Crystal nucleation from solutions--transition states, rate determining steps and complexity.

This introductory paper offers a contemporary view of crystal nucleation. We begin with a molecular interpretation of the transition state and then revisit the use of classical nucleation theory as a means of obtaining molecular scale information from kinetic data. Traditional physical organic chemistry has always utilised the combination of kinetics and thermodynamics in order to gain insight over reaction pathways. Here we demonstrate for the cases of sucrose and p-aminobenzoic acid how solution chemistry, crystallography and kinetics come together to provide self-consistent pictures of the molecular scale processes occurring during nucleation. In this and a number of other systems desolvation of specific functional groups is highlighted as the rate determining step. Finally we move on to discuss the question of complexity, both from a phase and molecular perspective.