Computational strategies for the accurate thermochemistry and kinetics of gas-phase reactions

This PhD thesis focuses on the theoretical and computational modeling of gas phase chemical reactions, with a particular emphasis on astrophysical and atmospherical ones. The ability to accurately determine the rate coefficients of key elementary reactions is deeply connected to the accurate determination of geometrical parameters, vibrational frequencies and, even more importantly, electronic energies and zeropoint energy corrections of reactants, transition states, intermediates and products involved in the chemical reaction, together with a suitable choice of the statistical approach for the rate computation (i.e. the proper transition state theory model). The main factor limiting the accuracy of this process is the computational time requested to reach meaningful results (i.e. reaching subchemical accuracy below 1 kJ mol−1), which increases dramatically with the the size of the system under investigation. For small-sized systems, several nonempirical procedures have been developed and presented in the literature. However, for larger systems the well-known model chemistries are far from being parameter-free since they include some empirical parameters and employ geometries which are not fully reliable for transition states and noncovalent complexes possibly ruling the entrance channels. Based on these premises, this dissertation has been focused on the development of new “cheap” composite schemes, entirely based on the frozen core coupled cluster ansatz including single, double, and (perturbative) triple excitation calculations in conjunction with a triple-zeta quality basis set, including the contributions due to the extrapolation to the complete basis set limit and core-valence effects using second-order Møller- Plesset perturbation theory. For the first time the “cheap” scheme has been extended to explicitly-correlated methods, which have an improved performance with respect to their conventional counterparts. Benchmarks with different sets of state of the art energy barriers, interaction energies and geometrical parameters spanning a wide range of values show that, in the absence of strong multireference contributions, the proposed models outperforms the most well-known model chemistries, reaching a subchemical accuracy without any empirical parameter and with affordable computer times. Besides the composite schemes development efforts, a robust protocol for disclosing the thermochemistry and kinetics of reactions of atmospheric and astrophysical interest, rooted in the so-called ab initio-transition-state-theory-based master equation approach have been thoroughly investigated and validated

Quantal calculations on the rotational excitation of NH(_3) and OH in collisions with H(_2)

Results are presented for quantal close coupled calculations of the rotational excitation of NH(_3) and OH in collisions with both ortho and para-H(_2). For the latter, these are the first calculations to include the rotational structure of the H(_2) molecule, whilst for the former, previous NH(_3) - ortho-H(_2) calculations have been subject to subsidiary approximations. The results from the NH3-H2 calculation show substantial qualitative changes in the cross-sections when ground state ortho-H(_2) (j = 1) replaces ground state para-H(_2) (j = 0) as the collision partner. In particular, cross-sections which were very small for NH(_3) - para-H(_2) collisions can be of a comparable magnitude with the other rotationally inelastic cross-sections for NH(_3) - ortho-H(_2) collisions. The changes in cross-sections are discussed in relation to the collisional pumping scheme for an astrophysical maser in the (jk = 33) inversion lines. From the OH-H(_2) calculations it is found that the propensities towards preferential excitation of a given component of the A doublets are reduced in strength when ortho-H(_2) replaces ground state para-H(_2) as the collision partner, similarly when (j = 2) para-H(_2) replaces ground state para-H(_2) the propensities are weakened. In both cases, the results are discussed in the context of crossed beam measurements at energies of 605cm(^-1)(NH(_3)-H(_2)) and 680cm(^-1) (OH-H(_2)). It is found that discrepancies between the experimental results and theoretical calculations for ground state para-H(_2) collisions can be explained, at least in part, by the neglect of the (j > 0) H(_2) rotational states in the latter

Advances in the theoretical determination of molecular structure with applications to anion photoelectron spectroscopy

