AutoCAD 2014

Exploring Reactivity via Electronic Reaction Coordinates

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Chemical reactions are usually described using approximate reaction coordinates that are expressed as a function of the ionic positions (such as bonds, angles, dihedrals). Whereas for some chemical reactions the definition of partial reaction coordinates can be a reasonable assumption, for many others, that for instance involve ionic collective motions or dynamic solvent effects, a proper reaction coordinate cannot be easily expressed via a trivial combination of ionic degrees of freedom.
Reactivity indexes such as hardness, softness and Fukui Functions may provide interesting insights into the reactivity pathways, especially in their Density Functional Theory formulation [Parr, R. G.; Yang, W. J.A.C.S.1984, 106, 4049-4050]. Recently, [L. Guidoni, U. Rothlisberger, J. Chem. Theo. and Comp., in press 2005 (pdf)], we introduced in the DFT framework reaction coordinates that only explicitly depends on the electronic degrees of freedom of the reactive system. This ‘Electronic Reaction Coordinate’, which is expressed in terms of a penalty function of the one-electron orbital energies, has been applied to study reaction pathways of the butadiene in vacuo.
Using only electronic degrees of freedom, three reactive channels have been identified in s-cis-butadiene: the s-cis/s-trans isomerization, the cis/trans isomerization, and the symmetry allowed cyclization. Interestingly for the latter case, despite the fact that Woodward-Hoffmann rules are guided by the butadiene frontier orbitals, the introduction of an electronic reaction coordinate which involves only these orbitals is not enough to drive the system towards cyclization. A low-lying valence shell orbital (see figure) needs to be included. Thermodynamical quantities like the activation free energy are also calculated along the electronic reaction coordinates in fair agreement with previous reports.
This Orbital Biased Molecular Dynamics has been also implemented within the QM/MM framework to explore different chemical pathways in enzyme catalysis.  
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 Within 1ps of first-principles MD, Electronic Reaction coordinates drive  butadiene
  towards ciclization across a 45 kcal/mol-hight energy barrier
 

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