Molecular Physics of Elementary Processes relevant to Hypersonics: atom-molecule, molecule-molecule and atom-surface processes.

In the present chapter some prototype gas and gas-surface processes occurring within the hypersonic flow layer surrounding spacecrafts at planetary entry are discussed. The discussion is based on microscopic dynamical calculations of the detailed cross sections and rate coefficients performed using classical mechanics treatments for atoms, molecules and surfaces. Such treatment allows the evaluation of the efficiency of thermal processes (both at equilibrium and nonequilibrium distributions) based on state-to-state and state specific calculations properly averaged over the population of the initial states. The dependence of the efficiency of the considered processes on the initial partitioning of energy among the various degrees of freedom is discussed

Machine Learning, Quantum Mechanics, and Chemical Compound Space

We review recent studies dealing with the generation of machine learning models of molecular and solid properties. The models are trained and validated using standard quantum chemistry results obtained for organic molecules and materials selected from chemical space at random

Towards High-Quality Black-Box Chemical Reaction Rates with System-Specific Potential Energy Surfaces

The calculation of highly-accurate reaction rate constants (k(T)) is one of the central topics in theoretical chemical kinetics. Two approaches for doing this are dominant in literature: application of heuristic corrections of the transition state theory (TST) and wave packet propagation with the aim to represent exact quantum mechanical dynamics. While the first approach is easy to handle but suffers from intrinsic approximations and limited accuracy, the second approach enables convergence towards the exact result, but at the expense of a complex handling and massive costs. This limits its application to a small circle of highly-specialized theoreticians. A new method that might be able to bridge the gap between easy application and convergence towards the exact result is the ring polymer molecular dynamics (RPMD) method. It is based on the isomorphism between quantum statistical mechanics and classical statistical mechanics of a fictitious ring polymer. With this, configurational state sums and free energy surfaces can be obtained from probabilistic samplings of the system's accessible phase space with classical MD of ring polymers. Based on these free energy surfaces reaction rate constants can be obtained that converge towards the results of wave packet propagations, if the size of the ring polymer is adequate. In order to conduct RPMD calculations, a sufficiently accurate representation of the thermally accessible potential energy surface (PES) of the system on which the ring polymers are propagated is needed. In principle, this surface could be represented by ad hoc calculations of energies and gradients based on quantum chemical methods like density functional theory (DFT) or second order Moller-Plesset perturbation theory (MP2). However, since many millions of single gradient calculations are needed to converge a free energy surface and the associated k(T) value, this approach is impractical. Instead, analytical representations of PESs that are fitted to DFT or MP2 results are commonly used. The parametrization of these representations is quite demanding, though, thus being a task for experts. The present thesis deals with new methods for the automated parametrization of analytical PES representations of reactive systems and the successive k(T) calculations based on RPMD. These representations are built on a combination of the quantum mechanical derived force field (QMDFF) method by \Grimme and the empirical valence bond method (EVB) by Warshel, being plugged together recently by Hartke and Grimme (EVB-QMDFF). In line with this thesis a crucial improvement of this combination of methods was done, complementing it with newly developed EVB concepts. For practical usage a new program package was developed, which enables the automated generation of an EVB-QMDFF-PES representation and calculations of RPMD-free-energy surfaces, recrossing corrections as well as k(T) values and Arrhenius parameters for comparison with experimental data, based on the preoptimized reaction path of an arbitrary thermal ground state system. The abilities of the new methods and the associated implementation were thoroughly benchmarked in different kinds of applications. These are calculations of k(T) values and Arrhenius parameters of arbitrary systems from a reaction data base and their comparison to literature values, theoretical molecular force experiments with quantitative investigations of force-dependent reactivities for different systems, a thorough study of urethane synthesis being part of our cooperation with Covestro AG and finally a combination of calculated rate constants of several elementary reactions for describing the dynamics of larger systems based on the kinetic Monte Carlo (KMC) method

Tunneling and Zero-Point Energy Effects in Multidimensional Hydrogen Transfer Reactions: From Gas Phase to Adsorption on Metal Surfaces

