Solution‐Phase Organic Chemistry

Abstract

Standard quantum chemical computations are performed on a single molecule or complex. This isolated species represents a molecule in the gas phase. While gas-phase chemistry comprises an important chemical subdiscipline, the vast majority of chemical reactions occur in solution. Perhaps most critical is that all of biochemistry takes place in an aqueous environment, and so if computational chemistry is to be relevant for biochemical applications, treatment of the solvent is imperative.Neglecting solvent effects is extremely hazardous. Equilibria and kinetics can be dramatically altered by the nature of the solvent. For example, the rate of nucleophilic substitution reactions spans 20 orders of magnitude in going from the gas phase to polar and nonpolar solvents. 1 -3 A classical example of a dramatic solvent effect on equilibrium is the tautomerism between 1 and 2. In the gas phase, the equilibrium lies far to the left, while in the solution phase, 2 dominates because of its much larger dipole moment. 4 Another classical example is that the trend in gas-phase acidity of aliphatic alcohols is reverse of the well-known trend in the solution phase; in other words, in the solution phase, the relative acidity trend is R 3 COH < R 2 CHOH < RCH 2 OH, but the opposite is true in the gas phase. 5,6 1 2Through the 1980s, computational chemists did neglect solvent effects, because they either hoped it would not matter or had no real way to effectively treat solvation. In terms of the former case, modeling reactions involving nonpolar molecules with nonpolar transition states (TSs) in nonpolar solvents as gas-phase chemistry might be appropriate. This, however, comprises a very small subset of chemistry, and even small changes in charge distribution can manifest themselves in large solvent effects.