Session Y32: Chemical Physics of Extreme Environments III

Complexities in Pressure Dependent Kinetics Across a Wide-Range of Temperatures and Pressures

Sample ab initio transition state theory based master equation calculations will be used to illustrate interesting features of the kinetics for a variety of reactions of importance in astrochemistry, atmospheric, and combustion chemistry. The calculations will explore the role of long-range interactions, angular momentum conservation, tunneling, radiative emission, roaming processes, torsional motions, and prompt dissociation of incipient molecules. Comparisons with experiment will be presented to illustrate the current accuracy of such calculations.   

Forming a Two-Ring Polycyclic Aromatic Hydrocarbon without a Benzene Intermediate: the Reaction of Propargyl with Acetylene

The reaction of acetylene (HCCH) with a resonance-stabilized free radical is a commonly invoked mechanism for the generation of polycyclic aromatic hydrocarbons (PAH), which are likely precursors of soot particles in combustion. In this work, we examine the sequential addition of acetylene to the propargyl radical (H$_{\mathrm{2}}$CCCH) at temperatures of 800 and 1000 K. Using time-resolved multiplexed photoionization mass spectrometry with tunable ionizing radiation, we identified the isomeric forms of the C$_{\mathrm{5}}$H$_{\mathrm{5}}$ and C$_{\mathrm{7}}$H$_{\mathrm{7}}$ intermediates in this reaction sequence, and confirmed that the final C$_{\mathrm{9}}$H$_{\mathrm{8}}$ product is the two-ring aromatic compound indene. We identified two different resonance-stabilized C$_{\mathrm{5}}$H$_{\mathrm{5}}$ intermediates, with different temperature dependencies. Furthermore, the C$_{\mathrm{7}}$H$_{\mathrm{7}}$ intermediate is the tropyl radical (c-C$_{\mathrm{7}}$H$_{\mathrm{7}})$, not the benzyl radical (C$_{\mathrm{6}}$H$_{\mathrm{5}}$CH$_{\mathrm{2}})$, as is usually assumed in combustion environments. These experimental results are in general agreement with the latest electronic structure / master equation results of da Silva et al. This work shows a pathway for PAH formation that bypasses benzene / benzyl intermediates.   

Threshold collision induced dissociation experiment for azobenzene and its derivatives.

In this study we investigated protonated azobenzene cation and properties of trans 2,2',6,6'-tetrafluoroazobenzene anion using the collision induced dissociation method and the results are compared with the results from ab initio electronic structure calculations. We measured the bond dissociation energies experimentally and found which theoretical quantum chemistry methods yield best results. Several high accuracy multi-level calculations such as CBS-QB3, G3 and G4 had been carried out to obtain reliable thermochemical information for azobenzene and several of its derivatives and their anion or cation. We also performed other experiments such as Raman spectroscopy to study these light sensitive molecules with promising applications such as photo-switching.   

A Compressed Sensing Approach to Select Optimal Atom-Centered Basis Functions for DFT and Beyond

The choice of the basis sets is one of the most important factors in quantum chemical calculations. It is particularly challenging for functionals that treat electron correlations. Commonly used basis sets for advanced exchange-correlation functionals are not sufficiently accurate. This leads to extended basis set, such as the most famous correlation-consistent basis sets by Dunning. However, such basis sets have been so far widely used mainly for light atoms and their molecules, since they are too expensive for transition metals. We have developed a new approach to select basis functions based on compressed sensing (CS), a recently developed signal processing technique. CS provides a simple and efficient framework for basis selection based on $l_{1}$ norm regularization techniques. As introductory example, we select via our CS-based approach Gaussian basis functions (GTO) from a large pool of various GTOs, in order to fit to the reference atomic orbitals. We calculate the total energy for atoms from H to O, and then extend to molecules, e.g., H$_{\mathrm{2}}$, N$_{\mathrm{2}}$, and O$_{\mathrm{2}}$. For H, He, and Li, our total-energy results are within 0.05{\%} compared with STO-6G energies. Starting from Be, CS selected basis set provide significantly better results than STO-6G, even when only 5 GTOs are considered. Our new approach enables us to determine optimal basis sets for heavier atoms and molecules.   

Accurately Predicting Complex Reaction Kinetics from First Principles

Many important systems contain a multitude of reactive chemical species, some of which react on a timescale faster than collisional thermalization, i.e. they never achieve a Boltzmann energy distribution. Usually it is impossible to fully elucidate the processes by experiments alone. Here we report recent progress toward predicting the time-evolving composition of these systems a priori: how unexpected reactions can be discovered on the computer, how reaction rates are computed from first principles, and how the many individual reactions are efficiently combined into a predictive simulation for the whole system. Some experimental tests of the a priori predictions are also presented.