.