Stepwise solvation

Recent gas-phase studies of proton-transfer reactions with stepwise solvation of the reactants i.e. incremental addition of solvent molecules to form supermolecular clusters) have demonstrated that the acid/base behaviour of isolated solvent molecules can be dramatically different from their performance as bulk liquids. Water, the classical amphiprotic solvent, shall serve as an example. 

Further examples of gas-phase measurements which reveal the influence of stepwise solvation of homo- and heteroconjugated anionic nucleophiles, A (AH)n and A (BH)n, on the kinetics of Sn2 discplacement reactions with halomethanes can be found in referenees [475, 486], In all cases, solvation of the reaetant anion with up to three solvent moleeules leads to a decrease in the rate constants by at least three orders of magnitude. 

Not only reaetion rates but also the kind of products obtained ean be changed in going from the gas phase to solution. For instance, the reaetion of the stepwise solvated hydroxide ion with aeetonitrile follows three distinct pathways bare and singly-solvated (in the gas phase) as well as bulk-solvated hydroxide ions reaet with aeetonitrile to give three different sets of produets as result of displacement, proton-transfer, and hydrolysis reactions [488] ... 

Bohme, D. K. Raksit, A. B. Gas-phase measurements of the influence of stepwise solvation on the kinetics of nucleophilic displacement reactions with chloromethane and bromomethane at room temperature, J. Am. Chem. Soc. 1984,106, 3447-3452. 

Only occasionally have salt molecules been vaporized for use as a reactant toward another species in matrix isolation studies. Devlin (24,25,26) conducted extensive experiments in which salt molecules were vaporized and condensed into argon matrices containing from 1% to 90% H2O or NH3, to study the effects of stepwise solvation of the salt molecule, as a model for solution studies. Margrave (27 ) and Snelson (28) each have used salt molecules as reactants, but most commonly toward another salt molecule to form a mixed salt dimer. The work described below, which was initiated at the University of Virginia and has been continued at the University of Cincinnati, employs alkali halide salt molecules as reactants toward a variety of species, including both Lewis acids and Lewis bases. The initial intent was to react a salt molecule such as NaCl with HCl in an excess of argon to bring... 

It is found that reaction (27) is more favorable for small n, but that the preferential solvation of the proton by methanol decreases as n increases. At n 9 water and methanol solvate equally well and for n > 9 water becomes the better solvent. The changes of energetics in the stepwise solvation of the corresponding negative ions,... 

The effect of stepwise solvation of molecules and ions on their acidity has been the subject of extensive investigation in the last decade [1]. Here we report [2] the observation of proton transfer reaction in isolated ultracold neutral clusters of an acid molecule HA with n base molecules B ... 

Simonin JP, Bernard O, Krebs S, Kunz W (2006) Modelling of the thermodynamic properties of imiic solutions using a stepwise solvation-equilibrium model. Fluid Phase Equil 242 176-188... 

Blom MN, Compagnon I, Polfer NC, von Helden G, Meijer G, Suhai S, Paizs B, Oomens J (2007) Stepwise solvation of an amino acid the appearance of zwitterionic structures. J Phys Chem A 111(31) 7309-7316... 

The preceding section on gramicidin S demonstrates how the combination of buffer-gas cooling and IR-UV double-resonance spectroscopy can simplify the spectrum of a decapeptide and allow its precise structural determination in the gas phase as well as elucidate the key structural changes upon stepwise solvation. As one pushes these techniques to larger biomolecules, the increasing number of stable conformations they can adopt increase the degree of spectral complexity,... 

Experimental and theoretically calculated values for the clustering energetics are not confined to the solvent S=H20, but for most solvents, only the addition of a single S molecule to an ion has been considered. Ammonia, acetonitfile, and methanol are exceptions to this situation and stepwise solvation of ions has been studied for these solvents (Table 2.7). Competition of such a solvent with water around an ion in a cluster is a readily available experimental method as suggested by Nielsen et al. [32]. 

D. K. Bohme and A. B. Raksit, Can. J. Ghent., 63, 3007 (1985). Gas-Phase Measurements of the Influence of Stepwise Solvation on the Kinetics of Sj 2 Reactions of Solvated F with CH3CI and CH3Br and of Solvated Cl with CH3Br. 

Dalleska, N. R, Honma, K., Armentrout, P. B. (1993) Stepwise solvation enthalpies of protonated water clusters collision-induced dissociation as an alternative to equilibrium studies. J. Am. Chem. Soc. 115, 12125. 

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