We have simulated the cluster dissociation reaction Arn→Arn−1+Ar (12≤n≤14) using molecular dynamics (MD) with well defined internal energy and total angular momentum. Reaction rates and kinetic energy release distributions are compared to the predictions of several statistical theories: Rice, Ramsperger, and Kassel (RRK), Engelking, and phase space theory (PST). We employ the Nosé prescription for constant temperature dynamics coupled with the multiple histogram method of Labastie and Whetten to obtain highly accurate vibrational densities of states for the clusters. The absolute densities are determined by the adiabatic switching method of Reinhardt. Incorporation of these accurate anharmonic vibrational densities of states into classical PST allows us to make a direct comparison with the simulation results and eliminates any parameters from the theory. Then PST predictions for the kinetics of evaporation are given for the low energy (long time scale) regime where MD simulations are prohibitively expensive. A critical evaluation of the approximate statistical theories is presented.

We have carried out a molecular dynamics (MD) simulation study of melting of Ar7. By periodically quenching trajectories, we are also able to follow the path of the cluster through configuration space. This procedure yields information about isomerization rates, isomerization dynamics, and the connectivity of the phase space as a function of energy. New criteria for melting and the coexistence of phases in small clusters are compared with the traditional T(E) curves and rms bond fluctuations available from time averages in MD simulations.

We have explored the potential energy surface for reactions of acrolein on a Mo 3 O 9 cluster model of the MoO 3 surface to investigate the thermodynamics and kinetics of hydrogenation and selective hydrodeoxygenation. In the presence of hydrogen, conversions of acrolein to allyl alcohol, 1-propanol, and propene are all thermodynamically favorable, but the selectivity is controlled kinetically to form the least favorable product, allyl alcohol. We propose a mechanism in which coordinatively unsaturated Mo sites (i.e., oxygen vacancies) selectively chemisorb acrolein. On the basis of experimental and theoretical evidence, the active phase of the catalyst is a reduced hydrogen bronze, H x MoO 3-y , and surface hydroxyl sites are occupied when x is in the range 1.1-1.2. Surface hydroxyls are important for both oxygen vacancy formation and as Brønsted acids. The reaction rate is essentially controlled by protonation of the C-1 carbon of chemisorbed acrolein. Additional reaction barriers for proton donation to the C-2 or C-3 sites are similar in magnitude (104-134 kJ/mol), limiting the rates of formation of propene and 1-propanol, respectively. In contrast, the selectivity toward allyl alcohol is determined by a smaller O-H bond formation barrier (33 kJ/mol). The estimated reaction rate is comparable to the rate of oxygen vacancy creation, so that operation in a continuous flow process appears to be feasible. The calculated reaction barrier for C-O scission is 104 kJ/mol; we discuss the advantages of other oxides, particularly WO 3 , that have stronger metal oxygen bonds and stronger Brønsted acidity of surface hydroxyls.

We present resonant two-photon two-color photoionization (R2P2CI) spectra of a series of Aniline-Ar, complexes (n --1 -6). An apparently anomalous blue shifted spectra for An-Ar 3 is explained by a modified spectral shift additivity rule which assigns different shifts to different relative positions of the Ar with respect to aniline. Evidence is presented for the existence of several isomers of clusters with n > 2. It is shown that, by changing the nucleation conditions, it is possible to control the relative populations of the various isomers.