Research Group Simulation of Chemical Reactions

Theoretical description of photoreactions is particularly challenging: since calculating electronically excited states is usually more demanding than calculating the ground states and since the reaction coordinates are often unknown.

Using the ROKS method (Restricted Open-shell Kohn-Sham), we perform molecular dynamics in the first excited singlet state.[7, 8, 9] There, the atomic nuclei move (according to the laws of classical mechanics) in an electronic potential which is calculated using quantum chemical methods. With this approach we investigated for example photoreactions of small, organic molecules (butadiene, cyclohexadiene), as well as the photoreaction of azobenzene, a molecular rotor and of rhodopsin (the latter in combination with QM/MM).

Movie 1 shows the photoreaction of oxirane: immediately after excitation the C-O bond breaks in agreement with experimental results, subsequent hydrogen migration leads to an aldehyde. A large amount of the released energy is transfered to the vibrational degrees of freedom.

Photochemical excitation of diphenyloxirane leads to cleavage of the C-C bond in a complicated disrotatory motion, as shown in movie 2.

An other reaction type induced by electronic excitation is the photodissociation. Movie 3 shows the cleavage of the N-H bond in the pyrrole molecule. The anti-binding orbital of the excited state is converted into a spherical hydrogen orbital.

The reaction is modified in the presence of solvents. The dissociation is prevented in a pyrrole•3H₂O cluster (movie 4). Instead an electron transfer to the water molecules is observed (orange orbital). Movie 5 shows a pyrrole•3NH₃ cluster. In this more basic system the electron transfer is followed by the transfer of the proton.

We investigated the material failure of siloxane materials under mechanical load in cooperation with the Wacker Chemie AG. In contrast to previous models which are based on radical bond rupture, the Car-Parrinello molecular dynamics simulations show an ionic bond rupture. This observation is in accordance with ESR-experiments. Hence the Si-O and C-C bonds exhibit a qualitatively different rupture behaviour while they have comparable bond strenghts. This affects the consecutive reactions: the ionic fragments react quickly with neighbouring chains. Due to this mechanism a crack propagates rapidly.
Movie 1 shows the tearing of a siloxane chain from a silica surface. In movie 2, four siloxane chains under mechanical stress are shown.

Motivated by single molecule AFM experiments, we are investigating chemical reactions induced by pulling a disulfide with first principles simulations. Both in the presence (reduction) and in the absence (simple bond rupture) of the reducing agent dithiothreitol, we investigate the force to break the disulfide bond.
The relevant localized orbitals are depicted in short movies. Movie 3 shows the disulfide in the absence of solvent and reducing agent, movie 4 shows the disulfide in ammonia and in the presence of dithiothreitol.

Hypergolic mixtures are compounds that react spontaneously when brought into contact with one another. This characteristic is deployed e.g. in the propellant for the upper stage engine of the European launch vehicle Ariane 5. The bipropellant that is used in this case consists of monomethylhydrazine (MMH) and nitrogen tetroxide (NTO). Complications in the upper stage procedure emerged during a mission to transport two satellites into space in July 2001. A possible reason for this could be undesired reactions in the bipropellant.
We investigate the reactions of the MMH/NTO mixture on a picosecond timescale with Car-Parrinello molecular dynamics.
We observe the sequence of electron (MMH→NTO) and proton transfers (e.g. MMH radical cation→MMH) over a few picoseconds. The proton transfers are shown in movies 1 and 2 which present a timeframe of two picoseconds. The first electron transfer of this sequence is shown in movie 3.

The electrolysis of water is a prime example of an electrolytic reaction. Water is decomposed into H₂ and O₂ by an electric current between two electrodes. Understanding the reaction mechanism at the atomic level gives insight into the basic concepts of electrochemical reactivity. With Car-Parrinello molecular dynamics simulations the elementary reaction steps of the electrolysis  of bulk water are investigated. We focus on the reactions in pure water without explicit consideration of the electrode surfaces. Our simulations illustrate the single reaction steps starting from the discharged solutions to the formation of the reaction products.

Movie 1 (the evolution of triplet molecular oxygen. Depicted are the spin densities) shows a simulation starting from eight OH^• radicals (four with spin up and four with spin down, as is evident from the spin densities shown). At the end of the movie the aqueous solution contains one triplet oxygen molecule (O₂), two OH^• radicals, and one hydrogen peroxide molecule (H₂O₂) in addition to hydroxide anions and hydronium cations.

Movie 2 ( the evolution of molecular hydrogen. The orbitals shown are the localized (Wannier) orbitals of the excess electrons) shows the formation of one hydrogen molecule (H₂) from two H₃O⁺ cations and two excess electrons.

We have implemented a sinusoidal potential which may change in phase and amplitude. In first applications electron transfer and ion migration are modeled.

Electron transfer in a single molecule junction in an external field.

Movie 1: Motion of the electrons shown as centers of localized orbitals.
Movie 2: Motion of a single orbital.