This Dissertation is focussed primarily on development of methods aiming at the determination of molecular structures with application to systems with intra and intermolecular hydrogen bonds. I have developed and demonstrated usefulness of Potential Energy Surface Scanning Tool (PESST) by performing a systematic search for the most stable structures of neutral and anionic phenylalanine and tyrosine molecules using electronic structure methods. I have found out that tautomers resulting from the proton transfer from the carboxylic OH to phenyl ring determine the structure of the most stable anions of phenylalanine, but double proton transfer from the carboxylic and hydroxyl groups determine structures of the most stable anions of tyrosine. The most stable conformer of these valence anions remained adiabatically unbound with respect to the canonical neutral in case of phenylalanine but bound in case of tyrosine. Valence anions identified in this report have recently been observed experimentally. Acetoacetic acid (AA), equipped with neighbouring carboxylic and keto groups, is a promising system for studies of intramolecular proton transfer. The results of my computational search for the most stable tautomers and conformers of the neutral and anionic AA were used to interpret anion photoelectron and electron energy-loss spectroscopy measurements. The valence anion was identi ed in photoelectron spectroscopy experiments and the measured electron vertical detachment energy is in good agreement with my computational predictions. My computational results allow rationalizing these experimental findings in terms of the co-existence of various conformers of AA. I considered stability of dimers formed by molecules that can exist in different conformational states. I have developed a protocol that allows the dissection of the total stabilisation energy into one-body conformational and deformational components and the two-body interaction energy term. Interplay between these components determines the overall stability of the dimer. The protocol has been tested on the dimers of oxalic acid. The global minimum stability results from a balancing act between a moderately attractive two-body interaction energy and small repulsive one-body terms. I have analysed zero-point vibrational corrections to the stability of various conformers of oxalic acid and their dimers. I have found that minimum energy structures with the most stabilising sets of hydrogen bonds have the largest zero-point vibrational energy, contrary to a naive anticipation based on red shifts of OH stretching modes involved in hydrogen bonds. My computational results demonstrated an unusual electrophilicity of oxalic acid (OA), the simplest dicarboxylic acid. The electrophilicity results primarily from the bonding carbon-carbon interaction in the SOMO orbital of the anion, but it is further enhanced by intramolecular hydrogen bonds. The well-resolved structure in the photoelectron spectrum has been reproduced theoretically, based on Franck-Condon factors for the vibronic anion!neutral transitions. The excess electron binding energies in the dimer and trimer of OA become very signi cant due to intermolecular proton transfer, with the corresponding vertical detachment energy (VDE) values of approximately 3.3 and 4.6 eV. I have postulated a mechanism of excess electron mobility along molecular linear chains supported by cyclic hydrogen bonds. Searches for the most stable molecular conformer are frustrated by energy barriers separating minima on the potential energy surface (PES). I have suggested that the barriers might be suppressed by subtracting selected force field terms from the original PES. The resulting deformed PES can be used in standard molecular dynamics (MD) or Monte Carlo simulations. The MD trajectories on the original and deformed PESs of ethanolamine differ markedly. The former gets stuck in a local minimum basin while the latter moves quickly to the global minimum basin.(US) National Science Foundation grant CHE-111169

Computed potential energy surfaces for chemical reactions

The work on the NH + NO system which was described in the last progress report was written up and a draft of the manuscript is included in the appendix. The appendix also contains a draft of a manuscript on an Ar + H + H surface. New work which was completed in the last six months includes the following: (1) calculations on the (1)CH2 + H2O, H2 + HCOH, and H2 + H2CO product channels in the CH3 + OH reaction; (2) calculations for the NH2 + O reaction; (3) calculations for the CH3 + O2 reaction; and (4) calculations for CH3O and the two decomposition channels--CH2OH and H + H2CO. Detailed descriptions of this work will be given in manuscripts; however, brief descriptions of the CH3 + OH and CH3 + O2 projects are given

Spectroscopic Analysis of Potential Astromolecules Via Quantum Chemical Quartic Force Fields

Astrochemistry has been substantially aided by computational techniques, particularly through the use of Quartic Force Field (QFF) analysis. Several methods have proven useful at correlating computed spectroscopic data with experimental observations. The F12-TZ QFF correlated well with experimental data for silicon oxide compounds, particularly those potentially involved in development from rocky bodies to planetary masses [27]. Compared to argon matrix experimental data, the vibrational frequencies for the molecules SiO2, SiO3, Si2O3, and Si2O4 become less accurate as the complexity of the molecules increases but should still be predictive of infrared characteristics of silicon oxides as they form clusters in space [27]. The CcCR QFF was found to be accurate for predicting B0 and C0 rotational constants within 35 MHz and vibrational frequencies within 5.7 cm-1 for many molecules, including those with heavy atoms [21]. When used in conjunction, the F12-TZ and CcCR QFFs produced parallel data for predicting the brightest vibrational frequencies in relatively complex molecules containing noble gases; rotational constants produced by the CcCR QFFs also present evidence for future identification of such molecules

Doctor of Philosophy

dissertationGuided ion beam tandem mass spectrometry (GIBMS) is used to probe the kinetic energy dependences of protonated hydrazine colliding with Xe, proton-bound hydrazine and unsymmetrical 1,1-Dimethylhydrazine (UDMH) clusters and protonated hydrazine and UDMH clustered with water colliding with Ar. The resulting cross sections are analyzed using a statistical model after accounting for internal and kinetic energy distributions, multiple collisions, and kinetic shifts to obtain 0 K bond dissociation energies (BDEs) for the threshold collision induced dissociation (TCID). The dominant dissociation pathways for protonated hydrazine (N2H5+) and its perdeuterated variant (N2D5+) were the observed endothermic non-adiabatic homolytic and heterolytic N-N bond cleavages forming NH 3+(2A2'') + NH2(2A1) and NH2+( 1A1) + NH3(1A 1), respectively. For the proton-bound clusters, the primary dissociation pathways for (N2H4)nH+ where n = (2-4) and (UDMH)2H+ consists of a loss of hydrazine or UMDH unit, followed by the sequential loss an additional hydrazine at higher energies for n > 2. As to be expected, a similar trend is observed for the primary dissociation pathways for (N2H4)H +(H2O)n where n = ( 2 and 3) and (UDMH)H+(H2O) where the losses of a water unit are followed by the sequential loss of a water unit for n ≥ 2. A larger GIBMS is used to probe the association reactions below 1 eV, of Fen+ + CO where n = 4-17. All clusters where n ≥ 4 form the FenCO+ association complex; the resulting cross sections are analyzed using a statistical model after accounting for internal and kinetic energy distributions, multiple collisions, and kinetic shifts to obtain 0 K binding energies for CO binding to iron cluster cations. The probability of this reaction increases with cluster size until the absolute cross sections equal the collision limit for n > 10, with those for n = 12 and 14 exceeding the collision limit. For the largest clusters, the binding energies approach that of an extended Fe(111) surface, whereas the prominent higher energy feature correlates to binding energies for dissociatively chemisorbed C and O on an iron surface

