This PhD thesis has been focused on the theoretical and computational modeling of gas phase chemical reactions and specifically on the accurate estimation of reaction rate coefficients. The ability to determine accurately the rate coefficients of key elementary reactions has become in fact gradually more important over the past years. In this field, transition state theory (TST) coupled with ab initio electronic structure methods has been a powerful tool to calculate the rate coefficient of gas phase elementary reactions due to the simplicity and efficiency by which it is characterized. However, TST is characterized by several limitations and this reduces the accuracy and reliability of its predictions. Moreover, also the precision of the electronic structure calculations play a relevant role on the accuracy of the predicted rate coefficient. Taking into account these factors, TST calculations proved to give rate coefficients that in general have an uncertainty factor of 3. In this scenario, the goal of the present work has been to address the major limitations of transition state theory and to develop and implement ad hoc methodologies in order to overcome these issues, with the intent of reducing the uncertainty from a factor of 3 down to less than a factor of 2. Specifically, we concentrated on the solutions of the following topics: spin forbidden reactions, hindered rotors, multi-well and multi-channel potential energy surfaces, tunneling corrections and a microcanonical description of bimolecular reactions. The methods adopted for each one of these issues have been validated against literature data and tested on chemical reactions of interest. The rate coefficients thus predicted were compared when possible with experimental rates. Otherwise, they were introduced in detailed kinetic schemes in order to reproduce experimental concentration profiles of the species of interest evolving in complex systems, such as toluene or cyclopentadiene pyrolysis. The results obtained allowed us to improve our understanding on the kinetic aspects of a number of reactions of scientific interest.
Questa tesi di dottorato è stata focalizzata sulla modellizzazione teorica e computazionale di reazioni chimiche in fase gassosa e in particolare sulla stima accurata di costanti cinetiche. La possibilità di determinare con precisione le costanti cinetiche delle principali reazioni elementari è diventata sempre più importante negli ultimi anni. In questo campo, la teoria dello stato di transizione (TST) accoppiata a metodi ab initio è un potente strumento per calcolare la costante cinetica di reazioni elementari in fase gas grazie alla semplicità e all'efficienza da cui è caratterizzata. Tuttavia, la TST è caratterizzata da diversi limiti e questo riduce la precisione e l’ affidabilità delle sue previsioni. Inoltre, anche la precisione dei calcoli di struttura elettronica gioca un ruolo rilevante sulla accuratezza della costante cinetica. Tenendo conto di questi fattori,risultati ottenuti tramite l’utilizzo della TST hanno dimostrato di dare costanti cinetiche che, in generale, hanno un fattore di incertezza di 3. In questo scenario, l'obiettivo del presente lavoro è stato quello di superare i limiti principali della teoria dello stato di transizione e sviluppare e implementare metodologie create ad hoc per superare questi problemi, con l'intento di ridurre l'incertezza di un fattore 3 fino a meno di un fattore 2. In particolare, ci siamo concentrati sulle soluzioni delle seguenti complicazioni: reazioni spin forbidden, rotori impediti, superfici di energia potenziale multi-well e multi-channel, correzioni di tunneling e una descrizione microcanonica delle reazioni bimolecolari. I metodi adottati per ognuno di questi problemi sono stati convalidati con i dati della letteratura e testati su reazioni chimiche di interesse. Le costanti cinetiche così calcolate sono state confrontate quando possibile, con costanti cinetiche sperimentali. Altrimenti, tali costanti cinetiche sono state introdotte schemi cinetici dettagliati per riprodurre profili di concentrazione sperimentali delle specie di interesse in sistemi complessi, come la pirolisi di toluene o ciclopentadiene. I risultati ottenuti ci hanno permesso di migliorare le nostre conoscenze sugli aspetti cinetici di un certo numero di reazioni di interesse scientifico.
Development of methodologies for the accurate estimation of reaction rate coefficients
POLINO, DANIELA
Abstract
This PhD thesis has been focused on the theoretical and computational modeling of gas phase chemical reactions and specifically on the accurate estimation of reaction rate coefficients. The ability to determine accurately the rate coefficients of key elementary reactions has become in fact gradually more important over the past years. In this field, transition state theory (TST) coupled with ab initio electronic structure methods has been a powerful tool to calculate the rate coefficient of gas phase elementary reactions due to the simplicity and efficiency by which it is characterized. However, TST is characterized by several limitations and this reduces the accuracy and reliability of its predictions. Moreover, also the precision of the electronic structure calculations play a relevant role on the accuracy of the predicted rate coefficient. Taking into account these factors, TST calculations proved to give rate coefficients that in general have an uncertainty factor of 3. In this scenario, the goal of the present work has been to address the major limitations of transition state theory and to develop and implement ad hoc methodologies in order to overcome these issues, with the intent of reducing the uncertainty from a factor of 3 down to less than a factor of 2. Specifically, we concentrated on the solutions of the following topics: spin forbidden reactions, hindered rotors, multi-well and multi-channel potential energy surfaces, tunneling corrections and a microcanonical description of bimolecular reactions. The methods adopted for each one of these issues have been validated against literature data and tested on chemical reactions of interest. The rate coefficients thus predicted were compared when possible with experimental rates. Otherwise, they were introduced in detailed kinetic schemes in order to reproduce experimental concentration profiles of the species of interest evolving in complex systems, such as toluene or cyclopentadiene pyrolysis. The results obtained allowed us to improve our understanding on the kinetic aspects of a number of reactions of scientific interest.File | Dimensione | Formato | |
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https://hdl.handle.net/10589/74722