Mechanism of Alkyne Alkoxycarbonylation at a Pd Catalyst with P,N Hemilabile Ligands: A Density Functional Study

A detailed mechanism for alkyne alkoxycarbonylation mediated by a palladium catalyst has been characterised at the B3PW91‐D3/PCM level of density functional theory (including bulk solvation and dispersion corrections). This transformation, investigated via the methoxycarbonylation of propyne, involves a uniquely dual role for the P,N hemilabile ligand acting co‐catalytically as both an in situ base and proton relay coupled with a Pd⁰ centre, allowing for surmountable barriers (highest ΔG^≠ of 22.9 kcal mol^?1 for alcoholysis). This proton‐shuffle between methanol and coordinated propyne accounts for experimental requirements (high acid concentration) and reproduces observed regioselectivities as a function of ligand structure. A simple ligand modification is proposed, which is predicted to improve catalytic turnover by three orders of magnitude.     

Conservation of mus-ms enzyme motions in the apo- and substrate-mimicked state

Solution NMR spin-relaxation experiments were used to compare mus-ms dynamics in RNase A in the apo form and as complexed to the substrate-mimic, pTppAp. The crystal structure of the RNase A/pTppAp complex was determined and demonstrates that this ligand binds at the active site and utilizes established substrate binding sites in its interaction with RNase A. Relaxation-compensated CPMG experiments identify flexible residues in and around the active site in both the apo and pTppAp-bound enzyme. Quantitative analysis of the NMR spin-relaxation dispersion curves show that the time scale of motion in RNase A is unchanged when pTppAp binds and is similar to the time scale for the rate-determining step of the catalytic reaction. Temperature-dependent measurements provide an activation barrier for motion of 5.2 +/- 1.0 kcal/mol and 4.5 +/- 1.2 kcal/mol for the apo and pTppAp forms of RNase A, respectively. These data indicate very similar motion exists in the free and bound enzyme. Additionally, chemical shift data suggests that the magnitude of motion is also similar for these two forms and that it is likely that apo enzyme interconverts to a structure that resembles a ligand-bound form. Likewise, it appears that the bound conformation samples the apo enzyme form even when ligand is present. Taken together the data imply that RNase A is in a preexisting dynamic equilibrium between two conformations that represent the open and closed enzyme forms. These data suggest that ligand binding stabilizes the bound conformer but does not induce it.     

Comment on ‘Nanoscale water capillary bridges under deeply negative pressure’ [Chem. Phys. Lett. 451 (2008) 88]

Yang et al. reported pull-off force measurements between an atomic force microscope tip and a silicon wafer. They deduced the pressure of liquid water inside the capillary bridge formed in humid air. They claimed that their ‘research shows that nanoscale water capillary bridges are metastable and have absolutely negative pressure approaching the limit of stability for water’ (around −200 MPa at room temperature). Indeed, pressures reaching −160 MPa were reported, establishing a world record. However, we show that the bridges are not metastable, that the analysis used suffers from internal inconsistency, and that several assumptions made are questionable.     

Edge-Coloring Bipartite Multigraphs in O(E logD) Time

Let V, E, and D denote the cardinality of the vertex set, the cardinality of the edge set, and the maximum degree of a bipartite multigraph G. We show that a minimal edge-coloring of G can be computed in O(E logD time. This result follows from an algorithm for finding a matching in a regular bipartite graph in O(E) time. Received September 23, 1999     

Branch-and-price-and-cut algorithms for solving the reliable <Emphasis Type=Italic>h</Emphasis>-paths problem

We examine a routing problem in which network arcs fail according to independent failure probabilities. The reliable h-path routing problem seeks to find a minimum-cost set of h ≥ 2 arc-independent paths from a common origin to a common destination, such that the probability that at least one path remains operational is sufficiently large. For the formulation in which variables are used to represent the amount of flow on each arc, the reliability constraint induces a nonconvex feasible region, even when the integer variable restrictions are relaxed. Prior arc-based models and algorithms tailored for the case in which h = 2 do not extend well to the general h-path problem. Thus, we propose two alternative integer programming formulations for the h-path problem in which the variables correspond to origin-destination paths. Accordingly, we develop two branch-and-price-and-cut algorithms for solving these new formulations, and provide computational results to demonstrate the efficiency of these algorithms.     

Dynamic Lagrangian dual and reduced RLT constructs for solving 0–1 mixed-integer programs

In this paper, we consider a dynamic Lagrangian dual optimization procedure for solving mixed-integer 0–1 linear programming problems. Similarly to delayed relax-and-cut approaches, the procedure dynamically appends valid inequalities to the linear programming relaxation as induced by the Reformulation-Linearization Technique (RLT). A Lagrangian dual algorithm that is augmented with a primal solution recovery scheme is applied implicitly to a full or partial first-level RLT relaxation, where RLT constraints that are currently being violated by the primal estimate are dynamically generated within the Lagrangian dual problem, thus controlling the size of the dual space while effectively capturing the strength of the RLT-enhanced relaxation. We present a preliminary computational study to demonstrate the efficacy of this approach.     

Isometries of certain operator algebras

To a given basis on an -dimensional Hilbert space , we associate the algebra of all linear operators on having every as an eigenvector. So, is commutative, semisimple, and -dimensional. Given two algebras of this type, and , there is a natural algebraic isomorphism of and . We study the question: When does preserve the operator norm?

     

Supramolecular architectures featuring stereoisomeric cluster complexes of the [Re6(μ3-Se)8]2+ core

The [Re₆(μ₃-Se)₈]²⁺ core-containing cluster complexes of the general formula [Re₆(μ₃-Se)₈(PEt₃)₄L₂]²⁺ (both trans- and cis-isomers) site-differentiated with inert PEt₃ and functional L ligands that are capable of hydrogen bonding or secondary (with respect to primary coordination with the cluster core) metal-ligand coordination interactions have been prepared. The applications of such stereospecific cluster isomers as building blocks for supramolecular construction have been studied. A great variety of multicluster arrays mediated by intercluster hydrogen bonding or cluster ligand coordination with secondary metal ions have been obtained and structurally characterized. The findings from this research clearly establish the superior utility of these novel building blocks for the creation of structurally sophisticated architectures and possibly functional materials.