A molecular beam apparatus for performing single photon initiated dissociative rearrangement reactions (SPIDRR) with transition metal cation bound organic clusters.

The study of gas-phase ion-molecule reactions has been influential in the investigation of transition metal mediated bond activation and catalysis. We have furthered this field by developing a new technique capable of measuring the microcanonical kinetics for reactions between transition metal cations and neutral organic molecules. This novel method has been designated as single photon initiated dissociative rearrangement reaction (SPIDRR) technique and provides a nearly direct measurement of microcanonical reaction rate constants. For this reason, SPIDRR offers unique insight into reaction mechanisms and dynamics by assessing the energy dependence of the microcanonical rate constant, as well as measuring product branching fractions and kinetic isotope effects. The following paper provides a detailed overview of SPIDRR and its advantages in the field of gas-phase catalysis research.

Submerged Barriers in the Ni(+) Assisted Decomposition of Propionaldehyde.

The reaction dynamics of the Ni(+) mediated decarbonylation of propionaldehyde was assessed using the single photon initiated decomposition rearrangement reaction (SPIDRR) technique. The exothermic production of Ni(+)CO was temporally monitored and the associated rate constants, k(E), were extracted as a function of activating photon energy. In addition, the reaction potential energy surface was calculated at the UCCSD(T)/def2-TZVP//PBEPBE/cc-pVDZ level of theory to provide an atomistic description of the reaction profile. The decarbonylation of propionaldehyde can be understood as proceeding through parallel competitive reaction pathways that are initiated by Ni(+) insertion into either the C-C or C-H bond of the propionaldehyde carbonyl carbon. Both paths lead to the elimination of neutral ethane and are governed by submerged barriers. The lower energy sequence is a consecutive C-C/C-H addition process with a submerged barrier of 14 350 ± 600 cm(-1). The higher energy sequence is a consecutive C-H/C-C addition process with a submerged barrier of 15 400 ± 600 cm(-1). Both barriers were determined using RRKM calculations fit to the experimentally determined k(E) values. The measured energy difference between the two barriers agrees with the DFT computed difference in rate limiting transition-state energies, 18 413 and 19 495 cm(-1).