No abstract

The time dependence of the gaseous unimolecular decomposition of the jet-cooled adduct ion, Ni+-OC(CH3)2, was monitored through selective detection of the Ni+CO fragment ion. Various resolved amounts of energy in the range 15600-18800 cm(-1) were supplied to initiate the dissociation reaction through absorption of laser photons by the title molecular complex. First-order rate constants, k(E), ranged from 113000 to 55000 s(-1) and decreased with decreasing amounts of internal excitation. The energy used to initiate the reaction is well below that required to fragment C-C sigma bonds and indicates the necessity of the Ni+ cation to induce bond activation and fragmentation. These measurements are carried out in a unique apparatus and represent the first direct kinetic study of such catalytic type reactions.

Rate constants for the low-energy Ni(+)-assisted C-C bond cleavage reaction of deuterium-labeled acetone have been acquired under jet-cooled conditions in the gas phase. The energies used to initiate the dissociative reactions of the precursor complex ion Ni(+)(d(6)-Ac) are well below that required to cleave C-C sigma-bonds in isolated organic molecules. The rate constants are compared to those acquired previously for the lighter Ni(+)(h(6)-Ac) isotope and result in a substantial kinetic isotope effect (k(H)/k(D) approximately 5.5). Arguments are made that implicate isomerization leading to C-C bond coupling as the rate-limiting step (not C-C sigma-bond activation) in the dissociative reaction.

Rate constants for the low-energy Ni+-assisted dissociative reaction of acetaldehyde have been measured under jet-cooled conditions in the gas phase. The rate constants are acquired through monitoring the time dependence of fragment Ni+CO formation. The decomposition of the precursor Ni+−acetaldehyde cluster ion proceeds via consecutive, parallel reaction coordinates that originate with the Ni+-assisted cleavage of either a C−C or an aldehyde C−H bond. The energies used to initiate these reactions are well below that required to cleave σ-bonds in the isolated acetaldehyde molecule. Direct measurement of the reaction kinetics over a range of energies indicates that the rate-limiting step in the dissociative mechanism changes at cluster ion internal energies = 17 200 ± 400 cm−1. Arguments are presented that this energy marks the closure of the dissociative coordinate that initiates with C−H σ-bond activation and thus provides a measure of the activation energy of this dissociative pathway.

The unimolecular decomposition kinetics of the jet-cooled Ni(+)-butanone cluster ion has been monitored over a range of internal energies (16000-18800 cm⁻¹). First-order rate constants are acquired for the precursor ion dissociation into three product channels. The temporal growth of each fragment ion is selectively monitored in a custom instrument and yields similar valued rate constants at a common ion internal energy. The decomposition reaction is proposed to proceed along two parallel reaction coordinates. Each dissociative pathway is rate-limited by the initial Ni(+) oxidative addition into either the C-CH₃ or C-C₂H₅ σ-bond in the butanone molecule. Ratios of integrated product ion intensities as well as the measured rate constants are used to determine values for each σ-bond activation rate constant. The lowest energy measurement presented in this study occurs when the binary complex ion possesses an internal energy of 16000 cm⁻¹. Under this condition, the Ni(+) assisted decomposition of the butanone molecule is rate limited by k(act)(C-C₂H₅) = (0.92 ± 0.08) × 10⁵ s⁻¹ and k(act)(C-CH₃) = (0.37 ± 0.03) × 10⁵ s⁻¹. The relative magnitudes of the two rate constants reflect the greater probability for reaction to occur along the C-C₂H₅ σ-bond insertion pathway, consistent with thermodynamic arguments. DFT calculations at the B3LYP/6-311++G(d,p) level of theory suggest the most likely geometries and relative energies of the reactants, intermediates, and products.