Classical hybrid Monte-Carlo simulation of the interconversion of hexabromocyclododecane stereoisomers

The interconversion of the six main stereoisomers of the flame retardant hexabromocyclododecane ( HBCD) is investigated by means of statistical thermodynamics using classical force- fields. (boolean AND)- a-, (boolean AND)- b- and (boolean AND)- g- HBCD interconvert by swapping of absolute configurations on the three different ( BrHC- CHBr)- moieties. The approach avoids saddle- point energy computations, but relies on classical thermodynamic simulation and pursues three consecutive steps. First, the application of classical hybrid Monte- Carlo simulations for quantum mechanical processes is justified. Second, the problem of insufficient convergence properties of hybrid Monte- Carlo methods for the generation of low temperature canonical ensembles is solved by an interpolation approach. Third, it is shown how free energy differences among stereoisomers are derived and how they can be used for the computation of interconversion rates. The simulation results confirm the experimentally observed interconversion rates and correctly identify a- HBCD as a thermodynamical sink in the oscillating mixture of stereoisomers.