| Gas phase experiments explore the intrinsic properties without the complications of solvation. Fourier-transform ion cyclotron resonance (FT-ICR) spectrometry was exploited to study ion equilibrium and ion reactivity in the gas phase. Quantum calculations were used to support the experiments and examine the ion properties that are not readily measurable, such as solvation energies. A combination of experimental and theoretical investigations of a wide range of ion thermodynamics and kinetics is presented, including...; Acidities of diols and nucleophilicities of diol anions. The intramolecular hydrogen bonding in diols and the corresponding anions is constrained by geometry; hydrogen bonds formed are with different strengths. For example, cis-1,3-cyclohexanediol is 11 kcal mol -1 more acidic than its trans isomer, because the cis anion is stabilized by a hydrogen bond. The diols with different hydrogen bonding, together with variously substituted alkoxides, serve as good models of microsolvated nucleophiles. Marcus theory was used to study their intrinsic nucleophilicities. We found that the intrinsic nucleophilicity of alkoxides is not significantly affected by polar group substitution, but hydrogen bonding significantly decreases the intrinsic nucleophilicity. For example, the non hydrogen-bonded diol anions have similar SN2 reactivities compared to fluoro-substituted alkoxides with similar basicities, while the hydrogen-bonded diol anions are much less reactive than those alkoxides with similar basicities. We also found that conjugation and steric effects decrease the intrinsic nucleophilicity by a different amount.; Steric effect and solvation effect on SN2 transition states. Alkyl substitution in SN2 transition states introduces steric hindrance. At the same time, solvation is affected as well. The two effects are inextricably linked. We propose that alkyl substitution has two consequences on solvation: a direct shielding effect with the increasing substitution that lowers the solvation energy, and an indirect ionic solvation effect that arises from the geometric perturbations introduced by the substitution and increases the solvation energy. The two effects mostly compensate each other when the substitution is on the beta-position. On the other hand, the ionic solvation effect appears to be more important in case of alpha-substitution and makes the steric effect apparently smaller in solution. |
