The hydrolysis mechanisms of phosphor-monoester monoanions NPP(-) (p-nitrophenyl phosphate) catalyzed by unsymmetrical bivalent dinuclear complexes are explored using DFT calculations in this report. Four basic catalyst-substrate binding modes are proposed, and two optional compartments for the location of the nucleophile-coordinated metal center are also considered. Five plausible mechanisms are examined in this computational study. Mechanisms 1, 2, and 3 employ an unsymmetrical dizinc complex. All three mechanisms are based on concerted SN2 addition-substitution pathways. Mechanism 1, which involves more electronegative oxygen atoms attached to the imine nitrogen atoms in the nucleophile-coordinated compartment, was found to be more competitive compared to the other two mechanisms. Mechanisms 4 and 5 are based on consideration of the substitution of the bivalent metal centers and the intrinsic flexibility of the ligand. Both mechanisms 4 and 5 are based on stepwise SN2-type reactions. Magnesium ions with hard base properties and more available coordination sites were found to be good candidates as a substitute in the M(II) dinuclear phosphatases. The reaction energy barriers for the more distorted complexes are lower than those of the less distorted complexes. The proper intermediate distance and a functional second coordination sphere lead to significant catalytic power in the reactions studied. More importantly, the mechanistic differences between the concerted and the stepwise pathways suggest that a better nucleophile with more available coordination sites (from either the metal centers or a functional second coordination sphere) favors concerted mechanisms for the reactions of interest. The results reported in the paper are consistent with and provide a reasonable interpretation for experimental observations in the literature. More importantly, our present results provide some practical suggestions for the selection of the metal centers and how to approach the design of a catalyst.

The solvolysis mechanisms of RNA phosphodiester model 2-(hydroxypropyl)-4-nitrophenyl phosphate (HpPNP) catalyzed by mononuclear zinc(II) complexes are investigated in the paper via a theoretical approach. The general-base-catalyzed (GBC) and specific-base-catalyzed (SBC) mechanisms are thoroughly discussed in the paper, and the calculations indicate a SBC mechanism (also named as the direct nucleophilic attack mechanism) when the cyclization of HpPNP is promoted by the Zn:[12]aneN3 complex ([12]aneN3 = 1,5,9-triazacyclododecane). The ligand effect is considered by involving two different catalysts, and the results show that the increasing size catalyst provides a lower energy barrier and a significant mechanistic preference to the SBC mechanism. The solvent medium effect is also explored, and reduced polarity/dielectric constant solvents, such as light alcohols methanol and ethanol, are more favorable. Ethanol is proven to be a good solvent medium because of its low dielectric constant. The computational results are indicative of concerted pathways. Our theoretical results are consistent with and well interpret the experimental observations and, more importantly, provide practical suggestions on the catalyst design and selection of reaction conditions.

In alkaline flooding, Interfaclal properties of acidic oil/aqueous caustic play an important role. Although lots of experimental results concerning the effect of various parameters upon interfacial tension and surface charge density have been reported, the mechanism is still not well clarified. C. J. Radke, T. F. Yen and D. T. Wasan have already presented their chemical models for interfacial phenomena of this system, but they all ignore the possibility of forming micelles in the aqueous phase. This article deals with an improved equilibrium chemical model of acidic oil/caustic system. Taking the effect of NaOH-NaCI concentration on the micelles formation into consideration, an expression of interfacial activity is proposed on the basis of Davies theory of the ionic monolayer. Using kerosene containing oleic acid/aqueous solution of hydroxide-sodium containing sodium chloride as the model system, the data of interficial tension (7) and electrophoretic mobility (U) with various concentration of sodium hydroxide and sodium chloride in aqueous phase and acid value of oil phase are presented. The critical micelle concentration of sodium oleate under the various ionic strength is determined and the relation between the micelle concentration and ionic strength is employed in the simulation calculation. γmin and U max thus obtained from the model calculations agree well with the experimental results in regard to the varying trend of parameters used. The mechanisms for the presence of γ min are also suggested. This model may be used in practical numerical simulation for alkline flooding with reasonable option of physicochemical parameters.

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