Reakciómechanizmus felderítés ab initio MD módszerekkel = Reaction mechanisms from ab initio MD methods

A 2007-2011 években az OTKA támogatásával a következő témák indultak el: 1) CO2 hidrolízise a) semleges és b) bázikus körülmények között. 2) A Wacker folyamat lépéseinek vizsgálata: a) a katalitikusan aktív [PdCl2(C2H4)(H2O)] komplex képződésének mechanizmusa; b) a hidroxipalladáció mechanizmusa; c) a hidridtranszfer mechanizmusa. 3) Pt(II) hidrátszférájának vizsgálata. 4) A Pechmann reakció mechanizmusának vizsgálata. 5) Intramolekuláris átrendeződések vizsgálata ab initio MD szimulációkkal. 6) Frusztrált komplexek reakciói a) gázfázisú számításokkal; b) oldatfázisú szabadenergia számításokkal. 7) Pirit felszín oxidációjának vizsgálata. A témákban megjelölt kémiai folyamatok reakciómechanizmusát felderítettük, és az egyes elemi lépésekhez tartozó szabadenergiagátakat megadtuk. A legérdekesebb és legközérthetőbb eredménynek azt tartom, hogy megmutattuk, a CO2 vízben nem a tankönyvekben szereplő szénsavat adja első lépésben, hanem egy ionpárrá alakul (HCO3- + H3O+), ami egy következő lépésben alakul át szénsavvá. Tehát így látszódna a szódavíz képződése egy atomi felbontású (vagyis szimulációs) mikroszkóp alatt. | Between 2007-2011 the following topics have been started with the present OTKA support: 1) CO2 hydrolysis a) in water and b) at higher pH. 2) Studies of the Wacker process: a) formation of the catalytically active [PdCl2(C2H4)(H2O)] species; b) the hydroxy-palladation step; c) mechanism of the beta-hydride transfer. 3) ab initio MD study of the hydrated Pt(II) cation in water. 4) Theoretical study on the mechanism of the Pechmann process. 5) ab initio MD studies of intramolecular rearrangements. 6) Properties and reactions of frustrated Lewis pairs in a) gas-phase; b) in solution. 7) Investigation of the oxidation of defective pyrite surface. The main motif of the present projects is to explore the reaction mechanism of the elementary steps constituting the processes and to determine the corresponding activation free energy barriers. In my opinion the most interesting and presumably most popular result obtained within the present OTKA support is what we have found for CO2: its dissolution in water does not follow the typical textbook picture of the H2CO3 formation in a single step. Instead first it forms an ion pair (HCO3- + H3O+), which in a subsequent step gives H2CO3. This is how a very precise microscope (like atomistic simulations) would show the process of preparing sparkling water

SO(2) hydrolysis: ab initio MD study of the formation of bisulfite ion

Ab initio molecular dynamics simulations have been performed to study the dissolution of SO2 in water. It has been obtained that the hydrated SO2 is surrounded by the water molecules without any S-H hydrogen bond, restraining the sulfonate anion formation but allowing the bisulfite isomer formation. The metadynamics method has been employed to explore the free energy surface of the SO2 + H2O reaction. The simulations revealed that the hydrated SO2 forms bisulfite anion and hydronium cation after overcoming a ca. 17 kcal/mol free energy barrier. Direct, one-step H2SO3 formation could not been observed, in sharp contrast with earlier cluster calculations. These findings indicate a step-wise H2SO3 formation in water. The presence of the sulfur lone pair represents an important constraint on the mechanism: the nucleophilic H2O attack can occur only from certain angles as shown by the reactive trajectories

Divided Saddle Theory: a new idea for rate constant calculation

We present a theory of rare events and derive an algorithm to obtain rates from postprocessing the numerical data of a free energy calculation and the corresponding committor analysis. The formalism is based on the division of the saddle region of the free energy profile of the rare event into two adjacent segments called Saddle Domains. The method is built on sampling the dynamics within these regions: auxiliary rate constants are defined for the Saddle Domains and the absolute forward and backward rates are obtained by proper reweighting. We call our approach Divided Saddle Theory (DST). An important advantage of our approach is that it requires only standard computational techniques which are available in most molecular dynamics codes. We demonstrate the potential of DST numerically on two examples: rearrangement of alanine-dipeptide (CH3COAla- NHCH3) conformers and the intramolecular Cope reaction of the uxional barbaralane molecule

Effect of temperature and substitution on cope rearrangement : A symmetry perspective

Many reactions feature symmetry variation along the reaction path on the potential energy surface. The interconversion of the point group symmetry of the stationary points can be characteristic of these processes. Increasing the temperature, however, leads to the loss of symmetry in its traditional yes-no language. We find that in such cases the instantaneous distance of the molecular structure from its symmetric counterpart is a suitable collective variable that can describe the reaction process. We show that this quantity, the continuous symmetry measure (CSM) has a positive linear relationship with temperature, implying that even highly symmetric molecules should be considered as asymmetric above 0K. Using ab initio molecular dynamics we simulate the temperature induced Cope rearrangements of several fluxional molecules and employ different CSM-s to follow the reaction progress. We use this methodology to demonstrate the validity of important concepts governing these reactions: Woodward-Hoffmann rules and TS aromaticity. Statistical analysis of the CSM distributions reveals that ligands connected to the carbon frame have profound effect on the reaction course. In particular our results show that lower temperatures tend to enhance the differences between the TS-stabilizing effect of the substituents

Pyrite in contact with supercritical water: the desolation of steam

The supercritical water and pyrite interface has been studied by DFT calculations. A surprisingly dry surface has been found which points to a new reactivity under extreme conditions which has relevance in the iron–sulfur world prebiotic chemistry of the early Earth.</p

Challenges in modelling homogeneous catalysis : new answers from ab initio molecular dynamics to the controversy over the Wacker process

The controversial reaction mechanism considering experimental results and theoretical treatment from static to ab initio molecular dynamic simulations is reviewed.</p

Efficient direct 2,2,2-trifluoroethylation of indoles via C-H functionalization

A novel highly C3 selective metal free trifluoroethylation of indoles using 2,2,2-trifuoroethyl(mesityl)-iodonium triflate was developed. The methodology enables the introduction of a trifluoroethyl group in a fast and efficient reaction under mild conditions with high functional group tolerance. Beyond the synthetic developments, quantum chemical calculations provide a deeper understanding of the transformation. This journal i