The Curtin-Hammett principle in mass spectrometry.
The Curtin-Hammett principle (CHP) is an important concept in physical organic chemistry and is often utilized in the investigation of reaction mechanisms. Two reactants, A and B, in rapid equilibrium, react to form products P(A) and P(B) with rates k(A) and k(B), respectively. If the reaction is under kinetic control and the rate of equilibration between the two reactants is much faster than the reactions to form products, then the branching ratio of products P(A) and P(B) depends solely on the difference in barrier heights for the two product channels. The CHP is based on the fact that the ratio of products formed is not determined by the reactant population ratio. However, the CHP also applies to studies in other areas of chemistry, including mass spectrometry. This Account describes work from our groups in which the results must be interpreted in light of the CHP. These studies illustrate two important implications of the CHP. First, they demonstrate how product distributions cannot be used to assess reactant structure in mechanistic studies in Curtin-Hammett systems. A recent investigation of the structure of hydroxysiliconate anions demonstrated that it was not possible to distinguish between the possible reactant ion structures. A second important implication of the CHP is that the structure of the reactant does not affect the product branching ratio and therefore does not need to be a consideration if the CHP applies. We address this aspect of the discussion through kinetic method studies of the acidities of amino acids and proton affinities of bifunctional compounds. Recently reported mass spectrometric studies illustrate how the CHP puts limitations on what conclusions can be drawn from product distribution studies but also allows experimental methods, such as the kinetic method, to be carried out for complicated systems without having to know all the details of the reactant ion structures. These studies show that although the CHP is most commonly applied in mechanistic studies in physical organic chemistry, it also applies to other areas of chemistry, including mass spectrometry. Although the CHP in some cases limits the conclusions that can be drawn from an experimental study, its proper application can often be used to greatly simplify very complicated chemical systems. Therefore, it is important in mass spectrometry, and indeed, in all areas of chemistry, to recognize those systems in which the CHP should and should not apply.