Microsolvation theoretical studies

Hu and Truhlar have recently reported a modeling transition state solvation at a single-water representation [295]. Recent experimental advances leading to the study of SN2 reactions of gas-phase microsolvated clusters which can advantageously been studied with ab initio electronic theory. These experiments and theoretical studies are quite relevant to chemical reactions in supercritical water. 

I. Dauster, M. A. Suhm, U. Buck, and T. Zeuch, Experimental and theoretical study of the microsolvation of sodium atoms in methanol clusters Differences and similarities to sodium water and sodium ammonia. Phys. Chem. Chem. Phys. 10, 83 95 (2008). 

Other theoretical studies discussed above include investigations of the potential energy profiles of 18 gas-phase identity S 2 reactions of methyl substrates using G2 quantum-chemical calculations," the transition structures, and secondary a-deuterium and solvent KIEs for the S 2 reaction between microsolvated fluoride ion and methyl halides,66 the S 2 reaction between ethylene oxide and guanine,37 the complexes formed between BF3 and MeOH, HOAc, dimethyl ether, diethyl ether, and ethylene oxide,38 the testing of a new nucleophilicity scale,98 the potential energy surfaces for the Sn2 reactions at carbon, silicon, and phosphorus,74 and a natural bond orbital-based CI/MP through-space/bond interaction analysis of the S 2 reaction between allyl bromide and ammonia.17... 

One interesting challenge that reactions in van der Waals and hydrogen-bonded clusters offer is the possibility of studying specifically how weak interactions or microsolvation bonds affect a chemical reaction or dissociation process. In that sense, theoretical studies of weakly bound clusters have proved to be useful in learning about the crossover in behavior from that of an isolated nonsolvated molecule in the gas phase to that for a molecule in a liquid or solid solvent. 

Theoretical studies of the microsolvation effect on SN2 reactions have also been reported by our coworkers and ourselves (Gonzalez-Lafont et al. 1991 Truhlar et al. 1992 Tucker and Truhlar 1990 Zhao et al. 1991b, 1992). Two approaches were used for interfacing electronic structure calculations with variational transitional state theory (VST) and tunneling calculations. We analyzed both the detailed dynamics of microsolvation and also its macroscopic consequences (rate coefficient values and kinetic isotope effects and their temperature... 

The theoretical treatment of cluster kinetics borrows most of its concepts and techniques from studies of smaller and larger systems. Some of the methods used for such smaller and larger systems are more useful than others for application to cluster kinetics and dynamics, however. This chapter is a review of specific approaches that have found fruitful use in theoretical and computational studies of cluster dynamics to date. The review includes some discussion of methodology it also discusses examples of what has been learned from the various approaches, and it compares theory to experiment. A special emphasis is on microsolvated reactions—that is, reactions where one or a few solvent molecules are clustered onto gas-phase reactants and hence typically onto the transition state as well. 

The dimer-based mechanistic pathways in the deprotonation of ketones and imines have been carefully studied by theoretical (MNDO) methods, particularly for LDA and pinacolone, microsolvated by various solvents (Scheme 6)47. The results suggest that, even if no general rule can be drawn, the open dimer mechanism is expected to dominate under all circumstances, and particularly with increasing amide and solvent steric... 

The Diels-Alder reaction is one of the most powerful carbon-carbon bond forming processes in organic synthesis [69]. Considerable experimental work has been carried out to improve the rate as well as the selectivity of Diels-Alder reactions [69]. Theoretical work in understanding this important reaction is relatively small compared to the huge amount of available experimental data (see references in Ref. 17). As a result, the Diels-Alder reaction is well studied, but not completely understood. From our research efforts accumulated over the last few years, we summarize the differences discovered between the computed transition structures of the Diels-Alder reaction in vacuum, microsolvated environments, and fully solvated systems for one of the simplest Diels-Alder reactions, acrolein, and s-cis butadiene, as schematically illustrated in Fig. 4. Molecular origins leading to the rate enhancement and selectivities are discussed, and then are related to the issues surrounding enzymatic catalysis. 

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