Reaction Dynamics of Zr and Nb with Ethylene

The reactions of transition metal (M) atoms Zr and Nb with ethylene (C 2 H 4 ) were studied using the technique of crossed molecular beams. Angular and velocity distributions of MC 2 H 2 products following H 2 elimination were measured at collision energies between 5 and 23 kcal/mol using electron impact and 157 nm photoionization mass spectrometry. Photodepletion studies identify that the atomic reactants are predominantly in their ground electronic states and that the observed MC 2 H 2 products result primarily from reactions of these ground-state atoms. Center-of-mass product angular distributions derived from the data indicate that reactions involve the formation of intermediate complexes having lifetimes longer than their rotational periods. Product translational energy distributions demonstrate that a large fraction of excess available energy is channeled into product internal excitation. Wide-angle nonreactive scattering of metal atom reactants following decay of long-lived MC 2 H 4 association complexes was also observed for both transition metal reactants at collision energies ≥ 9 kcal/mol, with approximately 36% of the initial translational energy converted into C 2 H 4 internal excitation. At collision energies of ≤ 6 kcal/mol, nonreactive scattering of Zr from ZrC 2 H 4 decay was found to be negligible, whereas this channel was clearly observed for Nb. RRKM modeling of the competition between decay of MC 2 H 4 complexes back to M + C 2 H 4 and C−H insertion forming HMC 2 H 3 indicates that there exists an adiabatic potential energy barrier for M + C 2 H 4 association in the case of Zr and that the transition state for this process is tighter than for the analogous process in Nb + C 2 H 4 . The barrier for Zr + C 2 H 4 association is attributed to the repulsive s 2 ground state configuration of Zr, whereas for Nb the s 1 ground state configuration results in no barrier for association. The absence of decay of ZrC 2 H 4 back to Zr + C 2 H 4 at low collision energies indicates that the barrier for C−H insertion forming HZrC 2 H 3 lies below the barrier for Zr + C 2 H 4 association. This opens up the possibility that direct C−H insertion without initial ZrC 2 H 4 formation may play an important role.