Ab Initio Constrained Molecular Dynamics

Density Functional Theory-Based Molecular Dynamics Simulation of Acid-Catalyzed Chemical Reactions in Liquid Trioxane

Journal of the American Chemical Society, 119(31):7218-7229
Curioni, A, Sprik, M, Andreoni, W, Schiffer, H, Hutter, J, and Parrinello, M (1997).

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 12 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 trioxaneformaldehyde 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.

URL: http://dx.doi.org/10.1021/ja970935o