EPR studies of the reactions of high-energy copper species with hydrocarbons using a rotating cryostat

Abstract

The rotating cryostat technique has been modified to allow the use of laser ablation and magnetron metal evaporation sources to complement our previous studies of ground-state metal-atom and metal-cluster reactions using resistively heated sources. Pulses of neutralized metal plumes ablated from a copper disc by absorption of photons from an Nd/YAG 532 nm laser (8 ms pulse duration and 10 Hz repetition rate) were directed onto the constantly renewed surface of the solids previously deposited onto the rotating drum. The continuous beam of high-energy species formed by argon ion bombardment of a copper disc in a magnetron (operating at 300 V potential) was similarly deposited onto the solids at 77 K.

Bombardment of benzene by copper from either laser ablation, [Cu(LA)], or magnetron sputtering sources, [Cu(MS)], gave composite EPR spectra at 220 K consisting of transitions from a copper–benzene complex (minor), cyclohexadienyl radicals, C₆H₇(B), and a strong isotropic sextet with g= 2.002 and a_H(5)= 3.4 G (A). Assignments were confirmed by using C₆D₆ and C₆H₅D. For resistively evaporated copper atoms, Cu(TS), and those from electron beam heating, Cu(EB), only the Cu–benzene complex was formed. Species A is not the expected phenyl radical; this new species has hyperfine interactions which suggest a π-type radical bonded to an unobserved copper moiety. The two possible radicals, C₆H₅˙ and C₅H₅˙, are discussed in terms of structure and reaction pathways. The occurrence of such reactions only for Cu(LA) and Cu(MS) and not for Cu(TS) probably indicates that the atoms from the former sources have high kinetic energy ( > 83 kJ mol^–1) and may also be electronically excited.

Similar metal-mediated hydrogen transfers are observed for Cu(LA) and Cu(MS) atoms reacting with cyclohexa-1,3-diene.