The reaction of triplet atomic oxygen O(3P) with terminal alkenes plays an important role in combustion, atmospheric and interstellar chemistry. The initial triplet adduct may either react on the triplet potential energy surface (PES) or undergo a spin transition to the singlet PES, called “intersystem crossing” (ISC). The reactivity of systems involving ISC is poorly understood, and therefore it is difficult to predict theoretically the fraction of the final products of the reaction, i.e. the “branching ratios” (BRs). However, in the past decades, the coupling of advanced experimental techniques with computational quantum chemistry allowed both a better understanding of ISC and a representative characterization of the overall kinetics of small systems, i.e. O(3P) addition to ethylene, propylene and 1-butene. In the present thesis, an efficient theoretical methodology to study reactions involving ISC was developed. Its strength is that kinetic constants of each reaction pathway are determined completely a priori, starting from electronic structures. First, the approach was validated on the most studied system in this reaction class, O(3P)+C2H4. Ab initio calculations of the structures and energies for the main triplet pathways and ISC were performed. The product branching ratios were then obtained solving a population balance, called “master equation”, where ISC was included using Landau-Zener statistical theory. The results were comparable to benchmark calculations and experimental data, proving the validity of the method. This also legitimates the use of kinetic laws derived ab initio to predict the unknown reactivity of alkenes with higher molecular weight (MW). In this respect, a scaling methodology for the rate laws was developed and applied to derive the product branching of O(3P)+1-butene from the reactivity of propylene, obtaining a good correspondence with theoretical and experimental data. Then, also the BRs of O(3P)+1-pentene were derived, showing how the unknown behaviour of longer chain alkenes can be predicted. This scaling approach may be extended to the singlet product branching, so as to obtain predictions for the full reactivity of higher MW alkenes.
La reazione di addizione di ossigeno tripletto ad alcheni terminali è rilevante nella chimica dei processi di combustione, atmosferici e interstellari. Formatosi l’addotto iniziale, il sistema può reagire sulla superficie di tripletto, oppure passare a quella di singoletto tramite una transizione di spin chiamata “intersystem crossing” (ISC). La reattività di sistemi che coinvolgono ISC non è compresa appieno, ed è dunque difficile predire con metodi teorici le frazioni dei prodotti finali, i.e. “branching ratios” (BRs). Ciononostante, grazie ad avanzate tecniche sperimentali supportate dalla chimica computazionale, il fenomeno dell’ISC risulta oggi meno oscuro, e la cinetica globale di piccoli sistemi quali O(3P) + etilene, propilene e 1-butene è stata riprodotta in modo piuttosto accurato. In questa tesi, è stato sviluppato un efficiente metodo teorico per lo studio di reazioni con ISC, determinando le costanti cinetiche di ogni percorso reattivo completamente a priori (ab initio) a partire dalle strutture elettroniche. Questo approccio è stato validato tramite lo studio dei principali cammini reattivi di tripletto e dell’ISC di O(3P)+C2H4, il più studiato in questa classe di reazioni. Una volta calcolate strutture ed energie ab initio, i BRs sono stati ottenuti da un bilancio di popolazione, detto “master equation”, includendo l’ISC con la teoria di Landau-Zener. La rispondenza dei BRs con dati teorici e sperimentali di riferimento prova la validità del metodo, e legittima l’utilizzo di leggi cinetiche derivate ab initio per predire la reattività di alcheni ad alto peso molecolare (PM), attualmente ignota. A questo proposito, è stata sviluppata una metodologia di “scaling” per leggi cinetiche. Essa è stata validata derivando i BRs dei prodotti di tripletto e dell’ISC di O(3P)+1-butene a partire dalla reattività del propilene, ottenendo un’ottima corrispondenza con dati sperimentali e teorici. Dunque, si sono predetti anche i BRs di O(3P)+1-pentene, finora ignoti. Estendendo questo approccio di scaling ai prodotti della superficie di singoletto sarà quindi possibile predire la reattività globale dell’addizione di O(3P) ad alcheni ad alto PM.
Investigation of the reaction of O(3P) with alkenes : from ab initio kinetic constants to rate rules
PRATALI MAFFEI, LUNA
2017/2018
Abstract
The reaction of triplet atomic oxygen O(3P) with terminal alkenes plays an important role in combustion, atmospheric and interstellar chemistry. The initial triplet adduct may either react on the triplet potential energy surface (PES) or undergo a spin transition to the singlet PES, called “intersystem crossing” (ISC). The reactivity of systems involving ISC is poorly understood, and therefore it is difficult to predict theoretically the fraction of the final products of the reaction, i.e. the “branching ratios” (BRs). However, in the past decades, the coupling of advanced experimental techniques with computational quantum chemistry allowed both a better understanding of ISC and a representative characterization of the overall kinetics of small systems, i.e. O(3P) addition to ethylene, propylene and 1-butene. In the present thesis, an efficient theoretical methodology to study reactions involving ISC was developed. Its strength is that kinetic constants of each reaction pathway are determined completely a priori, starting from electronic structures. First, the approach was validated on the most studied system in this reaction class, O(3P)+C2H4. Ab initio calculations of the structures and energies for the main triplet pathways and ISC were performed. The product branching ratios were then obtained solving a population balance, called “master equation”, where ISC was included using Landau-Zener statistical theory. The results were comparable to benchmark calculations and experimental data, proving the validity of the method. This also legitimates the use of kinetic laws derived ab initio to predict the unknown reactivity of alkenes with higher molecular weight (MW). In this respect, a scaling methodology for the rate laws was developed and applied to derive the product branching of O(3P)+1-butene from the reactivity of propylene, obtaining a good correspondence with theoretical and experimental data. Then, also the BRs of O(3P)+1-pentene were derived, showing how the unknown behaviour of longer chain alkenes can be predicted. This scaling approach may be extended to the singlet product branching, so as to obtain predictions for the full reactivity of higher MW alkenes.File | Dimensione | Formato | |
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https://hdl.handle.net/10589/142730