Computational exploration of α-lactone rearrangements and the cyclic halonium zwitterion from bromination of acrylate anion in water: implicit vs. explicit solvation

In memory of Jonathan Williams, a valued colleague of incisive intellect (and regarded by undergraduates as ‘the better-looking Prof. Williams’ in the Department). Bromomethyloxiranone has a much larger repertoire of molecular acrobatics than previously recognised: conformational isomerism, degenerate rearrangement that exchanges O atoms in the α-lactone ring, and epimerisation, all of which occur with lower barriers than dyotropic rearrangement to the more stable β-lactone. DFT calculations (B3LYP/6-31 + G∗) with implicit solvation (PCM) by water predict the cyclic bromonium zwitterion (formally derived from addition of Br ⁺ to acrylate anion) to be a transition structure but QM/MM simulations, combining the same DFT method with explicit solvation by many MM water molecules and using molecular dynamics to obtain free-energy profiles and surfaces, reconfirms the status of the cyclic bromonium as an intermediate.