A fluorinated phosphite traps alkoxy radicals photogenerated at the air/solid interface of a nanoparticle

Abstract

With interests in alkoxy radical formation on natural and artificial surfaces, a physical‐organic study was carried out with a Hammett series of triaryl phosphites (p‐MeO, H, p‐F, and p‐Cl) to trap adsorbed alkoxy radicals on silica nanoparticles. A mechanism which involves PhC (Me)2O• and EtO• trapping in a cumylethyl peroxide sensitized homolysis reaction is consistent with the results. The p‐F phosphite was able to indirectly monitor the alkoxy radical formation, and 31P NMR readily enabled this exploration, but other phosphites of the series such as the p‐MeO phosphite were limited by hydrolysis reactions catalyzed by surface silanol groups. Fluorinated silica nanoparticles helped to suppress the hydrolysis reaction although adventitious water also plays a role in hindering efficient capture of the alkoxy radicals by the phosphite traps.

Surface-Radical Mobility Test by Self-Sorted Recombination: Symmetrical Product upon Recombination (SPR)

“…Upon excitation, nanoparticle-adsorbed 1 transfers energy to the O–O bond of 2 resulting in its homolysis. Our previous work suggested this to be a Dexter (triplet) energy transfer process. In the present work, we found the following.…”

“…Cumylethyl peroxide 2 was synthesized in 74% yield and 82% purity by a literature procedure. 5 1 H NMR data were collected on a Bruker Avance 400 MHz instrument. HPLC data were collected on an Agilent Technology instrument (column: ZORBAX Eclipse XDB-C18).…”

“…68 Upon irradiation, the temperature of the particles was found to rise by ∼10 °C, as we detected in a similar system previously. 5 This was measured by a thermos couple probe attached to an IR thermometer. After photolysis, compounds were desorbed from the nanoparticle surface by stirring with 2 mL of acetonitrile for 20 min.…”

“…A second approach is theoretical; for example, a H 3 Si • diffusion activation barrier on silicon was found to be 3.7 kcal/mol by DFT and MD simulations. 17 A third approach is the use of 31 P NMR spectroscopy with phosphite traps 5,18 due to their oxophilicity to trap alkoxy radicals. In the third approach, alkoxy radicals on silica nanoparticles were trapped by phosphites to form phosphates.…”

“…However, mechanistic studies on surface-bound radicals, such as alkoxy radicals, are still challenging. While such information is typically found with EPR trapping − and 31 P NMR spectroscopy, expanding on methods to measure the radical migratory aptitude is an area in need of research. Here, we report a symmetrical product recombination (SPR) method that allows determination of the alkoxy radical surface mobility by a symmetrical product from an unsymmetrical substrate [R′OOR → R′O • + • OR → ROOR] (Figure ).…”