Determination of chemical rate constants in singlet molecular oxygen reactions by using 1,4-dimethylnaphthalene endoperoxide
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Thermal decomposition of 1,4-dimethylnaphthalene endoperoxide (DMNE) as a source of singlet oxygen was used to measure chemical rate constants, k(R), for reactions between singlet oxygen, O-2((1)Delta(g)), and various substrates. Time resolved O-2((1)Delta(g)) IR luminescence detection and steady-state experiments were used to monitor the decomposition of the endoperoxide and the rate of singlet oxygen production. Only 25% of oxygen from thermal decomposition of DMNE, in acetonitrile at 20degreesC, is detected as O-2((1)Delta(g)). Values of k(R) for reactions of O-2((1)Delta(g)) with 1,4-diphenylisobenzofurane (DPBF) and rubrene measured by this method are similar to values obtained by photosensitization. Values of k(R) for the chemical reaction of O-2((1)Delta(g)) with the antiinflammatory drugs piroxicam and tenoxicam, of (6.1 +/- 0.4) x 10(6) and (1.6 +/- 0.2) 10(7) M-1 s(-1), respectively, are close to those for the total singlet oxygen deactivation rate. Thermal decomposition of aromatic endoperoxides is a convenient source of singlet oxygen for measurements of rate constants in reactions of O-2((1)Delta(g)) where photosensitization cannot be employed. However, experimental conditions and approaches involved determine the method's limitations and applicability in a given system.