A new expression for the direct quantum mechanical evaluation of the thermal rate constant

Based on the formalism of Miller, Schwartz, and Tromp [J. Chem. Phys. 79, 4889(1983)], we derive a new expression for the thermal rate constant for a chemical reaction. The expression involves an unperturbed, i.e., reactant or product channel Boltzmann operator for the imaginary time propagation, making it possible to compute efficiently the rate constant for a range of temperatures. We illustrate numerical aspects with an extensive study of the one-dimensional Eckart barrier problem, as well as a study of the three-dimensional (J = 0) D + H2 problem.     

Second-order perturbation theory for the angular pair correlation function in molecular fluids

Second-order perturbation theory is used to calculate spherical harmonic coefficients of the angular pair correlation function g(rω₁ω₂) for a liquid in which the molecules interact with a pair potential that is the sum of Lennard-Jones and quadrupole-quadrupole parts. The theory is compared with both molecular dynamics results and with the predictions of the GMF ≡ LHNC, QHNC and first-order perturbation theories. Second-order perturbation theory gives excellent results for the harmonic coefficient g(224,r), but is poorer for g(222,r) and g(202,r).     

From solution-phase to solid-phase enyne metathesis: crossover in the relative performance of two commonly used ruthenium pre-catalysts

A crossover in the ability of two distinct ruthenium-based metathesis pre-catalysts to effect the synthesis of dialkenylboronic esters in solution and on the solid-phase was observed. Specifically, while the Grubbs 2nd generation pre-catalyst 3 affords a greater degree of conversion to product than the Hoveyda-Grubbs pre-catalyst 2 in a solution-phase enyne-metathesis reaction, this trend is reversed in the solid-phase variant. Systematic investigation showed this trend to be general, regardless of variations in the homoallylic alcohol and alkynylboronic ester components of the reaction, as well as in the type of solid support employed. Experiments to determine a mechanistic hypothesis for this trend highlighted the significance of the ruthenium remaining bound to the substrate after metathetic rearrangement and found the presence of phosphine ligand to be detrimental to the success of the solid-phase reaction. Therefore, these results suggest an expanded role for phosphine-free pre-catalysts, such as 2, in challenging solid-phase metathesis reactions.     

Monte Carlo study of the pair correlation function for a liquid with non-central forces

The pair correlation function g(R), giving the probability that the centers of two nonspherical molecules are a distance R apart irrespective of their orientations, has been studied by the Monte Carlo method. The pair potential model studied is of the type u_o + u_a, where u_o is the isotropic Lennard-Jones (12,6) potential and u_a is either a dipole-dipole, quadrupole-quadrupole, or anisotropic overlap interaction. Dipolar and quadrupolar forces are found to have a small effect on g(R), whereas anisotropic overlap forces have a large effect.     

The Crystal structure of 2-Diisopropylamino-3,7-dehydrotropone

Die erste Röntgenstrukturanalyse eines 3,7-Dehydrotropon ( 1 )-Derivates, nämlich von 2-Diisopropylammo-3,7-dehydrotropon ( 4 ) zeigt das Ringsystem in 4 als planaren Bicyclus mit nahezu C_2V-Symmetrie. Dementsprechend enthalten Dehydrotropone formell zwei trans-konfigurierte Doppelbindungen in einem 7-gliedrigen Ring; auch das O-Atom und das N-Atom mit seinen Ligandatomen liegen in derselben Ebene wie die Ringatome. Die Bindung zwischen den beiden C-Atomen, welche die Heteroatome tragen, (C(1) and C(2)), ist wesentlich langer (1,56 Å) als die anderen Bindungen im Ringsystem (1,37-1,46 Å). Dies impliziert eine ‘push-pull’-π-Elektronen-Delokalisierung, in der auch das O-und das N-Atom involviert sind, und macht eine ‘aromatische’ Ring-Delokalisierung weniger wahr-scheinlich. Im Gegensatz zu 2-(t-Butyl)-3,7-dehydrotropon ( 3 ) existiert das 2-Diiso-propylamino-3,7-dehydrotropon ( 4 ) im Kristall als Monomeres, was dem stabili-sierenden Einfluss der ‘push-pull’-Delokausierung zugeschrieben werden kann.     

Study of deposition offset in plasma spray of zirconia

Numerical modeling and experimental measurements have been performed to study the effects of powder carrier gas flow rates and powder sizes on the deposition offset in a plasma spray of yttria-stablized zirconia. The mathematical model involved simultaneous solution of the continuity, momentum and energy equations of the plasma gas, the dynamics and heat transfer of powder particles in the plasma, and the coupling effects between the plasma and panicles. Experiments included measurement of particle velocities by laser strobe technique and measurement of deposition offset. Calculated plasma temperatures and velocities are greater than 13,000 K and 2,000 m/s, respectively, in the vicinity of nozzle exit. For the plasma-particle momentum transfer, the drag coefficient was computed in two ways- with corrections accounting for the strongly varying plasma properties, and without these corrections. Calculated and experimental results, in respect to deposition offset, are in agreement to within 25% when calculated without varying properties corrections, and within about 40% with corrections; agreement in respect to average particle velocities is within 20% when calculated without varying properties corrections, and within the range 30–50% with corrections.