Density functional theory-based molecular dynamics simulation of acid-catalyzed chemical reactions in liquid trioxane

Abstract

Ab initio molecular dynamics simulation is used to investigate the kinetics and thermodynamics of some of the chemical reactions that occur during the induction phase of acid-catalyzed polymerization of 1,3,5-trioxane. In particular, the first ab initio calculation of-a free-energy profile in a condensed-phase system is presented. The introduction of an H+ ion to a sample of trioxane liquid initiates the complete protolysis of several trioxane molecules in a rapid succession of picoseconds. Subsequently, the re-formation of small formaldehyde oligomers is observed, which break up again after 1-2 ps. The fast kinetics is found to be consistent with the results of a constrained ab initio molecular dynamics evaluation of the free-energy profile for the formation of a protonated dimer. In the trioxane-formaldehyde mixture, this reaction is found to be barrierless with a reaction free energy in the thermal range (10 kJ mol-1). Solvation of the chemically active carbocation by formaldehyde molecules reduces the binding energy compared to that in the gas phase by 1 order of magnitude.