Theoretical Studies of OME-synthesis and Ammonia SCR in Zeolite Catalysis

Die Emissionen globaler und lokaler Schadstoffe sind ein wachsendes Problem im Hinblick auf den Klimawandel und die öffentlich Gesundheit. Treibhausgase bestehen zu mehr als 80% aus Kohlendioxid. Die Verwendung von Oxymethylendimethylethern (OMEs) als Kraftstoffe oder Kraftstoffadditive im Transportsektor hat den Vorteil geringerer CO$_2$- und Stickoxidemissionen (NO$_x$) ohne dass der Motor angepasst werden muss. NO$_x$-Emissionen, die zur lokalen Verschmutzung beitragen, können auch durch Abgasnachbehandlung reduziert werden. $\\$ Auf Dichtefunktionaltheorie (DFT) basierende Berechnungen werden oft zur Modellierung von Katalysatoren verwendet. In dieser Arbeit habe ich die Genauigkeit von DFT für säurekatalysierte Reactionen (Methanol zu Olefinen) un Redoxreaktionen (selektive katalytische Reduktion) in Bezug auf übergeordnete Methoden getested. Clustermodelle wurden verwendet, um einen Hohlraum des Zeolithen innerhalb des Chabazitgerüsts zu modellieren, indem 46 tetraedische Atome aus der periodischen Struktur extrahiert wurden. Die mittleren absoluten Fehler der DFT hängen von der verwendeten Funktion ab, die im Vergleich zu DLPNO-CCSD(T)-Berechnungen zwischen 10 und 40 kJ/mol für MTO-Reaktionen und zwischen 20 und 50 kJ/mol für SCR-Reaktionen variieren. $\\$ In dieser Arbeit habe ich die Reaktionsmechanismen für die Synthese von Polyoxymethylenether (POME) und die selektive katalysche Reduktion, die eine Abgasnachbehandlung ist, von NO$_x$-Gasen zu molekularem Stickstoff und Wasser unter Verwendung theoretischer Methoden wie der Dichtefunktionaltheorie, Möller-Plesset-Störungstheorie zweiter Ordnung (MP2) und "domain-based local pair natural orbital coupled cluster with single, double and perturbative triple excitations" (DLPNO-CCSD(T)) untersucht. Die Untersuchungen zeigten, dass für die OME-Synthese unter Verwendung des H-BEA-Zeolithen die Trioxanringöffnung mit einem Übergangszustand von 60 kJ/mol, der ratebestimmende Schritt ist. Es wurde gefunden, dass die OME-Synthese in der homogenen Katalyse während des Initiationsschritts ein ähnliches Gibbs-Profil der freien Energie aufweist, welches der OME-Protonierung entspricht und auf der Acidität des Katalysators basiert. Für die SCR wurden Reaktionsmechanismen untersucht, die auf dem schnellen SCR-Zyklus und dem NO-Aktivierungszyklus basieren, Hierzu habe ich den Cu-SSZ-13-Zeolith untersucht und gefunden, dass der geschwindigkeitsbestimmende Schritt die NO-Oxidation mit Übergangszuständen nahe 300 kJ/mol ist. Strukturen im NO-Aktivierungszyklus haben einen Multireferenzcharakter gezeigt und erfordern wahrscheinlich die Verwendung von CAS-Methoden (Complete Active Space)

Multireference approaches for excited states of molecules

Understanding the properties of electronically excited states is a challenging task that becomes increasingly important for numerous applications in chemistry, molecular physics, molecular biology, and materials science. A substantial impact is exerted by the fascinating progress in time-resolved spectroscopy, which leads to a strongly growing demand for theoretical methods to describe the characteristic features of excited states accurately. Whereas for electronic ground state problems of stable molecules the quantum chemical methodology is now so well developed that informed nonexperts can use it efficiently, the situation is entirely different concerning the investigation of excited states. This review is devoted to a specific class of approaches, usually denoted as multireference (MR) methods, the generality of which is needed for solving many spectroscopic or photodynamical problems. However, the understanding and proper application of these MR methods is often found to be difficult due to their complexity and their computational cost. The purpose of this review is to provide an overview of the most important facts about the different theoretical approaches available and to present by means of a collection of characteristic examples useful information, which can guide the reader in performing their own applications