Photochemistry of anhydrides. Part 3.—Primary processes in the photolyses of (CF3CO)2O and (C2F5CO)2O and the use of (CF3CO)2O and (C2F5CO)2O as gas phase actinometers

Abstract

The photolyses of the vapours of the anhydrides (CF₃CO)₂O and (C₂F₅CO)₂O are quantitatively described by the overall equation (R = CF₃ or C₂F₅), (RCO)₂O +hv→ CO + CO₂+ R₂. The quantum yield of CO is independent of anhydride pressure in the range 0.5–16 Torr and increases slightly with temperature. For (CF₃CO)₂O, ϕ_CO(25°C)= 0.24 and ϕ_CO(200°C)= 0.29: for (C₂F₅CO)₂O, ϕ_CO(25°C)= 0.26 and ϕ_CO(190°C)= 0.31. ϕ_CO is unaffected by the presence of mercury vapour and is virtually independent of whether photolysis is at 254 or 265 nm. Addition of CO₂ or olefins indicates weak vibrational quenching. No evidence of involvement of excited triplet states was obtained, using the Cundall and biacetyl techniques. Weak emission of light was observed with ϕ(emission)⩽ 2 × 10^–4 for (CF₃CO)₂O and ⩽ 2 × 10^–3 for (C₂F₅CO)₂O.

A proposed mechanism involves efficient internal conversion of excited singlet molecules of anhydride to the ground state, possibly via a valence isomerization. The anhydrides are recommended as useful vapour-phase actinometers at 254 and 265 nm.