A Two-Color Laser Photolysis Method for Determining Reaction Rates of Short-Lived Intermediates by Product Analysis:  Application to the <i>o</i>-Quinodimethane Problem

A time-delayed, two-color pulse laser photolysis technique was used for a kinetic study of short-lived transient species through product analysis, the determination of the rate constant of the cycloaddition of <i>o</i>-quinodimethane (<b>1</b>) and maleic anhydride (<b>2</b>) in room-temperature solutions. <i>o</i>-Quinodimethane (<b>1</b>) was generated from 1,2-bis[(phenylseleno)methyl]benzene (<b>3</b>) by the irradiation of a pulse of a KrF excimer laser (248 nm) in the presence of excess <b>2</b>, and a successive pulse of a XeCl excimer laser (308 nm) was irradiated to the reaction mixture after varied delay times from 0 to 0.1 s for the decomposition of the remaining <b>1</b> to quench the cycloaddition reaction. The rate constant of the cycloaddition of <b>1</b> and <b>2</b> was 2.1 × 10<sup>5</sup> M<sup>-1</sup> s<sup>-1</sup>, which was obtained by the analysis of the delay-time dependence of the product yields.