We present calculations for the Arg-H2O system and predict that the zwitterionic Arg is thermodynamically more stable than the canonical form in the gas phase under the influence of a single water molecule because of the strongly basic guanidine side chain. Canonical conformers of Arg-H2O are found to isomerize to the zwitterionic forms via a small barrier (approximately 6 kcal/mol).

In the catalysis of S(N)2 fluorination reactions, the ionic liquid anion plays a key role as a Lewis base by binding to the counterion Cs(+) and thereby reducing the retarding Coulombic influence of Cs(+) on the nucleophile F(-). The reaction rates also depend critically on the structures of ionic liquid cation, for example, n-butyl imidazolium gives no S(N)2 products, whereas n-butylmethyl imidazolium works well. The origin of the observed phenomenal synergetic effects by the ionic liquid [mim-(t)OH][OMs], in which t-butanol is bonded covalently to the cation [mim], is that the t-butanol moiety binds to the leaving group of the substrate, moderating the retarding interactions between the acidic hydrogen and F(-). This work is a significant step toward designing and engineering solvents for promoting specific chemical reactions.

ABSTRACT:Calculations are presented for the glycine-(H 2 O) 3 Ϫ cluster anion, with glycine in canonical or zwitterionic form. The zwitterionic anions are predicted to be considerably lower in energy than the canonical anions, and the latter forms are found to be prone to isomerization to the zwitterionic anions. Therefore, we predict that the zwitterionic anions would be observed predominantly in the gas phase at low temperature. In contrast, calculated stability of neutral glycine-(H 2 O) 3 clusters indicates that only the canonical forms of the anions would be observed in photoelectron experiments, if anions are produced from preformed neutrals.

A thermal reaction scheme for the rearrangement of trimethylsilylsilylene (Me3Si−S̈i−H) proposed in a previous study [Organometallics 1986, 5, 698] was studied by the MP2 and DFT methods. The reaction pathways were searched by intrinsic reaction coordinate analysis. We report structures and energies of various silicon species. Activation energies for the C−H insertions by the divalent silicon centers in Me2HSiCH2−S̈i−H and MeHSiCH2−S̈i−Me are predicted to be high, 121 and 110 kJ/mol, respectively, in excellent agreement with an experimental value of 121 kJ/mol. However, the energy barrier for the C−H insertion by the divalent silicon center in Me3S−S̈i−H, predicted to be relatively small, 53 kJ/mol, is lower than that for the 1,2-Me shift (78 kJ/mol) in Me3Si−S̈i−H.