Hydrogen transfer reactions play a significant role in many technological applications and fundamental processes in nature. Despite appearing to be simple reactions, they constitute complex processes where nuclear quantum effects (NQE) such as zero-point energy and nuclear tunneling play a decisive role even at ambient temperature. Moreover, the anharmonic coupling between different degrees of freedom that take place in realistic systems leads to hydrogen dynamics that, in many cases, are hard to interpret and understand. Systematic and quantitative ab initio studies of hydrogen dynamics were performed in systems ranging from gas phase molecules to adsorbates on metallic surfaces using state-of-the-art methodologies based on the path integral formulation of quantum mechanics in combination with the density functional approximation. In order to achieve this task, the construction of a general infrastructure that made the required ring polymer instanton simulations feasible was created, and a new approximation which considerably reduces the computational cost of including NQE on weakly bound systems was proposed and tested in the study of water dissociation at Pt(221) surface. Practical guidelines and limitations were also discussed to help the adoption of such methodologies by the community. The system of choice for most of the studies presented in this thesis was the porphycene molecule, a paradigmatic example of a molecular switch. The are a large number of experimental results in well-controlled environments available in the literature which have demonstrated the importance of NQE and multidimensional coupling for this molecule. Therefore, the porphycene molecule provides the unique possibility to theoretically address these effects and compare the theoretical predictions with experimental results in different environments. A portion of this thesis focuses on the study of porphycene molecule in the gas phase. For this purpose, the intramolecular double hydrogen transfer (DHT) rates and vibrational spectrum were calculated. The theoretical results showed a remarkable agreement with the experiments, and enabled the explanation of the unusual infrared spectra, the elucidation of the dominant DHT mechanism, and the understanding of their temperature dependence. In all the cases, the coupling between low- and high-frequency modes proved to be essential to get qualitatively correct trends. Another portion of this thesis examines molecules adsorbed on surfaces. Studies of porphycene molecules adsorbed on (111) and (110) metal surfaces showed that the stronger the surface-molecule interaction is, the more the molecule buckles upon adsorption, leading to an overall decrease of the DHT rates. The simulations identified different temperature regimes of the DHT mechanism, which was not possible by experimental measurements, and evidenced the importance of surface fluctuations on the DHT rates. In conclusion, this thesis provides a stepping stone towards the understanding of the impact of NQE, anharmonic effects, and multidimensional mode coupling on hydrogen dynamics, and also describes novel computational tools to approach their study by using first-principle calculations

Interactions and collisions of cold molecules: lithium + lithium dimer

There is at present great interest in the properties of ultracold molecules. Molecules are created in traps in excited rovibrational states and any vibrational relaxation results in the trap loss. This thesis provides a theoretical study of interactions and collisions in the spin-polarized lithium -b lithium dimer system at ultralow energies. Potential energy surface of the electronic quartet ground state of lithium trimer is generated ab initio using the CCSD(T) method and represented by an IMLS/Shepard fit. Long-range nonadditive interactions are modelled using a symmetric global form with coefficients taken from a fit to the atom-molecule dispersion coefficients. The surface allows barrierless atom-exchange reactions. It has a global minimum of ≈ 4000 cm(^-1) at equilateral geometries with r(_e) = 3.1 Å. The nonadditive interactions are very strong near equilibrium. They increase the well depth by a factor of 4 and reduce the interatomic distance by ≈ 1 Å. Another surface of À symmetry in C(_s) meets the ground state surface at linear geometries at short range. Part of the seam, near D(_ooh) geometries, is in an energetically accessible region for cold collisions. Inside the seam, the lowest À surface correlates with (^4)II rather than (^4)Σ state. Inelastic and reactive collisions are investigated using a quantum mechanical coupled channel method in hyperspherical coordinates. Bosonic and fermionic systems in the spin-stretched states are considered. The inelastic rate coefficients from the rovibrationally excited states of dimer at ultralow collision energies are large, often above 10-(^-10) cm(^3)s(^-1) The elastic cross sections are ≈ 3 orders of magnitude lower at 1 nK. Atom-molecule mixtures, at the densities found in Bose-Einstein condensates of alkali metal atoms that were recently produced, would last only a fraction of a second. Classical Langevin model describes semi-quantitatively the energy dependence of inelastic cross sections above ≈ 50 mK. No systematic differences between the bosonic and fermionic systems were found. Sensitivity of the results on potential was investigated. Reactions in isotopic mixtures of lithium may be exothermic even from the molecular ground state. The reactive rate coefficients are 1 - 2 orders of magnitude smaller than those in systems involving an initially vibrationally excited dimer

