Coupled large-amplitude motions in oxygenated biofuel candidates

• Gekoppelte anharmonische Bewegungen in sauerstoffhaltigen potentiellen Biokraftstoffen

Kopp, Wassja Alexander; Leonhard, Kai Olaf (Thesis advisor); Pischinger, Stefan (Thesis advisor)

Aachen (2018)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2018

Abstract

Biofuels may maintain human mobility if they are produced sustainably. Modern green production processes tend to use the synthesis power of nature by incorporating larger molecular groups of the biomass into the fuel. Biofuels therefore often contain oxygen atoms which also improves their combustion properties. The oxygen atoms can lead to hydrogen bonds in clusters of biofuel molecules and in transition states with oxygen-containing radicals. Oxygen-containing radicals like OH and HO2 play a key role in ignition and combustion processes. To model thermochemistry and kinetics of the production and combustion process it is therefore crucial to properly describe these hydrogen-bonded clusters and transition states. Hydrogen bonds lower both the energy and entropy of the molecular structure by restricting the structure to a small region of the configuration space of low energy. Widely used molecular models like the rigid-rotor harmonic-oscillator approximation and anharmonic polynomial expansions of higher order base the description of the molecular structure only on this rather small hydrogen-bonded region. Modeling the whole configuration space in turn, thereby modeling the hydrogen-bonded structure according to its share of the configuration space, is computationally very demanding and is so far limited to small molecules. In this work, I apply approximations as the rigid-rotor harmonic oscillator, the anharmonic oscillator, and a superposition of one-dimensional cuts through the configuration space to three molecular biofuel structures: transition states of n-butanol with HO2, transition states of n-butyl formate with HO2, and clusters of methanol. All approximations used herein show very different results and have a large effect on the resulting reaction rate constants and equilibria. I show how modeling of the whole configuration space can be accelerated by the use of specific features of a coordinate transformation from Cartesian to chemically more relevant internal coordinates. The transformation can be separated into steps that are easy to invert, and the elements within the transformation all show a form that allows for analytical integration. I outline how a ladder of approximations can be derived from this exact formulation, including approximations postulated in the literature like the three mentioned above. This allows to rank the approximations and estimate their errors. The approximations allow to treat larger biofuel clusters and transition states.

Institutions

• Chair and Institute of Technical Thermodynamics [412110]