Level Crossing Resonance Studies of the Muoniated Cyclohexadienyl Radical (MuC(6)H(6)) Interacting with Uncapped Gold Nanoparticles in Porous Silica Hosts

The study of metal nanoparticles (NPs) and gold NPs (AuNPs), in particular, has been a subject of considerable interest in recent years. The present paper reports on the formation of the muoniated cyclohexadienyl radical in the Mu + C6H6 -> Mu<(C)over dot>H-6(6) addition reaction in the same 8 and 10 nm AuNP samples, encapsulated in SBA-15 mesoporous silica, where the spin-relaxation rates.Mu were recently reported on [Fleming et al. J. Phys. Chem. C, 2019, 123, 27628, paper "P1"], but the final state was not identified in that study. The formation of this Mu<(C)over dot>H-6(6) radical and its interactions with the AuNP surfaces is investigated herein by the technique of avoided level crossing (ALC) resonance spectroscopy, for the same range of Bz loadings as studied earlier. The positions of the level crossings, giving the hyperfine coupling constants (hfcc) for the C-Mu and C-H bonds at the "ipso" position where Mu adds to the ring, are essentially the same as those seen in solid Bz and in bare silica, and hence these hfcc do not distinguish site locations for the surface-adsorbed benzene. The amplitudes of these resonances, which are a measure of the fraction of Mu forming the free-radical final state, do, however, provide such a distinction, being much less for Bz adsorbed in the AuNP/silica samples than in the bare silica. This loss in amplitude is attributed to competitive reaction channels, one forming the Mu<(C)over dot>H-6(6) free radical interacting with the AuNPs and the other due to formation of diamagnetic final states. An important signpost as to the nature of these competitive reaction channels is provided by the measured widths of the ALC resonances seen. These are surprisingly narrower in the presence of the AuNPs than for the bare silica, implying that the Mu<(C)over dot>H-6(6) radical is not in direct contact with the AuNPs. These differing possibilities and contrasting mechanisms are discussed.