Photoinitiated processes in functionally diverse organic molecules elucidated by theoretical methods

In this thesis reaction mechanisms of organic compounds with applications in different areas, such as kinetic studies, labeling, and battery electrolytes, are investigated with theoretical methods from quantum chemistry, quantum dynamics, and molecular dynamics. The variety of the investigated molecules ranges from polycyclic hydrocarbons, dyes, electrolytes to precursors of reactive species. The work was performed either in close collaboration with experimentalists or based on experimental results and in this way allows an in-depth look at the occurring chemistry. In the first part the concept of adapted reactive coordinates for reduced dimensional quantum dynamics is presented necessary for the studies in the following part. It relies on the Wilson G-matrix method as formulation of the kinetic part of the Hamiltonian and allows to include the relaxation of background coordinates besides the identified main reactive coordinates without optimizations for each grid point. The concept is shown for a photodissociation involving complex structural changes and the G-matrix elements and their influence on the dynamics are discussed. In the second part the photoinitated bond cleavage reaction for diphenylmethyl chloride and diphenylmethyl bromide is studied. Based on the reactive coordinate system presented before, quantum dynamical simulations enlighten the path of the wave packet, which passes through two consecutive conical intersections—a three-state and a two-state one—as decisive elements for the product splitting. In the case of chlorine, the experimental signal is modeled from the simulated data to further prove the mechanism. For the bromine case, additionally non-adiabatic mixed quantum-classical dynamics is used to clarify the role of vibrations during the bond cleavage, which are responsible for small amplitude oscillations of the experimental signals. Throughout this part the “our own n-layered integrated molecular orbital and molecular mechanics” (ONIOM) method is used to reduce the involved computational cost. The third part is dedicated to the photophysics of elongated π systems in organic molecules discussing two examples. The first one is the polycyclic hydrocarbon pyrene, the second one covalently linked constructs of DNA and the dye Cyanine 3. For pyrene, the ultrafast transition from the photo-accessible S2 state to the fluorescent S1 is simulated for the first time using two complementary dynamical methods. It is shown that both methods yield comparable results and demonstrate the strong coupling between the two states. The constructs in the second example are investigated experimentally and theoretically. Simulated spectra for a model system help attributing an occurring blue-shift to dimerization. Circular dichroism measurements and molecular dynamics simulations further characterize the formed dimers. The last part comprises a joint experimental and theoretical study concerning the chemical stability of two electrolytes commonly used in lithium-ion batteries towards singlet oxygen. It is shown that singlet oxygen is reactive towards the electrolyte ethylene carbonate. Ab initio calculations suggest a concerted double hydrogen abstraction by the singlet oxygen as mechanism, which is not possible for the second electrolyte dimethyl carbonate. It is an example for the unusual reaction of an alkyl group with singlet oxygen and yields hydrogen peroxide. Ground state mixed quantum-classical dynamics verify the further decay of the reaction intermediate vinylene carbonate to carbon dioxide, which is found experimentally. The theoretically predicted intermediate formation of hydrogen peroxide is detected colorimetrically proving the reaction mechanism and its detrimental effect is investigated experimentally

Making and Breaking of Chemical Bonds: Dynamics of elementary reactions from gas phase to condensed phase

The present thesis is concerned with the dynamics of elementary chemical reactions. In particular, the processes of bond formation (association) and of bond cleavage (dissociation) are studied. Both photo-induced and solvent-induced reaction mechanisms are elucidated. By embedding simple diatomic model systems in rare gas clusters and matrices, the transition of the dynamics of making and breaking of chemical bonds from the gas phase to the condensed phase is systematically investigated