Real-World Predictions from Ab Initio Molecular Dynamics Simulations - Multiscale Molecular Methods in Applied Chemistry - page 143

was thoroughly investigated from MTD. Free energy activations were calculated
for the C-H and C-C reductive elimination but also for the dissociation of one arm
of the diphosphine ligand. Thereby, Michel et al. estimated the free energy cost thus
including entropy effects and the Pt-P distance of the transition state structure. The
authors deduced that, from a mechanistic point of view, the C-C reductive elimi-
nation occurs through a two-step dissociative pathway with barriers of around 19
and 16 kcal mol 1 if the less rigid ligand dppe is used. From kinetic simulations it
was shown that this combination of values provides results comparable to a first-
order kinetics with a barrier of around 40 kcal mol 1 . If the more rigid ligand,
dppbz, was treated, the increase of the dissociation cost prevented the system from
being reactive. For C-H reductive elimination, two mechanisms were found, the
direct one previously postulated and a new one - the concerted mechanism discov-
ered from MTD. In the concomitant mechanism the platinum-phosphorus bond
formation occurred simultaneously with the C-H bond formation. Depending on
the cost of the phosphine dissociation, the direct or the concomitant mechanism was
observed. Thus, the strong influence of the phosphine ligand's basicity as well as
the influence of its intrinsic rigidity was detected. A subsequent study was
undertaken in 2008 [ 198 ].
4.4 Electrochemistry
This section of complex electrochemical reactions in solution and on electrodes is
divided into three parts regarding the following questions. First, how does the
solvent interact with the unbiased and biased metal surface? Second, how does
the oxidation/reduction of a single electrochemical active species work in pure
solvents? And finally, how does a complex electrochemical reaction proceed in
solution and on metal surfaces? Therefore, metal-liquid interfaces are discussed at
the beginning, followed by half cell reactions in solvents, and finally complex redox
reactions in metal-liquid interfaces are reviewed.
4.4.1 Metal-Liquid Interfaces
In 2001, Izvekov and coworkers investigated the Cu(110)-water [ 199 ] and the Ag
(111)-water [ 200 ] interface from AIMD simulations. In both simulations an
absorption of water on the surface and a bilayer structure of water was found, in
which water was tightly bound to the metal surface in the first shell. Exploration of
the interface's electronic structure showed strong coupling of the water molecules
and the metal. However, the metal surface remained almost undisturbed in the
presence of water, both geometrically as well as electronically.
In 2007 and 2008, Sugino et al. [ 201 ] and Otani et al. [ 202 ] investigated biased
platinum-water interfaces. Sugino et al. found that an orientation of the water
molecules emerged due to the negative bias potential of the water-Pt(111) interface