Selective vibronic excitation of singlet oxygen–furan reactions in cryogenic matrices

Abstract

The reactions of 2,5-dimethylfuran (DMF), 2-methylfuran (MF), and furan with molecular oxygen to form endoperoxides have been induced in Ar and O2 matrices at 12 K with selective vibronic excitation of O2 using near infrared light. Reaction was induced through excitation of the 1Δg(v′=0, v′=1) and 1Σg+(v′=0, v′=1) O2 states near 8000 and 13 100 cm−1, respectively, as well as by the (1Δg, 1Δg) simultaneous transitions of (O2)2 at 15 900 and 17 300 cm−1. The 1Δg ← 3Σg− vibronic progression in solid O2 was recorded by FTIR spectroscopy, whereas members of the 1Σg+ ← 3Σg− and (1Δg, 1Δg) ← (3Σg−, 3Σg−) progressions in Ar matrices were located by ‘‘reaction excitation’’ spectroscopy. The DMF+O2 reaction is a single photon process, apparently with unit quantum yield, for all vibronic levels excited. For MF+O2(1Δg, v′=0) and furan +O2(1Δg, v′=0), quantum yields were high, ∼0.6 and 0.4, respectively, but the reservoirs of reactive pairs were 10–20 times smaller than for DMF+O2. The furan+O2 reaction rate showed an 18O2 isotope effect 0.78±0.15, which can be interpreted in terms of quantum mechanical tunneling on the lowest singlet hypersurface. The IR matrix spectra of DMF endoperoxide and its peroxidic 18O counterpart are reported, and a partial vibrational assignment for this intermediate is given. The endoperoxide vibration that absorbs at 815.7 cm−1 is dominated by the bridge O–O stretching motion.