Spectroscopy and reactivity of size-selected Mg+ -ammonia clusters

Photodissociation spectra for mass-selected Mg(+)(NH(3))(n) clusters for n=1 to 7 are reported over the photon energy range from 7000 to 38 500 cm(-1). The singly solvated cluster, which dissociates primarily via a N-H bond cleavage, exhibits a resolved vibrational structure corresponding to two progressions in the intracluster Mg(+)-NH(3) modes. The addition of the second, third, and fourth solvent molecules results in monotonic redshifts that appear to halt near 8500 cm(-1), where a sharp feature in the electronic spectrum is correlated with the formation of a Mg(+)(NH(3))(4) complex with T(d) symmetry and the closing of the first solvation shell. The spectra for the clusters with 5 to 7 solvent molecules strongly resemble that for the tetramer, suggesting that these solvent molecules occupy a second solvation shell. The wavelength-dependent branching-ratio measurements show that increasing the photon energies generally result in the loss of additional solvent molecules but that enhancements for a specific solvent number loss may reveal special stability for the resultant fragments. The majority of the experimental evidence suggests that the decay of these clusters occurs via the internal conversion of the initially excited electronic states to the ground state, followed by dissociation. In the case of the monomer, the selective cleavage of a N-H bond in the solvent suggests that this internal-conversion process may populate regions of the ground-state surface in the vicinity of an insertion complex H-Mg(+)-NH(2), whose existence is predicted by ab initio calculations.     

Voltammetric Reductions of Ring-Substituted Acetophenones. 2. A Senior-Level Experiment Requiring Classification of an Electrochemical Mechanism as Stepwise or Concerted

An experiment suitable for upper-level undergraduates in which they determine the characteristics of a chemically coupled electron transfer (EC) mechanism is described. Students examine the free-energy barrier between an unstable radical anion and the products of its decomposition for ring-substituted and alpha-substituted acetophenones, acetanilides, and alkyl halides. Digital simulation is used to estimate the value of the standard potential, E°. Calculation of the electrochemical transfer coefficient, , allows a determination of whether the electron transfer occurs in a single step (concerted) or follows the generation of the intermediate radical anion (stepwise).