DENSITY-FUNCTIONAL THEORY-BASED MOLECULAR-DYNAMICS SIMULATION OF ACID-CATALYZED CHEMICAL-REACTIONS IN LIQUID TRIOXANE

Ab initio molecular dynamics simulation is used to investigate the kin etics and thermodynamics of some of the chemical reactions that occur during the induction phase of acid-catalyzed polymerization of 1,3,5-t rioxane, In particular, the first ab initio calculation of a free-ener gy profile in a condensed-phase system is presented. The introduction of an Hf ion to a sample of trioxane liquid initiates the complete pro tolysis of several trioxane molecules in a rapid succession of picosec onds. Subsequently, the re-formation of small formaldehyde oligomers i s observed, which break up again after 1-2 ps. The fast kinetics is fo und to be consistent with the results of a constrained ab initio molec ular dynamics evaluation of the free-energy profile for the formation of a protonated dimer. Hn the trioxane-formaldehyde mixture, this reac tion is found to be barrierless with a reaction free energy in the the rmal range (IO kJ mol(-1)), Solvation of the chemically active carboca tion by formaldehyde molecules reduces the binding energy compared to that in the gas phase by 1 order of magnitude.