Microsolvation theoretical calculations

Three papers on microsolvation have been published. The effect of microsolvation by 0,1,2, or 3 water molecules on the identity 5 2 reaction between r and CH3-I has been determined using IR photodissociation spectroscopy and theoretical calculations at the MP2/aug-cc VDZ level of theory. The activation energy changes from -1.0 to 6.0 to 15.8 to 27.0 kJ mol with respect to the reactants when 0, 1, 2, or 3 water molecules, respectively, are present. The results indicate that the reaction is exothermic in the gas phase but endothermic in solution. 

For many years microwave (MW) spectroscopy has been providing invaluable information about the structure of tautomers in the gas phase. However, more recently, MW spectroscopists have been able to identify several tautomers simultaneously four tautomers of guanine [6] two tautomers of 2-hydroxypyridine/2-pyridone microsolvated with one and two water molecules [7] two tautomers, H and ZH, of 4-vinylimidazole [8] and all five tautomers of cytosine [9]. These data are very useful for theoretical chemists, but reciprocally it should be remembered that MW spectroscopists use high-level theoretical calculations to assign their structures. A cautionary note should be added here about the problem of theoretically calculated entropic contributions and the use of the harmonic approximation [10]. Well-known aspects are the relationships between tautomerism and aromaticity due to the work of some pre-eminent authors Katritzky et al. [11], Schleyer et al. [12], and Krygowski et al. [13]. Another well-understood issue is the influence of intramolecular hydrogen-bonds (IMHBs) on tautomerism [14,15]. 

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... 

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... 

---