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 reformation of small formaldehyde oligomers is observed, which break up again after 1 to 2 ps. The fast kinetics is found to be consistent with the results of a constrained {\it 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 kJmol**(-1)). Solvation of the chemically active carbocation by formaldehyde molecules reduces the binding energy compared to that in the gas phase by one order of magnitude.
By: Alessandro Curioni, Michiel Sprik, Wanda Andreoni, Heinz Schifter, Jurg Hutter, Michele Parrinello
Published in: American Chemical Society. Journal, volume 119, (no 31), pages 7218-29 in 1997
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