FORMATE ESTER NORRISH TYPE II ELIMINATION - DIODE LASER PROBING OF GAS-PHASE YIELDS

摘要

Time-resolved infrared absorption spectroscopy was utilized to monitor the production of HCOOH, CO2, and CO following ultraviolet laser excitation of gas-phase formate esters. Excitation of ethyl formate at 227.5 nm resulted in formation of HCOOH and CO2. The CO2 quantum yield was estimated to be 0.5 +/- 0.1. No evidence for CO formation was obtained at this wavelength. Relative quantum yields for the Norrish Type II elimination of HCOOH from ethyl, n-propyl, n-butyl, isopropyl, isobutyl, and tert-butyl formate were obtained at 227.5 and 222 nm. Normalization of the observed HCOOH yields with respect to the number of gamma-hydrogen atoms resulted in reactivity trends at 227.5 nm of 1:3:9 for the abstraction of primary, secondary, and tertiary hydrogen atoms, respectively. At 222 nn, a similar reactivity trend was observed with yields per available gamma-hydrogen of 1:3:7 for abstraction of primary, secondary, and tertiary hydrogen atoms. Yields were found to be independent of ester pressure over the range 100-550 mTorr. Semiempirical and ab initio calculations of the excited state hydrogen abstraction step were performed and enthalpies of activation of 8-12 kcal/mol were obtained using AM1 with configuration interaction.