ELECTRONIC STATE-SELECTED REACTIVITY OF TRANSITION-METAL IONS - CO+ AND FE+ WITH PROPANE

Recently, we developed a ''chromatographic'' technique to determine the electronic state distribution for transition metal ions. This method allows the study of state-selected reactions. In this paper we report the quantitative determination of rate constants and branching ratios for state-selected Co+ and Fe+ reacting with propane. Observed rates for adduct formation as well as H-2 and CH4 elimination channels were strongly dependent on the electronic configuration of the metal ion. Co+ ions were formed by electron impact on either Co(CO)3NO or COCP(CO)2 or by surface ionization of CoCl2. The Co+ electronic state population is a function of both the electron energy and the precursor used, and can be varied from 36% ground state to 97% ground state. Thus, we can measure reaction rate constants over a wide range of ground- and excited-state populations and extrapolate to 100% ground- or excited-state Co+ to obtain the state-specific reaction rates. Under our experimental conditions (10(-5) Torr of C3H8 in 1.75 Torr of He), adduct formation is the dominant product for the a3F 3d8 ground state of Co+ with only small amounts of elimination products observed. The 4s3d7 excited states (a5F and b3F) show greatly reduced clustering (due to the repulsive 4s electron) and enhanced elimination channels. Fe+ was formed by electron impact on Fe(CO)5. Again the electronic state population was varied by varying the electron energy. Absolute rate constants were obtained for the 6D ground state as well as for the 4F and 4D excited states of Fe+ reacting with C3H8. Adduct formation is the dominant product for the 6D 4s3d6 ground state of Fe+ despite the repulsive 4s electron. This is due to a crossing from the ground-state surface to the Fe+(4F 3d7)C3H8 first excited-state surface where the adduct is more strongly bound. The Fe+ 4D 4s3d6 second excited state reacts similarly to the Co+ a5F and b3F 4s3d7 excited states.